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Pesticide Effects on Plant Physiology:
Integration into a Pest
Management Program
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SUMMER 19Ho
ABSTRACT Consideration of subtle, nonvisual pesticide effects on crop physiology and yield has generally been ignored
by entomologists responsible for developing pesticide recommendations.
The
literature since 1979 on nontarget pesticide effects was reviewed with respect to
plant physiology. Suggestions concerning
integration of these effects into present
pest management programs are made.
U
seof synthetic organic pesticides
in modern agriculture has allowed the use of crop varieties
that have a higher yield per hectare and
higher susceptibility to damage caused by
arthropods, weeds, and plant diseases (Pimentel et a!. 1978, McEwen 1978). During
the so-called "era of optimism" (19461962) (Metcalf 1980), the detrimental aspects of pesticide use were either ignored
or given little attention by many in the
entomological community. However, the
increasing frequency of insecticide resistance, pest resurgence (of both primary
and secondary pests), and a more general
awareness of environmental problems led
to the "era of doubt" 0962-1976)
(Metcal f 1980). This era was characterized by
a more skeptical attitude towards insecti-
Vincent P. jones is affiliated with the
Dep. of Biology, Utah State Univ., Logan,
UT 84322-5305. Nick C. Toscano and
Roger R. Youngman are with Cooperative
Extension, Univ. of Cahfornia, Riverside,
CA 92521. Marshall W johnson is with the
Dep. of Entomolog)', Univ. of Hawaii at
Manoa, Honolulu, HI 96822. Stephen
Welter is with the Dep. of Entomolog)J,
UI/iv. of Cahfornia, Berkeley, CA 94720.
cides and their use in agro·ecosystems,
The "era of integrated pest management"
C1976-present)
(Metcalf
1980)
has
brought insecticide use into a more reasonable perspective, where the insecticide is one of a variety of possible control
measures and positive and negative aspects of applying a particular pesticide
are considered for possible side effects
before application.
Numerous studies have documented
the ecological implications associated
with pesticide application to insect pest
populations. These have concentrated on
1) destruction of natural enemy complexes that can lead to secondary pest
outbreaks as well as an increased dependence on insecticide use for control of pest
populations (McMurtry et al. 1970, Doutt
and Smith 1971); 2) genetic selection that
leads to pesticide resistance (Georghiou
1972, Brown 1971); and 3) sublethal ef·
fects on pest populations that increase
their reproductive capacity or decrease
reproduction
in predator populations
(Huffaker et al. 1970, Maggi and Leigh
1983, Jones and Parrella 1984). These
problems have resulted in the establishment of pesticide use recommendations
based not only on pesticide efficacy
against the pest, but also with regards to
natural enemy conservation (Huffaker et
al. 1970, Hoyt and Caltagirone 1971, Croft
and Hoyt 1978, Huffaker and Smith 1980,
McMurtry 1982, Jones and Parrella 1983).
Despite increased awareness of pesticide-induced problems, some entomologists have remained generally unaware or
have ignored subtle, nonvisual pesticideinduced changes in plant physiology and
concomitant changes in plant yields. A
large number of studies concerned with
visible phytotoxicity of compounds (par-
103
ticularly on ornamental plants) have been
published in the Entomological Society of
America's publication, Insecticide and
Acaricide Tests. However, only six studies
concerned with the more subtle effects of
pesticides on crop physiology appeared
in Enl'ironmental Entomology and Journal of Economic E1ltomology from January 1980 to December 1984.
The purpose of this paper is to present
work published since the review of Ferree
(1979) (with the inclusion of two comprehensive
papers published
before
1979) and to indicate how pesticide-induced changes in plant physiology can be
incorporated into the economic threshold
concept of Stern et a]. (1959) and van den
Bosh and Stern (1962).
Pesticide Effects on Plant Physiology
Many insecticides/acaricides
may be
described as general biocides, with the
effects on nonarthropod organisms dependent on the concentration of the compound in question. Before development
for marketing, insecticidal compounds
are routinely screened for biological activity against many organisms including
weeds, fungi, and nematodes. These compounds may show herbicidal action at one
concentration and insecticidal action at
another concentration. Some compounds
may be excellent insecticides, but poor
herbicides and are, thus, marketed only
as insecticides.
