P.H. Vereijken
Feeding and multiplication
of three cereal aphid species
and their effect onyield of winter wheat
Proefschrift
terverkrijgingvandegraadvan
doctorindelandbouwwetenschappen,
opgezagvanderectormagnificus,
dr.H.C.vanderPlas,
hoogleraarindeorganischescheikunde,
inhetopenbaarteverdedigen
opwoensdag14maart1979
desnamiddagstevieruurindeaula
vandeLandbouwhogeschoolteWageningen
Centrum voor Landbauwpublikaties
Wageningen -1979
en LandbouudoaumentaHe
ABSTRACT
Vereijken,P.H. (1979)Feedingandmultiplicationofthreecerealaphid speciesandtheir
effectonyieldofwinterwheat.Agric.Res.Rep.(Versi.landbouwk.Onderz.)888,ISBN
9022006948,(vii)+58p.,13figs,14tables,53refs,Eng.andDutchsummaries.
Also:DoctoralthesisWageningenandMeded.Inst,plziektenk.Onderz.793.
SitobionavenaeF.multipliesatahigherrateonwinterwheatthanRhopalosiphum
padiL.andMetopolophiumdirhodumWlk.UnliketheothertwospeciesS.avenaeprefers
theear,whereitmultipliestwiceasquicklyasontheflagleaf.Infestationoftheear
ratherthanoftheleavesleadstoreductionoftheaveragegrainweight.Theseresults
suggestthatS.avenaeisthemostinjurious.In3field trialswith thisaphid fungal
growthonhoneydewcausedabouthalfofthedamage.Aftercorrectionforthefunguseffect,thelossinyieldcausedbyS.avenaefeedingontheeardidnotdiffermuchfrom
thatcalculatedfromitsenergybudget.Therefore,itisconcluded thatstyletinsertion
andsalivainjectionhavenoimportantinfluenceonthegrowthofgrains.
Fromresultswithlonglivingandhighlyproductivecropsitseemsthathoneydewcan
causeamoreseriouslossinyield ifthecropdoesnotprematurelyripenundertheinfluenceofvariousotheryield-reducingfactors.
Freedescriptors:SitobionavenaeF.,MetopolophiumdirhodumWlk.,RhopalosiphumpaliL.,
TriticumaestivumL.,hostplant,crowding,wingformation,mortality,reproduction,
migration,foodutilization,honeydew,fungi,damage,economicthreshold,warning
system,integratedcontrol.
Frontcover,
alatiformfour
thinstarnymphsof SitoUon avenae F.suckingonawheatear.
cover
Backcover:
artificialinfectionofwheatwithcereal „ „ W J .•
wereusefultoassessthedamagecausedbva tloT
" ^ " ^ ° fn y l o n S a u z e - Thesecages
stagesoftheplantu n d e r fullfieîdconditions! S P 6 C l e S a t t a c k i n S differentgrowth
ThisthesiswillalsobepublishedasAgriculturalResearchReports888
©Centre f orA g r i c u l t u r a l ^
^
^
^
^
^
^
^
^
^
WOORD VOORAF
Hetvooru liggendewerk isnietdoordeauteuralleentotstandgebracht;veleanderenhebbenopéénofanderewijzehunmedewerkingverleend. Ikbenhenhiervoor zeer
erkentelijk enwilmetnamedevolgendepersoneneninstantiesmethunrespectievelijke
bijdragenvernoemen.
Dr.Brader,voormalighoofdvandeafdeling entomologievanhet Instituutvoor
Plantenziektenkundig Onderzoek (IPO)steldemehetonderwerpvoor,gafmede gelegenheid
aanzijnafdelingmethetonderzoek testartenenvoorzagmevaneentijdelijksalaris.
Prof.deWildewasbereid alspromotor optetredenendeeddatopzijnbekendevaderlijkewijze.Zobehoeddehijmevoordemilitairedienst,zochtprof.Brouweraanom
alsco-promotorhetplantenfysiologisch deelvanhet'onderzoektebegeleidenenwistde
stichting ZuiverWetenschappelijk Onderzoek (ZWO)tebewegenhetonderzoekvoor3jaar
tesubsidiëren.
Dedirectievanhet IPOwasbereidmevoordezeperiodeonderdakendebenodigdeonderzoeksfaciliteiten teverschaffen.Devriendschappelijke enidealistische sfeerinde
afdelingentomologievanhet IPOzorgdevooreeninspirerendwerkklimaat.
WimMantel leverde talrijkegegevensenwasvaakbehulpzaambijdeproevenendeuitwerkingvan deresultaten.
DestudentenvandeLandbouwhogeschool KlaasWagenaar,FonsGiesen,ItavanBeusekom,
LiesbethFaber,MarianStapperenAndréBeekmanbewerktendoctoraalonderwerpen dietelkenseenimpulsgavenaandevoortgangvanhetonderzoek.
AdaHoogendijkverleende onsregelmatighulpbijdeanalysevandebladluis-engewasmonsters aan Landbouwscheikunde.
RoyvandeVeerenir.TissevanhetInstituutTNOvoorWiskunde,InformatieverwerkingenStatistiek (IWIS)gavenaanwijzingenbijdeopzetvanproevenenvoerdeneengedeeltevandevariantieanalyses uit.RudieRabbinge enPietRuardijsteldenmeermalenhun
vaardigheidmetdecomputerbeschikbaarvoordestatistischebewerkingvandegegevens.
Dr.Ankersmit,FransDielemanenRudieRabbingevoorzagendeeersteversiesvanhet
proefschriftvanwaardevolle kanttekeningen.
WinoldNoordinkvertaalde daarophetconcept inhetEngels,enwistdaarbijmenige
onduidelijkheid oftegenstrijdigheid aanhet lichttebrengen.Mevr.BrounsvanhecPUDOC
corrigeerde deeerste tweehoofdstukkenophetEngels,SteveWrattennamde laatste2
hoofdstukkenvoorzijnrekening engafbovendien opmerkingenoverdeinhoudvanhettotaaL
DeprofessorendeWilde enBrouwerwistentoennógeenaantal zwakkeplekken intekst
eninhoudaantewijzen,maaruiteindelijkkonLiesLansinkophaarvoortvarendewijzede
definitieve versieuittikken.Defoto'sdiemenhierinaantreft zijnvanAlbertKoedamen
degrafiekenkomenvandevastehandvanFritsScheffel.
Deeindredactievanhetproefschrift tenslotteberusttebijdehr.Aalpol.
CURRICULUM VITAE
DeauteurwerdgeboreninHelmondop31januari1948.AanhetCarolusBorromeus
Collegealdaarbehaaldehijin1966hetdiplomaGymnasiumB. In1972werdaandeKatholiekeUniversiteitteNijmegenmetlofhetdoctoraalexamenbiologieafgelegd (metonderwijsaantekening).Hoofdvakwas zoölogie,waarineenfysiologischeneenecologischonderwerpwerdenbewerkt.Bijvakkenwarenchemischecytologie,enurbaneenruralesociologie.
Van1972t/m 1973washijleraarbiologieaanhetChristelijkAtheneum teArnhem.
In1974starttehijinWageningenalsgastmedewerkervandeLandbouwhogeschoolenhetInstituutvoorPlantenziektenkundigOnderzoeko.l.v.Prof.dr.deWildeenDr.Braderhet
huidigeonderzoek,datvanseptember 1975totoktober 1978isgesubsidieerd doorde
stichtingZuiverWetenschappelijkOnderzoek.
Per1januari1979ishijaangesteldaanhetProefstationvoorAkkerbouwenGroenteteeltindeVollegrondalscoördinatorvanhetvergelijkend onderzoeknaargangbare,geïntegreerdeenbiologisch-dynamische landbouwopdeDrieOrganischeStofbedrijvente
Nagele,gemeenteNoordoostpolder.
CONTENTS
1 General introduction
1
2 Influenae of the host plant on the population development of Sitobion avenae F.,
Metopolophium dirhodum Wlk. and Rhopalosiphwn padi L.
2.1
2.2
2.2.1
2.2.2
2.3
2.3.1
2.3.2
2.4
Introduction
Materialsandmethods
Aphidculture
Chemicalanalysisanddeterminationofhoneydewproduction
Results
The influenceofleafandearassubstrateandofplantage
Theinfluenceofthenitrogendressing
Discussion
3 Population development of Sitobion avenae F., especially
2
2
4
4
5
s
5
8
^
in relation to crowding
3.1
Introduction
3.2
Materialsandmethods
3.3
Results
3.3.1 Fieldobservations
3.3.1.1 Migrationtothecrop
3.3.1.2 Migrationwithinthecrop
3.3.1.3 Influenceofmigrationandtemperatureonpopulationgrowth
3.3.1.4 Analysisofpopulationdevelopmentbasedonageandmorphcomposition
3.3.1.5 Wing formation
3.3.2 Climateroomtrials
3.4
Discussion
15
"
15
16
'"
'°
''
18
19
20
23
4 Injury and damage to wheat, caused by Sitobion avenae F., Rhopalosiphwn padi L.
and Metopolophium dirhodum Wlk.
4.1
Introduction
4.2
Materialandmethods
4.2.1 Laboratorytrials
4.2.1.1 Plantculture
4.2.1.2 Aphidculture
4.2.2 Fieldtrials
4.3
Results
4.3.1 Climateroomtrials
4.3.2 Fieldtrials
2S
26
27
27
28
31
4.4
4.4.1
4.4.2
4.4.3
4.4.4
4.4.5
4.4.6
4.4.7
Discussion
Thethreecomponentsofyield
Ncontentofthewheat
Timeandsiteofinfection
Nitrogendressing
Aphidspecies
Honeydewdamage
Feedingdamage:styletinsertion,salivainjectionandsapuptake
5 Quantitative aspects of attack by Sitobion avenae F. and its effect
their relevance to supervised and integrated control
5.1
Introduction
.
5.2
Trialsandresults
5.2.1 Methodsinobservationandwarning
5.2.2 Theeffectofchemicalcontrolonyield
5.3
Discussion
35
35
35
36
37
37
37
38
on yield;
39
39
40
40
41
42
5.3.1
Methodsinobservationandwarning
42
5.3.2
Economic injuryandeconomicthreshold
44
6 General conclusion
47
Summary
49
Samenvatting
52
References
56
1 GENERAL INTRODUCTION
Thegeneralconcernofgrowers abouttheoccasionally explosive increase ingrain
aphids inpastyears resulted inworld-wide researchprogrammes.Currentresearchstartedin1974attheResearch Institute forPlantProtection inWageningen.Itaimedto
clarify themainproduction-physiological andproduction-ecological aspectsoftheaphid-wheatrelationship. Itwasconsidered thatsuchfundamentalresearchcouldyieldknowledgenecessary todevelopschemes forsupervised and integratedcontrol.Inalaterstage
researchers andstudents oftheAgriculturalUniversity co-operated.Theresearchprogrammewasorganized toanswerthefollowingquestions.What isthesignificance inaphid
populationdevelopmentoftemperature,parasites,predators andpathogens,hostplant
anddensity-dependent effects?What factorsareinvolved inlossofyieldandwhatare
thequantitative aspectsofyield loss?Rabbingeetal. (1979)studiedaphidpopulation
dynamics inrelation totemperature,naturalennemies andpopulationdensityandpreparedpreliminary simulationmodelsforpopulationdevelopmentandlossinyield.Istudiedthe influenceofageoftheplant,nitrogen dressingandfeedingsiteonpopulation
development (Chapter 2). Ideterminedtheinfluenceofthesefactorsandalsothatof
aphidspecies andhoneydewon lossofyield (Chapter 4). With Sitobion avenae (F.),which
appearedtobethemost injurious cerealaphidspeciestheeffectofdensityonpopulationdevelopment (Chapter 3)and loss inyield (Chapter5)was studied.
2
INFLUENCE OF THE HOST PLANT ON THE POPULATION DEVELOPMENT OF SITOBION
AVENAE F . , METOPOLOPHIUM DIRHODUM WLK. AND RHOPALOSIPHUM PADI L .
2.1 INTRODUCTION
Sitobion avenae ( F . ) , Metopolophium dirhodvm (Wlk.) and Rhopalosiphum padi (L.) are
known a l l over the world as pests in cereals (Kolbe, 1969). The 3 species differ much in
their feeding behaviour. S. avenae has a marked preference for the ear and feeds on the
rachis and on the bases of the s p i k e l e t s . M.dirhodvm usually s e t t l e s on the abaxial surface of the leaf and is seldom found on the e a r . R. padi prefers the pseudostem of
plants in the vegetative stage, j u s t above the ground and where the leaf blade changes
into the leaf sheath. When the plant reaches the stage of stem extension, the stem seems
less suitable as e feeding s i t e and R. padi disperses over the whole p l a n t . In Great
Britain (Dean, 1973) and in the Netherlands (Hille Ris Lambers, p e r s . comm.) R. padi i s
less abundant in cereals, compared with the other 2 species, than could be expected from
suction trap observations. In western Europe S. avenae i s generally the most numerous
species, reaching important numbers only after flowering (Dean, 1974a; Kolbe, 1973;
Latteur, 1976; Remaudière et a l . , 1976; p e r s . o b s . ) . Wheat and barley are the main cereals m this region. From the scarce information available, i t seems t h a t barley i s a t tacked less than wheat in the Netherlands.
Based on these considerations, i t was decided to study the performance of the 3 species on winter wheat from the stage of flowering on. In view of the marked differences in
feeding behaviour between the 3 species, the influence of leaf and ear as s u b s t r a t e , the
age of the plant and the nitrogen dressing were taken into consideration. For S. avenae
Z\fl TT' °ft h e l a S t 3 f a C t 0 r S ° n l t S emC±™y
° f f 0 0 d u t i l i z a t i o n i n general
P a r t i C U l a r WaS S t u d i e d
ZL«
T
"
'
- Apart from the increase in biomass, also
hon y ew production and the N, P and Kcontents of honeydew and aphids were determined.
« k n t n T l ° ? P h i d S P e r t i l l e r ° r P S r m m d t h e e f f i c i e ^ ° f fo°d u t i l i z a t i o n
are toown, l t should be possible to estimate the amount of dry matter the crop has l o s t
mak&
ZVc o Z r T
° £ P h l ° e m SaP by tHe apMdS (Rabbi ^ e & Vereijte,, 1979).
GStmated Wlth the aCtUal l0SSeS
I ras
' * Can be e s t e b l i s h e d aether the
by Styl6t
ama a
oÜIT
"JZ
toocompensate
losses
(Chapter 4 ) . * °
^
SaliVa injeCtion
' - ^
P l - t i s able
J
*ï
y*
,*
*
•4
V\
îi
. i
ok' : \ a
»A™. ^ j —
,_{ i
rfj|
a g i ' ifiMWihutff
Fig. 1. Left:Perspexcageusedforinfectionoftheear.See2.2.1.Right:
avenue F.,feedingonanear.
Sitobion
"w^^gl'**g**WIBy',ET*'w;13V-•"i'-H
A
ƒ
itJ
/
~
v
\
1 \ /'/¥•'
iL
\
Fig.2. Perspexcageusedforinfectionof
theleaf.See2.2.1.
2.2 MATERIALSANDMETHODS1
2.2.1 Aphid culture
Theaphidswererearedonearsoronleavesin2mmthickPerspexcagesof
100mmx30mmx25mmand 180mmx25mmx20mm,respectively (Fig.1and2).Theleaf
cageshad2ventilationholesof1cmdiameter,coveredwithnylongauze (0.3mmmesh)on
3sides.Theearcageshad3ventilationholesin2opposite sides.Thebottomandtopof
thecageswerecoveredwiththesamenylongauze.Inthegauzecoveringtheendofthe
leafcages,a2cmgroovewasmadethroughwhichtheleafcouldbepassed.With smallcottonwads,thegroovesweremadeaphidtight.Thegauzeatthebottomoftheearcageswas
onlypartiallyglued.Intheloosepartaholewasmade,bigenoughtocontaintheear
stem.Fromtheedgeofthelooseenduptothishole,thegauzewascut.Viathebottom
theearwasinserted intothecageandtheloozegauzewas fastenedwith adhesivetape.
Thegauzeatthetopofbothtypesofcagewas alsokeptpartially loosetoputtheaphids
ontheencagedplantparts.Theloosegauzewas thengluedtothePerspexwith adhesive
tape.
Afterincubationoftheearstheadhesive tapeatthebottomoftheearcages
wasloosened,thecageswere takencarefullyfromtheearandtheaphidswerebrushed from
theearintoPetridishesforquantification.Withtheleafcagestheadhesive tapeatthe
topofthecagewasloosenedtobrushtheaphids fromtheleafandthecagewiththe
aphidswastakenfromtheleaf.
Thecageshadtheadvantage thatthehoneydewproductioncouldbequantifiedandthat
emigrationofalataecouldbeprevented.Thedisadvantageswere thatapplyingandremoving
thecageswasverytimeconsumingandthattheybecameovercrowdedanddirtyifmore than
2generationswererearedinthem.Thereforethecageswerenotusedintrialstostudy
thepopulationdevelopmentofS. avenae ontheearfor4weeks,i.e.3generations.Unlike
alatae,fewnymphsandapteraeappearedtomigrateinthissituation.Most alatae settled
onthelightceilinganddiedwithinafewdays,ordisappeared throughtheventilation
system.
Theaphidswithwhichthepopulationswere startedcamefromcloneswhichhadbeen
conditionedintheclimateroomincoloniesof10to50individualsfor2ormoregenerations.Inthetrialwithoutcagesapteraewereused.Inthetrialswithcages3rdinstar
nymphswereused,whichlaterappearedtobemostlyalatae.Fourtosixdaysafterrearing
started,theybecameadultsandbeganreproducing.Mortalityoccurred onlyduringthe
nymphal stage.After14daysrearingwasstopped.Thefirst-bornnymphswereatthattime
10daysoldandnotyetabletomultiply.Thenumberandweightofthesurviving adults
andnymphswasestablished.Inthismannerthedifferencesinnymphaldevelopmentrate
andadultreproductionratewerenotestablishedseparately,butcombined.
Thetrialswere setupasasplit-plotscheme.Sotheseparate effectsandmutual
1.DetailsonplantculturearegiveninChapter4.
interactionsofseveralfactorscouldbetestedatthesametimebyanalysisofvariance.
Inviewofthelargevariationintheperformanceofthecolonies,alltreatmentswere
donewithasmanyreplicatesaspossible,consideringtheavailabletimeandmanpower.
2.2.2 Chemical analysis and determination
of honeydew production
AfterthetrialtheaphidcoloniesandthePerspexcagesweredriedfor3daysat
70Candweighed immediatelyafterhavingbeentakenfromtheoven.Thecageswere
cleanedandweighed againafter3daysofdryingat70°C.Thusthehoneydewproduction
couldbecalculatedinmgdrymatterpermgofweightincreaseoftheaphidpopulation.
FromthecoloniesofS. avenae ontheearandontheflagleafabout25mgofhoneydew
wascollectedandanalysedtogetherwiththeaphidsandkernelsofuninfectedearsonN,
PandKcontent (Novozamskyetal., 1975;Schouwenburg&Walinga,1967and1975).
