Journal of the American Mosquito Control Association, ll(2):260-269, lgg5
Copyright @ 1995 by the American Mosquito Control Association, Inc.
THE FUTURE OF MICROBIAL INSECTICIDES AS
VECTOR CONTROL AGENTS'
BRIAN A. FEDERICI
Department of Entomology and Graduate Program in Genetics,
University of California, Riverside, CA 92521
ABSTRACT. Insect vectors of human diseasesare subject to diseasesof their own caused by viruses,
bacteria, fungi, protozoans,and nematodes.Over the past 30 years,many members ofthese groups have
beenevaluatedas vector control agents,particularly for mosquito control. Most pathogensand nematodes
occur primarily in larvae, and are only effective against this stage.The principal candidate control agents
studied include iridescent and nuclear polyhedrosis viruses, thebacteia Bacillus thuringiensis and Bacillus
sphaericus, the fungi Lagenidium giganteum, Culicinomyces clavosporus, and species ofthe genus Coelomomyces, the protozoan Nosema algerae, and the mermithid nematode Romanomermis culicivorax.
Of these, the only one considered an operational successis the bacterium, Bacillus thuringiensis subsp.
israelensis(B.t.t.), which has proven usefirl for control of both mosquito and blackfly larvae in programs
where larviciding has been traditionally employed as a vector control tactic. The reasonsfor the success
of B.t.i. are its cost-efectivenessand relative easeof use, which are due, respectively,to the ability of
B.t.i. to be grown on artificial media and the development of formulations that can be applied using
conventional insecticideapplication technology.Becausefew microbial insecticidesare cost-effective,and
those that are are only effective against larvae, these agentswill likely play only a minor, but in some
casesirnportant, role in most future vector control programs.
INTRODUCTION
The use of synthetic chemical insecticides for
vector control is in decline due to high costs,the
development of resistancein many target populations, and perceivedrisks to the environment
and human health. Althoueh chemical insecticides will remain important in vector control
programs, problems they have caused,and the
paucity ofnew types under development, have
for some time stimulated interest in alternative
control agents. In seeking replacements for
chemicals, the pathogens and nematodes that
causediseasesfatal to vectors have receivedconsiderable study over the past 30 years, particularly with respectto theirpotential for controlling
the mosquito vectors of malaria and filariasis,
and the blackfly vectors ofonchocerciasis (Federici 1981, Chapman 1985, Lacey and lJndeen
t This paper was prepared for oral presentation as
part of a symposium, and thus my treatment of the
subject has been necessarily brief, particularly with respectto the data that serveasthe basisfor my opinicns"
For those readers interested in a more detailed treatment ofeach ofthe groups I dealt with, and references
to numerous original papers,I recommend the following reviews. For viruses, Federici (1974); for bacteria,
Guillet et al. (1990) and Mulla (1990); for fungi, Federici (1981), Hall and Papierok (1982), and McCoy et
al. (1988); for protozoa, Henry (1981) and Brooks
(1988);and for nematodes,Petersen(1982). The review on microbial control of mosquitoes and blackflies
by Lacey and Undeen (1986) is also very worthwhile,
as is the book on bacterial control of mosquitoes and
blackflies edited by de Barjac and Sutherland (1990).