The mode of action of most pesticides
that affect plant physiology was broadly
characterized by Boger (1979) as "acting
quite specifically on certain components
of the photosynthetic redox chain" and
more specifically by Murthy (1983) as
"either inhibitors of photosynthetic electron flow at the site of the primary
quencher (Q) of photosystem II and before the reduction of plastoquinone or
inhibitors of electron flow after the functional site of plastoquinone, inhibiting the
reoxidation of the plastoquinol pooL"
While orientation of most entomologists to pesticide-induced
reduction in
crop yields does not require knowledge
of the eXact site of action, information on
the physiological aspects can be obtained
from Anonymous (1979) and the review
of Murthy (1983) and the references contained therein.
Various types of selective activity
caused by photosynthesis-inhibiting
herbicides were characterized by van Oorschot (979). He indicated that cuticle
structure and composition, size, and distribution of stomata, and additives to the
spray solution may affect herbicide pen104
etration into the leaf. This means that the
formulation or adjuvants may be responsible in some cases for the noted effect
on plant physiology. Differences in transpiration rate can also be important because they may affect absorption and
translocation of the herbicide throughout
the plant.
To qual1lify a pesticide's effect on plant
physiology and because of the difficulties
in chemical determination of herbicide
(or other pesticide) metabolism, van Oorschot (1979) stated " . . . In the case of
inhibitors of photosynthesis, inactivation
in the leaves can be readily established
with equipment suitable for continuous
measurement of photosynthesis. Usually
a distinct inhibition of photosynthesis in(Hcates a toxic amount of herbicide has
entered the leaf. When uptake is stopped,
subsequent recovery from inhibitors is a
measure of herbicidal inactivation (from
plant metabolism) in the leaves provided
the contribution of new leaf growth is
negligible during a test period of sufficiently shon duration .... " These methods were most commonly used in the
studies reviewed in following sections,
where either some form of infrared gas
analyzer or quantification of the uptake of
radio-labeled isotopes was used to quantify photosynthesis rates.
Orchard Crops
Apples. The most comprehensive study
of pesticides on a single crop was per·
formed by Ayers and Barden (975) on 1year-old apple seedlings, Malus pumila
Mil!., in a greenhouse environment. Only
12 of the 33 pesticides studied (which
included insecticides, acaricides, and fungicides) induced changes in photosynthetic rate. Four increased photosynthetic rate and the other eight caused a
reduction. It is interesting to note that
of the 12 pesticides that changed the
photosynthetic rate (maximum change
indicated in brackets) (carbaryl 50WP
[+16%], chloropropylate
2EC [-56%],
maneb 80W [+7%J, dikar SOW [-12%],
karathane 25W [-10%], formetanate hydrochloride 92SP [+17%], parathion 8EC
[-11 %], superior oil [70s] [-73%], phosalone 3EC [-47%], cyhexatin 50W [+16%],
oxythioquinox 25W [-8%], and sulfur
95W [-6%]), all are insecticides or acaricide/fungicide combinations except maneb. Photosynthetic rate determinations
were made at 0, 1, 6, and 11 days posttreatment, thereby quantifying the rate of
recovery from pesticide-induced changes.
With most insecticides, the peak reductions were found 6 days posttreatment,
with a trend towards recovery of normal
photosynthetic activity occurring 11 days
posttreatment.
Sharma et al. (978) also tested several
pesticides on l-year-old apple seedlings,
M. domesticus Bork. They found that of
the 37 pesticides tested, S increased photosynthetic rate and 20 decreased photosynthetic rate by >S% 28 h following application. The insecticide/acaricides
diazinon SOWP,dicofoI42%EC, and superior
oil (70 s) caused a respective 26.6, 23.6,
and 66.3% reduction in photosynthetic
rate while Ieptophos 30.6%EC and propargite 30WP caused 13.3 and 1S% reduction, respectively.
Pecan. The importance of pesticide formulations was demonstrated by Wood and
Payne (1984) working on 'Curtis' pecan
seedlings CalJ'a illilloellsis
(Wang.) K.