2.3 RESULTS
2.S.1 The influence
of leaf and ear as substrate
and of plant age
Table1showshowfarthenatureofthesubstratei.e.leaforearcombinedwith
plantageinfluencethepopulationofR. padi andS. avenae overaperiodof2weeks.
Ofthe6third-instarnymphswithwhichthecolonieswerestarted,anaverageof
191ofS. avenae and37'.ofR. padi diedbeforetheyreachedtheadultstage.Onthe
youngear,fromfloweringuntilwateryripe (10.5.1 till 10.5.4,Feekes,1941)mortality
of S. avenae washardlyhigherthanontheflagleafbeforeflowering,althoughtheaverTable1. Thesignificanceoffeeding siteandplantgrowth stageforthepopulationdevelopmentof Rhopalosiphum padi (L.)andSitobion avenae (F.)onwinterwheat (cv.Clement).
DataofRandSrepresentthemeanperformanceduring 14daysof54colonies,startedwith
6 thirdstagenymphs.Mortalityintheinitialnumbermainlyoccurredduringtheprereproductiveperiod.Abbreviations;Fe=growth stageoftheplantaccordingtoFeekes (1941).
RS=meanperformanceofbothaphid species.LSD=LeastSignificantDifference(p=0.05),
onlylistedifexceeded accordingtotheanalysisofvariance.
Feedingsiteand
plantgrowthstage
Mean% Meanmultiplicationin14days Mean
mortality r~7Z
individual
number
biomassm
in
b i o m a s s i„
mg/mg
initial
mg
aphids
.
R S RS R S RS R S RS
R
Mean%
of•
winged
morphs
RS
R S RS
flagleaf:Fe10.1-10.5.1 231418 12.611.812.2 14.215.314.80.190.350.27 3081 56
ear:Fe10.5.1-10.5.4
ear:Fe10.5.4-11.1
totalmean
LS
D fortotalmean
forRS
LSD
LS
D
forRandS
4 1 1 9 30 15.016.315.7 17.130.123.6 0.180.360.27 155133
482436
10.0 13.,1 11,.6 9.2 26.,5 17,.9 0.15 0.310.23
3719 .
12.5 13.,8 .
13.5 24,.0 •
0.17 0.34.
2366 .
5
1.3
2.6
6
1.7
2.5
3.3
notanalysed
3
5
4.3
notanalysed
•
10
notanalysed
agecolonyontheearwasmuchlargerthanonthe leaf.Moreover,theavailablefeeding
areaoftheleafwas4timesaslargeasthatoftheear.Mortalityof R. padi was,on
theyoungear,almosttwiceashighasontheflagleafbeforeflowering.Mortalityin
bothspeciesincreasedsomewhatastheplantsgrewolder.
S. avenae increasedinnumbersmorerapidlythan R. padi, especiallyontheolder
earfromwateryripetomilkyripe (Fe10.5.4till 11.1).Consideringthebiomass,
S. avenae multipliedtwiceasquicklyas R. pädi. Themeanindividualweightof S. avenae
after2weeks,wastwiceashighasthatof R. padi. Bothspeciesmultiplied innumbers
andinbiomassmorequicklyontheyoungearthanontheflagleafbefore floweringand
alsomorequicklythanontheolderear. S. avenae multipliedontheoldearataboutthe
samerateasontheyoungearanddevelopedworstontheflagleafbeforeflowering:in
biomassevenlessthanhalfthanontheyoungear. R. padi, however,multipliedonthe
flagleafbeforefloweringjustaswellasontheyoungearanddevelopedworstonthe
oldear:inbiomassalmosthalfasquicklyasontheyoungear.Apartfromthat,there
waslittledifferenceinthereproductionratesbetweenthetwospeciesontheflagleaf
beforeflowering.
Ontheyoungearthepercentagesofnymphswithwingbudsremainedmarkedly lower
thanontheyoungleaf,whereasthereversewasexpectedinviewofthecolonydensities.
S. avenae appearedtobemorequicklyinclinedtoformwings than R . padi.
Table2showstowhatextentthenatureofthesubstrate,leaforear,andtheage
oftheplantinfluencedthepopulationdevelopmentof S. avenae and M. dirhodum. Mortalityof S. avenae ontheearwasagainnotmuchhigherthanontheflagleaf,althoughthe
earcolonieswerealmosttwiceaslargeasthoseontheflagleaf.Mortalityof M. dirhodum
ontheflagleafwastwiceashighasthatof S. avenae. Onanaverage,mortality inboth
speciesduringmilkyripewashigherthanduringtheperiodbetweenflowering andmilky
ripe.
Asadults S. avenae weighed 20*.andasnymphs401moreontheearthanontheflag
leaf Du r i n g^ milkyripestage>^ ^ . ^ Q £^ ^ ^ ^ ^ ^ ^ ^ ^
remained
25*belowthatduringthepreviousperiod.Adultsof M.. dirhodum becamejustasheavyon
theflagleafasadultsof S. avenue ontheear.
Beforemilkyripe S. avenae multipliedmorequicklyontheearthanontheflagleaf,
üviceasquicklyinnumbersandmorethan3timesasquicklyinbiomass.Ontheflag leaf
M. dvrnodun multipliedduringthisperiod twiceasrapidlyinnumbers andinbiomassalmost3tunesasrapidlyas S. avenae.
Duringthemilkyripestage s . avenae multipliedmoreslowlyontheearandsomewhat
morequicklyontheflagleafthanintheprecedingperiod.Consequentlymultiplicationin
biomassontheearprogressedonlyhalfmorequicklythanontheflag leaf. M. dirhodusn
mulUp ie dd u n n g^ ripeontheflagleafmore
than^
Mo_
« s sothatthedifferencefrom S. avenae ontheflag leafdiminished,
P r d U C t i 0 nb y S
onth 7
°
- ™ena° e *P-ssed*•*drymatterpermgaphidbiomasswas
tasef?
" *~* " " * "»** «*'*»faCt* « thebiomassontheear
differelinI T 7 ***** " ^ **^ ^ iS" * » « * *e l a t e dwiththe
L r i H-
°n°feSSentialmtrientSb6tWeen ^
leafandear,assuming
thatthefeedingratesonbothplantpartsareequal.Theageingflagleafwisofbetter
W
6-5 T )
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C 60
to C
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Table3. Thesignificanceofplantgrowth stageandfeedingsiteforS. avenae, R. padi
and M. dirhodwn: Summaryoftables1and2.Dataareexpressedaspercentagesoftheperformanceof S. avenae ontheflag leafbeforethemilky ripestage.
S.
S.
S.
S.
avenae
avenae
avenae
avenae
R.
R.
R.
R.
padi
padi
padi
padi
,, Multiplicationin„
,
...
Nymphal
Honeydew Wing
mortality numbers biomass production formation
onflag leafbeforeFe11.1(I) 100
100
100
100
100
onflag leafduringFe11.1(II)95
115
125
60
onearI
120
175
270
40
80
onearII
170
135
180
55
onflag leafI
onflag leafII
onearI
onearII
M. dirhodwn onflag leafI
M. dirhodwn onflagleafII
165
105
.
290 290 125
345
180
180 195
205
95
.
110
125
85
.
.
110.
60
265
195
155
265
70
160
40
.
.
•20
•
70
45
65
feedingqualityfor S. avenae thantheyoungflagleaf.
M. dirhodwn reactedmoreslowlywithwingformationoncrowding than S. avenae.
Again S. avenae developedmorealataeontheleafthanontheear,eventhoughhigher
densitieswerereachedontheear.
Table3summarizesthemainresultsofTables1and2.
2.3.2 The influence of the nitrogen
dressing
ThepopulationdevelopmentofS. avenae wasstudiedonearsofplants,whichhad
grownon3levelsofnitrogendressing.Thebasicdressingwas30mgofNpertiller(N1),
whichisequivalenttoabout135kgofN/ha.Duringthestageofstemextension,the
plantsreceivedanextra5mgofN(N2)or10mgofN(N3)pertillerbyaddingNH 4 N0 3via
thewatersupply.AfterthecrophadripenedtheaverageyieldpertillerwasforN1,N2
andN3:1.58,1.52and1.51gofdrymatter,withanNcontentof1.45,1.64and1.881
(LSD=0.20°O,respectively.InpracticetheNcontentvariesbetween1.70and2.00?».
As S. avenae feedsonlysomemillimetresinfrontofthedeveloping grains,itwas
assumedthattheingestedphloemsapwouldcontainN,PandKinthesameratioasthey
occurinthegrain.Fig.3showstowhatextentthisvariationinthefood influencedthe
populationdevelopmentinS. avenae, starting fromoneapterous adult.Themortalityof
theadultswasforN1,N2andN3duringthefirstweek:8,4and201 (LSD=10»,with
p=0.05),respectively.Duringthe4thweekmortalitywas 10Î,37».and57°«(LSD=14°*
withp=0.05),respectively.Undoubtedlytheincreaseinmortality duringthefirstweek
wascausedbyachangeinfoodquality,butduringthe4thweekthedifferencesinaverage
populationdensityundertheinfluenceofthevariousNlevelsalsoplayedanimportant
role.Assumingthatthereproductionrateduringthefirstweekwas constant (Wratten,
1977),thatmortalityoftheinitialadultsduringthesecondweekwasnegligible andthat
100-1
«H
'A'"
'm~—
o
10.3
X
7
10.5.1
14
10.5.3
21
10.5.4
28 days
11.1 Plant growth stage
Fig.3. Population developmentof Sitobion
nitrogendressing:Nl (•)N2(*) N3(O).
presentstheaverageof24colonies.
avenae from1adultperearat3levelsof
Adults.
Totalnumber.Eachpointre-
thenymphsafter 10.5daysbecameadults,itappearedthattheapterouspopulationafter
14daysconsistedofsurviving initialadultsandyoungadultsbornduringthefirsthalf
ofthefirstweek.So,theratioofwingdevelopmentinnymphsthatwerebornduringthe
first3.5dayscanbeexpressedas1-(A-,4-A7)/(0.5 L 7 ) ,whereL 7isthenumberofnymphs
onthe7thdayandAisthenumberofapterousadults.
OnplantsdressedwithN1,N2andN3:59°6,45«„and0°softhenymphsbecamealatae.
Theaveragenumberofobservedapteraewasattheendofthe2ndweek 2.7,3.6and5.2
(LSD=1.7withp=0.05),respectivelyandattheendofthe3rdweek3.0,5.0and5.6
(LSD= 1 . 2withp=0.05),respectively.
Nearlyallalatae lefttheearaftertheirlastmoult.Thenumberofalataeremaininginthecolonywas largestattheendofthe3rdweek.Atthattimeanaverageof0.9,
1.2and0.6alataeperearwerepresent.Therepressionofwingformationinthe1stgenerationbytheextranitrogenandconsequentlythedevelopmentofmoreapteraefinally
resultedinanincreaseinthetotalnumberofaphidsinthe2ndgeneration.Attheend
ofthe3rdweek,thetotalnumberofaphidsperearinN1,N2andN3was64,85and104
(LSD=29withp=0.05),respectively.
Table4showstowhatextent leafandearassubstrateandalsothenitrogendressinginfluencedtheproductionofbiomassandhoneydewandalsofoodutilizationin
S. avenae. From5alataepertiller,2.4timesasmuchbiomassconsistingof31.5+1.51
drymatterdevelopedin3weeksontheearasontheleaf.Permgdrymatteraphidproduction2.4timesasmuchdrymatterhoneydewwasproducedontheleafasontheear.
Againthisindicatedthattheincreaseinbiomassofaphidsiscloselycorrelatedwith
theconcentrationofnutrientsintheingestedphloemsapandthatsapfromthe rachis
containstwiceasmuchnutrientvalueasthatfromtheflagleaf.Theefficiencywith
whichtheaphidstakeupNandPcompoundsfromthesapappearedtobesimilaratboth
feedingsites,consideringtheequalamountsofNandPinthehoneydewfromflagleafand
ear.Thenitrogendressingdidnot influence this.TheextranitrogendressingdideffectuateastrongdecreaseinhoneydewproductionviaenhancementoftheNandPconcentrationandpossiblybyadecreaseintheKconcentrationinthephloemsap.Asthecages
Table4. Productionofhoneydewandbiomass (31.5 +_ 1.5%drymass)andfoodutilization
by Sitobion aoenae onwinterwheat (cv.Cyrano),asinfluencedbyfeedingsiteandnitrogendressing.Nl=basicnitrogengiftof30mgN/tiller;N2-3=extranitrogengift
duringstemextensionandheading5-10mgN/tiller.Aphidpopulations,incages,started
with5alatae/tillerafterflowering,weresampledafter3weeks.DataofNl-3represent
theaveragesover5tillers (colonies).Betweenbrackets:standarddeviation
Drymasspertillerinmg
(forhoneydew:drymass
pertiller/mgaphids)
Kernelyieldofuntreated tillers
1415
Nl
1091
N2
1170
N3
190
LSD(p-0.05)
1225
N123
Aphidproductionon
flagleaf
Nl
8.9
N2
10.6
N3
12.9
N123
11.01 (2.9)
ear
Nl
30.6
N2
25.4
N3
23.6
N123
26.51 (9.5)
Honeydewproductionon
flagleaf
Nl
10.2
N2
7.9
N3
8.9
N123
9.12 (2.2)
ear
4.83 (1.2)
Nl
N2
3.6
N3
3.03 (1.0)
N123
3.82 (1.4)
Utilization:m SNrp>Ki°aphids
mgN,P,Kinaphids+honeydew'
flagleaf
Nl
N2
N3
N123
ear
Nl
N2
N3
N123
N
1.51
1.57
1.81
0.20
1.63
%P
0.38
0.43
0.44
0.16
0.41
0.67
0.59
0.54
0.04
0.60
seeanalysisaphidsonear
6.46
6.59
6.72
6.59
0.88
0.89
0.88
0.88
0.99
1.00
1.03
1.01
0.50
0.40
0.30
0.40
0.09
0.11
0.12
0.11
0.96
1.02
1.10
1.03
0.39
0.37
0.34
0.37
0.19
0.16
0.16
0.17
1.11
1.09
0.94
1.05
48
51
44
48
9
11
10
10
x 100%on
56
68
72
65
78
49
83
61
87
65
83
58
1),2)and3)Correspondingdatashowingsignificantdifferenceat5%levelaccording
toStudentst-test.
10
K
16
20
27
21
avoidedmigration,therestrictedwingformationundertheinfluenceoftheextra
nitrogenappliedwasnotexpressed inahigherbiomassproductioncomparedwiththenormaldressing.Theincreaseinmortalityandpossiblyalsothedecreaseinkernelyield
broughtaboutbytheextranitrogendressingevenledtoasomewhatlowerbiomassproductionontheear.FromTable 4itcanbededucedthatfromthe3elementsinthesap,N,
PandK,thefirstwasutilizedmost.Thustheconcentrationoforganicnitrogencompoundslimitsthenutrientvalueofthephloemsap.
2.4 DISCUSSION
Thenumberofreplicates inthevarioustrialswasnotexcessivebecauseevenconsiderabledifferencesbetween2averageswereoftenhardlylargerthantheLSD.
Halfthenumberofnymphsof R. padi thatwereontheeardiedbeforetheyreached
thereproductivestage.Itwassurprisingthatyoungalataedidnotseemtobeableto
settleagainafteraflightperiodofsomedays.Thisseemstoconfirmthesupposition
ofDean(1973) that R. padi, atleastasalata,hasan"aversion"tocerealsandonly
settlesonthiscroponasmallscale.
ContrarytothefindingsofDean (1974b),whorearedhisaphidsonyoungbarley,
the3speciesdidnotmultiplyatthesamerateatthegiventemperature. M. dirhodum
multipliedontheflagleafbyfarthemostquickly,inspiteofitshighnymphalmortality.Still S. avenae isbestadaptedtowheat,becausethisspeciessettles inthefield
mainlyontheear,whereithasalowernymphalmortality,ahigherefficiencyoffood
utilizationandaquickermultiplicationbasedonbiomass,than M. dirhodum ontheleaf
(Table 3).Moreover, S. avenae reactssoonerwithwingformationoncrowdingthanthe
othertwospeciesandisthusbetteradaptedtotherapidsenescenceoftheplantinthe
courseofthemilkyripestage,whenthepopulationsusuallyreachtheirpeak.Astheear
generallyremainsphysiologicallyactive longerthantheleaf, S. avenae canmaintainitselflongestinthecrop (Latteur,1976;pers. obs.).Theseconclusionsareconfirmedby
thegeneralobservationthatinWesternEurope S. avenae is usuallythemostnumerous
aphidinwheat (Dean,1974a;Kolbe,1973;Latteur,1976;Remaudièreetal.,1976;pers.
obs.).
Thefactthat S. avenae hasbecomeaseriouspestinlargepartsoftheworldduring
recentyears,couldhavebeencausedtoalargeextentbytheenhancednitrogendressing.
Anextranitrogenapplicationtotheplant,generallyappliedduringthephaseofstem
extension,appearstostimulatethemultiplicationrateof S. avenae bylimitingwing
formationandconsequentlytheemigrationofalatae.Moreover,senescenceoftheplantis
sloweddownbytheapplicationofnitrogenincombinationwithfungicides,thusgiving
theaphidstheopportunitytoprolongtheirmultiplication,which increasesthechance
ofreachingtheeconomicthreshold.
Mittler (1958)assumedonthebasisofhisresultswith Tuberolachnus
salignus
(Qnelin),thatanincrease inN contentinthephloemsapenhancesthegrowthrateofthe
aphidanddiminishesitsexcretionrate.Hertel&Kunkel (1976)foundthat4thinstar
nymphsof Myzus persiaae
(Sulzer),feedingonanartificialdietcontaininglessthanhalf
theconcentrationofaminoacidsofthestandarddiet (Mittleret al., 1970),excreted
11
22-40%morecarbohydratesandlost35'.ingrowthrate,comparedwiththestandarddiet.
Llewellynetal. (1974)showedwith Tuberolachnus
salignus
thathoneydewproductionhas
anegativecorrelationwithgrowthrate.
ExtranitrogendressingcausedanincreaseinNandPcontentandadecreaseinK
contentinthegrainsandconsequentlyinthephloemsapintherachis.Atthesametime
thehoneydewproductiondecreasedsharplyin S. avenae. However,theincreaseinefficiencyoffoodutilizationwasnotfollowedbyanenhancedbiomassproduction.Thiscould
havebeencausedbyanincreaseinmortalityandadecreaseinplantproductionunderthe
influenceoftheextranitrogendressing,whichenabledtheplanttokeepformingnew
sidetillersuntilafterflowering.Becauseofthesuboptimalradiationintensityinthe
growthroom,thishappenedtothedetrimentofthekernelcontentintheexistingtillers.
Inprevioustrials,wheretheplantproductionremainedthesameorevenincreaseda
littleundertheinfluenceofanextranitrogendressing,fastergrowthoftheaphids
againcouldnotbeestablished.Probablyothergrowth-limitingfactorsplayalsoarole.