1986, Guillet et al. 1990, Mulla 1990). These
studies have included the search for pathogens
and nematodes,the laboratory and field evaluation of those that appeared to have the best
potential for operational use, the development
and assessmentof methods for massproduction,
and finally implementation in operational control programs followed by evaluation of costeffectivenessbasedon control ofthe target pests
or vectors. Overall, these studies have resulted
in 3 principal findings: l) pathogensand nematodes are only effective against the larval stages
ofvectors, 2) effective control typically requires
repeatedrather than a single application of the
agent during the breeding season,and 3) to be
used cost-effectivelyin vector control programs,
a method for massproduction of the control agent
in vitro must exist. The first finding is simply a
biological property of most of the pathogensand
nematodesdiscovered to date; they may occur
in adult stages,but seem to take their greatest
toll againstlarvae. The secondresults from natural diminution, or what might be considered
poor recycling potential from the standpoint of
vector control. The third results from the relatively large areas that must be treated, and derives from the high costsof mass rearing control
agentsin organisms,suchas mosquito or blackfly
larvae, in comparison to the lower costsof mass
production for a control agent grown on an inexpensive artificial medium. The latter is only
possible at present for some of the bacteria and
fungi. As a result of the combination of these
findings, only the bacterium, Bacillus thurin'
giensis sttbsp.israelensis(B.t.i.), has been used
260
261
VBcron CoNrnol wrrHoLIT Cnwnclrs
JuNe 1995
in operational vector control programs,and only
in programs where larviciding has been a traditional method of vector control. Progressin developing microbials for vector control has been
slow, and this situation is unlikely to improve
soon. Thus, it can be concludedthat bacteria are
likely to be the only microbial agents used for
some time to come in vector control programs,
and these will only be used in programs where
larviciding has been a successfulvector control
strategy.
Predictions about the future use of microbial
insecticides and nematodes in vector control
progxamsare best arrived at by an examination
ofwhat we have learned over the past 30 years
from the study ofthese potential control agents.
Thus, in this paper I will review the key findings
for eachof the major pathogengroups and nematodesthat led to our current view oftheir probable use in the future.
tality rates typically being less Ihan 25o/ofor I st
and 2nd instars, even when larvae were placed
in water containing virions at concentrationsof
severalg.g/ml.The NPVs and CPVs were shown
to only infect the midgut epithelium, where the
NPVs replicated in the nuclei throughout the gut,
and the CPVs in the cytoplasm of infected cells
in the gastriccecaand posterior stomach.As with
the iridescent viruses, the mortality rates obtained with NPVs rarely exceeded25o/o,and although higher rates of infection were achieved
with CPVs, evenlarvae heavily infectedwith this
virus type could often survive the infection, pupate, and emergeas adults. Owing to their poor
efrcacy and lack of suitable methods for mass
production, seriousresearchon the development
and evaluation of virusesas mosquito or blackfly
control agentswas discontinued in the 1970s.
BACTERIA
Bacteria are relatively simple unicellular microorga.nismsthat lack internal organellessuch
All viruses are obligate intracellular parasites, as a nucleus anC mitochondria, and reproduce
and as such must be grown in living hosts. With
by binary fission. With few exceptions,most of
insect viruses, this means either in insects (i.e., those discovered in vectors grow readily on a
in vivo) or in cultured insect cells (in vitro) be- wide variety of inexpensive substrates or artificauseno methods exist for growing viruses on cial media, a characteristicthat greatly facilitates
"artificial"
media.
their mass production. The bacteria currently
Despite these difficulties, searcheswere con- used or under development as microbial control
ducted for viruses pathogenicto mosquitoesand agentsfor vectors are spore-forming members of
blackflies becauseat the time this researchwas the bacterial family Bacillaceae.Two specieshave
initiated in the 1960s,it was thought that viruses received considerable stldy, Bacillus thuringienmight work as classicalbiological control agents srs subsp. israelensis(B.t.i.), originally discov(i.e., agentsthat would lead to long-term control ered in Israel by Goldberg and Margalit (1977)
once introduced against previously unexposed and the only speciescurrently usedoperationally,
populations), or that it might be possible to de- and Bacillus sphaericus,which has good larvivelop suitable methods for the mass production cidal activity against certain mosquito larvae,
of viruses using colonized insects.The searchfor but whoseoperational utility hasnot beenclearly
viruses,carried out primarily by members of the demonstrated.
