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BIOLOGICAL CONTROL OF
MOSQUITOES
Culex , Aedes, Anopheles, etc. -- Diptera, Culicidae -----Please CLICK on desired category; Depress Ctrl/F to find Subject Matter: Detailed
Biological Control Measures [Please refer
also to Related Research #1, #2, #3 ] Introduction
Interest in biological control of medical pests and vectors had
its modest beginning prior to the turn of the last century (Lamborn
1890). At that time the possible use
of dragonflies as natural enemies for the control of mosquitoes was clearly
recognized. However, as is true even
today, the enormous difficulties associated with the colonization and
management of these insects quickly extinguished any idea for the practical
use of these predators for mosquito control.
Shortly after the turn of the century the mosquitofish, Gambusia affinis (Baird & Girard), came to the forefront of
biological control. This small fish,
being much easier to deal with than dragonflies, was quickly utilized and transported
throughout the world during the early decades of this century in attempts to
control mosquitoes. The mosquitofish, G.
affinis, <PHOTO> along with several other natural controls, was
employed with some enthusiasm during the first 40 years of the century. All of these control measures were
curtailed sharply with the introduction of synthetic organic insecticides
after World War II. The convenience
and quick killing power of these chemicals was so dramatic for such insects
as mosquitoes, flies and lice, that other control tactics were quickly
reduced to a minor role.
Nevertheless, interest in alternative methods of control, especially
biological, was to arise again when the succession of chemicals developed
during the 1940s and 1950s began to fail due to the development of widespread
genetic resistance in vector and pest populations. Although the biological control of medically important pests
and vectors has made some progress since its revival, it has been rather slow
and is still well behind that which has occurred in agricultural systems
(Service 1983). This disparity is
partly due to the problems of fixing pest tolerance levels, but more
importantly because of the temporary unstable habitats exploited by medically
important pests (Legner & Sjogren 1984,
Legner & Warkentin 1989). As Service (1983) pointed out,
the successful widespread use of biological control agents against mosquitoes
will require a much better understanding of the ecology of predator/prey and
pathogen/host relationships. The
opportunistic characteristics of many species (i.e., their ability to exploit
temporary habitats, coupled with their short generation time, high natural
mortality, great dispersal potential, and other R-strategist characteristics)
pose difficult problems for any biotic regulatory mechanism. Mosquitoes, in general, exploit a wide
breadth of different aquatic habitats.
Consequently, under many conditions a biological control agent will
have a much narrower range of environmental activity than that of the target
species. Thus, in many situations a
number of different biological control agents and/or appropriate methods will
be necessary if we expect to control even a single species of mosquito across
its range of exploitable breeding sources. The most important nonarthropod invertebrate predators to draw
attention for mosquito control are the turbellarian flatworms and a
coelenterate. Flatworm species which
were shown experimentally to be excellent predators of mosquito larvae in a
variety of aquatic habitats are Dugesia
dorotocephala (Woodworth)
and Dugesia tigrina (Girard) (Legner &
Medved 1974 , Yu & Legner 1976, Collins & Washino 1978,
Case & Washino 1979, Legner 1977 , 1979, Ali
& Mulla 1983, George 1978, Meyer 1981a,b; George et al. 1983). Several biological and ecological
attributes of flatworms would seem to make them ideal candidates for
manipulative use. Among them are ease
of mass production, an overwintering embryo, effective predatory behavior in
shallow waters with emergent vegetation, on site exponential reproduction
following inoculation (Medved & Legner 1974,
Tsai & Legner 1977, Legner
& Tsai 1978, Legner 1979) and
tolerance to environmental contaminants (Levy & Miller 1978, Nelson
1979). Collins & Washino (1978) and Case & Washino (1979)
suggested that flatworms, particularly Mesostoma,
may play an important role in the natural regulation of mosquitoes in some
California rice fields because of their densities and their predatory attack
on mosquito larvae in sentinel cages (Garcia & Legner 266. 99 ). Preliminary analysis using extensive sampling showed a
significant negative correlation between the presence of flatworms and
population levels of Cx. tarsalis and An. freeborni (Case & Washino 1979). However, these workers cautioned that an
alternative hypothesis related to the ecology of these species may have
accounted for the correlations. Later
investigations by Palchick & Washino (1984), employing more restrictive
sampling, were unable to confirm the correlations between Mesostoma and mosquito
populations. However, the enormity of
the problem associated with sampling in California rice fields, coupled with
the complexity of the prey and predator interactions (Collins & Washino
1979), make further studies necessary before the role of this group of
flatworms in rice fields can be clearly established. Nevertheless, in a detailed study by Legner (1977), which was later duplicated by Ali
& Mulla (1983), the relationship between the number of Dugesia dorotocephala applied per m2 of water surface
and the average density of mature Culex
spp. larvae was highly significant and lineal, the latter assuming densities
of <1 per standard dipper at the higher Dugesia application rates. It was emphasized that the use of D. dorotocephala
for direct mosquito suppression as an alternative to insecticides or Gambusia is desirable because
natural predator densities appeared unaffected, and a single application resulted
in a prolonged and increased suppression of mosquito larvae as the planarians
reproduced in the environments to which they had been introduced. Although D. dorotocephala
is widespread in North America (Kenk 1972, McConnell 1967), some strains are
very cannibalistic and might be unsuitable for mass rearing. The production of adequate numbers of the
noncannibalistic strain is possible if cultures are stockpiled during winter
months (Legner et al. 1976) and rapid
mass production can be obtained through carefully controlled culture with
filtration, optimum temperature and dissolved oxygen and food (Legner et al. 1976, Tsai & Legner 1977, Legner & Tsai 1978).