OftheN,PandKpresentinthefood, S. avenae utilizedNbestbyfar:for65?»
fromthesapoftheflagleafandfor83°sfromthatoftherachis.Otheraphidspecies
showthefollowingN-utilizationpercentages. Aphis fabae Scop,in Vicia faba (L.)
50-705»(Banks&Macaulay,1965), Uegovœa vioiae Buckt.on Vicia faba (L.)morethan501
(Ehrhardt,1962), Myzus persicae
(Sulzer)onanartificialdiet S0%(Mittler,1970)and
64-70«.(Kunkel&Hertel,1975).
S. avenae multipliestwiceasquicklyontheearasontheflagleaf.Thisfactis
coupledwithahoneydewproductionpermgbiomassproductionontheear,whichistwice
aslowasontheleaf.Sothephloemsapstreamintherachishasanutrientvalue
whichistwotimeshigherthanthatintheflagleaf.Wecaneasilyunderstand thisobservationbecausethewheatplantaccumulatesinthegrainin4weeksabout503,ofits
drymatterand90°sofitsNandPsupply.
Alataeaswellasapteraeshowamarkedpreferencefortheear (Chapter3).Inview
oftheabovedata,thisadaptationhasagreatsurvivalvalue.Thefactthat S.
avenae
usuallyreacheshighnumbersafterflowering(Chapter3 ) ,seemstomeverymuchcorrelatedtothehighnutrientvalueoftheearanditspreferenceforthisfeedingsite.
Itiscommonpracticetoestimatetheefficiencyoffoodutilizationonthebasis
oftheenergybudget.Thusforaphidsthecaloricvalueofthefoodtakenupisequalisedbythecaloricvalueofhoneydewandbiomass (includingexuviae)andthemetabolic
heatloss(respiration).
Llewellyn (1972)foundfor Eucallipterus
tiliae
acaloricvalueof5.6cal/mgdry
matter andLlewellyn&Quereshi (1978)foundfor Aphis fabae Scop,acaloricvalueof
5.5cal/mgdrymatter.FromresultsofKunkel&Hertel (1975)itcouldbecalculated
that3rdinstaralatiformsof Myzus persicae
contained5.8cal/mgdrymatter.Fromtheir
dataitcouldalsobecalculatedthattheratiobetweenincreaseinbiomassandmetabolic
heatlossat15°Camountsto1:2,approximately.
Assumingthatonthebasisofdrymattertheenergycontentof S. avenae is6cal/mg
andthatofphloemsapandhoneydewis4cal/mg (wheatenflourandsaccharose:4cal/mg)
2. 1cal=4,1868J.
12
andthattheheat lossis3cal/mgofaphidproduction,thenwecanestimatethe
energybudgetof S. avenae asfollows. S. avenae ontheear:caloricvalueofingested
sap=caloricvalueofbiomassproductionandofhoneydew (contaminatedwithexuviae)
plusmetabolicheatloss=6+ ( 3 x 4 ) +3=21Cal.Theefficiencyoffoodutilization
is6/21 =29%.Fortheproductionof1mgofaphid21/4~5mgofphloemsapisneeded
ofwhich3mgisexcretedashoneydew. S. avenae onleaf:caloricvalueofingestedsap
=6+ (9x4)+3=45cal.Theefficiencyoffoodutilization is6/45 =131.Forthe
productionof1mgofaphid45/4» 11mgofphloemsapisneededofwhich9mg isexcreted
again.Fortheseestimates thehoneydewproductionatthehighestnitrogenlevel(Table4)
isconsidered tobe themostrepresentative forthehoneydewproductioninthefield.At
thisfertilisinglevel,theNcontentofthegrainswas 1.9%,inthefieldanNcontent
of1.7-2.0% isnormal.
M. dirhodum (Table2)producesontheflagleaf2.1/2.5x 100%= 84%oftheamount
ofhoneydewthatisproducedby S. avenae. Sofor M. dirhodum itcanbestated thaton
theflagleafthecaloricvalueoftheingestedsap= 6 + (7.5x4) + 3 = 39cal.Theefficiencyoffoodutilizationis6/39 =15%.Toproduce 1mgof M. dirhodum 39/4*10mg
ofphloemsapisneeded,ofwhich 7.5mg isexcretedagain.
Table5presentsareviewoftheenergybudgetsof9speciesofaphids.Itappears
thatwithregard toefficiency offoodutilization,grainaphidstakeanintermediate
position.Thevalueofthedatashowninthistableshouldnotbeoverestimated,however.
Factorslikeageoftheplantand leaf,feedingsiteandnitrogendressingundoubtedly
wereofstronginfluence.
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3
3 POPULATION DEVELOPMENT OF SITOBION AVENAE F., ESPECIALLY IN RELATION TO
CROWDING
3.1 INTRODUCTION
Sitóbion avenae F. livesonmanyhostplants,almostallbelonging totheGramineae
andshowsaspecificpreference forthepanicle (HilleRisLambers,1939;Latteur, 1976).
Inspringandinsummer,reproduction isparthenogenetic andviviparous.At temperatures
between15Cand20C,nymphaldevelopment (4stages)iscompleted inabout 10days.One
apterousadultcanproduce 20to40nymphs in2weeks,dependingonitssize.
"Crowding"("effectdegroupe",Bonnemaison,1951)canresultintheproductionof
alatae,whichisveryimportantforthesurvivalofthespecies.Overpopulationisthus
avoidedandwhenthequalityofthehostplantdeteriorates,theaphidsarecapableof
migratingtoanotherhost.With Aphis fabae Scop. (Way&Banks,1967;Dixon&Wratten,
1971)itwasfoundthatcrowdingdecreases thesize,whichresults inalowerfecundity
oftheindividuals.According toTaylor (1975)size influences fecunditymainlythrough
reproductionrateandlongevity.
Thissectionfirstdescribes thepopulationdevelopmentof S. avenae inthefield
ingeneraltermsandalateproductionandmigrationindetail.Thenitdiscusses trials
ontheeffectsofcrowding,whichweredoneinthelaboratorywherenaturalenemiesare
absentandclimate isundercontrol.
3.2 MATERIALSANDMETHODS3
Fieldtrialsweredonenear theResearch Institute forPlantProtection (IPO)in
Wageningen.In1974and 1975,thepopulationdevelopmentwas studiedbytakingrandom
samplesweekly.Theaveragenumberofaphidspertillerandperleafwasrecorded.The
rapidpopulationincreaseanddecline,thepatternofdispersionandthechange inage
compositionrequiredmoredetailedobservations.Therefore in1976and 1977sampleswere
akentwiceaweekandthenumbersofaphidswerequantifiedaccordingtosevenageand
morphclasses:1st,2ndand3rdinstar,4thinstaralatiformorapteriform,alateor
Perousadult.Thevariousnymphal instarsweredistinguishedbysizeandwingdimorP ism.Apteraeweredistinguished from4th instarnymphswithoutwingbudsbythelength
ecaudaorbydorsalpigmentation.Fromclimate roomobservations itwas assumed
thosealataethatdidnotsuckwiththeirheadsdownontherachisoronthebaseof
spikelets,asallotheraphidsdid,wouldeventuallymigrate.As theywouldnot
ofaphid andplant culture are given inChapters 2and 4,respectively.
15
functionasreproductiveadultsinthecolony,theywereclassifiedas4thinstaralatiforms.Inthiswayarealisticevaluationcouldbemadeofthetotalnumberofreproducingadultsandoftheratiobetweenalataeandapterae.
Theprimarypurposeoftheobservationswastoobtainaninsightinthefactorsdeterminingpopulationdevelopment,especiallythoseinvolvedindensity-dependenteffects.
Thereforeitwasconsideredtobesufficienttofollowthelevelandcompositionofthe
2
populationon100tillers,welldistributedinafieldof2500m .Toavoidlossof
aphids,theearswerenottouchedduringcounting.
3.3 RESULTS
Z.Z.I Field observations
3.3.1.1Migrationtothecrop
Alataewereobservedforthefirsttimeduringtheperiodofstemextension.In1976,
(Fig. 4)theirnumberincreasedbeforefloweringstarted (Fe10.5.1)andremainedmoreor
lesssteadyuntilgrowthstagewateryripe (Fe10.5.4). In1977,(Fig.5)astrongmigra10*-
number
of aphids
on 100 tillers
35
42 46 days
11.1 plant growth stage
,*%'
, " M , P ° P u l a t ^ o n developmentof Sitobion avenae. Observationswereniaueon
100markedtillers,twiceaweekfrom24Mai (Fe.9)till9July (Fe.11.2).(O)Total
numberonears.(•)Totalnumberonleaves.(+)Apterae.(* )Alatae.(x)Nymphsof4thstage,
16
number
of aphids
on100 ears
103-
/
\
/
\
102-
/
10-
'
1
\
*•- /
X*""""^
.*
**
*
*
1-
3
10.5.3
10
10.5.4
17
11.1
24days
11.1plantgrowthstage
Fig. 5. 1977.Populationdevelopmentof Sitdbion avenae. Observationsweremadeon
100marked ears,twiceaweekfrom 17June (Fe.10.5.2)till 11July (Fe.11.1).(O)Total
number.(+)Apterae.(*)Alatae.(X)Nymphsof4thstage,alatiform.
tionhadalreadytakenplacebefore floweringandthenthenumberofalataedecreased
rapidlyuntilthebeginningofthemilkyripestage(Fe11.1).In1976,thenumberof
alataeincreasedstronglyfromFe10.5.4andin1977fromthebeginningofFe11.1.Probablymostofthesealataeoriginatedfromwinterwheat,whereusuallyamassivemigration
occursinthisperiod (Figs4and5,4thinstar,alatiform).Thenumberofalatae,however,remainedmuchsmallerthanthenumberofapterae,sothatthissecondmigration
wavecanbeconsideredrelativelyunimportant.In1976,anaverageof1.4aphidswaspresentperearduring flowering,ofwhich31werealataeand37«.oftheearswereoccupied.
In1977,thecorrespondingfigureswere0.9,321and78«„,respectively.Thusthespeed
withwhichtheavailableearsarecolonisedseemstodependontherateofimmigration
ofalatae.
3.3.1.2Migrationwithinthecrop
Itwasestablishedinlaboratorytrialsthatnymphshardlymigrateiftheyarenot
disturbed.Onlyafterthelastmoultdotheybecomerestiveandmoveinthedirectionof
thelightsource.Alataesoonstarttoflyandapteraesettledownatthetopofthe
plant.Oncehavingstartedreproduction,theadultsusuallystaywheretheyare.
Theobservationsin1976confirmedthesefindings.FromFig.6itappearsthatapteraehadalreadycolonisedtheears,whentheywereonlypartlyvisible(Fe10.4).
17
No3rdand4thinstarnymphswerepresentyet,soreproductionhadjuststarted.At the
beginningofflowering,thepreferencefortheearofbothalataeandapteraeresulted
inasharpdecrease inreproductionontheleavescausingthetwooldestnymphalstages
tobethemostnumerous.Atthesametimemoreapteraethan3rdand4th instarnymphs
werepresentontheears.Thismigrationfromtheleavestotheearsresultedin1976in
adecreaseinpercentageoftillerswith leafinfectionfrom17beforeflowering to10
attheendofflowering,whereasinthisperiodthepercentageoftillerswithearinfestationincreasedfrom0-535..Fig.4showsthattemporarilythetotalnumberofaphidson
theleavesevenslightlydecreased.Afterfloweringin1976,thepercentageoftillers
withleafinfestationincreasedtoalmost 1001attheendofthemilkyripestage.Inmy
opinionthismainlyresultedfromdisturbanceofearcolonies,causedbywind (rainwas
absent),coccinellidsandnymphsofsyrphids.Manytimes Isawthatthepresenceofa
predatorinadenseearcolonycausedespecially4thinstarnymphsandadultstodrop
down.Undoubtedlyanumberofthefallenaphidssettledagainontheleaves.Afterfloweringin1976,theaveragenumberof1stinstarnymphsperadultwas3.3 inleafpopulationsand5.0 inearpopulations.Thesevalues indicatethattherateofreproduction
onleavesislowerthanonears,whichwasalsofoundinthelaboratory (Chapter 2). The
growthrateoftotalleafandearpopulations,however,wasthesame (Fig. 4). Thissuggeststhattheaphidspresentontheleavesafterfloweringmainlyoriginatefromthe
3.3.1.3Influenceofmigrationandtemperatureonpopulationgrowth
In1976,thepopulationshowedanexponentialgrowthduringthe3weeks from
Fe10.4toFe11.1 (Fig.4 ) .Inthisperiodthepopulationkeptpacewiththenumberof
adults,mostlyapterae.FromFe10.5.4thenumberofadultsstartedtogrow lessrapidly
becauseanincreasingproportionofthenymphsdevelopedwingsandmigratedassoonas
theyreachedtheadultstage.Fig.4showsthatthepopulationasawholekeptongrowing
for10dayswithapproximatelythesamespeed,apparentlybecausethereproductionrate
increasedstrongly.Thisprobablyhappenedundertheinfluenceofasharprise intemperature.In10daysthetemperaturerosesteadilyfrom14.5°Ctot24°C.
In1977,thepopulationgrowthduringfloweringwashardlynoticeable (Fig.5)becauseofthesharpdecreaseinthenumberofalatae,whichwere infactthe largest
fractionofreproducingadults.However,thenumberofapteraedidincreaseduringflowering,sothatbytheendofflowering (Fe10.5.3)theyoutnumberedthealatae.From
thatmomenton,thepopulationstartedtogrowmorerapidlyandkeptpacewiththenumberofapterae.Eventhoughthenumberofadultsstartedtodecreasebeforemilkyripe
becausemoreandmorenymphsdevelopedwingsandmigratedaftertheirlastmoult,the
populationstillincreasedforafewdaysbecausethereproductionrateincreased,probablyinducedbyariseintemperaturefrom15°Cto20°C.
J! .f,^
3
J
Fe10.1 (7)
17%
10.4(11)
24
Tt
^
10.5.1(15)
27
10.5.2(18)
44
IL Ik ik^Jib^
10.5.3.(211
62
(35)
(251
77
11.1(38)
10.5.4 (28)
94
(42)
11.1 (32)
100
11.2(46)
11.2(46)
80
Fig. 6. 1976.Agecompositionofthepopulationatsuccessivedates.Frequenciesof
the5ageclassesarepresentedaspercentageofthetotalnumber.Symbolsused:
L,E=Leaf,Earpopulation.1-4= 1st-4thnymphalstage.A=adult.( )=Dayof
observation. 7, =Percentageofcolonized tillers.CZ3Bi Unwinged,wingedaphids.
3.3.1.4Analysisofpopulationdevelopmentbasedonageandmorphcomposition
Thevariousphasesinthedevelopmentofaphidpopulationsbecomevisibleinthe
ratioofnumbersbetweentheseparateageclassesonsubsequentdates.InFig.6an
overallpictureofthepopulationdevelopmentin1976isshown.OnDay8(Fe10.1)the
firstearswereoccupied,andtheproductionofthe1stgenerationstarted.Themajority
oftheparentgenerationontheearwasapterousandthereforeoriginatedfromtheleaf.
Becausebeforeandafterfloweringlittlemigrationoccurred,colonisationofthecrop
progressedgradually.OnlyonDay30(Fe10.5.4)allearswereoccupied.OnDay15
(Fe10.5.1)forthefirsttime4thinstarnymphsofthefirstgenerationwerepresenton
theear.OnthebaseoftheresultsofDean (1974)itcouldbeestimatedthatthefirst
nymphsofthesecondgenerationwerebornaroundDay19(Fe10.5.2)andreachedthe4th
instararoundDay25(Fe10.5.4).Onthisdayforthefirsttime4thinstaralatiform
nymphswereobserved.Inallprobabilitythesewerethefirstnymphsofthesecondgeneration,becausetheybelongedtotheoldestandtherebylargestcolonies,wherethe
strongestincentivetowingformationexisted (seewing formation).So,migrationof
secondgenerationalataestartedafterDay25,whichisalsoindicatedbythelessrapid
increaseinthenumberofapteraeafterthisday(Fig.4 ) .Itkeptonincreasing,hawever,for2weeksandeventripledduringthisperiod.Thissteadyincreasewascausedby
theverygradualcolonisationofthecrop.Becausethelastearswerecolonizedonly
aroundDay28(Fe10.5.4),apteraefromthe1stgenerationcouldjointhepopulation
leastuntilDay38(seewingformation).
In1977,populationdevelopmentontheearprogressedmainlyinasimilarnanner
andcanbedescribedasfollows (Fig.7 ) .OwingtoanintensiveintógraUonbefore owering,80.oftheearswerecolonisedatthebeginningofflowering.Asa« e " e '
themajorityoftheparentgenerationadultswerewinged.Thepresence « * * ™ £
nymphsofthefirstgenerationonDay0(Fe10.5.2)showedthatreproducti onthecar
haTstartedabout8daysearlier(Fe10.5).AroundDay3(Fe10.5.3)thefirstnymphs
19
&_
Fe.10.5.2(0l 10.5.3(31
78%
80
11.1(17)
93
(7)
77
(21)
93
h,
10.5.4(10)
70
11.1(24)
77
Fig.7. 1977.Agecompositionofthepopulationatsuccessive dates.Forfurther details
seefig.6.
ofthe2ndgenerationwerebornandreachedthe4thinstararoundDay10(Fe10.5.4).At
thatmoment 5%ofthe4thinstarnymphswere alatiformand4days laterthis percentage
was80.Soagainthesecondgenerationwasmainlywinged.
3.3.1.5Wing formation
Onlyatthe4thnymphalstageiswing formation clearlyvisiblewiththenakedeye.
In1976andin1977 (Figs6and7)thefirstalatiformaewere observed7days laterthan
apteriformae.Thusbothalataeandapteraehadunwingedprogeny duringthefirstweek
afterthestartoftheearcolony,notwithstanding theirprevioushistory.Apparently
wingformationinS. avenae dependsonthecircumstancesprevailing justbeforebirthor
duringthefirstdaysofnymphal life.Resultsofclimate roomtrials (3.3.2)confirmed
this.For1datein1976andfor2datesin1977separateearcolonieswere studiedtofind
acorrelationbetweenthepercentagesofnymphswithwingbudsinthe4thinstarandthe
sizeofthecolonyjustaftertheirbirth,so7days earlier.Onthese3datesthepercentagesofalatiformaewerewelldistributedbetween0and100andmore than10colonies,
aminimumforcorrelationanalysis,satisfiedthefollowing2conditions:(a)there
shouldbeatleast2nymphsofthe4thinstarand(b)1adultshouldhavebeenpresent
7daysearlieroratleast2nymphs shouldhavebeeninthe1stinstar.These conditions
wereappliedtoexcludecolonieswhichweredisturbedduringthelast7daysorwere
foundedbyfallenaphids.Therankcorrelation coefficient accordingtoSpearmanwas calculated,becauseitislesssensitivetorandomvariation.Theresultswere 0.63,0.68
and0.82,respectively.Everytimetheprobability thatnocorrelationexistedwas less
than0.01.Inviewofthesereliable indicationsforadirect influenceofthesizeof
thecolonyonwing formation,theircoherencehasbeen testedinallobservation datain
1976andin1977untilthepopulationmaximumwasreached.Astrongcorrelation (r=0.95)
wasapparentbetween thepercentageofnymphswithwingbudsinthe4thinstar converted
intoprobitsandthe logarithmofthecolonysize7daysearlier (Fig.8 ) .Thesecalculationsshowedthat s. avenae formswingsatvery lowpopulationdensities.Accordingto
Ankersnut (pers.comnu)thegreen S. avenae isapttodevelopwings earlierthanthe
dark,brownone.Inthecurrenttrials themajorityofaphidsweregreen.
