BIOLOGICAL CONTROL OF
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Biological resistance among arthropods to chlorinated hydrocarbon, organophosphate and carbamate insecticides in the late 1950's resulted in an expected turn toward suitable alternatives and especially to biological control. Attention was directed to the control of Diptera of medical and veterinary importance at a 1960 symposium in Washington, D.C. (Anonymous 1960), where biological control possibilities were emphasized. Jenkins (1964) reviewed the literature listing the known natural enemies of all arthropods of medical and veterinary importance, noting over 1,500 parasites, pathogens and predators. Renewed research emphasis on natural enemies followed the Washington symposium and Jenkins' review, and by 1999 there has been a substantial increase in research treating of the existence and biologies of natural enemies, as well as further reviews of the subject (Laird 1971a,b,c; Bay 1974; Brown 1973, Chapman 1974, Bay et al. 1976, Legner et al. 1974, Federici 1981, Murdoch 1982, Service 1983, Legner and Sjogren 1984, Laird 1986, Garcia and Legner 1999). The American Mosquito Control Association has maintained a quarterly accounting of publications pertaining to mosquito biological control agents since the Jenkins (1964) review, and the World Health Organization began issuing a series of reports in 1979 which described the characteristics of specific proven biological control organisms.
Interest in biological control of aquatic Diptera actually began in the late 1800's (Lamborn 1890). At that time the possible use of dragonflies as natural enemies for the control of mosquitoes was clearly recognized. However, to the present day the difficulties associated with the colonization and management of these insects has discouraged their practical use in mosquito control. In the early 1900's the small mosquitofish, Gambusia affinis* (Baird and Girard) (Microcyprini: Cyprinodontidae), was stressed for biological control, and being much easier manipulate than dragonflies, it was quickly utilized and transported throughout the world during the early decades of the 1900's in attempts to control mosquitoes (Legner and Sjogren 1984).
The mosquitofish, and a few other natural enemies were employed with some enthusiasm until the mid-1940's, when all biological control measures were curtailed sharply with the introduction of synthetic organophosphorus insecticides after World War II. Their rapid killing power was so dramatic for flies and mosquitoes, that other control tactics were temporarily dismissed to a minor role. Interest in biological control resumed when the succession of insecticides developed during the 1940s and 1950s began to fail, due to the development of biological resistance in vector and pest populations and in the 1990's when environmental contamination became an increasing public concern. Progress in the biological control of Diptera has been uninterrupted since its revival, even with problems of establishing pest tolerance levels, and the temporary unstable habitats exploited by Diptera (Legner and Sjogren 1984).
Bay et al. (1976) noted that dipterous pests are usually in the adult stage, which is of some advantage for control because it allows the control action to be taken against the immature stages, thus eliminating the adult before it can cause problems. However, it is difficult to establish tolerance levels for such pests. For example, an individual mosquito can be extremely annoying, which may lead to a reaction for control; and low population levels of a vector may still transmit a disease. However, reductions of any kind are desirable in the absence of more effective strategies, even though such partial controls may seem unacceptable (e.g., Service 1983). Setting tolerance levels for veterinary pests is comparatively more practical than for humans. The frequently temporary habitats utilized by aquatic Diptera poses a problem for biological control in that natural enemies cannot always coexist with pests to thus regulate their populations. Also, the habitat exploited by the pests is often only an undesirable extension of human activity, such as in the cultivation of rice, where the production of mosquitoes is usually of little concern to the rice producer.
As studies on biological control agents progressed, it became evident that their practical application for control would not be simple. The classical biological control approach involving the introduction of exotic natural enemies followed by substantial and sustained declines in host population densities have been reported in only a few cases. Often significant decreases in the pest population density were still not acceptable to the general public or health authority that desired an even lower population threshold, or investigations were terminated early before long-term benefits could be recorded. Problems of mass production, packaging and distribution of biological control agents have burdened commercial involvement. However, not until the 1990's did the desire for expedient and thorough effectiveness of commercial insecticides begin to give way to the slower and usually less potent biological controls.