Progeny require about one month to reach maturity. The important attributes for manipulative use of flatworms
mentioned above raises the question of why they have not been developed
further for use in mosquito control.
Perhaps the contemporary development of Bacillus thuringiensis
var. israelensis DeBarjac
(H-14), a highly selective easily applied microbial insecticide, may have
been at least partially responsible for slowing further work and development
of these predators. Their mass
culture must be continuous and demands skilled technical assistants (Legner
& Tsai 1978). Their persistence in field habitats may
also depend on the presence of other organisms which can be utilized for food
during low mosquito abundance, such as ostracods (Legner et al. 1976). The coelenterates, like the flatworms, showed great promise for
further development and use in selected breeding habitats. Chlorohydra
viridissima (Pallas) is
efficient in suppressing culicine larvae in ponds with dense vegetation and
this species also can be mass produced (Lenhoff & Brown 1970, Yu et al. 1974a, 1974b.
1975). However, like the flatworms, work on these predators has waned,
perhaps for similar reasons as speculated for the flatworms. Microbial pesticides can be employed over
an extensive range of different mosquito breeding habitats. Also, commercial production of flatworms
and coelenterates would be much more costly, and storage of viable cultures
all but impossible. Recent emphasis on the fungal genus Lagenidium which is capable of infecting and killing
several genera of mosquito larvae (e.g., Anopheles,
Culex, Aedes, and Psorophora)
encourages the continued quest for biological control agents as alternatives
to pesticides (McCray et al. 1973, Christensen et al. 1977, Glenn & Chapman
1978, Washino & Fukushima 1978,
Washino 1981, Axtell et al. 1982, Domnas et al. 1982, Jaronski & Axtell
1982, 1983a,b). The potential of such
fungi for operational mosquito control is nevertheless no greater than for
some of the flatworms or hydra. This
recent switch in attention to fungi is probably due to the existence of a
greater number of mycologists in the research force than specialists in the
other groups. Problems of mass
production, dissemination of an acceptable fungal stage and adaptability to
polluted water habitats have placed their immediate deployment in doubt. Similar problems were either nonexistent
or minimal with the Dugesia
flatworms, so that their integrity as effective and available biological
control agents is undiminished. The successful widespread use of biological control agents
against mosquitoes will require a much better understanding of the ecology of
predator/prey and pathogen/host relationships (Service 1983). The opportunistic characteristics of many species
(i.e., their ability to exploit temporary habitats, coupled with their short
generation time, high natural mortality, great dispersal potential, and other
R-strategist characteristics) pose difficult problems for any biological
control agent. Mosquitoes typically
exploit many aquatic habitats. Often
a biological control agent will have a much narrower range of environmental
activity than the target species.
Thus, in many situations a number of different biological control
agents and/or appropriate methods will be necessary to control even one
species of mosquito across its range of exploitable breeding sources. Detailed
Biological Control Measures
Fish.--Several
species of fishes are used for the biological control of mosquitoes, and these
species together form the major successes in biological control. Unfortunately, their usefulness is limited
to more permanent bodies of water, and even under these situations their
impact on the target species has been only partially successful. Bay et al (1976) point out that many
species of fish consume mosquito larvae, but only a few species have been
manipulated to manage mosquito populations.
[Please see Research] The mosquitofish, G. affinis, <PHOTO> is the best known agent for mosquito control. This fish, which is native to the
southeastern United States, eastern Mexico and the Caribbean area, was first
used as an introduced agent for mosquito control when it was transported from
North Carolina to New Jersey in 1905 (Lloyd 1987). Shortly thereafter it was introduced to the Hawaiian Islands to
control mosquitoes which had been introduced during the 19th century. During the next 70 years, the mosquitofish
was transported to over 50 countries and today stands as the most widely
disseminated biological control agent (Bay 1969, Lloyd 1987). Many of these introductions were aimed at Anopheles species that were
transmitting malaria. Hackett (1937)
described its usefulness in malaria control programs in Europe. He commented that its effects were not
sufficient by themselves, but that the fish had a definite impact on the
suppression of the disease.
Tabibzadeh et al. (1970) reported a rather
extensive release program in Iran and concluded that the fish was an
important component in malaria eradication.
Sasa and Kurihara (1981) and Service (1983) believed that the fish had
little impact on the disease and that most evidence is circumstantial. Gambusia
no longer is recommended by the World Health Organization for malaria control
programs, primarily because of its harmful impact on indigenous species of
fish (Service 1983, Lloyd 1987). The biological attributes of G.
affinis, namely a high
reproductive capability, high survivorship, small size, omnivorous foraging
in shallow water, relatively high tolerance to variations in temperature,
salinity and organic waste, would seemingly make this species an excellent
biological control agent (Bay et
al. 1976, Moyle 1976). However, whether this fish leads to
effective mosquito control at practical costs in many situations is still
debated. Kligler's (1930) statement
that "... their usefulness as larvae-destroyers under local conditions
where vegetation is abundant and micro fauna rich enough to supply their
needs without great trouble, is limited.