USDA's mosquito researchlaboratoriesat Lake
Both B.r.i. and B. sphaeriao kill larvae through
Charles,LA, and Gainesville, FL, resultedin the the action of insecticidal proteins that destroy
discovery ofnumerous viral pathogensoflarvae
the larva's midgut epithelium shortly after in(Federici 1974). These included several iridogestion,usually within a few hours. Theseinsecviruses of floodwater mosquitoes belonging to ticidal proteins are containedwithin a parasporal
the genera Aedes and Psorophora, cytoplasmic body produced when the bacterium sporulates
polyhedrosis viruses (CPVs), and baculoviruses (Fig. l). In B.t.i.,1heparasporalbody is spherical,
of the nuclear polyhedrosis virus (NPVs) type, and contains 4 proteins with molecular masses
the latter 2 types being isolated primarily from
of 27, 72, I 28, and I 34 kDa (Federiciet al. I 990).
Aedes and Culex species.Iridoviruses and CpVs
These proteins interact synergistically to yield
have also been reported from larvae of several toxicity comparable to that of chemical insectiblackfly species.
cides such as DDT. Bacillus thuringiensis israeStudies of the iridoviruses. or iridescent vilensis h;asbeen shown to be toxic to most diprusesas they are more commonly known, showed terans of the suborder Nematocera, including
that they infected and replicated in most larval mosquitoes (Culicidae), blackflies (Simuliidae),
tissueswith the exception of the midgut epithe- craneflies (Tipulidae), and midges (Chironomilium (Anthony and Comps l99l). However, in
dae).One ofthe advantageouspropertiesof.B.r.i.
Iaboratory trials these viruses were shown to be is that it is highly insecticidal for many vector
only moderately infectious for larvae, with morspecies, including species of Aedes, Culex,
VIRUSES
263
VEcroR CoNTRor wITHour Cn*ncers
Juxe 1995
Table L Comparison of the efrciency of production relative to application rates for
representative bacterial and fungal pathogens evaluated as microbial insecticides for control of
mosquito larvae.
LCrolhectare
Agent
Bacteria
BaciIlus t huri ngiensis
subsp. israelensis
Bacillus sphaericus 2362
Fungr
Culicino myces cIavosporus
Maarhizium anisopliae
Production
yield
Medium/hectaret
200 g technical
powder2
100 g technical
powder2
l0 g/liter
20 liters
10 g/liter
l0 liters
l0t'conidia
2 kg conidia
l0rt conidia/liter
1,000 liters
I kg25 kg rice
50 kg ofrice
t Refers to the amount of liquid or solid substrate needed to produce the amount of technical matcrial required to achieve
population reductions of 90% for I ha.
, Technical powder rcfers to the unformulated dried solids obtahed after fermentation. Of this weight, only approxinarely
25% consists of insecticidal proteins in B. thuringiensis, and 5-10% in B. sphaerias; the rcst is fermentation solids, bacterial
spores, and rcmnants ofbacterial cells.
B.t.i. usedin vector control programsare Acrobe
(American Cyanamid), Skeetal (Novo Nordisk),
Teknar (Sandoz), and Vectobac (Abbott l^aboratories). Although not used widely for vector
control, these products have proven useful and
cost-effective in programs that employed larvicides as a vector control tactic. In addition, they
are commonly used in industrialized countries
for the control ofnuisance mosquitoesand blackflies, particularly in environmentally sensitive
areas.
With respect to B. sphaericas, there are 3 insecticidalproteins, with molecular massesof 43,
52, and 100 kDa, but thesedo not alwavs occur
in all isolates@aumannet al. I 991, Porter et al.
I 993). Interestingly, theseproteins are only toxic
to mosquitoes, and tend to exhibit the highest
toxicity against Culex species.No significant toxicity for B. sphaericus has been reported against
blackflies or other dipterans. Though the spectrum of activity for 8. sphaericusis not as broad
as for B.t.i., B. sphaericushas greater residual
activity, and works better in highly polluted waters, although it requires high rates of application, generallygreaterthan I g oftechnical powder/m2.It is still consideredto hold potential for
the species of Culex that vector filarial worrns
(Hougard 1990, Yap 1990),however, no commercial products based on B. sphaericus are currently marketed. This does not mean that for-
Table 2. Summary of property ratings for selected pathogens and nematodes evaluated for
veclor controt.