Koch. They investigated the effect of monocrotophos 5EC, methomyl I.HEC and
90SP, phosalone 3EC and 2SWP, carbaryl
50WP, azinphosmethyl 2EC, fenvalerate
2AEC, petroleum oil (70 s), dimethoate
25WP and 2.67EC, and a combination of
fenvalerate and petroleum oil on the photosynthetic rate in three sepawte experiments. In all three experiments, the photosynthetic rate observed 1 day after tre:ltment of treated plants was significantly
lower (the reduction ranged from 1S to
30% depending on the treatment and experiment) than the untreated controls. In
the first experiment at 9 days postapplication, the photosynthetic
rate of the
plants treated with the wettable powder
formulations of methomyl and phosalone
recovered to control levels, while the respective rates of emulsifiable concentrate
formulations were still ca. 20% below that
of the untreated control. In the second
experiment at 9 days postapplication, the
carbaryl (WP) and fenvalerate (EC) treatments were 20% below the control rates
and petroleum oil waS 40% below that of
the controls, but the plants treated with
azinphosmethyl (EC) had recovered. The
third experiment
demonstrated
that
plants treated with the WI' formulation of
dimethoate recovered by 7 days postspray,
while those treated with the dimethoate
(Ee), fenvalerate (EC), and the fenvalerate-oil combination required another 7
days to recover to the control levels.
Wood et a\. (19H4) investigated the effect of a single application of fungicides
on 'Curtis' pecan seedlings. The fungicides propiconazole
3.6EC, etaconazol
1.125EC, benomyl 50WP, triphenyltin hydroxide 30WP and 4L, fenarimol lEC,
chlorothalonil 50H, and dodine 6SWP
were evaluated 1 and 9 days following
application. One day following applica-
BULLETIN OF THE ESA
tion, all treatmL'nts except propiconazole
and etaconazole reducL'd leaf photosynthesis rates significantly, with the average
redul'lion ranging from 2'i to 3'5%. The
Iwnomyl-treated plants rL'l'(wered to control levels by 9 days, but the triphenyltin
(both formulations), fenarimol, chlorothalonil, and dodinL' trL'atments were still
2·+ ·+0%below the untreated control levt'ls at that time. Based on the recovery
time observed by Wood and Payne (198--1)
working with insecticides, and the results
of their study on fungicides, Wood et al.
( 198·+)concl uded that pecan physiology
was mort' sensitive to fungicide application than to insel'licide application.
\X'ood and PaynL' 098·+) and Wood et
al. (198·+) also pointed out the possible
effel'l of pesticides on tree crops the year
following IWsticitk application. This was
hypmlwsized to occur because the reduction in net photosynthL'sis can affect the
:1I110untof stored carbohydrates. This type
of delay in expression of spider mite feeding damage has been shown to occur on
walnuts (Barnes and Moffitt 1978) and
almond (Barnes and Andrews 1978, Welter l't al. 198·+).
Citrus. Jont's et al. (983) compared
the effects of propargite 30WI>, dicofol
1.(lE, and NI{·++O oil on the photosynthet ic rate of I 'i-year-old 'Lisbon' lemons,
Citrus lill/on (I..) Burmann, and fenbutatin-oxide 'iOWP, dicofo( I.GE, oX)1hioquinox 2<;\X', and NR-+-10on the phmosynthelie rate of 17-year-old 'Valencia' or:lIlges, C'. sil/el/sis (I..) Osheck. They
found that the k'mons were rather insensitive to pesticide-induced changes in
photosynthetic rate, hut the propargite
treatl1wnt showed an increase of 26.8 and
2(l.6% in mesophyll conductance (which
is a nwasure of intL'rnal CO2 transport)
and photosyllthetic rate, respectively, 12
days postappl ieation.
'Valencia' oranges were much more
sensitive to :Icaricide appl ication. Three
days following pesticide appl ication, the
fenbutatin-oxide treatment showed a 12
and 9% increase in stomatal conductance
(which is an indicator of stomatal opening) and photosynthetic rate, respectively,
and was significantly higher than the
other compounds tested. Leaves treated
with oxythioquinox
and dicofol were
slightly lower in stomatal conductance
(16.' and 22.'1%, respectively) and photosynthetic rate (12.3 and 21.8%, respectively). The trees in the NR-++O oil
treatment showed a 31.8% drop in photosynthetic rate and were significantly
lower than the water control.