In moderately clear canals, on the other hand, or in pools having a
limited food supply, they yielded excellent results ..." is probably one
of the most accurate. In California this fish had been used extensively for control of
mosquitoes in various habitats (Bay et
al. 1976). Many mosquito abatement districts in the
State have developed systems for culturing, harvesting and winter storage of
the mosquito fish to have enough available for planting early in the spring
(Coykendall 1980). This is
particularly important in the rice growing areas of California where early
stocking appears to be of critical importance for build-up of fish populations
to control mosquitoes during late summer.
The results of the use of G.
affinis in California rice
fields will be summarized below as an illustrative example of the mixed
successes achieved in the field. Rice cultivation in California continuously poses one of the most
difficult control problems for Anopheles
and Culex species. Hoy & Reed (1970) showed that good to
very good control of Culex tarsalis Coquillett could be
achieved at stocking rates of about 480 or more females per hectare, and Stewart
et al (1983) reported excellent control with a similar stocking rate against
this species in the San Joaquin Valley. Although Cx. tarsalis appears to be
controlled effectively by G.
affinis, the control of its
frequent companion in northern California rice fields, Anopheles freeborni
Aitken, is less apparent. Hoy et al. (1971) showed a reduction of An. freeborni
populations at various stocking rates of about 120 to 720 fish per hectare,
but the reduction was not nearly as striking as for Cx. tarsalis. These workers surmised that improvement in
control could be achieved by earlier season stocking, possibly multiple
release points in fields and a reliable source of healthy fish for stocking. Despite an extensive research effort in
mass culture, management and storage for G.
affinis by the State of
California (Hoy & Reed 1971), a mass production method has not been
satisfactorily achieved (Downs et
al. 1986, Cech and Linden
1987). Studies of G. affinis for control of
mosquitoes in wild rice show that relatively high stocking rates can
effectively reduce An. freeborni and Cx. tarsalis populations within a three-month period (Kramer et al. 1987a). The
commercial production of wild rice, which is a more robust and toller plant
than white rice and requires only 90 instead of 150 days to mature, has been
increasing over the last few years in California (Kramer et al.
1987). In the above study, stocking
rates of 1.7 Kg/ha (ca. 2400 fish/Kg) released in 1/10 ha wild rice plots
failed to show a significant difference in reduction of mosquitoes from plots
with no fish. A decrease in numbers
of larvae was noted just prior to harvest which suggested that the fish were
beginning to have an impact on mosquito numbers (Kramer et al.
1987). Numbers of fish in these
plots, based on recovery after drainage, was about 100,000 individuals per
hectare (ca. 32 Kg/ha) or a density of about 10 fish per square meter. However, significant control was not
achieved. During 1987 this study was repeated at the rates of 1.7 and 3.4
Kg/ha of fish. Results showed an
average suppression of larvae (primarily An.
freeborni) of <1 and 0.5
per dip for the low and high rate respectively, compared to control plots
which averaged >4.5 per dip. Fish
densities in the 1987 study surpassed those of 1986 by about two fold at the
1.7 Kg/ha rate and three fold at the 3.4 Kg/ha rate. It is believed that these greater fish
numbers accounted for the control differences observed in the second year,
although mosquitoes were not eliminated.
Differences between test plots and control plots were first observed
eight weeks after the fish had been planted and mosquitoes remained under
control until drainage of the fields (Kramer et al.
1988). Davey & Meisch (1977a,b) showed that the mosquitofish at inundative
release rates of 4,800 fish per hectare, was effective for control of Psorophora columbiae (Dyar & Knab) in Arkansas rice fields. Fish released at the water flow inlets
dispersed quickly throughout the fields.
This is an important attribute for controlling species of Psorophora and Aedes, whose hatch and larval
development are completed within a few days.
A combination of 1,200 G.
affinis and about 300
sunfish (Lepomis cyanellus Rafinesque) gave
better control than either four times the amount of G. affinis
or L. cyanellus used separately. This synergistic effect reduces logistic problems associated
with having enough fish available at the times fields are inundated. Blaustein (1986) found enhanced control of
An. freeborni by mosquitofish in California rice fields after
the addition of green sunfish. He
speculated that the increased control was the result of the mosquitofish
spending more time in protected areas where mosquitoes were more abundant and
the green sunfish was avoided. The
availability of fish for stocking fields either inundatively, such as in
Arkansas or for control later in the season as practiced in California, has
been a fundamental reason why fish have not been used more extensively in
rice fields. A unique use of the mosquitofish by inundative release was
reported by Farley & Caton (1982).