Property
Broad host/target
spectrum
Mass production
In vitro
In yiyo
Efficacy/unit weight
Safety to nontargets
Formulation
Storage
Residual activity
Virus
Iridovirus
or
baculovirus
++
N/P'
++
++
++++++
+++++
+++
+
Bacteria
Fungus
Bacillus
thuringiensis
subsp.
israelensis
Culicinomyces
clavosporus
Nematode
Nosema
algerae
Romanomermis
calicivorax
Protozoan
+++++
++++
+++
++++
++++++
+++
++
++
++++
+++
++
+
N/P
++
+
++++
+++
+++
+
N/P
++
+++
++++
++++
+++
++++
N/A2
++++++
+++++
++++++
++++++
+
I N/P : Not practical or cost-effective with
existing technology.
' N/A : Not applicable becausetechnology
already exists foicost-effective mass prod.uction in vilro.
264
JounNer op rxs Ar,tsRrcANMosqurro CoNrnol AssocrarroN
VoL 11,No. 2
mulations based on B. sphaericuswould not be
Although the difrculties encountered with
useful in filariasis control programs, but rather Coelomomycesmake it unlikely that this fungus
that the market is consideredtoo small to merit will ever be developed as a biological control
commercial development until more cost-effec- agent, there is still considerable interest in Lagenidium giganteum. This oomycete fungus has
tive formulations are devised.
2 important advantagesover Coelomomyces;it
is easily cultured on artificial media and it does
not requirean alternatehost (Federici 1981,JaFUNGI
ronski et al. 1983). In the life cycle, a motile
Three types of aquatic fungi that attack mos- zoospore invades a mosquito larva through the
quito larvae have been studied for use as bio- cuticle. Once within the hemocoel, the fungus
logical control agents,speciesof Coelomomyces colonizes the body over a period of 2-3 days,
(chytridiomycete fungi), Lagenidium giganteum producing an extensive mycelium consisting
(an oomycete fungus), and Culicinomyces cla- largely ofnonseptate hyphae. Toward the end of
vosporus(a deuteromycetefungus).
gowth, the hyphae become septate,and out of
The genusCoelomomycescomprises more than each segmentan exit tube forms that grows back
70 speciesof obligately parasitic fungi that un- out through the cuticle and forms zoosporangia
dergoa complex life cycleinvolving an alteration at the tip. Zoosporesquickly differentiate in these,
of sexual (gametophytic) and asexual (sporo- exiting out through an apical pore to seekout a
phytic) generations(Couch and Bland 1985. new substrate.In addition to this asexualcycle,
Whisler 1985). In all speciesstudied, the sexual thick-walled resistant sexual oospores can also
phaseparasitizesa microcrustaceanhost, usually be formed within the mosquito cadaver.
a copepod, whereasthe asexualgeneration typTechniques have been developed to produce
ically develops in a mosquito larva. In the life both zoosporangia and oospores in vitro, and
cycle, a biflagellate zygosporeinvades the hem- methodsare currently being developedto modify
ocoel of a mosquito larva where it produces a existing technology so that the fungus can be mass
sporophyte that colonizes the body and forms produced. Severalyears offield trials in Califorresistant sporangia. The larva dies and subse- nia and North Carolina have shown that the zoquently the sporangiaundergo meiosis, produc- osporangia are too fragile for routine use in oping uniflagellatemeiosporesthat invade the hem- erational control programs (Merriam and Axtell
ocoel of a copepod host where a gametophyte 1982,Kerwin and Washino 1987).The oospore,
develops.At maturation, the gSmetophytecleaves however, is quite stable, although germination
forming thousands of uniflagellategametes. remains unpredictable. Nevertheless, freld reCleavage results in death of the copepod and sults indicate that germination of even a small
escapeof the gametes,which fuse forming bi- percentageofoosporescan result in the initiation
flagellatezygosporesthat seekout another mo$- of epizootics that lead to season-longmosquito
quito host, completing the life cycle. The life control. The fungus is, however, quite sensitive
cyclesofthese fungi are highly adapted to those to ammonia and chloride ions, and thus is not
of their hosts. Moreover, as obligate parasites efrcaceousin polluted or brackish waters. Nevthesefungi are very fastidious in their nutritional ertheless,although severaltechnicalproblems rerequirements,and, as a result, no speciesof Coe- lated to mass production and formulation relomomyceshas been cultured in vitro.