The I] -day postspray samples showed
that orange leaves treated with fenbmatin-
SUMMER
19H6
oxide and dicofol exhibited a significantly
higher photosynthetic rate compared with
the other treatments. Oxythioquinox and
NR-440 caused ca. 21% reduction in photosymhetic rate compared with the untreated trees. However, based on the differential rate of recovery, NR-440 may be
a better choice of pesticide for control of
a given mite species given equal activity
against the pest species (Jones et al.
1983).
I
.~
100
I.
I.
OO.!!
',..
2
O£HSlTY
1L.LnM
•
3
TREATMENT LEV[L
LatillCeo •••• t)
.•
p.,
Ornamental Crops
Fig. I. Lettuce )'ield re:,jJOIlse ill relaChrysanthemum. Baun and Peterson tioll to IlarioliS lePidopterous lanla DTL 's
and lIlltreated
check plots (UNT) alld
(1981) investigated the effects of aldicarb,
potelltial
ecollomic
alld pesticide threshbenomyl, capmn, dichlorvos, dicofol,
olds (after Toscallo et al. 1982h).
dienochlor, mancozeb, and piperalin (formulations not available) on potted,
pinched 'Bright Golden Anne' chrysanthemums, Chr)'salllhemum
morifolium
plant physiology when pesticides were
Ramat, at various growth stages. Interacappl ied at noon (stom:lta open) and midtion of plant phenology and number of
night (stomata closed). They therefore,
applications
influenced
height, fresh
concluded that penetration through the
weight, and dry weight of plant lOps but
leaf cuticle was the mode of pesticide
only time of application influenced numuptake. In addition, they found that the
ber of breaks (= vegetative shoots). Dephotosynthetic rate of plants treated with
velopmental rate and number of flowers
methoxychlor (50WP) did not.exhibit sigwere also influenced by pesticides. In
nificant differences in either stomatal conaddition, single or multiple applications
ductance or photosynthetic rate compared
of the above pesticides changed the phowith untreated plants. However, plants
tosyntlietic rates compared with the untreated with methomyl (50WP) exhibited
treated control.
a 9% reduction in stomatal conductance
Jones et al. (198"1),working with potted
and an 8% reduction in photosynthetic
'Tip' chrysanthemums, found that after
rate 1 day following treatment; after 1
five applications, plants treated with cyromazine 75W or permethrin
3.2E week, a 20% reduction in photosynthetic
rate was recorded. Melhyl parathion
showed a significant increase in photo(.:tEC) induced a 17% reduction in stosynthetiC rate compared with plants
matal
conductance and a 10% reduction
treated with aldicarb lOG or a combinain photosynlhelic rate in lhe I-day samtion of permethrin and micro-encapsuhued methyl parathion 2M. However, at ples, but this increased lO a respective 27
and 18% reduction 1 week later. Permethshipping time they did not ohserve differrin (2EC) exhibited a 14% initial reducences in mean flower head weight, mean
tion in photosynthetic rate, which inflower head diameter, mean plant height,
creased to 18% 1 we<:k lat<:r.
mL':lIl dry weight of the above ground
In additional studies, yield measurefoliage + flowers, mean number of open
ments were taken from plots where the
flowers, or mean number of flowers +
various treatments menlioned above were
buds between treatments. They attributed
applied during the period from germinathe disparity of results between photosynthetic measuremeI1lS and yield measure- tion to thinning (ca. weeks 1-3) and from
head formation to harvest (ca. weeks 8ments to application of the growth regu11) in contrast to plots lrealed throughout
lator B-9 (succinic acid 2,2 dimethyl hythe entire season. Treatments were apdrazide).
plied either once or twice a week in each
case. In general, lettuce treated weekly
Vegetables
throughout the whole season produced
Lettuce. The most intensive field inves- lower yields than lettuce not treated during the growth period from thinning to
tigations of pesticide effects on a single
head formation (ca. weeks 4-8). The
crop were performed on lettuce, Lactuca
satil'a 1.., in California by Toscano et al. methyl parathion treatments caused sig(1982a,b) and Johnson et al. (983). Tos- nificant reductions in mean head weight
(24% reduction for the weekly spray and
cano et :11.0982b) did not observe sig30%
reduction for the plots treated twice
nificant differences in pesticide effects on
105
t
>
!::
en
2
w
CI
•...
en
w
a.