20
%wing developing
nymphs
/
99.8-
/•
•
99-
•/*
95 :
•/
90-
•
80-
50-
• /
20-
/•
10-
/ ».
5-
T3
1
51
10
20
30
colony size
Fig.8. 1976and 1977.Percentageofwingdevelopingnymphsinthe4thstageasinfluencedbycolonysizeattimeofbirth (oneweekbefore).Eachpointrepresentstheaverage
over 100markedearsatacertaindate,n= 11,r=0.95.Regressionformula :probity=
4.53 logx +1.36.
3.3.2Climate room trials
Table6showsthattherewerebigdifferencesinthedevelopmentof S. avenae populationsinPerspexcagesstartingwith4and8individualsofthe3rdinstarperear.
After14daystheincreaseinbiomassfromstartingdensity8was491lessthanthatfrom
startingdensity4.Approximately 10°softhiseffectwascausedbyahighermortalityin
theinitialnumber,60°sbyalower increaseinnumbersand301byalower averageweight
ofthenymphs.
In2trialsthepopulationdevelopmentwasstudiedduring4weeksstartingwiththe
lowestpossibleinitialdensitiesi.e.1,2and3apteraeperear (Figs9and 10). Inthe
firsttrial,smallerandconsequentlylessproductiveadultswereusedthaninthesecond
trial.Duringthefirstweekmorethan10°softheadultsdiedinTrial1,butinTrial2
Table6.Thesignificanceofinitialdensityforthepopulationdevelopment
of Sitobion avenae ontheearofwinterwheat (cv.Clement).Datarepresent
themeanperformanceof27coloniesinperspexcages,startedwith3rdinstarlarvae,over 14daysduringtheperiodfrom 'flowering'until'watery
ripe'.Alldifferencesbetweenthetwodensitiesaresignificantat5%.
Betweenbrackets:relativeperformanceofinitialnumber8.
Initial
number
Initial
biomass
Meansurvivalin
initialaphids
Meanmultiplicationin
biomass
num ber
correctedformortality
-
+
-
+
4
0.78mg
91%
19.5 20.4
41.0 43.0
8
1.68mg
76%
13.6 15.5
21.1 24.0
(70%)(76%) (51%)(56%)
21
100
o
10.:3
7
10.5.1
14
'0.5.3
21
10.5.4
28 days
11.1plant growth stage
F i g . 9. Lab.exp.1.Populationdevelopmentof Sitobion avenae from3initialdensities
1 (•) ,2(*) and3(O)adultsperear.
Adults.
Totalnumber.Eachpointrepreseiits theaverageover72colonies.
100^
-8•
34
-o'//
2
1-4
0
.'
7
14
21
10.5 10.5.2 10.5.4 11.1
28days
luplantgrowthstage
Fig.10. Lab.exp.2.SeeFig.9fordetails.
somemortalityoccurredonlyatdensity3.After1weektherewere501lessnymphspresentinTrial1thaninTrial2.Correctedformortalitythereproductionrate
(nymphs/adultperday)duringthefirstweekwithdensity1wasinbothtrialssignificantlyhigherthanwiththeotherdensities.Onanaverageoverthetwotrialsthereproductionratesfordensities1,2and3were1.66,1.22and1.13nymphs/adultperday,
respectively.Theleastsignificantdifferencewas0.23 (p=0.05).Tendaysafterthe
startofthepopulationdevelopmentthefirstnymphsbecameadults.Twodayslaterthe
firstalataewereobservedagainstthelightceilingoftheclimateroom.Migrationincreasedduringthenextweeks.Itwasstrikingthatthemajorityofthealataedidnot
returntothecropanddiedafterafewdays.Onlyinthemilkyripestagedidanuifcer
ofalataesettleontheyellowingleavesandformcolonies.
22
FromFigs 9and 10iscanbededuced thatalthoughduringthefirstweek inTrial1
lessthanhalfthenumberofnymphswereborncomparedwith thatinTrial2,thenumber
ofyoungapteraewasnearlyequal inbothtrialsafter 14days.Apparentlyastronger
wingdevelopment occurred intheyoungnymphs inTrial2owingtohighercolonydensities.
Assumingthatthereproductionrateduringthefirstweekwasconstant (Wratten,1977),
thatmortality oftheinitialadultsduringthesecondweekwasignorableandthatthe
nymphsafter 10.5daysbecameadults,itappeared thattheapterouspopulationafter
14daysconsistedofsurviving initialadults andyoungadultsbornduringthefirsthalf
ofthefirstweek.So,theratioofwingdevelopment innymphs thatwerebornduringthe
first3.5 days canbeexpressedas 1- (A14-A7)/(0.S L 7 ),whereL 7 isthenumberofnymphs
onthe7thdayandA isthenumberofapterous adults.
Thefirst trialyielded 341and thesecond 56%alatae.Fromthetwotrialstogether,
amarkedrelationseemed toexistbetween theaveragepercentagesofwingformationand
theaverage colonysizeinthemiddleofthefirstweek:thecorrelationcoefficientwas
0.98.Assuming that theremaining initialadultsalldiedinthe3rdweek,thenumberof
apteraeofthesecondgenerationincreasedduringthe3rdweekby 40%inTrial 1anddecreased inTrial2by 261.Ofthenymphsborninthesecondhalfofthefirstweekapproximately655oinTrial 1and 1001inTrial 2werewinged.Crowdingmostlikelycaused
mortalityandpossibly somemigrationofthe2ndgenerationapteraeduringthe3rdweek.
Owingtothedecrease innumberofapterae,thetotalnumberofaphidsinTrial2increasedonlyslightlyduring the3rdweek,sothatthemaximumnumberafter21dayswas
equalinbothtrials.Only inTrial 1waslevellingbetweenthetotalnumbersofthe
3densitiesnotyetcompleteatthis time.Duringthe4thweekthetotalnumberofaphids
decreased inbothtrialsasaconsequenceofageing,mortalityofapteraeandcontinuing
migrationofalatae.
3.4 DISCUSSION
Untilnow ithasgenerallybeenassumed (Latteur,1976;Sparrow,1974;Wetzeletal.,
1975)that,apartfromactionbyEntomophthora,thedecrease inpopulationiscausedby
ageingofthehostplanttogetherwithanincreasingactivityofnaturalenemies.Thereproductionrateof S. avenae ontheeardoes indeeddecreaseundertheinfluenceofhost
plantageing (Chapter2)butthisoccursonlyinthesecondhalfofthemilkyripestage,
whereasthepopulation inthefieldfrequentlyreachesitsmaximumbefore that.Thefact
thataphidsusuallyattaintheirmaximumdensityafewdaysbeforetheirnaturalenemies
doesnotonly indicate thatthesecondphenomenoncauses thefirstone,thereversecan
alsooccurandmaybeevenmorerelevant.Thecurrentresultsshowthatthetypicalpopulationdevelopment consistingof:buildup,rapidmultiplication,slowdown,stagnation
andquickdecrease is inthefirstplaceregulatedbyintraspecificfactors.
Development ofwinged formsdependingonthesizeofthecolonyfollowedbyemigrationinitselfcannotonlyrestrainthepopulationgrowth,butcanevenreduceitto
lessthanhalf.Inpractice thenaturalenemiesandthesenescenceofthehostwillsubsequentlyenhanceafurtherdecrease.Thestrongpreference forthepanicleandthesharp
reactiontocrowdingnormallyseemtoresultinanearpopulationofnotmorethan2ge23
nerations,ofwhichthelastoneisalmostentirelywinged.
Itwasstrikingthatthereproduction rateofasingleadultontheearwas 3(H
higherthanthatof2or3adults living togetherononeear.So,contrary toTaylor's
(1975)findingswith Aphis fabae Scop,itseems thatwith S. avenae thedensity doesnot
onlyinfluencetherateofreproductionbyway ofthebodysizebut alsodirectly.The
effectofdensityontheindividualsizeand longevity seemshardlyrelevant tothepopulationdevelopment incereals.Nymphswitharetarded growthasaresultofcrowding
willundoubtedly formwingsatanearlystageandmigrate after the lastmoult.Inthe
laboratoryeventheadultstageofsmall individuals inadensecolony lastsat least
7days.Periodicobservationsofmarked tillersrevealed that inthefield the duration
oftheadultstage isusuallyshorter than7days,becauseofnatural enemies andclimate.
Thus inthefieldmostaphidsdonotgetthechance todie fromphysiological ageingor
fromtheeffectsofoverpopulation.
24
4 INJURY AND DAMAGE TO WHEAT, CAUSED BY SITOBION AVENAE F., RHOPALOSIPHUM
PADI L. AND METOPOLOPHIUM DIRHODUM WLK.
4.1 INTRODUCTION
Strickland &Bardner (1967)defined injuryanddamageasfollows."Injury istheeffectofinsectfeedingorotheractivities onthegrowthorappearanceofcropplants.
Damage isinjuryresulting inmeasurable lossofyieldorreduction inquality".Uptake
ofphloemsap,injectionoftoxicsaliva,pollutionoftheplantwithhoneydewandtransmissionofvirusarethemostobviouscausesofaphid injury.Towhatextentinjuryleads
todamage isacomplexmatter.According toBardner &Fletcher (1974)itdependsonthe
nature,thesiteand intensityofthe injury,theconditionoftheplantandenvironmental
factors.To investigate therelationbetween injuryanddamagetheyrecommendedastudy
ofcropphysiology,especiallyanalysisofplantgrowthintermsofeffectivephotosyntheticarea,productionofdrymatterand itsdistributionbetweenthevariouspartsof
theplant.
Fromseveralstudies itappearsthatthedrymatterofthegrains isformedmorethan
905»afterfloweringmainly intheflag leaf,peduncleandear,andthattheyield level
dependsonthesizeanddurationofthephotosynthetic areaandenvironmental factorssuch
asradiation,temperatureandmoisturecontentofthesoil (seereviewsbyThome,1974;
Yoshida, 1972).
Particularly intheperiodafterfloweringaphidsusuallyattainlargenumbers
(Chapter 3). Thereforetheeffectofaphidsonyieldwasonlydeterminedinthisphase.
Inclimateroomtrials Iinvestigated towhatextentaphidspecies,siteandtimeofinfestationandnitrogendressinginfluenced thedamage.Thelightandtemperatureregirae
andthemoisturecontentofthesoilwerenotvaried,althoughthese factorsarepossibly
relevanttothedamage.Thenitrogendressingwasvariedbecause itstronglydetermines
thesizeanddurationofthephotosynthetic areaandtherebytheyield level.Moreover,
theefficiencyoffoodutilizationseemedtoincrease throughtheextra fertilization
withnitrogen (Chapter2 ) .
Fromaknownquantityofbiomassof S. avenae itcouldbeestimatedonthebasisof
itsenergybudget,howmuchdrymatterwaswithdrawnfromtheplant.Ifthecalculated
lossinyieldwouldappear higherthantheestablished lossinyield itwould indicate
thattheplantisabletocompensatefortheloss.Thereversewould indicatethatstylet
penetrationandsalivainjectionintheplantalsocontribute todamage.Intheseclimate
roomtrials foulingoftheplantbyhoneydewandblackmouldscouldbeneglectedbecause
thehoneydewdriedoutandremainedassmallandfungus freecrystalsontheplantasa
resultoftheintensiveventilationandlowhumidityoftheair.
However,inthefieldhoneydewdepositedontheplantcancauseaseverefungusinfestation.As thisphenomenonoftenoccursduring theperiod inwhich thegrains-are
25
stilldeveloping,itwasassumedthatitwouldaffecttheyieldnegatively.Bydetermining
theeffectofaphidsonyieldinthefieldwithandwithout treatingthecropwithabroad
spectrumfungicide,Itriedtoestablishtheratiobetween"feeding damage"and"honeydew
damage".ThepossibledirectinfluenceofhoneydewonplantproductionisbeinginvestigatedbyRabbingeetal.
Thesignificanceoftheintensityofinfestationforthedamagewillbediscussed
togetherwiththepossibilitiesofcontrol (Chapter5 ) .
Becausetransmissionofvirusseems onlylocallyimportantinwheat culturein
westernEurope,thissourceofinjuryhasnotbeenincludedinthisstudy.
4.2 MATERIALANDMETHODS
4.2.1 Laboratory
trials
4.2.1.1Plantculture (Fig.11)
Germinatedgrainsofwinterwheatcv.Cyranowereputonmoist filterpaperandvernalisedinPetridishesfor6weeksat2°C.Theywere thenplantedin2rowsof25grains
eachinboxesofSOcmx20cmx20cm,inasoilmixtureof10kgofsand,8kgofpottingcompost,5kgoflightclayand41ofwaterwhichsaturated thewatercapacityfor
m
Ï-W
-tMj
y
/
*^'î
L/
26
È
Fig.11.Wheatcultureintheclimateroom.
See4.2.1.1.
60°6.Ineachbox3porous filter candleswithatotalvolumeof600mlwereputata
depthof10an.Theywere connectedtoawaterreservoirunder theboxwitharubbertube
havinganinnerdiameterof2mm.Lossofwaterinthesoilmixturewasconstantlycompensatedbythecapillary actionofthetube.Toeachboxthefollowingquantitiesof
"slowrelease fertilizers"were added:5gofCaC0 3 ,5gofMgO,10gofP 2 0 5 ,2gof
K 2 0and3.5gofN.
Thefirst6weeks,duringthetilleringphase,theclimatewasregulatedasfollows:
daylength15h,dayandnight temperature 15°Cand9°Crespectively.After thatdaylength
was16handdayandnight temperatureswere 18°Cand12°C.Relativehumiditywas 701.
Thelightintensityatthetopofthecropwasconstantlykeptat20.000lux(62W/m)by
adjustingthedistancebetweenthecropandtheTL-33ceiling.Underthese circumstances
50maintillersand60to80sidetillersperboxstartedflowering after13weeks.The
available quantityofnitrogenper tillerwas aboutthesameasinthefield.Theaverage
yield/tillerwas1.75gwithamoisture contentof17°,andthemeankernelweightwas
44mg,whichequalstheaverageinpractice.
4.2.1.2Aphid culture
Detailsonaphidculturewere giveninChapter 2.Beforeeachtrialasampleofthe
aphidswasputonoatseedlings,toseeiftheywerestillfreeofBarleyYellowDwarf
Virus.Beemster,virologistattheResearch InstituteforPlantProtection,taughtmethe
symptomswhichnormallybecomevisibleaftersome10days.Unlikesamples fromthefield,
samples fromthelaboratoryneverreactedpositive.
4.2.2 Field
trials
Thefieldtrialsweredonewithwinterwheatcv.Cariboontheexperimentalfields
oftheResearch InstituteforPlantProtectioninWageningen.Themaintaimwastofind
outtowhatextent fungalgrowthonhoneydewledtodamage.Instatistical termsthe
questionwas:isthereaninteractionbetweenanaphicidalandafungicidaltreatment.
Tosolvethisproblemasplit-plot schemeseemedmost appropriate.AsIwasmoreinterestedintheeffectsofaphidsandtheircontrolthanintheeffectsoffungicides,it
wasdecidedtoconsidertheaphicidal treatmentasasplitfactor.Basedonexperience
frompreviousyearsandontheexpected extentoftheeffect,thetrialsweresetup m
10blocks.Eachblock consistedof2sub-blocksofwhich onewastreatedwithafungicidemixture comprisingmaneb 42.5%andcarbendazim, 6%a.i.4kg/ha,atthestartot
flowering.Inthe 1976trialcagesmadeofnylongauze(.3mmmesh)wereused,withadiameterof44cmand150cmhigh.Thenylongauzewasputaroundaframeofcoarseplastifiedmetalgauze,whichwas fixedtothegroundwith ironpins.Earthwaspiledup
againstthelowerpartofthecagetoprevent insectstocrawlthrough.Allplants 15cm
aroundthecageswereremovedallowingtheencagedplantsmore light,whichappearedto
compensateforthe10-20",reflectionandabsorptionbythegauze.Ineachcage40t o /
tillerswerepresentandtheiryieldwassimilartothatofthefullfieldtillers THe
nylongauzewasattachedatthetimetheears justbecamevisible.Thesamedayand
27
4dayslatertheplantsinthecageswere treatedwithashorttermphosphoric esterto
killtheaphidsandthenaturalenemiespresent.Atthestartoffloweringineachsub-blockwhichcomprised4cages,4treatmentswereallocatedatrandom: S. avenae,
U. dirhodum, R. padi andcontrol.Ineachcage404thinstarnymphsandadultswerereleased,originating fromalaboratoryculture.Fourweeks later,attheendofthemilky
ripestage,thegauzewasremoved.Theaphidspresenton10tillerspercage,randomly
chosen,werecountedandtheplantssprayedwithashortterminsecticide.Afterthecrop
hadripenedthetotalgrainyield,thenumberoftillersandtheaveragegrainweightper
cagewasdeterminedandtheaveragenumberofgrainsperearwas calculated.Onthetotal
yieldandoneachofitsthreecomponentsananalysisofvariancewasmade.
In1977another2trialsweremade likewise.Onecomprised10blockswitheach2sub-blocks.Asub-blockconsistedof2plotsof6x5m.Againperblockonesub-blockwas
treatedwithafungicide.Ineachsub-blockoneplotwas treatedwiththeselective aphicidepirimicarbatthebeginningoffloweringandagain10days later,toavoidanatural
infestationwith S. avenue. Intheuntreatedplotsthepopulationdevelopmentwasestablishedbycountingthenumberofaphidson10earsperplot twiceaweek.Toavoidmigrationandincasethenaturalpopulationremainedtoolow,another trialwassetupin
cageswithartificial infestation.Thiswouldpermitthecalculationofthelossofdry
weightoftheplantaccordingtobiomassproductionbytheaphidsandtheirenergybudget.
Thetrialconsistedof8blocksandagainineachblockonesub-blockwas treatedwitha
fungicideatthebeginningofflowering.Asub-block comprised2cages,oneofwhichwas
infestedbeforefloweringwith1004thinstarnymphsandadultsofS. avenae andtheother
remaineduninfested.After3weeks10tillerspercagewereexaminedforaphidandfungus
infestationandforthepercentageofphysiologicalyellowing (Ubels&vanderVliet,1979).
Theseobservationswerealsodoneinthefullfieldtrial.