main to be overcome, L. giganteurn is a promCoelomomycesis the largest genus of insect- ising candidate for successfulcommercial deparasitic fungi, and has been reported worldwide velopment and use in freshwater habitats such
from numerousmosquito species,many ofwhich as rice fields. Its principal advantage over B.t.i.
are vectors of important diseasessuchas malaria is that if efective formulations of the oospore
and filariasis. In some of these species,for ex- can be developed, it appearsthat in many habample, Anophelesgambiae Giles in Africa, epi- itats only a single application would be required
zootics causedby Coelomomyceskill more than per season.Even less frequent application may
950/oof larval populations in some areas(Couch be possiblein somehabitats, asevidencesuggests
and Umphlett 1963). Such epizootics led to ef- that the oosporescan overwinter, initiating epforts to develop severalspeciesas biological con- izootics the following seasons.The extent to which
trol agentsover the past 30 years(Federici I 98 I ). this occurs and can be relied upon for effective
However, theseefforts have largely been discon- mosquito control remains to be determined.
tinued due to the discovery that the life cycle Large-scalefield trials have not yet been conrequired a 2nd host for completion, the inability ducted with L giganteum, so production costs
to culture these fungi in vitro, and the develop- and rates ofapplication cannot be related to the
ment of B.t.i. as a bacterial larvicide for mos- level and length ofvector control. Thus, it is not
quitoes.
possibleto assessthe cost-effectivenessof L. gi-
JUNB1995
Vscror Coxrnor, wrrHour Cnnacets
ganteum as a mosquito or vector control agent
at this time.
In addition to Coelomomycesand L. giganteum, the aquatic deuteromycete fungi, Culicinomyces clavosporw and Tolypocladium cylindrosporum, and the terrestrial deuteromycete
fungus,Metarhizium anisopliae,have been considered for mosquito control (Federici 1981,
Sweeneyet al. 1983,Soaresand Pinnock 1984).
These fungi produce conidia, which adhere to
the cuticle of larvae, and then invade and colonize the larva via a germ tube. However, high
production costs resulting from inefrcient production methods. lack of cost-effectivecontrol
in the field, and the discovery and developmenl
of B.t.i., have eliminated these fungi as serious
candidatesfor development as microbial control
agents.
PROTOZOA
"Protozoa" is a generalterm applied to a large
and diverse group of eucaryotic unicellular motile microorganisms that belong to what is now
known as the kingdom Protista. Members of this
kingdom can be free-living, saprophytic, commensal,symbiotic, or parasitic.The cell contains
a variety of organelles,but has no cell wall, and
cells vary greatly in size and shape among different species. Feeding is by ingestion or more
typically by absorption, and vegetative reproduction is by binary or multiple fission. Both
asexual and sexual reproduction occur and the
latter can be very complex, and is often useful
for taxonomic purposes.Many protozoa produce
a resistant spore stage that is also used in taxonomy. The kingdom is divided into a seriesof
phyla based primarily on the mode of locomotion and structure of locomotory organelles,and
includes the Sarcomastigophora (flagellates and
amoebae),Apicomplexa (sporozoa),Microspora
(microsporidia), Acetospora (haplosporidia), and
Ciliophora (ciliates).Sometypesofprotozoa, such
as the free-living amoebaeand ciliates, are easily
cultured in vitro, whereas most of the obligate
intracellular parasiteshave not yet been grown
outside ofcells.