T1ME-
Fig. 2. Relationship between tbe clas·
sical economic injury lel'els, economic
tbresbold, and pest density.
weekly) and mean head diameter (6%
reduction) compared with the untreated
control plots. From this, Toscano et a!.
(1982b) concluded that the effects of pes·
ticicle·induced changes in plant physiol·
ogy and yield were important enought to
justify a "pesticide threshold" that would
indicate the maximum number of applications permissible
at recommended
rates on certain crops. This threshold
would vary with compound, formulation,
plant condition, and environmental conditions.
further evidence of pesticide-induced
reductions in yield were given by Toscano
et a!. (1982a). The experiments were not
specifically designed to measure pesti·
cide effects, but instead were designed to
determine
a density treatment
level
(OTL) (= economic threshold when eco·
nomics are ignored) for the lepidopterous
pests of lettuce. They found that plots that
were assigned to the moderate OTL
(= 0.5-1.0 larvae per plant) produced a
higher yield than plots where a low OTL
(= 0 larv;le per plant) was maintained by
an average of three additional sprays of
methomyl or orthene. They felt that the
additional three sprays were responsible
for the reduced yield and constructed a
yield-response curve based on the three
studies showing the relationship between
the pesticide-induced
yield reductions
and yield reductions caused by insect
feeding damage. From this chart they calculated that a OTL of ca. 0.5-1.0 larvae
per plant would not reduce yield signifi·
cantly (Fig. 1).
Johnson et ;11. (983) further investigated the effects of methomyl and methyl
parathion on lettuce yield. They varied
the number of pesticide treatments be·
tween thinning and head formation,
which was the period when growers have
some leeway in their pest management
program. Mean lettuce head weight, mean
106
head density, and percent of bolted plants
were linearly correlated with the number
of methyl parathion treatments. Pesticide·
induced reductions in total yield were
attributed to reduced head weight and
increased bolting of the plant. Only an
increase in the percentage of bolted
plants was associated with the use of
methomyl. A change in application rate
produced a curvilinear relationship in
photosynthetic rate, mesophyll conduct·
ance, and stomatal conductance. Based on
these results, Johnson et a!. (1983) rec·
ommended a "pesticide threshold" of no
more than three applications of methyl
parathion to avoid a yield reduction of
>20%.
Strawberry. The effects of acaricides on
growth, yield, and physiology of Fragaria
chiloensis (L.) was investigated by LaPre
et a!. (1982). Plants were sprayed with
cyhexatin (50WP, every 2 weeks; total of
six sprays), fenbutatin·oxide
(SOWP,
monthly; total of three sprays at 1- and 2fold rates), formetanate hydrochloride
(92SP, monthly; three sprays), and pro·
pargite (30WP, every 2 weeks; six sprays
or monthly; two sprays). Early in the sea·
son, the propargite and formetanate hy·
drochloride treatments caused reductions
in stomatal conductance, mesophyll con·
ductance, and photosynthetic rate. Reduc·
tions in photosynthetic rates were 7 and
21% for the respective 1- and 2-fold rates
of formetanate hydrochloride and 20 and
24% for the respective biweekly and
weekly propargite
treatments.
Plots
treated with fenbutatin·oxide and cyhex·
atin showed no physiological differences
from the control plots.
Late in the season, it was found that
only formetanate hydrochloride caused
reductions in photosynthetic
rates be·
cause of its lasting effect on mesophyll
conductance. Total fruit yield and number
of fruit were significantly higher in the
fenbutatin·oxide
treatments
compared
with the high formetanate hydrochloride
and both propargite treatments.