The fish were released in subterranean urban storm drains to control Culex quinquefasciatus Say breeding in entrapped water at low
points in the system. Fish releases
were made following the last major rains to avoid having them flushed out of
the system. Fish survived for more
than three months during the summer and were found throughout the system. Gravid females produced progeny. However, no mating occurred, and after the
initial increase in numbers populations of fish diminished as summer
progressed. Reductions of mosquitoes
from 75 to 94% were observed for three months compared to untreated areas
(Mulligan et al. 1983). This control practice is now conducted on
a routine basis by the Fresno Mosquito Abatement District (J. R. Caton 1987,
pers. comm.). Although G. affinis
has been useful for control of mosquitoes in a number of situations, clearly
there are drawbacks to its use. In
fact, if today's environmental awareness existed at the turn of the century,
this fish probably never would have been intentionally introduced into exotic
areas (Pelzman 1975, Lloyd 1987). The
major objection to this fish has been its direct impact on native fishes through
predation, or its indirect impact through competition (Bay et al. 1976, Schoenherr 1981, Lloyd 1987). More than 30 species of native fish have
been adversely affected by the introduction of Gambusia (Schoenherr 1981, Lloyd 1987). Gambusia,
a general predator, can also substantially reduce zooplankton and thus lead
to algal blooms in certain situations (Hurlbert et al.
1972). Introductions of Gambusia have also reduced
numbers of other aquatic invertebrates coinhabiting the same waters (Hoy et al. 1972, Farley & Younce 1977, Rees 1979, Walters
& Legner 1980, Hurlbert &
Mulla 1981). In California
where native pup fishes in the genus Cyprinodon (PHOTO-1,
#2 ) may afford a greater potential for
mosquito control under a wider range of environmental stresses than Gambusia (Walters & Legner 1980), the California Department of Fish and
Game has discouraged their use on the basis that unknown harmful effects
might result to other indigenous fishes. There is also the concern that
certain rare species of Cyprinodon
might be lost through hybridization. The next most widely used fish for mosquito control is the common
guppy, Poecilia reticulata (Peters) <PHOTO>. It has been
deployed successfully in Asia for the control of waste water mosquitoes,
especially Cx. quinquefasciatus. Like its poeciliid relative Gambusia, it is native to the
Americas (tropical South America).
But, rather than being intentionally introduced to control mosquitoes,
it was taken to other parts of the world by tropical fish fanciers. Sasa et
al. (1965) observed wild
populations of this fish breeding in drains in Bangkok and concluded from
their observations that it was controlling mosquitoes common to that
habitat. The practical use of guppies
is primarily restricted to subtropical climates because of an inability to
tolerate temperate-zone water temperatures (Sasa & Kurihara 1981). However, their most important attribute is
a tolerance to relatively high levels of organic pollutants, which makes them
ideal for urban water sources that are rich in organic wastes. In Sri Lanka, wild populations have been
harvested and used for the control of mosquitoes in abandoned wells, coconut
husk pits and other sources rich in organics (Sasa & Kurihara 1981). The fish occursin in India, Indonesia and China
and has been intentionally introduced for filariasis control into Burma (Sasa
& Kurihara 1981). Mian et al
(1985) evaluated its use for control of mosquitoes in sewage treatment
facilities in southern California and concluded that guppies showed great
potential for mosquito control in these situations. Exotic fish have also been used for clearing aquatic vegetation
from waterways which has resulted in excellent mosquito control. In the irrigation systems of southeastern
California, three species of subtropical cichlids, Tilapia zillii (Gervais), Oreochromis mossambica (Peters) and Oreochromis
hornorum (Trewazas) <PHOTO> were introduced and have become established over some 2,000
ha of Cx. tarsalis breeding habitat
(Legner & Sjogren 1984). In this situation,
mosquito populations are under control by a combination of direct predation
and the consumption of aquatic plants by these omnivorous fishes (Legner
& Medved 1973, Legner 1978a, 1983; Legner & Fisher 1980; Legner & Murray 1981, Legner & Pelsue 1983). As Legner &
Sjogren (1984) indicate, this is a unique example of persistent biological
control and probably only applicable for relatively sophisticated irrigations
systems where a permanent water supply is assured, and water conditions are
suitable to support the fish (Legner et
al. 1980). There is a three-fold
advantage in the use of these fish:
(1) clearing of vegetation to keep waterways open, (2) mosquito
control and (3) a fish large enough to be caught for human consumption. Some sophistication is necessary when
stocking these cichlids for aquatic weed control, which is often not
understood by irrigation districts personnel (Hauser et al. 1976, 1977; Legner 1978b). Otherwise competitive
displacement may eliminate T.
zillii, <PHOTO> the most efficient weed consuming species (Legner 1986). Household storage of water in open containers has frequently been
the cause for outbreaks of human disease transmitted by Aedes aegypti
(Linnaeus) in less developed parts of the world. While conducting Ae.
aegypti surveys in Malaysia
during the mid 1960s, Dr. Richard Garcia of UC Berkeley observed what were
apparently P. reticulata being utilized by
town residents for the control of mosquitoes in bath and drinking water
storage containers. The origin of
this control technique was not clear but it appeared to be a custom brought
to the area by Chinese immigrants.
Not all residents used fish, but those that did had no breeding
populations of Ae. aegypti. Neng (1987) reported on the use of a catfish, Claris sp., for the control of Ae. aegypti in water storage tanks in coastal villages of
southern China. This fish was
considered appropriate since it was indigenous, edible, consumed large
numbers of mosquito larvae, had a high tolerance for adverse conditions and could be obtained from the local
markets. One fish was placed in each
water source and later checked for its presence by larval survey teams about
every 10 to 15 days. If fish were not
found on inspection the occupant was told to replace the fish or be
fined. The investigation was
conducted from 1981 to 1985, and surveys over this period showed a sharp
initial reduction in Ae. aegypti followed by a low
occurrence of the mosquito over the four-year study period. Outbreaks of dengue were observed in
neighboring provinces during this period, but not in the fishing villages
under observation. The cost of the
program was estimated to be about 1/15 that of indoor house spraying (Neng
1987). Alio et al. (1985) described another
use of a local species of fish for the control of a malaria vector similar to
the method reported by Kligler (1930).