As might be expected from such a large and
diversegroup of organisms,many protozoansare
parasites of vectors. Those that have been reported most commonly in vector populations are
microsporidia (phylum Microspora), and thus it
is these, especially members of the generaNosema and.Amblyospora. that have received the
most study. Thesestudieshave shown that most
microsporidians have the generalfeatureofcausing chronic diseases.Thus, most cannot be used
as fast-acting microbial insecticides.This characteristic is not detrimental in the caseof most
265
vectors becauseit is the adults that are responsible for diseasetransmission.However, the microsporidia are all obligate intracellular parasites, and there is no easyor cheapway to mass
produce them in vitro. M'oreover, the few field
trials that have beenconductedindicate that very
high rates of application are required to obtain
significant reductions in vector populations, at
least in mosquito populations, the only vectors
they have been tested against.The lack of commercially suitable methods for mass production
and poor efrcacy have significantly diminishec
interest in developing protozoansas vector control agents.Fairly extensivestudieswere carried
out with Nosemaalgeraein the 1970sto evaluate
its potential for controlling anophelines,with the
aim of reducing the prevalence of malaria, and
thus these studies are worth reviewing briefly
here.
Nosema algerae was originally isolated from
AnophelesstephensiListon,but has a broad host
range against anophelinesand is capable under
experimental conditions ofinfecting a wide range
of other mosquito species.It is also capable of
reproduction in the lepidopteran Heliothis zea,
and this host has been used to produce spores
for field trials (Anthony et al. 1978b). The infectious stageis a spore,within which is a coiled
tube, the polar filament, that is responsible for
injecting the sporoplasm into larvae after ingestion. Oncewithin the body of alawa,the Nosema
cells undergo numerous merogonic cyclesin larval tissues,and then a sporogoniccycle that results in the development of new spores.The larvae typically die at the end ofthe diseaseifthey
are infected during the early instars. Ifinfected
during the 4th instar, larvae will often pupate
and adults emerge.These adults have lower fecundity and decreasedcapacityfor malaria transmission (Anthony et al. 1978a). Although the
latter results initially indicated some potential
for having an impact on diseasetransmission,
field trials carried out in Panama againstAnopheles albimanus Wied. in the 1970s showed that
very high and uneconomic application rates for
sporeswould be required to have any significant
impact on diseasecontrol. To achieve mosquito
infection rates of 860/o,4 applications of spores
at a rate of 2.2 x lOe/m2had to be made. To
treat a hectare at such a rate would require a
minimum of approximately 40,000 lepidopteran
larvae to produce the neededamount ofspores.
Members of the genusAmblyspora also occur
commonly in mosquito populations and have
received considerable study. Studies of several
speciesof Amblyospora have shown that they
have complex life cycles,much like thoseof Coelomomyces,requiring an alternate crustaceanhost
(Sweeneyet al. 1985).As a result, theseparasites
266
Jounrqeror rxr ArraunrceN
Mosquro Covrnol AssocnrroN
are also thought to have little potential for development as vector control agents.
V o u 1 1 ,N o . 2
(960/o)were reported by kvy and Miller (1977)
when they applied preparasitesat a rate of 3,600/
m2 against floodwater mosquitoes in Florida.
When
introduced into suitable habitats, R. culMERMITHID NEMATODES
icivorax can recycle,providing some level of conMermithid nematodes are among the largest trol in subsequentgenerationsofmosquitoes, but
nematodes attacking insects, and the adult fe- this level is typically not sufficient to interrupt
males typically measure from 5 to over 20 cm. the vector potential of mosquitoes or alleviate
In parasitized insects with a translucent cuticle, the annoyance problem. In addition, both labsuch as the larvae of mosquitoes and blackflies, oratory and field studies showed that R. culicithe advanced stagesofdeveloping nematodes can vorex was quite sensitive to chloride ions, and
often be observed within the hemocoel where the methods developed for mass production,
they appear as long, thin white worrns. Mermistorage,shipment, and use were found not to be
thids are obligate parasites and have been re- cost-effective, particularly in comparison to
ported from many different orders of insects as commercial formulations of the bacterial insecwell as from other arthropods such as spiders ticide B. thuringiensis subsp. rsralensls, which
and crustaceans.However, the only ones seri- was discoveredwhile the development and evalously considered for use as biological control uation of R. culicivorexwas underway. As a reagentsare the speciesRomanomermis culicivo- sult of its limitations, and the availability of
rax and Romanomermis iyengari, which are ca- cheaperand more effectivealternative biological
pable of parasitizing many speciesof mosquito methods of control, there is little interest today
larvae (Petersen1982). The life cycle of R. cul- in the further development of R. culicivorax, or
icivorax, which is quite typical of mermithids, other mermithids for that matter, as biological
will be used to illustrate the properties that re- control agents.
sulted in studies ofthe biological control potential of this nematode type.