4.3 RESULTS
4.3.1 Climate room trials
UsingPerspexleafandearcages,theeffectofS. avenae and R. padi ontheyield
ofwinterwheatcv.Clementwasdeterminedwithavariationinthefollowingfactors:
(a)infectionsitecombinedwiththeinfectionperiod (aphidsontheflag leaffrom
Fe10.1toFe10.5.1,ontheearfromFe10.5.1toFe10.5.4orfromFe10.5.4toFe11.1)
and (b)thenitrogendressingoftheplantat3levels:40,50and60mgofN/tiller.On
anaverageinbothaphidspecies,mostbiomasswasproducedontheearduringthe2weeks
thetriallasted:601morethanontheflagleafbeforefloweringand301morethanon
themoredevelopedear.Thishadnodemonstrableeffectonthelossofyield.Atthe
3N-levelstheaveragekernelyieldperearwas 1630mg,1810mgand1840mg,respectively.Thisdidnotinfluencetheaphidproductionorthelossinyield.Table7shows
thataphidproductioninnumberswasthesameinbothspecies,butthatbiomassproduction (31.5%+1.51drymatter)ofS. avenae wasmorethantwotimeshigherthanthatof
if. padi. Nevertheless,nosignificantdifferencescouldbedetectedintheeffectthetwo
specieshadontheyield.Table7alsoshowsthatonanaverageinbothspeciesonlythe
28
Table7. Theeffectof S. avenae andR. padi ontheyieldofwinterwheat (cv.Clement)
C=untreated;R= R. padi; S= S. avenae; RS=meanofRandS.Theaphidswerekeptin
cagesaround theflagleaforearduring2weeksstartingwith6thirdstagenymphsper
tiller.DataofC,RandSrepresentthemeanof 162tillers.Onethirdofthesehadaphids
ontheflagleafduringFeekes 10.1- 10.5.1,anotherthirdhadaphidsontheearduring
Fe10.5.1- 10.5.AandtheremainderhadaphidsontheearduringFe 10.5.4- 11.1.
LSD=LeastSignificantDifference (p=0.05),onlylistedifexceededaccordingtothe
analysisofvariance.
mean
LSD
Aphidproduction
pertiller
Plantdrymatter
production pertillerinmg
number
drymatter
inmg
numberof
kernelsperear
kernelsper
earinmg
average
kernelweightperear
R
S
R
S
C
C
C
75
81
4.3
9.2
47.3 46.9
0.9
RS
RS
18001730
38.1
RS
36.9
0.7
meankernelweightperearhaddecreasedsignificantly,namelywith3»,by78aphidswith
atotaldryweightof6.8mg.Permgofaphid,lossofdryweightperearwas
{(38.1-36.9)x46.9}/6.8=8.2mg.
Inasecondtrial,usingPerspexcages,theeffectofS. avenae and M. dirhodum on
theyieldofwinterwheatcv.Cyranowasstudiedwithvariationsininfectionsite
(flagleaforear)andintheperiodtheinfectiontookplace (fromFe10.5.2toFe11.1
orfromFe11.1toFe11.2).Onanaverageinbothspecies,aphidproductioninthefirst
infectionperiodwas 20°*higherinnumbersand501higherinbiomassthaninthesecond
infectionperiod.Neverthelessnoeffectcouldbefoundonlossofyield.Table8shows
that S. avenae ontheearproducedtwiceasmuchbiomassasS. avenae and M. dirhodum on
theflagleaf.Thedifferencesbetweenthese3treatmentsinquantitiesofdrymatter
extractedfromtheplant,weremuchlessbecause S avenae ontheearutilizeditsfood
betteraswasappearantfromhoneydewproduction.Perspexcagesontheearcompared
withcagesontheflagleafsignificantlyreducedthenumberofgrains/earby51andthe
averageweightofthegrainsby 1%.Similartotheprevioustrial,aphidinfestationhad
nodetectableeffectonthenumberofgrainsperearnorontheyieldperear.A51reductioninaveragegrainweightoccurredonlywhentheearwasinfestedwith S. avenae.^
Permgofdryaphidmatter,thelossofdrygrainmatterwas{(37.9-35.9)x36J/8.-9.0mg. S. avenae ontheearhardlyreducedtheamountofNpergrain.Itwasstriking
thatundertheinfluenceofaphid infestation,thedryweightoftheinfectedleaforear
(Table8columns9and10)increasedsignificantly. M. dirhodum andS. avenae, feedingon
theflagleafincreasedthedryweightofthatleafby8and9*. respectively,andS. avenae
ontheearenhancedthedryweightofthatplantpartwithoutgrainswith 91.
InathirdtrialIdeterminedtheeffectofS. avenae ontheyieldofwinterwheat
cv.Cyrano,whentheaphidswereallowedtodevelopontheearduring4weeksfromFe10.3
toFe11.1.Theearswereinfestedwith2apteraeperearandnocageswereused.Inthis
trialagaintheinfluenceofNdressingoftheplantontheeffectofaphidinfestation
wasestablished.Asbasicdressing30mgofNpertiller (N1)wasused.Duringthephase
ofstemextensiontheplantsweregivenanextradressingof5mgofN(N2)or10mgofN
(N3).Afterthecropwasharvestednosignificantdifferencesinaverageyieldofdry
29
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Table9. Theeffectof S. avenae ontheyieldofwinterwheat (cv.Cyrano)at3levelsof
nitrogendressing, (N1-N3).C=untreated;S= S. avenae. Aphidswerekeptontheearduring4weeks (Fe10.5-11.1),startingwith2apterae/ear.DataofCandSrepresent themean
of144 ears.LSD=LeastSignificantDifference,only listedifexceededaccordingtothe
analysisofvariance.
Levelof
Aphid production
nitrogen in mg dry matter
dressing perear
S
C
Nl
N2
N3
mean
LSDmean
LSDCS
_
-
28
32
35
32
•
Average kernel Nin
weight perear
kernels
in %
inmg
C S
S
C
Numberof
kernels
perear
Drykernels
perear
inmg
C
C
S
41.939.5
36.338.6
39.038.2
39.138.8
1500
1308
1341
1383
•
,
•
1106 35.8
1020 36.0
995 34.4
1040 35.4
1.9
S
•
28.0
26.4
26.1
26.8
1.48 1.72
1.61 1.86
1.85 2.06
1.65 1.88
0.08
0.14
Nper
kernel
inmg
CS
c •S
1.60 0.53 0.48
1.74 0.58 0.49
1.96 0.64 0.54
0.580.50
notanalysed
notanalysed
grainmatterperearnorinaveragegrainweightperearinthe3Nlevelscouldbefound.
Table9showsthattherewasasignificantdifferenceinNcontentofthegrains,where
N 3 > N 2 > N 1 .AphidproductionincreasedsomewhatwiththeincreasingamountofN,butthis
effectwasnotsignificant.TheNdressinghadnodemonstrableeffectonthedamagewhich
was24°6oftheaveragegrainweight.Aphidinfestationhadagainnoinfluenceonthenumberofgrainsperear.InarandomsampleIfoundthattheaveragedryweightofthe
aphidswasabout0.3mg.Permgofaphidproduction,theyieldofdrygrainmatterper
earwasreducedintheN1-3levelswith 11.-mg,11.6mgand9.1mg,respectively.
Aphid-infestedearshadanNcontentinthegrainswhichwas 141higherthaninthecontrol.ConsequentlythereductioninthetotalamountofNpergrainwaslimitedto13*o.
Thespeedwithwhichtheflagleafyellowedwasnotinfluencedbyaphidinfestation.
4.3.2 Field
trials
Table10showstheresultsofatrialin1976.AtFe10.1eightynylongauzecages
wereplacedinthecropdistributedover10blocks.Atthestartoffloweringhalfof
eachblockwasrandomlytreatedwithfungicides.Threedayslatereachofthe4cagesin
onehalfblockreceivedoneofthefollowingtreatments;noaphids, S. avenae, M. dirhodum
and R. padi. After4weeks,attheendofthemilkyripestage,theaverageaphidpopulationhaddevelopedinto14,154,85and5aphidspertillerrespectively.Ofthesenumbers861,741, 0%and19%werepresentontheear.Thefungicidetreatmentdidnotseem
tohavedemonstrablyaffectedtheultimatenumberofaphids.Duringmassrearinginthe
laboratoryanEntomophthorainfectionhaddevelopedinR. padi, whichcausedthisspecies
toremainataverylowlevelduringthefieldtrial.Solowinfact,thatitwasnot
furtherinvolvedintheobservations.Ofthe3yieldcomponents,thenumberofearsper
area,thenumberofgrainsperearandtheaveragegrainweight,onlythelastone
seemedtobeinfluencedbytheaphids.aswasalsofoundintheclimateroomtrials.Over
allplots,only S. avenae reducedtheaveragegrainweightsignificantly,by9.51.
M. dirhodum andthefungicidehadnodetectableinfluenceontheyield.Therewasaclear
interactionbetweenfungicideandaphidtreatment.Inthecropnottreatedwithfungicide,
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S. avenae andM. dirhodwn reducedtheaveragegrainweight significantlywith121and41
respectively.Inthefungicide treated areathelossyieldwas 7%and0% only.Onthe
whole S. avenae andM. dirhodwn reducedtheNcontentofthegrainsbymorethan4t.A
randomsampleof100aphidsfrom3cagesshowedthattheaveragedryweightofS. avenae
attheendofthetrialwas0.20mg.Sointhefungicidetreatedcropanaverageof
0.74x157x0.20mg=23.2mgofdryaphidmatterhadbeendeveloped.Correctedforthe
lightinfestationinthecontrol,thisfigurebecomes 20.8mg.Yield losswascalculated
tobe71=84mgperear.Sopermgofaphidproductionontheear,4.0mgofdryplant
matterwaslost.InthesamemannerIcalculated thatinthecropnottreatedwithfungicides,7.8mgofdryplantmatterwas lostpermgofaphidproduction.Consequentlythe
fungicideshadreducedthelossespermgofaphidsby49°s.
Table 11showstheresultsofatrialin1977.AtFe10.1,fortynyloncageswere
distributedover10blocks.AtFe10.5.1halfofeachblockwastreatedwithafungicide.
At10.5.2onecageineachhalfblockwas infestedwithS. avenae. After3weeksat
Fe11.1,Ifoundthatinmeaninthecropnottreatedwith fungicide165aphidsperear
haddevelopedandinthetreatedcrop122aphids.Thenumberofaphidsontheleaveswas
estimatedtobe10to15pertillerinaverage.Overthewholetrial S. avenae signifiTable11.1977. Theeffectof S. avenae ontheyieldofwinterwheatcv.Caribo,asinflu
encedbyfungicidetreatment,(maneb/carbendazim,42.5/6%a.i.,4kg/ha) Roundnyloncages
(diameter44cm),8peraphidtreatment,wereplacedoverthepantsat **\°-\" » " " ""e
treatedwithTEPPatFe 10.4,withfungicideatFe 10.5.1 andwith 100*Phids/cage<«thin
starlarvaeandadults)atFe 10.5.2.After3weeks,atFe 11.1,cageswereremoved aphids
werecountedandplantsweretreatedwithTEPP.Plantswereharvested afterripening.The
tarepresenttheaverageof8cages.Symbolsused:seeTable 10.
^^
Production
of S. avenae
perear
0
F
OF
Drykernels
percage
ing
Ears per
cage
O
0
F
OF
F
OF
Kernelsper
ear
Average
kernelweight
inmg
0
0
F
OF
F
OF
t.l 48.2 47.647.9
1.6 43.6 46.244.9
44.244.044
93.095.494.2
45.,5 44 .144.8
46.,9 43 .445.2
mean
95.093.9.
46.,2 43 .8.
44.945.8.
45.9 46.9.
LSDmean
.
.
•
.
•
•
•
•
••'
•
control
S. avenae
96.992.494.7
165 122144
LSDOF
LSD0andF
Drymatter inmgof
topinternodiumwith
flagleaf
ear
without
kernels
0
control
S. avenae
mean
F
465462
482499
474481
OF
0
464
542526534
491
F OF
546560553
544543
45.547.646
1.6
2.3
0 F OF
20 1
2 2 2
18 1718
0 F OF
373536
17712
221217
12 1212
303332
12 7
15 15
3434
0 FOF 0 F OF
104
3
LSDOF
3
3
4
4
LSD0andF
34
.—
3
LSDmean
18
.
Observationondiseases ;atFe :11.1-11.2
%deadoi •yellow
%coveredwith
blackmoulds
flagleaf
ear
ear
flagleaf
2
33
cantlyreducedthemeankernelweightwithanaverageof6.5tandfungicidetreatmenthad
nodetectableeffect.Againtherewasaclearinteractionbetweenfungicideandaphid
treatment.Inthefungicidetreatedcrop s. avenae reducedgrainweightby T>% only,but
thisfigureisnotsignificant.However,intheuntreatedcrop S. avenae reducedgrain
weightsignificantlyby9.SU Inthecropnotinfestedwithaphids,fungicidetreatment
hadnodemonstrableeffect.Itdidenhancegrainweightsignificantlyby 6%,however,
inaphidinfestedcrop.Onthewhole,aphidinfestationcausedasignificantincrease
of61inthedryweightoftheearwithoutgrains.Inthefungicidetreatedcropaphid
infestationalsocausedasignificantincreaseof6.5tinthedryweightofthetopinternode (peduncle)plusflagleaf.
AttheendofFe11.1,itappearedthattheaphidsstronglyhadenhancedthedevelopmentofblackmouldonflagleafandearandthatthefungicidehadreducedit.Thetwo
treatmentsobviouslyinteracted.Thefungicidereducedtheblackmouldsignificantlyin
thepresenceofaphidsonly.Theaveragedryweightof S. avenae attheendofthetrial
was0.09mg,calculatedfromarandomsampleof100aphidstakenfrom5cages.Sointhe
fungicide-treatedcropanaverageof 11.-mgofdryaphidmatterperearhadbeendeveloped,whichledtoalossinyieldof3Ï=56mgperear.Thisimpliesthatpermgof
aphidproductionontheear,5.1mgdrygrainmatterwaslost.Inanalogyitwascalculatedthatthecorrespondingfigureintheuntreatedcropwas 12.3mg,sothefungicide
hadreducedthelossespermgofaphidsbyS81. •
_Table12showstheresultsofafieldtrialin1977.Theexperimentaldesignwas
similartotheonedescribedpreviously.AtFe10.5.1halfoftheplotwastreatedwith
anaphicideagainstnaturalpopulationsof s. avenae. Atthistimetherewasanaverage
ofonly2aphidsperear.AtthebeginningofFe11.1 thepopulationintheuntreated
Plotreachedamaximumof15aphidsperearinthecropwithoutfungicideand9aphids
perearwhereithadbeentreatedwithafungicide.Thissignificantdifferencesuggests
aphicidalpropertiesofthefungicide,whichisalsofoundbyVickermann (1977).Overall
îullir \AL f I h e ' £ f c c t . o £ S- ^ e n a e ° ntheyieldofwinterwheat cv.Caribo,asinfluencedby
S e s e n t '""ment, (maneb/carbendazin,42.5%/6%a.i.,4kg/ha).Netplots:6x5m.Data
Iphlcidet ™ e a ? , ' he ,!°"PlicatesPertreatment.Fungicide treatmentatFe 10.5.1.
AphicidetreatmentatFe 10.5.1andFe 10.5.4.Symbolsused:seeTable10.
'Max.number Drykernels
of
inkg/ha
S. avenae/ear
Averagedry
weightof
kernelsinmg
Observations ondiseasesatFe 11.1~''•_
%coveredwith
blackmould
ear
flagleaf ear
O F OF O F OF
control
15
9 12
%deadoryellow
O F OF
flagleaf
O F OF
432545004415
38.340.839.6
pirimicarb -
441047254570
41.441.941.7
302528
mean
43704615
39.941.4 .
3226 .
LSDmean
LSDOF
LSDOandF5
34
0.9
1.3
342731
plotsthefungicide treatmenthadnosignificantlypositiveeffectontheaveragegrain
weight {3.5%), contrarytoaphicide treatment (5.51).Fungicideandaphicideinfluenced
eachotheragain.Inplotswherenoaphicidewasused,fungicidetreatmentdidresultin
asignificantincreaseinyield (6.5°Ô)andwhere fungicidewasused,aphicidetreatment
couldnotenhanceyieldsignificantly (2.5%).
AttheendofFe11.1Ifoundagainthatthedevelopmentofblackmouldswasclearly
stimulatedbytheaphids.Wherenoaphicidewasusedthe fungicidesrestrainedthedevelopmentoftheblackmoulds significantly.
Thefungicidereducedthelossinyieldby65°«.Correctedfortheslightinsecticidalactionofthecompoundused,thereductioninlosspermgofaphidsamountedto44%.
4.4 DISCUSSION
4.4.1 The three components of yield
InwesternEurope, S. avenae, themostnumerousaphidspeciesincereals,usually
attainseconomically importantnumbersonlyafterfloweringofthecrop(Dean,1974a;
Latteur,1976;Breniauxetal., 1976;unpubl.pers. obs.). Fromtrialswithwinterwheat
intheopenfieldandinclimaterooms,itwas foundthatinthisphaseofplantgrowth,
theaphidscanonlycauseadecreaseinaveragegrainweightandnotinnumbersofgrains
perearnorinnumbersoftillersperarea.ThisagreeswiththeresultsofWratten&
Redhead (1976)andFreier&Wetzel (1976).However,Rautapaa (1966)andHinz&Daebeler
(1976)fouldinfieldtrialswith S. avenae incagesandonencagedears,thatbecause
ofinfectionfromthetimetheearsappearedorfromthetimethefloweringstarted,the
numberofgrainsperearwasalsoreducedinwinterwheat.Itseemstomethattheykept
theaphidsforsuchalongtimeinthecagespreventingemigration,prédationandparasitization,thattheirnumbersreachedunnaturallyhigh levels,causingareductioninyield
between40*.and90%.Withsuchasevere lossinyieldanimportantpartofthegrainsis
sosmallthatitislostduringharvesting.Possiblyalsoabortionofseedstookplaceby
sucha heavynutrientdrain.
f
Still,Inoticedthatinpracticeamoderateattackby S. avenae maycauselosso±
grainsonanindirectmanner.Whenkernelweightisloweredsome5'.to10$bytheapni
andthecropisnotverydryyetthecombineharvestermayloseanimportantfraction
oftheminorgrains.