Oreochromis sp., a
tilapine, was introduced into human-made water catchment basins called
"barkits" in the semi arid region of northern Somalia. These small scattered impoundments served
as the only sources of water during the dry season for the large pastoral
population of the area. Anopheles arabiensis Patton, the vector of malaria in that area, is
essentially restricted to these sites.
Release of fish into the "barkits" dramatically reduced both
the vector and nonvector populations of mosquitoes rather quickly. Treatment of the human population with
antimalarial drugs during the initial phase of this two-year study, combined
with the lower vector population reduced the transmission rate of malaria to
insignificance over a 21 month period whereas the control villages remained
above 10 percent. Alio et al (1985)
commented that the added benefits of reduced vegetation and insects in the
water sources was also recognized by the local population. This resulted in community cooperation and
was expected to further benefit the control strategy by providing assistance
in fish distribution and maintenance as the program expanded to other
areas. The last two examples involve the use of indigenous over exotic
fish where feasible in vector control programs. There are other examples where native fishes have been used in
specialized circumstances (Kligler 1930, Legner et al. 1974, Menon & Rajagopalan 1978, Walters & Legner 1980
,
Ataur-Rahim 1981 and Luh 1981). Lloyd
(1987) argued that only indigenous fish should be employed for mosquito
control because of the environmental disruption induced by exotics such as G. affinis. However,
he suggested that native fish should be analyzed carefully for prey
selectivity, reproductive potential and effectiveness in suppression pest
populations before attempting their use.
Lloyd (1987) also pointed out that a multidisciplinary approach
involving fisheries biologists and entomologists should be employed when
developing indigenous fish for mosquito control. However, in California where native pup fishes in the genus Cyprinodon
<PHOTO> may afford a greater potential for
mosquito control under a wider range of environmental stresses than Gambusia (Walters & Legner 1980 ),
the California Department of Fish and Game discourages their use on the basis
that unknown harmful effects might result to other indigenous fishes. There is also the concern that certain
rare species of Cyprinodon
might be lost through hybridization. Perhaps China's example of a multipurpose use of native fish for
mosquito control and a human protein source is the most resourceful
strategy. This application for
mosquito control is not new. Kligler
(1930) used a tilapine fish to control Anopheles
sp. in citrus irrigation systems in old Palestine, where farmers cared for
the fish, consuming the larger ones.
According to Luh (1981), the culture of edible fish for the purpose of
mosquito control and human food is not widely encouraged in China. The old Chinese peasant custom of raising
edible fish in rice fields has received greater attention in recent times
because of the benefits made possible through this practice. The common carp, Cyprinus carpio
Linnaeus, and the grass carp, Ctenopharygodon idella Valenciennes, are most
commonly used. Fish are released as
fry at the time rice seedlings are planted.
Fields are specially prepared with a central "fish pit" and radiating
ditches for refuge when water levels are low. Pisciculture in rice fields, as noted by Luh (1981), has three
major benefits: (1) a significant
reduction in culicine and to a lesser extent anopheline larvae, (2) fish are
harvested as food and (3) rice yields are increased apparently by a reduction
in competitors and possibly by fertilization of the plants by fish excreta. Another group of fishes, the so-called "instant" or annual fishes, (Cyprinodontidae),
which are native to South America and Africa, have been considered as
possible biological control agents for mosquitoes (Vanderplant 1941, 1967;
Hildemann & Wolford 1963; Bay 1965, 1972; Markofsky & Matias
1979). The relatively drought
resistant eggs of these cyprinodontids, which allows them to utilize
temporary water sources as habitat, would seem to make them ideal candidates
for mosquito control. There is also
some evidence that they do impact mosquito populations in native areas
(Vanderplant 1941, Hildemann & Wolford 1963, Markofsky & Matias
1979). Research on the biology and
ecology of several species has been conducted; however, there are no
published accounts on the successful use of these fish in field
situations. In California the South
American species Cynolebias nigripinnis Regan and Cynolebias bellottii (Steindachner) <PHOTO>,
survived the summer in rice fields, but no reproduction was observed over a
three-year period (Coykendall 1980).
It was speculated that they may play a future role in California's
mosquito control program in temporary pools and possibly rice fields. C.
bellottii was observed to
reproduce repeatedly and to persist in small intermittently dried ponds in
Riverside, California for eleven consecutive years, 1968-1979 (Legner &
Walters unpubl.). Four drying
flooding operations over two months were required to eliminate this species
from ponds that were to be used for native fish studies (Walters & Legner
1980 ). It seems logical, given the biological
capability of surviving an annual dry period, that these fish could be
successfully integrated into mosquito control programs, especially in newly
created sources in geographic areas where they naturally occur (Vaz-Ferreira et al. 1963, Anon 1981, and Geberich 1985). Arthropods.--Numerous species of predatory arthropods have been observed
preying on mosquitoes, and in some cases are believed to be important in
controlling mosquitos (James 1964, Service 1977, Collins & Washino 1979,
McDonald & Buchanan 1981).