Tomato. In a 2-year slLldy on the effectiveness of systemic insecticides for controlling Colorado potato beetle on tomato
('Petro Mech'), Romanow et a!. (1984)
found that the insecticides disulfoton,
phorate, and thiofanox delayed and reo
duced seedling emergence at all rates
used. At the higher rates, carbofuran and
aldicarb delayed seedling emergence but
did not reduce the crop stand. The carbofuran treatment resulted in a significant
increase in the number of flowering
plants 41 days following planting.
The second year of the study, 'UC-82B'
was planted. Again, plug mixes (seedlings
are planted with a "plug" of soil) of disulfoton, phorate, and thiofanox reduced
seedling emergence compared with the
untreated control. However, this same effect was not observed in banded treatments of the same pesticides at the same
dosages. A single plug application of aldicarb and two banded-and· incorporated
applications of aldicarb significantly accelerated seedling emergence. Aldicarb
was also found to increase percentage of
flowering plants 23 and --i8 days after
planting
regardless
of
application
method. Carbofuran had a similar effect
in the lower plug-mix dosages in the April
planting and at all dosages in the May
planting.
Field Crops
Cotton. In experiments attempting to
establish OTL's for bollworms on cotton
in South Africa, van Hamburg and Kfir
(1982) found that weekly sprays of cypermethrin caused significant reductions in
yields compared with plots where a DTL
of one larvae per plant was maintained.
They concluded that the pesticide effect
was important enough to preclude treat·
ment on a prophylactic basis. Their graph
of yield versus larva density is similar to
that obtained by Toscano et a!. (1982b)
on lettuce (Fig. 1).
Incorporation
into Pest Management
Programs
The economic threshold concept is described by Stern et a!. (1959) and van den
Bosh and Stern (962).
Briefly, pest
suppression activities are initiated on the
basis of the relationship between pest
population density and sufficient reduc·
tions in plant quality to result in economic
damage (economic injury level). The
point at which suppression
activities
should be initiated to prevent the increasing population from reaching the economic injury level (ElL) is called the economic threshold (ET) (Fig. 2). Although
some entomologists tend to use the idea
of "pest-days" (Hussey and Parr 1963, Allen 1976, Hoyt et a!. 1979, Sances et a!.
1979, Ruppel 1983, Welter et a!. 198'-i) to
quantify plant damage and reduction in
photosynthetic rates instead of population
density, the basic concepts of Stern et a!.
(19S9) are still valuable. Only slight modifications are necessary to incorporate
pest days and changes in the definition of
ElL and ET.
Classically, ElL is dependent on the
stage of crop development, season, projected value of the crop, and cost of the
BULLETIN OF THE ESA
W
I-
covery rate of the plant to the particular
pesticide (or combination of pesticides)
in question must be known.
Once an estimate of pesticide effects
has been obtained, the following simple
model can be used to estimate the combined effect of pest and pesticide on
yield:
100
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SUPERIOR
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12
14
=
Y,,, - P, - I,
management system (for monitoring pest
populations, pesticide, and appl ication).
ET is then normally considered to be a
fixed distance below the ElL with the
difference between them related to the
lag time between recognition of the problem and when control actions reduce population levels (Stern et al. I959).
When considering the effect of pesticides on plant physiology and yield, the
relationship between E1' ::Ind ElL becomes more complex. ElL is redefined
slightly and becomes the total amount of
physiological damage that a plant can
withstand and not cause economic loss. It
is a function of both pest and pesticide
damage, crop phenology, cost of the management system, and projected economic
value of the crop. The ET, therefore, becomes a function of pest days, pesticide
effects on plant physiology, and the lag
time between recognition of the problem
and when control actions reduce population levels. Because pesticides may have
either a positive or negative effect on
plant physiology, the redefined ET can be
either> (if stimulation occurs), = (if no
effect), or < (if reduction occurs) the
classical ET.