SUMMARY AND
The femalesof R. culicivorax arefound in wet
CONCLUSIONS
soil at the bottom of aquatic habitats in which
mosquitoes breed. Here, after mating, the feDuring the past 3 decades,in responseto the
males lay thousandsof eggs.The embryo devel- problems causedby synthetic chemical insectiops into a lst-instar juvenile over a period of cides,a considerableeffort has beenmade to find
about I wk, and afterwards molts to a 2nd-stage and develop pathogensand nematodes as biojuvenile still within the egg.This 2nd stage(pre- logical control agents,and microbial insecticides
parasite)then hatchesout and swims to the water for control of vectors. This research has prosurface where it seeks out and, with the aid of duced a wealth ofknowledge about the occurthe stylet, invades an early instar mosquito larva rence,basic biology, and control potential ofnuvia the cuticle. Once within the hemocoel, the merous viruses, bacteria, fungi, protozoa, and
immature larva growsover a period of 7-10 days nematodes that parasitize vector insects. This
by absorbing nutrients through its cuticle, and knowledgeis of value in its own right and serves
molts once during this period. Upon completion as a foundation against which to measure the
of this parasitic phase, the 3rd-stagejuvenile potential ofnew control agents.However, studies
punctures its way out through the cuticle of its of the various organismsthat have been discovhost, thereby killing the mosquito, and then drops ered and evaluated as control agents, in combito the bottom ofthe pond where it matures with- nation with an assessmentof how they might be
out feedingover another 7-10 days,finally molt- used in vector control programs, have demoning to the adult stage.The adults then mate and strated that most of these agents are not suffithe femaleslay eggs,thus completing the life cy- ciently cost-effectiveto be used in such control
programs. Moreover, there is no reason to becle.
Using several different species of colonized lieve this assessmentwill changeover the nexl
mosquitoes and simple rearing techniques, Pe- 10-20 years,barring some sort ofunanticipated
tersen(1982) and his colleagueswere able to de- breakthrough.
The primary reason for the lack of cost-effecvelop massrearing techniquesfor R. culicivorax,
and conducted a wide range of trials in which tivenes$for most pathogensand nematodesthat
they evaluated this nematode by applying the have been evaluated is that methods are not
preparasitic stagein the laboratory and field. In available for their mass production at an acthe field studies,ratesof parasitism (: mortality) ceptable cost. In essence,to achieve levels of
for anopheline larvae as high as 850/owere ob- vector control that will have a significant impact
tained when preparasiteswere applied at a rate on the prevalenceof human disease,the amounts
of 2,400/m2. Even higher levels of parasitism of vector control agents that would have to be
JUNE1995
VBcron CoNrnol wITHour CnslurcAls
267
1978a. Fecundity and longevity ofAnopheles albi'
manus exporcd at each larval instar to spores of
Nosemaalgerae.Mosq. News 38:l l6-121.
Anthony, D. W., K. E. Savage,E. I. Hazard, S. WAvery, M. D. Boston and S. W. Oldacre. 1978b.
Field tests with Nosema algeraeYarva and Undeen
(Microsporida, Nosematidae)againstAnophelesalbimanus Wiedemann in Panama. Misc. Publ. Entomol. Soc. Am. ll:.17-27.
Baumann, P., M. A. Clark, L. Baumann and A. H.
Broadwell. 199l. B acillus sphaericusas a mosquito
pathogen:properties ofthe organism and its toxins.
Microbiol. Rev. 55:425-436.
Brooks, W. M. 1988. Entomogenousprotozoa, pp.
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