However, among the several hundred predatory species observed, only a
few have been used in a manipulative way to control mosquitoes. Dragonflies, sometimes referred to as
mosquito hawks, were one of the first arthropods to be examined. Difficulties in colonization, production
and handling have restricted their use to experimental observation. It is unlikely that they will ever be used
extensively (Lamborn 1890, Beesley 1974, El Rayah 1975, Riviere et al. 1987a). There are a few cases where the difficulties associated with the
manipulative use of arthropods has been at least partly overcome. More than 50 years ago, in a classic use
of biological control, the mosquito Toxorhynchites,
whose larvae are predators of other mosquitoes, was released on several
Pacific Islands in an effort to control natural and artificial container
breeding mosquitoes such as Ae.
aegypti and Aedes albopictus (Skuse) (Paine 1934, Bonnet & Hu 1951,
Petersen 1956). The releases were not
considered successful, but the mosquitoes did establish in some areas
(Steffan 1975). Several reasons to
explain why these releases failed were low egg production, lack of synchrony
between predator and prey life cycles, and selection of only a relatively
small number of prey breeding sites (Muspratt 1951, Nakagawa 1963, Trpis
1973, Bay 1974, Riviere 1985). Although not apparently a suitable predator in the classical
sense, there is still interest in the use of various Toxorhynchites spp. for
inundative release (Gerbert & Visser 1978). Trpis (1981) working with Toxorhynchites
brevipalpis (Theobald)
showed that the high daily consumption rate and long survival of the larvae
without prey made it a prime candidate for biological control use. Observations on adult females indicated a
50% survivorship over a 10-week period with a relatively high oviposition
rate per female. All the above
attributes suggest that this species would be useful for inundative release
programs against container breeding mosquitoes. Studies by Focks et al (1979) in Florida, working with Toxorhynchites rutilis rutilis Coquillett, showed that this species had a high
success rate in artificial breeding containers. In a 12.6 hectare residential area, about 70% of the available
oviposition sites were located over a 14-day period by two releases of 175
females. Mass culturing techniques
have been developed for this species and Toxorhynchites
amboinensis (Doleschall)
(Focks & Boston 1979, Riviere et
al. 1987b). Focks et al (1986), working with Toxorhynchites amboinensis,
reported that release of 100 females per block for several weeks, combined
with ultra low volume application of malathion, reduced Ae. aegypti
populations by about 96% in a residential area of New Orleans. The Toxorhynchites
releases and not the insecticide treatment apparently accounted for most of
the reduction. These workers noted
that the procedure could be further refined by reducing both the number of
predators and malathion applications without lowering efficacy. Mosquitoes such as Ae. aegypti
and Ae. albopictus, which breed in and whose eggs are dispersed
via artificial containers, pose major health hazards as vectors of human
diseases throughout much of the warmer climates of the world. The massive quantities of containerized
products and rubber tires which are then discarded without care or
stockpiled, have given these mosquito species a tremendous ecological
advantage. The recent establishment
and extensive spread of Ae. albopictus in the United States
underlines this point (Sprenger & Wuithironyagool 1986). The apparent inability of governments to
appropriately control disposal of these containers and difficulties in
location once they are discarded makes inundative releases of Toxorhynchites, either alone or
in combination with other control tactics, a much more plausible approach
(Focks et al. 1986, Riviere et al. 1987a). Notonectids are voracious predators of mosquito larvae under experimental
conditions (Ellis & Borden 1970, Garcia et al.
1974, Hazelrig 1974), and in waterfowl refuges in California's Central Valley
(Legner & Sjogren, unpub. data). Notonecta undulata Say and Notonecta
unifasciata Guerin have been
colonized in the laboratory. In
addition, collection of large numbers of eggs, nymphs and adults is feasible
from such breeding sites as sewage oxidation ponds (Ellis & Borden 1969,
Garcia 1973, Hazelrig 1975, Sjogren & Legner 1974, Muira 1986). Some studies have been conducted on
storage of eggs at low temperatures, but viability decreased rapidly with
time (Sjogren & Legner 1989). At present, the most feasible use of these
predators appears to lie in the recovery of eggs from wild populations on
artificial oviposition materials and their redistribution to mosquito
breeding sites. Such investigations
were carried out in central California rice fields by Miura (1986). Floating vegetation such as algal mats and
sometimes duck weed (Lemna
spp.) form protective refugia for mosquito larvae, and consequently
populations of mosquitoes can be high in the presence of notonectids (Garcia et al. 1974). It
appears that colonization and mass production costs, coupled with the
logistics of distribution, handling and timing of release at the appropriate
breeding site, are almost insurmountable problems for routine use of
notonectids in mosquito control. In addition to insect predators, several crustaceans feed on
mosquito larvae. Among these are the tadpole shrimp, Triops longicaudatus (LeConte), and several copepod species. Mulla et
al. (1986) and Tietze &
Mulla (1987), investigating the tadpole shrimp, showed that it was an
effective predator under laboratory conditions and speculated that it may
play an important role in the field against flood water Aedes and Psorophora
species in southern California.