4.4,2 N content of the wheat
TheNcontentofwheatgrainsisimportanttoitsbakingquality.Wratten (1975,
1976,1978)foundthat infestationofleavesbelowtheflagleaf,whereinthefield
mainly M. dirhodwn isinvolved,canreducetheNcontentofthegrains.According o
thisauthor,thelossofNasaresultofsapuptakebytheaphidsisdetrimentalto
thetransportofNtothegrains.Moreover,theuptakeofNfromthesoilislimited
asaresultofprematuresenescenceoftheinfectedleaves.(Inmyopinionthisonly
happens'whenprédation,parasitizationandmigrationarepreventedbycagesana
35
aphidsreachabnormallyhighnumbers.)Transportofcarbohydratestothegrainsisnot
inhibitedbythiskindofinfestationbecauseittakesplacefromtheflagleaf,peduncleandear(Thome,1974;Yoshida,1972).Soonlyinfestationofthelowerleaves
leadstoareductionoftheNcontentofthegrains.IntheopinionofWratten,infestationofflagleafandear,whereinthefieldmainly S. avenue isinvolved,keepsthe
Ncontentofthegrainssteadybecausetheuptakeofphloemsapbytheaphidsequally
reducestheflowofcarbohydratesandNcompoundstotheerains.Thisdoesnotexplain
theincreaseinNcontentthatWrattensometimesfound.Sofieldetal. (1977)showedthat
duringthefirstdaysofgraingrowth,theratioofincorporationbetweenNcompoundsand
carbohydratesisclearlyhigherthaninthesubsequentperiod.Ingrainscollected1week
afterflowering,IfoundindeedaNcontentwhichwas40!higherthanthatingrainsa
weekolder.Theseresultsareunderstandablewhenonerealisesthattheperipherallayers
ofthegrain,especiallythealeuronlayerandtheouterpartoftheendosperm,whichare
firstfilledup,haveamuchhigherNcontentthanthecentrallylocatedendosperm
(Pomeranz,1972).Withanearinfestationmostofthedrymatterwillbewithdrawnduring
themilkyripestagebecauseatthattimetheaphidswillhaveattainedtheirhighest
numbers.Asthegrowthofthecentralendospermwillthenespeciallybelimited,the
Ncontentoftheinfestedgrainwillremainhigherthanthatof-heuninfestedgrain.A
climateroomtrialshowedindeedthatthedevelopmentof105specimensofS. avenaeon
theearintheperiodfromFe10.3toFe11.1,limitedtheaccumulationofdrymatterin
theendospermtosuchanextent(241),thattheNcontentoftheresultinggrainwas14».
higherthaninthecontrol(Table9).Aninfestationoftheflagleafafterfloweringby
65specimensof M. dirhodum and38of S. avenae andaninfestationoftheearby67
S. avenae didnothaveadetectableeffectontheNcontent(Table8).Inthistrialthe
averagegrainweightwasalmostequaltothatofthecontrol.Inafieldtrial(Table10)
itwasfoundthataninfestationof155specimensof S. avenae and85of M. dirhodum per
tiller,ofwhichnumbers25and1001werepresentontheleaves,reducedtheNcontent
significantlybyabout6».Theaveragegrainweightwasreducedby9.5and21respectively.
TheseresultsagreewiththosefoundbyWratten.Theeconomicimportanceof M. dirhodum
enphasizedbythisauthor,however,shouldnotbeoverestimated.Toreducetheprotein
contentofthegrainsfrom1 Hto10%some70aphidsshouldbepresentpertiller
(Wratten,1975),apopulationlevelIseldomobservedorheardofintheNetherlands
duringthepast5years.
4.4.3 Time and site of
infection
Itcouldnotbeconcludedfromthetrials,thatfromFe10.on,thecharacterand
rateoflossm yielddependsontheperiodinwhichtheinfectionoccurs.Itisclear,
(TZ"',
l?T i n f e C t i ° n1CadSt 0heaVler ***** **» * « * 1 * * or leafinfection
tfabes8and10). Wratten(1975,1976,1978)concludedalreadythatflagleafandear
u^IrsT/Tr et0Mgherl0SS6SttolinfeCti0nS
«otheri-ves.™ » *«***>
™ T ^
° P i M O n° f** Pl3nt ********* - takenintoaccount,thatthedry
E f e " 7 1 T ^ m a i n l yPTOdUCedbyflSgleaf'P ^ 1 6 - d « * •* *thendoesflagleaf
infectionresultinlesslossofyieldthanearinfection?Rawson&Evans(1970)found
36
thatsterilizationoffloretsinthemiddle spikeletsledtolargernumberofgrainsper
ear,biggergrainsintheupperandlowerspikeletsandamarkedly improvedyieldper
ear.Accordingtotheseauthorsthegrainsinthemiddle spikeletswould adverselyinfluencetheformationandsizeoftheothergrainsbecause theydevelopsoonerandmore
quickly.Theyfoundthatassimilatesfromtheflag leafaccumulateespeciallyinthese
grains.Inmyopinion uptakeofphloemsapfromtheflagleafcouldalsohaveabénéficiaieffectonthegrowthofthegrainsintheupperandlowerspikelets,whichcould
compensateforthelossinweightofthegrainsinthemiddlespikelets.
4.4.4 Nitrogen
dressing
InSection2itwas concludedthatextraNdressing leadstolargernumbersof
aphidsbecausewingformationisreducedandmaybe alsobecausetheplantagesless
quickly,providedfungicideapplicationchecksthedevelopmentoffungi.Moreover,the
efficiencyoffoodutilizationof S. avenue ontheearincreaseswiththeextranitrogendressing.FromthecurrentresearchitcouldnotbeconcludedthattheNdressing
influencestherateoflossinyield.Itshouldbekeptinmind,however,thatitwas
notpossibletoobtainasignificantriseinyieldintheclimateroomwithanextra
Ndressing,duetosub-obtimallight intensity.
4.4.5 Aphid species
Intheonlytrialwhere R. padi andS. ahenae werecompared,nodifferenceintheir
effectonyieldwas found (Table 7). Therewasalsonodetectabledifferencebetween
M. divhodwn ands. avenue intheireffectonyield,whentheflagleafwas infested
(Table 8).Itseemsthat M. divhodwn causessomedamageonlywhenatleast50aphidshave
developedpertillerandthecrophasnotbeenprotectedwithafungicideagainstgrowth
offungionhoneydew (Table10).
4.4.6 Honeydew damage
Fromtheresultsof3fieldtrialsitwasconcludedthat49'.,59'.and441ofthe
losseswerecausedbyfungigrowingonhoneydewwhichcouldbepreventedby
t r e
* ™
ofthecropwithafungicide (Table 10-12).Inparticularfungiofthesocalledblac
mouldcomplexthriveonhoneydew.LaboratoryresearchofUbels&Fokkema ( " " P ^ ™ "
sults)showed,however,that Septoria
nodorum andredandwhiteyeastalsogrowwe
onhoneydew.
In1976itrainedonly1dayintheperiod.floweringuntilmealyripe,soall
honeydewremainedontheleaves,whereasin1977itrainedfrequentlyinthesameperiodandhardlyanyhoneydewontheleaveswasvisible.Notwithstanding ^ ^ .
factthatin19763timesasmanyaphidsdevelopedpertillerasin1977, e r
aswellastheabsoluteshareinthedamageoffungalgrowthonhoneydewremaineda• w t thesame (Table10and11).In1976honeydewwasstronglycrystallized,coverng
onlyasmallpartoftheleaves.Onthecrystalsnoblackmouldsdeveloped.
^
37
honeydewpossiblyremainedontheplantasathinfilm,especiallyontheremoteparts
whereitsconcentrationwashighenoughtopromoteattachmentandgerminationofspores
andthefirstgrowthofmycelia.Honeydewandfungicoveringgreenarea,could lower
thephotosyntheticratebyreflectionandabsorptionofradiationandbyreductionof
transpirationandcarbondioxidediffusionviathestomata,duetothehighosmotic
valueofthehoneydewwhichmaycauselossofturgororevenplasmolysisandnecrosis
otplantcells.Waterstress,hotandsunnyweatherandtheactivityofmicroorganisms
ontheleavesdamagingcuticulaandepidermiscouldenhancethelattereffect.Through
thedeveloping lesionsfungicouldfurthermorepenetrate intotheplant.Thefinalresultofalltheseactionswouldbethathoneydewincombinationwith fungialsoreduces
leafareaduration.
Feedingontheear, S. avenae canshootawayitshoneydewpellets over4-8cm.
Basedonanaverageof500tillersperm 2 ,whichstandinrowslyingasdistanceof
25cmfromeachother,anestimated301ofthehoneydewdropsontheear,351onthe
flagleaf,20*onthesecondleafand10%onthethird leaf.Honeydewdamageby
M. dirhodm seemslessimportantsincethisaphidmainly feedsatthebottomofthe
second,thirdandfourthleaf.
4.4.7 Feeding damage: stylet
insertion,
saliva injection
and sap uptake
_ From3fieldtrials (Tables 10-12)itcanbeconcluded thatatthecurrentlevelof
yield,damagecausedbyfeedingisaboutasimportantasdamagecausedbyfoulingthe
plantwithhoneydewwhichstimulatesthedevelopmentoffungi.Fromlaboratory trialsit
couldbededucedthat,dependingontheNcontentofthegrainswhichwasinthesecases
. »,1.571and1.811, s . avenae tookup7.1mg,5.9mgand5.3mgofdryphloemsap
matterpermgofaphidproduction (Chapter 2). In3climateroomtrials (Table7-9)where
grainsofthecontrolplantscontained 1.61%,1.541and1.65»N,thelossinyieldper
m ofaphidproductionof S. avenae ontheearwas8.2mg,9.0mgand10.6mgofdrymaterrespectively.Sothelosseswereabout 50%higherthanwasexpected fromtheefficiencyoffoodutilizationoftheaphids.In2fieldtrials (Tables 10and11)wherethe
cropwastreatedwithafungicideandthegrainsfromcontrolplants contained 1.94and
.7« thelossinyieldcausedby S. avenae infestationontheearwas,basedondry
edf II ^- S ' 1m gPer^ ° faphldP r o d u c t i o n '^ s e lossesagreedwith thoseexpectedfromtheefficiencyoffoodutilizationoftheaphids.Theseresultsjustifytheconclusionthatstyletinsertionandsalivainjectionby S. avenae donotmarkedly influence
^physiologyofthegrains.Itwasremarkable,however,thatafterripeningtheinbs^
8 V r 6arS(With0Ut8rainS)hadaM g h e r ^ W e i ^ thanthecontrols(Tain L be
IT" ° 9 7 8 )f0Und^ fafectod leaV6Sh a dah ^ e rNcontentatdeath.
2 veI t T
"aStagnati0nln*" P h l ° e m t r a n S P ° r t- a ***** «*P i ™ ° f«he
P U n r e d Si6VetUb6S(EVert6 ta l 1 9 6 8
hi
?
"
>- * « » s t ù - L ï g effect
aphidsalivaonthegrowthanddifferentiationofplantcells (Miles,1968).
38
5 QUANTITATIVE ASPECTS OF ATTACK BY SITOBION AVENAE F . AND I T S EFFECT ON
YIELD; THEIR RELEVANCE TO SUPERVISED AND INTEGRATED CONTROL
5.1 INTRODUCTION
Ecologicallyacropformswithitspestsanddiseases,aswellastheirantagonists,
parasitesandpredatorsonedynamicbiosystem,which,inturn,isinclose relationwith
theculturalconditions.
Inthelatter,climateandsoilarecombinedwith culturalpracticestoformacomplex,whichissubjecttomanipulation.From thispointofviewtheconceptofintegrated
controlhasbeendeveloped (Sternetal., 1959). Itisbasedontheideathatregulation
ofpestsanddiseases firstofallcanandshould takeplacebytheplant itself,byantagonists,pathogens,parasitesandpredatorsandalsobytheuseofappropriate cultural
techniques:choiceofcrot>andvariety,croprotation,soilcultivation,sowingdate,
quantityofseed,qualityandquantityofmanure etc.Chemical controlisonlyallowedas
alastresort,ontheconditionthatthechemical sparesasmuchaspossiblethenatural
limitingfactorsoftheharmful organism.
Onthebaseofearlyresearchdatathegapbetweenpreventive chamicalcontroland
integratedcontrolcanbebridgedbyasystemofsupervised control.Thiscanbrieflybe
characterizedaschemicalcontrolguidedbyexperts,aimingattheuseofchemicals m
themostefficientway.
Inbothsupervisedandintegratedcontroltheuseofthechemicalshouldbebased
onagoodunderstandingofthephenologyoftheinsectandonaprognosisofitspopulationdevelopmentandthedamageitmaycause.Aschemeofregularobservationsonthe
populationdynamicsofthepestisrequired.Incaseofanintegratedcontrolprogramme,
thepopulationdynamicsofparasitesandpredatorshavetobeincluded.Thedecisionto
sprayistakenassoonasthepopulationreachestheeconomicthreshold,i.e.thepopulationlevel,resultingineconomic injurytothecrop.Economic injuryisthedamage,
expressedintermsofmoney,clearlyhigher thanthecostsofacontrol treatment
(costsofthechemical,applicationandpossibledamagebythetreatment).
Thepurposeofthepresentworkwastodevelopasystemofsupervised controlof
cerealaphids,especially S. avenue inwinterwheat.Aprerequisitetothesuccessful
introductionofsuchasysteminpracticeisthatitfulfilsthefollowingdemands:it
hastoofferthegrowersclearadvantages overapplyinganinsecticidepreventlyincombinationwitharoutine treatmentwith fungicides,theremustbenoriskforimportant
economicdamageandthemethodofobservationhastobesimpleandnottimeconsuming.
OnmeetingsandinprofessionalmagazinesofgrowersandtheextensionserviceImentionedtheadvantagesofsupervised control;thedevelopmentofgreatnumbersofpolyPhagousnaturalenemiesalsoreducingtheneedforcontrolinothercrops,lesschance
forresistanceofthepestagainsttheinsecticides and
fer
anexplosive increaseinthe
39
numberofaphidsinprematurelytreatedfields.Thesepositiveeffectsbalancethepossibleextracosts.
Tomeetthegrower'sdemands,itwasnecessarytostudyextensivelytherelation
betweenattackanddamageandtofindsafeandquickmethodsofobservation,warningand
control.
5.2 TRIALSANDRESULTS
5.2.1 Methods in observation and warning
Attackbycerealaphidscanbequantifiedon.severalways.A fieldtrialwassetup
tofindabothquickandreliablemethod.ItsresultsaresummarizedinTable13.
Themeandrygrainweightpertillerof100tillerskeptaphidfreewithpirimicarbwas
42.1+1.8mgwhichwas 6%higherthantheweightoftheinfectedtillers.Becausethe
averageeffectoftheaphidswashardlybiggerthantherandomvariationinyieldper
tiller,littlecoherencewasfoundbetweenattackandyield.Thenumericintegralofthe
populationlevelinthecourseoftimeshowedthepoorestcorrelation.Probablythiswas
duetoprédationbysyrphidsandcoccinellidswhichwerepresentinlargenumbersespeciallyjustaftertheaphidpopulationshadreachedtheirpeak.Predatorsandtracesof
prédationwerepresentatthattimeinmorethanhalfoftheearcolonies.Fortheears
whereprédationhadtakenplacetheintegralcalculationunderestimatedtheintensityof
attack,whereasthemaximumnumberwasmuchlessinfluencedbyprédation.Itisobvious
fromTable13thatthisparametercorrelatedmostwiththeyield.Themaximumbiomass
correlatedlessbecauseitisamoremomentaryobservationandconsequently lessrepresentativeforthepopulationdevelopmentthanthemaximum.number.Themaximumnumberper
tillerwasnotmorecorrelatedwiththeyieldthanthemaximumnumberperear,because
leafinfestationwasrelativelylowandmoreoverleadslessquicklytodamagethanear
infestation (Chapter4).
IntheNetherlandsawarningsystemhasfunctionedfrom1974on.Everyyeartime
Table13. Correlationbetweenmeandrygrainweightpertillerof40tillersofwinter
wheatcv.Cariboanddifferentparametersfortheintensityofattackby S. avenae and
M. dirhodwn. Populationdevelopment includingagedistributionoftheaphidswasobserved
twiceaweekfromFe9tillFe 11.2.Onthebaseoflaboratoryobservationsthebiomass
ofthe4nymphalstagesandadultswasdetermined at0.1 mg,0.2mg,0.A mg,0.8mgand
1.0mgrespectively.Numericintegralswerecalculatedwithacomputerfollowingtime
stepsof1day.Afterripeningthemeandrygrainweightpertillerwasestablished,averaging40.2mgoverthe40tillerswithastandarddeviation (SD)of2.1.Allcorrelations
weresignificantat5%.
Intensityofattack
parameter
Coefficientofcorrelationwith
meanpertiller SD meandrygrainweightpertiller
maximumnumberontheear
51aphids
36 -0.45
max.numberonearandleaves
64aphids
38 -0.46
max.biomassontheear
21mg
13 -0.39
max.biomassonearandleaves
25mg
15 -0.38
numericintegralnumbersonear
520aphiddays 370 -0.35
num.int.numbersonearandleaves 685aphiddays 405 -0.33
40
%infected ears
99-1
50
0.1
I
10
mean
number
TTTTi of aphids
100/ear
Fig.12.Thepercentageofinfected earsasafunctionoftheaveragenumberof
aphids/earbeforethepopulationmaximum. Correlation coefficientR=0.94. Regression formulas;probity=1.58+1.52log100x,log100x=-0.69+0.59
probity.Calculations weremadeon49samplesof100ears, takenintheyears
1973-1977.
andintensityofinfestation appearedtodifferstronglyfromregiontoregion,probably
becauseofdifferencesinclimateandlandscape.However,alsowithinthevarious regions
largedifferenceswere found,evenbetweenadjacent fields.Sofactors likesiteofthe
field,varietyofwheatandfertilizationcouldalsoplayarole.These factsemphazise
theimportanceoftheregionalizationofthewarning systemandoftheprinciple that
afterawarningtheintensityofattackineachfieldoughttobedeterminedbeforea
decisionregarding controlistaken.Ineachregionof10-30.000hawheatabout 10-30
welldistributed fieldsaresampledweeklyfromthemomenttheearsbecomevisible.Regionalspecialists fromtheextensionservicecountineveryfieldtheaphidson25ears
takenatrandom.Thesizeofthesamplesisbasedontheavailabletimeandmanpower.
Thecalltothegrowerstoinspectthecropbeforestartingtospray,whichwas
broadcast togetherwitharadiowarning,seemednottohavehadtheexpectedresult.A
largepartofthegrowersconsidered counting aphidson25earsperfieldtoodifficult
andtimeconsuming.Theseobjections couldbemetbyrelatingthecurrentthresholdof
controlof15aphidsperearto95$oftheears infected.Thisnewcriterionisbasedon
thestrongcorrelation(r=0.94)betweenthemeannumberofaphidsperearandthepercentageofinfectedears,whichwas foundin49samplesof100earsperfieldtakenin
theyears 1973-1977 (Fig.12).
5.2.2 The effect
of chemical control on yield
Between 1973and1977eleven trialswereperformedtoinvestigatethebenefitof
controlwiththeselectiveaphicidepirimicarb,whichisrecommendedbytheextension
serviceandincreasinglyusedbythegrowers.Atrialcomprised 10blocksof2fields
each,oneofwhichwaskeptaphid free.Intheuntreated fieldstheearinfestationwas
determinedweekly.Thedifferenceinyieldbetweentreatedanduntreatedwaswellcorrelatedwith themaximimnumberofaphidsperearduringthemilkyripestage(r=0.61)
contrarytothenumberofaphidsatthestartofflowering (r=0.04).Thispopulation
wasnotconnectedtothemaximmnumber3or4weeks later(r=0.06)becauseofthe
varying influenceofclimate,migrationandnaturalenemies.Thecorrelationbetween
41
loss in yield
in kg/ha
500-
•
400
' ,
/ .