Because the effects of insecticides that
change photosynthetic rates seem to be
characterized by an initial change in photosynthetic rate and a later recovery period related to the weathering and detoxification of the residue (van Oorschot
1979), the pesticide effect on plant physiology cannot be considered to be a linear
relationship but instead appears to be curvilinear (Fig. 3). To quantity the effect of
a pesticide on plant physiology, the reSUMMER 1986
1000
800
::
UJ 600
;:
400
200
(1)
where Y" = net yield, Y", = maximum yield
under the given cultural practices (for this
example yield has been used, but either
percentage of reduction in yield or reduction in yield could be used), P, = total
pest damage, and It = total pesticide damage. Mathematically this can be expressed
as a multiple linear regression equation:
DA YS SINCE APPLICATION
Fig. 3. Nate (!l reCOl'ery of 'Delicious'
apple treated u'itb selected insecticides
(dl/ta from Ayers and narden [J975j).
1200
where Po = accumulated pest days, P" =
pest damage coefficient, Ip = the pesticide
impact coefficient, N = the number of
pesticide applications, Po X P" = Pt, and
It = Ip X N P" is generally <0 (some
studies have shown that a low level of
damage can stimulate plant growth [see
Sances et al. 1981]), and Ip can take on
values >, =, or <0 depending on the
pesticide effect on plant physiology (Fig.
4). The treatment decision can be guided
by consideration of the projected value of
the commodity, and whether the pest effect will reduce final dollar return more
than the costs associated with the pesticide and its effect on plant physiology.
Examination of equations 1 and 2 reveals four cases of interest to entomolo·
gists (in the following hypothetical examples, Y,,, = 1,000 kg/ha, ET = the dam·
age that would reduce the yield to 800
kg/ha, Ell. = the damage that would reduce yield to 700 kg/ha, P" = -0.5 kg per
ha per pest-day, and I" = -50 kg per ha
o
o
2
NUMBER
1
2
3
OF PESTICIDE
456
APPLICATIONS
per application; therefore, yield decreases
5% of Y,,, with each pesticide application
or every 100 pest-days). The first case
occurs when a highly "aesthetic" value
crop such as cut flowers is considered.
Since virtually no pest damage can be
tolerated, pesticide applications are prophylactiC and (assuming 100% efficacy of
the pesticide) pest· days do not accumulate. Therefore, equation 2 simplifies to:
Y"
=
Y,,, -
tp(N)
(3)
or
Y" = 1,000 - 50(N)
(4)
and yield changes are due entirely to the
number of pesticide applications (e.g., a
"pesticide threshold"). Consequently, the
maximum number of applications
to
reach the ET is four and economic damage occurs at six applications (Fig. 5, Case
1). Using equation 3, the data of Johnson
et al. (I983) indicate that the ", for methyl
parathion and methomyl on lettllce is
-1.2 and -0.13, respectively.
The second case is similar to the first,
except that the classic ET has already been
reached and immigration of pests from
neighboring fields keeps pest population
pressure high. Again, no more pest damage can be tolerated and equation 2 becomes:
(Y", -
ET) - ",(N)
(5)
or
APPLICATIONS
Hypothetical relationship between yield and number of pesticide ap·
plicatiOllS if pesticide impact coejjlcient
is>, =, or< 0.
Fig. 4.
6
PESTICIDE
Fig. S. Hypothetical relationship between yield (in kg/haY and the numher
of pesticide applications. Case 1, 'aesthe tic , /lallie crop where 110 insect damage Call he tolerated. Case 2, the ecOnomic i1~jury le/lel is reached before
spraying hegins and pest pressure remains high from surrounding areas.
Y" = Y", NUMBER
4
OF
Y" = 800 - 50(N)
(6)
and yield changes are again primarily dependent upon the number of pesticide
applications. The maximum number of
107
1200
1000
600
~
UJ
;:
600
400
200
0
400
200
600
600
PEST-DAYS
Fig. 6. Hypothetical relationship between yield (ill kg/ ba) alld pest-days
whell a sillgle application ofpesticide will
control tbe pest for tbe entire season.
applications
possible without exceeding
the EI I. is two, demonstrating
an instance
where adherence
to the classic ET will
result in economic loss if pesticide effects
are not considered
(Fig. 5, Case 2).
The third situation
occurs frequently
with indirect pests that can be easily controlled for the entire season with only one
pesticide
application.