Drought resistance in predator eggs is an appealing attribute for egg
production, storage and manipulationin field situations against these
mosquitoes. However, synchrony in
hatch and development between the predator and the prey is crucial if this is
to be a successful biological control agent for the rapidly developing Aedes and Psorophora spp. In
addition, the tadpole shrimp is considered an important pest in commercial
rice fields. Miura & Takahashi (1985) reported that Cyclops vernalis Fisher was an effective predator on early
instar Cx. tarsalis larvae in the
laboratory. These workers speculated
that copepods could have an important role in suppressing mosquito
populations in rice fields because of their feeding behavior and abundance. Another crustacean that has shown promise for more extensive
application is the cyclopoid predator, Mesocyclops aspericornis Daday (Riviere
et al. 1987b). This
work has shown reductions of Ae.
aegypti and Ae. polynesiensis Marks by more than 90% after inoculative
release of the organism into artificial containers, wells, treeholes and land
crab burrows. Although not able to
withstand desiccation, the rather small cyclopod predator has persisted
almost 2.5 years in crabholes and up to five years in wells, tires and
treeholes under subtropical conditions.
This species can be mass produced, but its occurrence in large numbers
in local water sources allows for the inexpensive and widespread application
to mosquito breeding sites in Polynesia (Riviere et al.
1987a,b). The species is also very
tolerant of salinities greater than 50 parts per thousand. The benthic feeding behavior of Mesocyclops makes it an
effective predator of the bottom foraging Aedes,
but limits effectiveness against surface foraging mosquitoes. Riviere et al.
(1987a,b) believed that the effectiveness against Aedes is due to a combination of predation and competition
for food. Perhaps the greatest
utility of this Mesocyclops
will lie in the control of crabhole breeding species, such as Ae. polynesiensis in the South Pacific. Further investigations may uncover
additional cyclopods that can impact other mosquito species. The most important nonarthropod invertebrate predators to draw attention
for mosquito control are the turbellarian flatworms and a coelenterate. Several flatworm species have been shown
to be excellent predators of mosquito larvae in a variety of aquatic habitats
(Legner & Medved 1974, Yu &
Legner 1976, Collins & Washino
1978, Case & Washino 1979, Legner 1977, 1979, Ali & Mulla 1983, George et al. 1983). Several
biological and ecological attributes of flatworms would seem to make them
ideal candidates for manipulative use.
Among them are ease of mass production, an overwintering embryo,
effective predatory behavior in shallow waters with emergent vegetation, on site
exponential reproduction following inoculation (Medved & Legner 1974, Tsai & Legner 1977, Legner & Tsai 1978, Legner 1979) and tolerance to
environmental contaminants (Levy & Miller 1978, Nelson 1979). Collins & Washino (1978) and Case & Washino (1979)
suggested that flatworms, particularly Mesostoma, may play an important role in the natural
regulation of mosquitoes in some California rice fields because of their
densities and their predatory attack on mosquito larvae in sentinel
cages. Preliminary analysis using
extensive sampling showed a significant negative correlation between the
presence of flatworms and population levels of Cx. tarsalis
and An. freeborni (Case & Washino 1979). However, these workers cautioned that an
alternative hypothesis related to the ecology of these species may have
accounted for the correlations. Later
investigations by Palchick & Washino (1984), employing more restrictive
sampling, were not able to confirm the correlations between Mesostoma and mosquito
populations. However, the enormity of
the problem associated with sampling in California rice fields, coupled with
the complexity of the prey and predator interactions, make further studies
necessary before the role of this group of flatworms in rice fields can be
clearly established. The important attributes for manipulative use of flatworms
mentioned above raises the question of why they have not been developed
further for use in mosquito control.
Perhaps the contemporary development of Bacillus thuringiensis var. israelensis DeBarjac (H-14), a
highly selective easily applied microbial insecticide, may have been at least
partially responsible for slowing further work and development of these
predators. Their mass culture must be
continuous and demands skilled technical assistants (Legner & Tsai 1978).
Their persistence in field habitats may also depend on the presence of
other organisms, such as ostracods, which can be utilized for food during low
mosquito abundance (Legner et
al. 1976). The coelenterates, like the flatworms, showed great promise for
further development and use in selected breeding habitats. Chlorohydra viridissima
(Pallas) is efficient in suppressing culicine larvae in ponds with dense
vegetation and this species also can be mass produced (Lenhoff & Brown
1970, Yu et al. 1974a,b, 1975). However, like the flatworms, work on these
predators has waned, perhaps for similar reasons as speculated for the
flatworms. Microbial pesticides can
be employed over an extensive range of different mosquito breeding
habitats. Also, commercial production
of flatworms and coelenterates would be much more costly, and storage of
viable cultures all but impossible. Fungi.--The
most promising fungal pathogen is a highly selective and environmentally safe
oomycete, Lagenidium
giganteum Couch. First tested
for its pathogenicity to mosquitoes in the field by McCray et al. (1973), it is applied by aircraft to rice fields
(Kerwin & Washino 1987). Lagenidium develops asexually
and sexually in mosquito larvae, and is capable of recycling in standing
bodies of water. This creates the
potential for prolonged infection in overlapping generations of
mosquitoes. Lagenidium may also remain dormant after the water source
has dried up and then become active again when water returns. The sexually produced oospore offers the
most promising stage for commercial production because of its resistance to
desiccation and long-term stability.