300
200-
•
^r
100-
0-100-
' ••
•
5 10 15 20 25 30 35 40
i aphids/e
'
Fig.13.Thelossinyieldasafunctionofthepeaknumberofaphidsperear.Correlationcoefficientr=0.75.Regression formula;y=15.51+10.97x.Calculations were
madewiththeresultsofIItrials,doneintheyears 1973-1977.Atrial comprised
10blocksof2plotsofwhich1waskept aphid freebyIor2treatmentswithaselectiveaphicide.Aphidpopulation developmentinuntreated plotswasfollowed weeklyby
random sampling 10earsper plot.
attackandyieldwasconsiderablyhigher (r=0.75,seeFig.13)whenitwasnotexpressedasthemeandifferenceintotalyieldinkg/habetween treatedanduntreated
fields,butasthemeanproportional differenceinaverage grainweightmultipliedby
theaveragetotalyieldinkgperuntreatedha.InChapter4ithas alreadybeenshownthat
theaverage grainweightistheonlycomponentofthetotalyield thatisnormallyinfluenced.So,theincreaseincorrelationisduetothelossinrandomvariation,caused
bydifferencesinthenumberofearsperareaandthenumbersofkernelsperear.
5.3 DISCUSSION
5.3.1 Methods in observation and warning
Rautapaa (1966)andlaterLatteur (1976)expressedtheintensityofattackinthe
so-called aphid-index,whichisthenumeric integralofthenumberincourseoftime.
Table13demonstrates thatthisparameter correlated lesswithyield thanthemaximal
numberofaphidsperear,which alsointhe11trialswithpirimicarbwaswellcorrelatedwiththelossinyield (r=0.75,Fig.13).Ifinthesetrialstheintensityof
attackwasexpressedintheaphid-index,thecorrelationcoefficient remainedthesame.
Latteur (1976)foundin32trialswiththeaphicidemenazon,thattheaphid-index calculatedonthenumbersofs. avenae ontheearwasobviously relatedwith lossinyield
(r-0.74).Fromhisdata,however,Icouldcalculate thatthemaximal'numbersperear
wereequallywellcorrelatedtothelossinyield (r=0.73).Basedontheseconsiderationsitseemstomethatthemaximalnumberperearisamore suitableparameterfor
aphidattack thantheaphid-index.Themaximalnumberdoesnothavetobecalculated
andisequallyaccurate.
Becauseaclearrelationcouldbedemonstratedbetweenthepercentageofears
colonizedbys. avenae andtheaveragesizeofthecolonies (Fig.12)thethresholdof
42
controlof15aphidsperearcouldbereplacedby95°softheearsinfected,whichis
moreeasilyandquicklydeterminedandassuchmoreacceptabletothegrowers.Asthe
methodisquick,thesamplesizecanbebrought from25to50earsperfieldwhichenhancesthereliabilityoftheestimates.Fig.12alsoshows thatthereisstillaconsiderablevariationintherelationbetweenthetwoparameters:forexamplethenumber
ofaphidsperearcorrespondingwith 90%oftheears infectedmayvary from6to20approximately.Sincetheregionalwarnings arebasedontheaverageof10ormoreobservations,thisvariationonlyplaysarolewiththeobservationsbytheindividual growers.Inmyopinionitisofrelative importanceasthenumbersperearmaydoublein
3days (Chapter 3 ) ,thegrowersgenerally count inaccuratelyandthebenefitofcontrol
depends less-onthenumberofaphidsatthetimeofspraying thanonthefurtherincreaseoftheaphidsincasethecropwasnotsprayed!
FreierandWetzel (1978)also foundaclearcorrelation (r=0.89)betweenthe
percentageofcolonized earsandtheaveragecolonydensity.Theirresultsareina
longrangeofinfestationinmarked agreementwithours (Table 14).Fromouranalysis
itseemed thatasigmoid curverepresentstherelationbest.Abovethelevelof5
aphidsperearFreier&Wetzeldisposedofrelatively littleobservationsandsupposed
thettherelationkeptbeingpurelyexponential,sofromheretheirresults diverge
increasingly.ThegrowingdivergenceoftheresultsoftheDutchextensionservice from
oursisprobablybasedonthephenomenon that lessexperienced observers aremore likelytounderestimatethenumbersthehighertheybecome.Nevertheless,Table14demonstratesclearly,thatintherelevantrangeofinfestationthepercentageofcolonized
earsisonaverageareliable indicationofthenumberofaphidsperear.
Because infestationvaries stronglyfromregiontoregionandevenwithinthe
regions from fieldtofieldthewarning systemisregionalizedandwitheverywarning
thefarmersareaskedtochecktheirfieldsfirstbeforetheydecidetospray.Forthe
samereasonitcouldbeusefultowarnevenwhenthecontrol thresholdisreachedin
onlyhalfoftheobservationfields.Thisreducesthechancethattheworst infected
fieldsaretreatedtoolate.Moreover,thegrowerswillbelessinclinedtospray
withoutapreceeding observation.
Table,4. Theaveragenumberof 8. aVenaeper• » " ^ « ^ . ^ 1 5 ^ 1 «
infectedears,acomparisonoftheresultsfrom3 \™™**£°™*™^"and
1\1and
was50,100and25earsrespectively.Thenumberofsamples ™*29b'
thecoefficientofcorrelationwas0.89,0.94and0.83respectively.
^
T- • «.tj„,-,»i
Author
Dutchextensionservice
&
"fecte^arf
0978) ^ ^
S »
<*
*«•« *"">
50
60
70
80
85
90
95
2.2
2.9
3.8
4.8
5.3
5.9
6.5
.
'-f
2.6
3 7
5.7
7.5
'O-*
16.9
'•*
2.1
l'\
4-3
5.4
'•'
"°^
43
S.3.2 Economic injury and economic threshold
Ingeneralathresholdofcontrolof15aphidsperearisrecommendedinwestern
Europe,whichismainlybasedontheworkofLatteur (1976).Heconsiders controlonlyjustified ifaphidsthreatentocauseadamageofat leasttwicethecostsofcontrol
becausenormally inhiscountrythecapitaloutput inwheatistwotimesaslargeas
theinput.ThushestatesthatinBelgium theeconomicdamagevariesbetween 260and
570kg/ha,dependingonthecostsofcontrol.Astatisticmodel allowedhim tocalculateforinfestationduringthewateryripestage,theprobability ofapopulation
developmentresulting ineconomicdamage.Hefound thatinSO«,ofthecasespopulations
of S. avonaa consistingatthisstageof6to18specimensperearexeeded the loweror
upperthresholdofeconomicdamage.Onthesegroundshehasput theeconomic threshold
at15aphidsperear.Thisissomewhataboveaverage,but itenhances thechancefora
remunerativecontrol.Somepoints inLatteur'sideasaredebatable.Thetremendousvariationintheeffectoftheaphids isnottakenintoaccount.Hestates thataloss
of570kg/haiscausedbyanaphid-indexof280,but itseemsfromhisdatathatthis
infestationleadstoalossof570+340kg/ha (p-0.1).Notmanygrowerswillaccept
sucharisk.Currentpracticeinchemicalcontrolshowsthatmostgrowersprefertoinvestarelativelysmallamountofmoneywith thechancetoloseitpartlyorall,than
doingnothingwiththechanceoflosingamultipleofthisamount.Anotherpointis,
thatmanygrowersarecontentwithalowerratiobetweenoutputandinputandwillconsequentlyputtheeconomicthresholdatamuch lowerlevel.Forthisreasontheeconomicthreshold inGreatBritain (George,pers.commun.)andintheD.D.R. Oïetzeletal.,
1978)hasbeenputat5aphidsperear.IntheD.D.R.itisraised to8aphidsperear
duringthemilkyripestage.ThethresholdofcontrolintheNetherlandswasratherarbitrarilyputat10aphidspertillerin1974andwasatmyproposal increased to15
aphidsperearin1976.Itappeared fromtheavailabledatathatsuchaninfestation
causesalossinyield thatstayswithinthelimitsofthecostsofcontrol equalling
100-250kg/ha.Fig.13showsthat15aphidsperearcauseadamageof180+230kg/ha
(P-0.1),which isestimated tobereducedby SO*however,ifthecrop istreatedwith
fungicidesbeforeflowering (Chapter 4). Inourexperience thislevelofinfestation
wouldnormallynotbereached,sothisthresholdofcontrolwould strongly reducethe
useofinsecticides.
M myopinionnorealeconomicthresholdcanbedrawnatthemoment,becausepopulationdevelopmentandsubsequentdamagecanasyetnotbepredicted accurately.Possiblysimulationmodels likethatofRabbingeetal. (inp r e S s )willprovidesuchpredictionsinthenearfuture.Inparticularthedamageneedsmuchmoreresearch. Its
strongvariationappearsfromthefactthatS. avenae populationswithamaximumnumberof1 perearcausealossinyieldof460•340kg/ha(P-0.1) accordingto
Latteur (1976)and 180•230kg/ha(p=o.l)according tous (Fig.13). The3main
causesforthisdiscrepancyareinthefirstplacethattheregression iscalculated
m e ral m i t e dnumbero fobservations .Jn^ ^ ^ ^
^ ^ ^ ^
rf
ZIZ ll
'""'JGStimate° f**nUmber° fa p M d s *" r e a l l y develop tothedetritheyield.Inparticularmigration,parasitismandprédationcauseavariable
44
numberofaphidstoeludeobservation.Tnemostimportantcauseinmyopinionisthat
thedamageperaphidisnotconstant.Thedamageresulting fromfeedingisusually
slightandmoreorlessstable (Chapter 4),buttheeffectofhoneydewseemshighly
variable.Theoreticallyhoneydewincombinationwithfungicanreducetherateof
photosynthesis,theavailablegreenareaandthedurationofthisarea.Sincealso
factorslikevariety,availabilityofwaterandminerals,weatherandattackbydiseasesinfluencetheseyielddeterminingmagnitudes,thehoneydewdamagedependsonthe
timeandtheintensitywithwhich itandtheotherstressfactorsoccur.Itcanbereadilysupposedthatthehoneydewdamage ismaximalwhentheotherproductionlimiting
factorsareabsent.ThiswasindicatedbytheresultsofateamofDutchresearchers
investigatingthepossibilitiestobridgethegapbetweentheaverageyield inpracticeof5.5 t/haandthepotentialyieldofapproximately 10t/haofwinterwheat.TV
fieldtrialsin1978,ayearwhenstressfactorsasdrought,heat,lodgingordiseases
wereonthewholeunimportant,yieldedeach7t/hawithoutanyuseofpesucidcs Cving
tothecoldandwetweather S. avenae developedslowlyandjustreachedamoderatemax
imumnuniberof 12and20perear (Mantel,pers.commun.) However,intheplotskept
aphidfreewithpiriMcarbyieldwas 1.4and1.1 t/hahigher (deVos,pers co«-«.).
Inpresent-daycropshoneydewactionnormallycoincideswithanumberofotherproductionreducingfactors,soitsultimateeffectislimited (Chapter4andPig.13).
Wheninthefuturegrowingtechniques,especiallycontrolofdiseasesareopt»,cd.
themeanproductionlevelwillriseconsiderablyandgrainaphidswill
**»*>«**
becomemore important.Notonlybecausetheyremainoneofthelastyieldreducn
factors,butalsobecausetheywillhavethechancetoreachhighernumbers m thec
long-growingandhighlyproductivecrops.Moreover,aspecieslike H. * > * * £ * » *
isstSl relativelyuni^ortantinourregions,mayalsoreachapest
*™**™
itseemstofavourcropswithhighdensitiesoftillersand leaves.In19 Rjsdijk
(pers.con^n.)foundinasampleof120wneatfieldswithanaverageyxeldof
Z t/ha thatinsome 15»ofthefieldsaninsecticidalt r e a t y was < — «
theroutineapplicationoffungicidesbeforefloweringw M c h leadtoamean nrase
inyieldofal.st 1t/ha.Considering^ fact^hatthe=
n^er.^s
^
perearwas0to20(Mantel,pers.commun.)itmustbecone»«..
Toldofcontrolof15aphidsperearistoohigh.Theonlysolutionseems be »
lowerthethresholdofcontroltoreducetheriskforseriousyield los.I n n r e n n
ionathresholdof801oftheearsinfected,equallinganaverageo
hul P* _
willbesatisfactoryfortheperiodbeforethemilkyripestage.Aftor *.t hc t h « .
holdmustberaisedto90»oftheearsinfected,equalinganeanof* » 10 £ n d .
perear (Table 4).A thresholdof5aphidsperearduringthe*.geo^
npc
willbeexceededalmosteveryyearoverthe«holecountryand„ 1 1 certainlybr g
mostgrwers totheconclusionthattheymightaswellmixthe i n s c U « ^
f u n g i S L m i s willenhancetheriskofreinfectionandtheneedfo ^ s ^ d j r t
ment.Itmayalsoleadtoresistanceoftheaphidsand increaseofothernox.ous
sects,becausenaturalenemiesarewipedoutoverlargeareas.
1978alsoshovedthatitistooriskytocountonthecollapseofth, ph,d
populationsaroundtheniddleofthemilkyripestage(Chapter3)andtoadvSsc
gainstcontrolfromthenon.Migrationofalataeappearedtobereducedbythelong
periodofcoldandwetweatherandsincecropdevelopmentwas alsoretarded theaphid
populationskeptincreasinguntiltheendofthemilkyripestage.Possiblyalsowing
inductionwasreducedintheseexceptional circumstances.Still itseemsuseful to
avoid treatmentsagainstpopulationswhichareneartheirtoporarealreadydeclining,becausetheyareawasteofmoneyand leadtothewell-knownsideeffects.Therefore,treatmentsfromthetimethethirdandsecondleafstartyellowingareill-adviced,asatthattimeonecanbesurethattheproductivity ofthecrop israpidly
decreasingandaphidswillnotbeabletoincreasemuchmore.
46
6 GENERAL CONCLUSION
Currentresults suggest that Sitobion avenae isthemost injurious cerealaphid
speciesasaconsequence ofitsstrongpreference fortheearincontrast totheother
twospecies.Thehighnutrientvalueofthephloem sapintherachisenables S. avenae
tomultiply atahigherratethantheothers,whichexplainswhy itisgenerallythe
mostnumerous species inwheat.Secondly,infestationoftheearleadstolossin
yieldatloweraphiddensities thaninfestationoftheleaves.Thirdly,thehoneydew
excretedby S. avenae ismainlydeposited ontheearandupperleaves,whicharethe
veryorganstoproduce thedrymatter forthekernels.Honeydewcaused inthreefield
trialsabouthalfofthedamagebystimulating fungalgrowth.
Aftercorrection forthefunguseffect,theestablished lossescausedby S. avenae
feedingontheeardidnot differmuch fromthosecalculatedonthebasisofitsenergy
budget.Therefore,itwas concluded thatstylet insertionandsalivainjectionhaveno
importantinfluence onthegrowthofthegrains.
Until1978Iassumed thatthedamageby S. avenae wasmoreorlesspredictable
basedonbiomassproduction andefficiency offoodutilization.The funguseffectcould
beneglectedbecause afungicide treatmentofthewheatatthebeginningofflowering
wasaroutinemeasure intheNetherlands.TheresultsofaDutchresearchgrouponthe
improvementofwheatyield (deVos,pers.commun.)andtheresultsoftheDutchgrowers in1978 (Rijsdijk,pers.commun.)changedmypointofvue.ForthefirsttimeI
wasconfrontedwithcrops thatdidnotripenprematurelyundertheinfluenceofdiseases,heatorlackofminerals andwater andtherebyyielded 2-3 t/hamorethantheaverage5-6 t/ha.Only inthese long livingcropshoneydewwasallowed thetimetorealize
itsfullyield-reducing capacity,considering theunexpectedhighlossescausedbybut
moderate infestations.Thiseffectsuggested thathoneydewhasalsoadirecteffecton
production.Inmytrialswith theusualcrops thehoneydeweffectwasapparentlyneutralized forthemajorpartby thecombinedeffectofvariousotheryield-depressing
factors.
Theconclusion isthataphiddamage isinterrelatedwithacomplexofotherstress
factors.This implicates thatthedamagewillonlybepredictable ifexceptaphidpopulationdevelopment alsocropgrowthcanbepredicted,dependingonfactorssuchas
availability ofwaterandminerals,weather,diseasesandhoneydew.Thesimulationmodelapproachseemsverypromising inforecastingbutisstill lackingmuch quantitative
information.
Growing techniques areimprovingrapidlyandsoisyield.Consequently,anincreasingsignificance ofhoneydewcanbe expected.Therefore,Ifinallysuggestthatthecurrentcriterionofcontrolshouldbe loweredfrom 15aphidsperearto5and 10aphids
perearequalling 801and90Softheears infected fortheperiodbeforeandduringthe
47
milkyripestage,respectively.Until theabovemodels are available,thisadvicemay
serveasapreliminary compromisewith thedemand ofthegrowers,not toriskaserious
lossinyieldandthedemandoftheenvironment tomakeminimal useofpesticides.
48
SUMMARY
Influenae of the host plant on the population development of Sitobion avenae F.,
Metopolophium dirhodum Wlk.j and Rhopalosiphum padi L.
Intrialsinaclimateroomthepopulationdevelopmentof3cerealaphidspecieson
wheatwasstudiedfromthetimetheearbecamevisible.Atthesametimethepossibleinfluenceofflag leafandearassubstrate,theageoftheplantandnitrogendressingwas
established.SpecialattentionwaspaidtotheefficiencyoffoodutilizationbyS. avenae.
Beforethemilkyripestage, M. dirhodum multipliedontheflagleafinnumberstwice
asquicklyandinbiomass3timesasquicklyasS. avenae and R. padi. Ontheear,however, S. avenae multiplied twiceinnumbersandmorethan3timesinbiomassmorequickly
thanontheflagleaf.OntheearR. padi multipliedinbiomasslessthanhalfasquickly
as S. avenae. Inthecourseofthemilkyripestage,themultiplicationrateontheflag
leafdecreasedstronglywith M. dirhodum andhardlychangedwith S. avenae. Ontheear,
themultiplicationratedecreasedbothwith R. padi andwith S. avenae, butwith R. padi
thedecreasewasstronger.Inthesecondhalfofthemilkyripestage,themultiplication
rateofS. avenae ontheearhaddiminishedbyhalf.
In M. dirhodum and R. padi alateproductioninreactiontocrowdingwas lessrapid
thaninS. avenae, whichproducedalataesoonerontheflagleafthanontheear.Nymphal
mortalityinM. dirhodum and R. padi wastwiceashighasinS. avenae.