In this case equation 2 becomes:
or
}~,= 950 - O.S(P,,)
(8)
and only 300 pest-days are allowed before
the ET is reached and 500 before the Ell.
is reached (Fig. 6).
The last case occurs when the pest population develops
resistance
to the COI11pound used for control. Once the economic threshold
is reached,
equation
2
becomes:
}~,= }~"- CY,,, -
ET)
- PAP,,) - ~}(N)
(9)
or
}~,= 800 -
O.S(P,.) - SO(N)
(10)
and plant yield is decreased
because of
the increased number of sprays necessary
to achieve control and the continued
accumulation
of pest-days (Fig. 7). This situation is also important
in considering
t\Vo compounds
that have different efficacy. If the least efficacious material has
1/2 the effect on yield of the most efficacious material, the least efficacious material would be considered
for use if two or
less sprays would be used to achieve control.
Discussion
In actual practice,
damage are probably
108
pesticide
and pest
not linearly related
to yield reductions
when the entire range
of possible values are considered.
In addition, use of binary mixtures,
different
application
rates and methods, frequency
of application,
crop phenology,
and combinations of pests (with their corresponding different
types of damage)
all undoubtably affect yield in possibly a nonlinear or nonadditive
manner. However,
for the purpose
of demonstrating
how
pesticide effects can be included into an
economic decision model, the linear fit is
sufficiently accurate. For simplicity's sake,
we have chosen to use the broadly recognized model of Stern et a!. (1959), but
the same inclusion of pesticide effects on
plant yield could be done with any of the
current economic
decision
models discussed by Mumford' and Norton (1984).
When used in conjunction
with knowledge concerning
the effect of pesticides
on natural enemies, this concept provides
a framework that can be used to aid the
construction
of a pest management
system. The basis of this framework
is a
broader perspective
on a control tactic
that affects all three trophic
levels involved, instead of the one or two levels
currently recognized.
Ferree (1979) concluded
that insecticides should be routinely
screened
for
their effect on plant physiology before use
recommendations
of that particular pesticide are made. Jones et al. (1983) agreed,
and in addition, stated that an understanding of the effects of pesticide-induced
reductions
on growth and yield of plants
is crucial to the development
of pest management
systems.
Toscano
et
al.
(1982a,b)
and Johnson
et al. (1983) favored
the establishment
of pesticide
thresholds
for insecticides
shown to reduce crop yields. We reiterate these suggestions and, in addition,
feel that data
concerning
pesticide-induced
changes in
yield of indicator
crops should
be required for pesticide
registration.
In the
laboratory, future research in the areas of
quantitative
structure
activity
re[ationships, and optimizing
pesticide
formulation may yield pesticides
that have a
maximal effect on the pest species and a
minimal effect on plant physiology.
Field
experiments
that utilize portable
photosynthesis-measuring
instruments
may also
be used to help elucidate
the effects of
pesticides on plant physiology and aid in
providing
more informed
pesticide
recommendations.
Acknowledgment
We thank M. P. Parrella and V. M. Stern of
the Dept. of Entomology, Univ. of Calif. at
'000
YiELD
334
NO.
7
APPLICATIONS
21
700
PEST-OAYS
Fig. 7. H)potbetical relationsbip hetween yield (in kg/ba) and tbe numher
of pesticide applications, and pest-days
wben tbe pest displays tolerance for tbe
insecticide.
Riverside for critical review of earlier drafts of
this manuscript.
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van Oorschot,). L. P. 1979. Types of selective
action hy herhicides which inhibit photosynthesis. Z. Naturforsch, Teil C: 34: 900-90-1.
Welter, S. c., M, M. Hames, I. P. Ting, and J. T.
Hayashi. 1984. I mpact of various levels of
late-season spider l11ite (Acari: 'T'etranyehidae) feeding d:ull:tge on almond growth and
yield. Environ. Entomol. 13: 52-';5.
Wood, B., and). Payne. 1984. 1nfluence of
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photosynthesis of pecan. lIortic. Sci. ] 9: 265·
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Wood, B., T. Gouwald, and J. Payne. 1984.
I nfluence of single appl ications of fungicides
Oil net photosynthe.~is of pecan. Plant Dis.
68: ;27--128.
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