However, problems in production and activation of the oospores still
remain (Axtell et al. 1982, Merriam & Axtell
1982a,b, 1983; Jaronski & Axtell 1983a,b,c, Kerwin et al.
1986, Kerwin & Washino 1987).
Field trials with the sexual oospore and the asexual zoospore indicate
that this mosquito pathogen is near the goal of practical utilization. Kerwin et al (1986) reports that the asynchronous
germination of the oospore is of particular advantage in breeding sources
where larval populations of mosquitoes are relatively low, but recruitment of
mosquitoes is continuous due to successive and overlapping generations, as in
California rice fields. The
germination of oospores over several months provides long-term control for
these continuous low level populations.
In addition, the asexual zoospores arising from the oospore infected
mosquito is available every two to three days to respond in a density
dependent manner to suppress any resurging mosquito population. This stage survives about 48 hours after
emerging from the infected host. Kerwin et al. (1986) indicate that
laboratory fermentation production of the asexual stage of Lagenidium for controlling
mosquitoes in the field is approaching the development requirements and costs
for the production of Bacillus
thuringiensis israelensis. A distinct advantage of this pathogen over
the Bacillus is its
potential to recycle through successive host generations. The disadvantage of the asexual stage is
that it is relatively fragile, cannot be dried and has a maximum storage life
of only eight weeks (Kerwin & Washino 1987). Thus, the focus of attention for commercial production is on
the oospore, which is resistant to desiccation and can be easily stored. Axtell & Guzman (1987) have recently
encapsulated both the sexual and asexual stages in calcium alginate and
reported activity against mosquito larvae after storage for up to 35 and 75
days, respectively. Further
refinement in techniques of production and encapsulation might make this
approach a viable option for future commercial production and application. Limitations on the use of this pathogen include intolerance to
polluted water, salinity and other environmental factors (Jaronski &
Axtell 1982, Lord & Roberts 1985, Kerwin & Washino 1987). However, there are numerous mosquito
breeding sources where these limitations do not exist and therefore one would
expect to see this selective and persistent pathogen available for routine
mosquito control in the near future. The fungus Culicinomyces clavosporus
Couch, Romney & Rao, first isolated from laboratory mosquito colonies and
later from field habitats, has been under research and development for more
than a decade (Sweeney et al. 1973, Couch et al. 1974, Russell et
al. 1979, Frances et al. 1985). The
fungus is active against a wide range of mosquito species and also causes
infections in other aquatic Diptera (Knight 1980, Sweeney 1981). The ease of production with relatively
inexpensive media in fermentation tanks is an extremely desirable trait. However, problems in storage must be
overcome if this fungus is to be widely used. Perhaps a drying process, now being investigated, will solve
storage requirements (Sweeney 1987).
Although the fungus has shown high infection rates in field trials,
dosage rates have been high and appreciable persistence at the site has not
been demonstrated (Sweeney et
al. 1973, Lacey & Undeen
1986, Sweeney 1983, 1987). Various species of Coelomomyces have been studied over the last two
decades for use in mosquito control.
Natural epizootics with infection rates in excess of 90% have been
recorded. These fungi persist in certain
habitats for long periods; however, factors triggering outbreaks in these
situations are not well understood (Chapman 1974). Some field testing has been done, but results have been highly
variable (Federici 1981). In general,
difficulties associated with the complex life cycle of these fungi have
encumbered research on them. Federici
(1981) and Lacey & Undeen (1986) have reviewed the potential of these
fungi for mosquito control. Nematodes.--Among the various nematodes pathogenic for mosquitoes, Romanomermis
culicivorax Ross & Smith,
has received the most attention (Petersen & Willis 1970, 1972a,b, 1975;
Brown et al. 1977, Brown & Platzer
1977, Poinar 1979, Petersen 1980a,b, Brown-Westerdahl et al.
1982, Kerwin & Washino 1984).
This mermithid, which is active against a wide range of mosquito
species, has been mass produced (Petersen & Willis 1972a) and utilized in
a number of field trials. The
nematode was commercially produced and sold under the name Skeeter Doom TMR,
but according to Service (1983) eggs showed reduced viability in transport
and the product currently is no longer sold.
However, the nematode's ability to recycle through multigenerations of
mosquitoes and overwinter in various habitats, including drained, harvested,
stubble-burned, cultivated and replanted rice fields, are strong attributes
favoring its further research and development for biological control
(Petersen & Willis 1975, Brown-Westerdahl et al.
1982). Several field applications
have shown good results and have included both the preparasitic stage and
post parasitic stages with the former more applicable to the "quick
kill" and the latter for more long-term continuous control such as in
California rice fields (Petersen et
al. 1978a,b, Levy et al. 1979, Brown-Westerdahl et al.
1982). Some drawbacks to its widespread
use include intolerance to low levels of salinity, polluted water and low
oxygen levels, predation by aquatic organisms and the potential for
development of resistance by the host (Petersen & Willis 1970, Brown
& Platzer 1977, Brown et
al. 1977, Petersen 1978,
Brown-Westerdahl 1982). However,
these environmental problems are not generally an issue for anopheline
control. For control of these species
the cost of in vivo mass production clearly
stands as the major drawback for this pathogen. Perhaps its most plausible use will be in specialized habitats
integrated with other control strategies (Brown-Westerdahl et al. 1982). |