Ontheflagleaf, S. avenae produced2to3timesasmuchhoneydewasontheear.Of
thenitrogen,phosphorusandpotassiumpresentinthefood,nitrogenwasutilizedbest:
for651fromtheflag leafsapandfor83°«fromtherachissap.Inviewoftheenergy
budgetofS. avenae, itwasestimatedthatfortheproductionof1mgofaphid,5mgof
phloemsapfromtheearand11mgofphloemsapfromtheflagleafwasneeded,basedon
drymatter.Basedoncaloricvalue,291and13?ofthissapwouldbeconvertedintoaphid
drymatter,respectively. M. dirhodum feedingontheflagleaf,utilizeditsfoodabout
asefficientlyas S. avenae atthesamefeedingsite.Efficiencyoffoodutilizationby
S. avenae ontheearincreasedandalateproductionandconsequentlymigrationdecreased,
whentheplantwasgivenanextranitrogendressing.Thoughtherewasalsoariseinmortality,theincreaseinthenumberofapterousadultsinthecoloniesledtoamarkedly
higherpopulationlevelinthenextgeneration.
Population development of Sitobion avenae F., especially
in relation to crowding
Intheriver-clayareaintheenvironmentofWageningen,fieldobservationsweremain
the
years
1974
to
1977.
Migrants
of
Sitobion avenae
avenaecolonized
colonized
winter
wheat
especialdein
the
years
1974
to
1977.
Migrants
of
Sitobion
winter
wheat
espec
j.._.__
,
,.
u-j
,
•
*
,
.
™
,
t
in
the
weeks
after
flowerlyduringheadingandflowering.Immigrationreacheditspeakintheweeksafterflower
49
ing.Inviewofthelargenumberofapterous adults subsequently borninthecrop,immigrationinthisphasecouldhardlycontributeanymoretopopulation growth.
Fromfieldobservationsandlaboratory trials,thefollowing general conclusions
couldbedrawn.Assoonastheearsarevisible,theimmigrantsprefertosettleonthem.
Apterous adultsdevelopingontheleavesalsomovetotheear.Ontheleavesarelatively
smallnumberofaphidsremains. S. avenae isverysusceptibletocrowding:incolonies
withanaverageof5or15individuals, 50%or95%ofthenewbornnymphs developwings.
Consequently thesecondgenerationdevelopingontheearisalreadymostlywinged.The
majorityoftheyoungalataeemigratesothenumberofadults increases less strongly.
Atacertainmomentmoreadultsdiethanareaddedtothecolonywhich reaches.its
maximumdensitywhentherapidlyaccumulatingsumofemigrationandmortalityisbalanced
bythedecreasingbirthrate.Afterthat,aquickpopulation decrease takesplace.
Fromlaboratory research,itseemsthatcrowding alsoadversely effects longevity,
weightandtherebyrateofreproduction.However,therateofreproductionisalsodirectlyreduced.Iftheinfluenceofnatural enemiesandtheripeningoftheeararenot
takenintoaccount,wing formation followedbyemigration doesnotonly limitpopulation
growthbutcaneveneffectapopulationdecreaseofmore thanhalf.
Injury and damage to wheat, caused by Sitobion avenae F., Rhopalosiphum padi L. and
Metopolophium dirhodum Wlk.
Fieldandlaboratory trialsoninjuryanddamagewereperformedwithwheat fromthe
stageofflowering onwards.Aphid infestationcanleadtoadecreaseintheaverageweight
ofthegrains.Normally thenumberofgrainsisnotinfluenced. Comparingtheeffecton
yieldofR. padi andS. avenae bothfeedingontheear,ortheeffectofM. dirhodum and
S. avenae feedingontheflagleaf,nodifference couldbedemonstrated. Infestationof
theearleadstolossinyieldatlowerpopulationdensities thaninfestationoftheflag
leaf. S. avenae livesmainlyontheear,incontrasttothetwootherspecies,andis
thereforecomparatively themost injurious.Therewasnoeffectoftimeofinfectionand
nitrogendressingonthedecreaseinyieldatcomparable densities.
Honeydewontheplant stimulatesthedevelopmentoffungi,especiallyblackmoulds.
In3fieldtrialswith S. avenae thiscaused aboutasmuchdamageastheuptakeof
sap.Aftercorrectionforthemouldeffect,theestablished losses causedby S. avenae
didnotdiffermuchfromthosecalculatedonthebasisofitsenergybudget.Therefore,
itcanbeconcluded thatstylet insertionandinjectionofsalivahavenoimportantinfluenceonthegrowthofthegrains.
Quantitative aspects of attack by Sitobion avenae F. and its effect
relevance to supervised and integrated
control
on yield;
their
Theso-called aphid-indexofRautapaaislesssuitabletorelatedamagetothe
intensityofattack.Itisbettertousetheaphidpopulationmaximum,whichiscorrelatedtodamagetothesameextentbutdoesnothavetobecalculated.
Inasystemofsupervisedcontrolthecriterionfortreatment canbestbeexpressed
50
asthepercentageofinfectedears. Itisstronglycorrelatedwiththeaveragenumber of
aphidsperearandcanbedetermined relatively simplyandquickly.
Aphiddamageshowsastrongvariation,whichmainly seemstobe causedbythe effect
ofhoneydewdependingonthepresence andintensity ofanumberoffactors,which also
reduceplantproduction.Becausemuchquantitative informationonthepopulation dynamics
oftheaphids,theproductionofthecropand the influence ofthehoneydew isstill lacking,nopredictions concerningdamagecanbemadeyet.Hencenosatisfactory economic
thresholdcanbedrawn.
Aninsufficientwarning systemmay leadto lossofgoodwill totheextensionserviceandtosupervised control.Thereforea lowriskcriterion forcontrol seemstobe
themostappropriate solutionforthetimebeing. 801and901oftheears infected are
safethresholds fortheperiodsbefore andduring thestageofmilkyripe,respectively.
51
SAMENVATTING
Dealgemenebezorgdheid onderdetelersoverdesoms explosieve toenamevangraanbladluizenindeafgelopen jarengafoverdegehelewereld aanleidingtotonderzoek.
Mijnonderzoekstarttein1974aanhetInstituutvoorPlantenziektenkundig Onderzoekte
Wageningen.Hetwasbedoeldomdevoornaamsteproduktiefysiologische enproduktieecologischeaspectenvandebladluis-tarwe-relatieoptehelderen.Eendergelijk fundamenteel
onderzoekwerd geachtdekennistekunnenopleverendienodigisvoordeontwikkeling
vangeleideengeïntegreerdebestrijdingsschema's.IneenlaterstadiumkwamsamenwerkingtotstandmetonderzoekersenstudentenvandeLandbouwhogeschool.
Hetonderzoekprogrammawerd opgezetronddevolgendevragen.Watisdebetekenisvan
temperatuur,parasieten,predatorenenpathogenen,waardplantendichtheidsafhankelijke
effectenvoordepopulatieontwikkelingvandebladluizen?Welke factoren zijnbetrokken
bijhetverliesaanopbrengstenwat zijn"dekwantitatieve aspectenvandeopbrengstderving?Rabbingeetal. (1979)bestudeerdendepopulatiedynamiekvandebladluizenmet
betrekkingtottemperatuur,natuurlijkevijandenenpopulatiedichtheid,enmaaktensimulatiemodellen voordepopulatiepntwikkelingendeopbrengstderving. Ikonderzochtde
invloedvanstikstofbemesting,groeistadiumvandeplantenplaatsvanaantastingopde
populatieontwikkeling (hoofdstuk 2). Ikbepaalde tevensdeinvloedvandeze factorenen
ookvandesoortbladluisendehoningdauwafscheiding ophetverliesaanopbrengst
(hoofdstuk 4).Uitditonderzoekbleekdatdegrotegraanluis (SitobionavenaeF.)de
meestschadelijkesoortwas.Daaromwerdvandezebladluisheteffectvandedichtheid
opdepopulatieontwikkeling (hoofdstuk 3)enopdeschade (hoofdstuk 5)nader bestudeerd.
De invloed van de waardplant op de populatieontwikkeling
Metopolophim dirhodum Wik. en Rhopaloeiphvm padi L.
van Sitobion avenaeF.,
Inklimaatkamerexperimenten werddepopulatieontwikkelingvande3soortenbladluizen
opwintertarwebestudeerd.Hierbijwerdnagegaan inhoeverrevlagbladenaaralssubstraat,deleeftijdvandeplantendestikstofbemestingvaninvloedwaren.VanS.avenae
werdhetvoedingsrendement onderzocht.
V66rhetmelkrijpestadiumvermeerderdeM.dirhodum zichophetvlagbladinaantal
tweemaalzosneleninbiomassabijnadriemaalzosnelalsS.avenaeenR.padi.Opde
aarvermeerderde S.avenae zichechterinaantaltweemaaleninbiomassa ruim driemaal
zosnelalsophetvlagblad.OpdezevoedingsplaatsvermeerderdeR.padi zichinaantal
evensnelalsS.avenae,maar m biomassabijnadehelft langzamer.
52
Indeloopvanhetmelkrijpestadiumbleefdevermeerderingssnelheidophetvlagblad
bijS.avenaevrijwelongewijzigdennamdezebijM.dirhodumsterkaf.DevermeerderingssnelheidnambijS.avenaeenR.padiopdeaareveneensaf,maarbijR.padiwasdeafnamegroter.AanheteindevanhetmelkrijpestadiumwasbijS.avenaeopdeaardevermeerderingssnelheidtotdehelftgedaald.
R.padienM.dirhodumblekenveelmindersnelmetproduktievangevleugelde adulten
tereagerenoptoenameindichtheiddanS.avenae.S.avenaewerdophetvlagbladeerder
totvleugelvorming geprikkelddanopdeaar.BijR.padienM.dirhodumwasdelarvale
sterftetweemaalzohoogalsbij.S.avenae.
OphetvlagbladproduceerdeS.avenaetweeàdriemaal zoveelhoningdauwalsopde
aar.Vandestikstof,fosforenkaliuminhetvoedselwerddestikstofhetmeestbenut:
voor65«.uithetzeefvatsapvanhetvlagbladenvoor 831,uithetzeefvatsapindeaarspil.
OpgrondvanhetenergiebudgetvanS.avenaewerdgeschatdatopbasisvandrogestof
voordeproduktievan1mgbladluis5mgzeefvatsapuitdeaaren11mgzeefvatsapuit
hetvlagbladbenodigd is.Vanhetonttrokken zeefvatsapzouopbasisvandeenergetische
waarde29resp. 13%wordenomgezetinbladluisbiomassa.NaarschattingwasophetvlagbladhetvoedingsrendementvanM.dirhodumongeveer gelijkaandatvanS.avenae.
ExtrabemestingvandeplantmetstikstofverhoogdebijS.avenaehetvoedingsrendementopdeaar,remdedevleugelvormingenalsgevolgdaarvanookdeemigratie.Alhoewel
ookdesterftetoenam,leiddedestijgingvanhetaantalongevleugeldeadultentochtot
eenduidelijktoenameinhettotaalaantalbladluizenvandevolgendegeneratie.
Depopulatieontwikkeling
van Sitobion avenae F. met name de invloed van de dichtheid
Indejaren 1974-1977werdenveldwaarnemingenverrichtinhetrivierkleigebiedinde
omgevingvanWageningen.Migrantes koloniseerdendewintertarwevoornamelijk gedurende
deperiodevanhetin-de-aar-komenendebloei.
Deimmigratiebereikteeentopindeperiodenadebloei.Gezienhetreedsaanwezige
aantalterplaatsegeborenongevleugeldeadultendroegindezefaseimmigratienauwelijks
meerbijtotdepopulatiegroei.
Medeopgrondvanlaboratoriumexperimentenkondendevolgendealgemene conclusieswordengetrokken.Zodradearenzichtbaarworden,vestigenzichdeimmigrantesbijvoorkeur
hierop.Deongevleugelde adultendiezichopdebladerenontwikkelenbegevenzichooknaar
deaar.Opdebladerenblijfteenrelatiefkleinaantalbladluizen aanwezig.
S.avenaeiszéérgevoeligvoortoenameindichtheid:inkoloniesvan5respectievelijk
15individuenontwikkelt 50%respectievelijk 951vandenieuwgeborenlarvenvleugels.Daardoorisreedsdetweedegeneratiediezichopdeaarontwikkeltgrotendeels gevleugeld.
Jongegevleugeldeadultenhebbendeneigingteemigreren.Daardoorneemthetaantaladultenindekoloniesteedsminder toe.Opeengegevenmomentverdwijnenermeeradultendan
erbijkomen.Depopulatiebereikteenpiekwanneerdesnelgroeiendesomvanemigratieen
sterfteinbalansisgekomenmetdeafnemende geboorte.Daamavolgteensnelleafname.
Uitlaboratoriumonderzoek bleekverderdattoenameindichtheidtotgevolgheeftdat
delevensduur,hetgewichtendesnelheidvanvermeerdering afnemen.Devermeerderingssnelheidwordt zowelviahet gewichtalsrechtstreeksbeïnvloed.Onafhankelijkvandern53
vloedvannatuurlijkevijandenendeafrijpingvandeaarblijktdeproduktievangevleugeldedierenonderlaboratoriuraomstandighedennietalleentoteenverminderdepopulatiegroeimaarzelfstoteenpopulatieafnamevanmeerdandehelfttekunnenleiden.
Schade aan de tarweopbrengst als gevolg van aantasting
door Sitobion avenae F.,
Metopolosiphum dirhodum Wik. en Rhopalosiphum padi L.
Deexperimentenwerdenuitgevoerdmetwintertarwevanhetstadiumvandebloeiaf.
Aantastingdoorgraanbladluizenbleektekunnenleidentoteenverlagingvanhetgemiddeldgewichtvandegraankorrels.Hetaantalkorrelsperaarwerdnietbeïnvloed.Erkon
géénverschilinschadelijkheidwordenaangetoondbijaantastingvandeaartussen
S.avenaeenR.padienevenminbijaantastingvanhetvlagbladtussenS.avenaeen
M.dirhodum,dienietopdeaarvoorkomt.
Aantastingvandeaarbleekbij lageredichthedentotopbrengstverliesteleidendan
aantastingvanhetvlagblad.OmdatS.avenaeintegenstellingtotdeanderetweesoorten
voornamelijkopdeaarvoorkomtisdezesoortnaarverhoudingdushetmeestschadelijk.
Debetekenisvantijdstipvaninfectieenstikstofbemestingvoordeschadebijgelijkblijvendedichtheidkonnietwordenaangetoond.
Honingdauwopdeplantstimuleertdeontwikkelingvanmetname zwartschimmels.Dit
veroorzaakteineendrietalveldproevenmetS.avenaeongeveerevenveelschadealsde
opnamevansap.Nacorrectievoordeschimmelinvloedblekendegeconstateerde verliezen
weinigaftewijkenvandeverliezenberekendopgrondvanhetenergiebudget.Hieruit
konwordengeconcludeerddathetaanprikkenvandeplantendeinjectievanspeekseldoor
S.avenaeopdeaargeenbelangrijkeinvloedhebbenopdegroeivandekorrels.
Kwantitatieve aspecten van aantasting en schade door Sitobion avenae F. hun betekenis
voor geleide en geïntegreerde
bestrijding
De zogenaamdebladluisindexvanRautapaaismindergeschiktomdeschadeinverband
tebrengenmetdematevanaantasting.Menkanhiervoorbeterhetmaximumaantalvande
bladluispopulatiegebruiken,dateenevengoedesamenhangmetdeschadebezitmaarniet
hoefttewordenberekend.
Ineensysteemvangeleidebestrijdingkanhetbestrijdingscriteriumhetbestewordenuitgedruktinhetpercentageaangetastearen.Hetissterkgecorreleerd aanhetgemiddeldaantalbladluizenperaarenkannaarverhoudingeenvoudigensnelwordenvastgesteld.
Debladluisschadeiszeerwisselend.Devoornaamsteoorzaakhiervanlijktheteffectvandehoningdauw,datafhangtvandeaanwezigheidenintensiteitvaneenaantal
anderefactorendieookopbrengstverlagendwerken.Omdaternogonvoldoendekwantitatieveinformatieoverdepopulatiedynamiekvandeluizen,deproduktievanhetgewas
endeinvloedvandehoningdauwbeschikbaaris,kunnennoggéénvoorspellingenoverde
schadewordengedaan.Daaromkanooknoggéénbevredigendeeconomische schadedrempel
wordenaangegeven.
54
Eenonvolkomenwaarschuwingssysteemkanleidentotverliesaangoodwillvoorde
voorlichtingsdienstengeleidebestrijding.Omdezeredenlijkteenbestrijdingscriterium,datweiniggevaarinhoudtvooropbrengstdervingvoorlopigdemeestgeschikteoplossing.Eenaantastingvan 80°svandeareniseenveiligbestrijdingscriteriumvoor
deperiodevóórhetmelkrijpestadium.Tijdenshetmelkrijpestadiummageenaantasting
van901vandearenwordengetolereerd.
55
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58
STELLINGEN
1.Vandedriemeest algemenesoortengraanbladluizenisdegrotegraanluis
{Sitobion
avenae F.)hetschadelijkst.
Ditproefschrift.
2.Overbemestingvantarwemetstikstofstimuleertdegrotegraanluistothetbereiken
vanhogereaantallen.
Ditproefschrift.
3.Graanbladluizenmoetmenmeervrezenomhunhoningdauwafscheidingdanomhunvoedingsgedrag.Ditgeldttemeernaarmatedekorrelvullingsperiodevanhetgewaslangerwordt.
Ditproefschrift.
4.Devoorkeurdiedegrotegraanluisvertoontvoordeaarisvangrotewaardevoorhet
voortbestaanvandezesoort.
Ditproefschrift.
5.Hetefficiëntbestrijdenvangraanbladluizenheefttotgevolgdatdebestrijdingvan
afrijpingsziektenminderwinstoplevert.
6.Preventievebestrijdingvangraanbladluizen tijdensdebloeivanhetgewashoudtbelangrijkeecologischeenfinanciëlerisico'sinenmoetderhalvewordenafgeraden.
7.Geïntegreerdebestrijdingmaaktdelandbouwminderkwetsbaarvoorschaarsteaanenergieengrondstoffenencreëerttegelijk"schone"werkgelegenheidvoordemiddelbareen
hogereopleidingsniveau's.
8.Alshetdeoverheidernstismethetbevorderenvangeïntegreerdebestrijdingdient
zijzichnietalleenmethetonderzoekmaarookmetdeintroduktievandezebenadering
bezigtehouden.
9.Hettoepassenvanhetprincipe"devervuilerbetaalt"opdevervaardigingenhetgebruikvanbestrijdingsmiddelenindelandbouwverdientalsbeleidsinstrument aanbevelmg.
10.Alternatievelandbouwmethodenbehorenalleenalinstandtewordengehoudenomdatze
onmiskenbaareenbronvaninspiratieengelukvormenvoordevelendiezebeoefenen.
11.Hetponerenvanstellingeniseenachterhaaldevormvanimponeergedrag,zekeralsde
promovendusnietinrokverschijnt.
ProefschriftvanP.H.Vereijken
a n d f ^ r % f f*****?»«»«KWF.,Metopolophium dirhodumWik.and Rhopalosiphwn padi L.,
anathexreffectonyieldofwinterwheat.
Wageningen,IAmaart1979