The present review includes pertinent literature of major dipterous taxonomic groups where some success has been achieved or where work is currently being conducted on species breeding in aquatic habitats (mosquitoes, chironomids, blackflies and tabanids). Emphasis is on biological control agents that can be manipulated, that have been used successfully, that are being researched and which show at least some promise for successful deployment.
While progress in the development of biological control agents has been substantial and current work is expanding, a present overall evaluation is that biological control will continue to be implemented only gradually for Diptera of medical and veterinary importance. The majority of research is still driven by economic forces in the search for marketable products, especially evidenced by the disproportionate attention given to fungal and bacterial pathogens. However, the importance of maintaining maximum impact of resident natural enemies is almost universally accepted, and with continued effort, biolological control should become a major component in the overall strategy for the control of these important pests (Legner and Sjogren 1984, Garcia and Legner 1999).
The successful widespread use of biological control agents against mosquitoes requires a precise understanding of the ecology of predator/prey and pathogen/host relationships. The opportunistic characteristics of many species, including their ability to take advantage of 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 (Garcia and Legner 1999). Mosquitoes typically exploit many aquatic habitats. Often a good 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 are necessary to control even one species of mosquito across its range of exploitable breeding sources.
Insectivorous Fish.--Various species of fishes are used for the biological control of mosquitoes, which together constitute the major successes in biological control. However, their usefulness is limited to relatively permanent bodies of water, where their impact on the target species is usually only partially successful. Bay et al. (1976) remarked that many kinds of fish consume mosquito larvae, but only a few species have been manipulated to manage mosquito populations.
The mosquitofish, G. affinis <PHOTO> , is the best known biological mosquito control. Native to the southeastern United States, eastern Mexico and the Caribbean area, it was first used as an introduced agent for mosquito control when transported from North Carolina to New Jersey in 1905 (Lloyd 1987). Later it was introduced to the Hawaiian Islands to control mosquitoes, and during the next 70 years to over 50 countries. The mosquitofish ranks as the most widely disseminated biological control agent (Bay 1969, Lloyd 1987). Many of these introductions were to control Anopheles species that were transmitting malaria. Hackett (1937) described its usefulness in malaria control programs in Europe, noting that the fish had a definite impact on the suppression of the disease. Tabibzadeh et al. (1970) reported an expansive release program in Iran and concluded that the fish was an important component in malaria eradication. Nevertheless, Sasa and Kurihara (1981) and Service (1983) judged that the fish had little impact on the disease and that most evidence was circumstantial. Gambusia* spp. no longer are recommended by the World Health Organization for malaria control programs, primarily because of their harmful interference with indigenous species of fish (Service 1983, Lloyd 1987).
The biological attributes of G. affinis are a high reproductive capacity, high survivorship, small size, omnivorous foraging in shallow water, relatively high tolerance to variations in temperature, salinity and organic waste, which make this species an excellent biological control agent (Bay et al. 1976, Moyle 1976, Moyle et al. 1982). Whether this fish leads to effective mosquito control at practical costs in many situations is still debated, however. Probably an accurate assessment is revealed in a statement by Kligler (1930) 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 ...."
In California this fish had been used extensively for control of mosquitoes in various habitats (Bay et al. 1976). Many mosquito abatement districts in California have developed technology for culturing, harvesting and winter storage of the mosquito fish in order to facilitate stocking early in the spring (Coykendall 1982, 1986). This is particularly important in the northern rice producing areas of California where early stocking appears to be of critical importance for build-up of fish populations to control mosquitoes during late summer. Some results of the use of G. affinis* these rice fields illustrate the mixed successes achieved in the field. Rice cultivation in California continuously poses one of the most difficult control problems for Anopheles spp. and Culex species. Hoy and Reed (1971) showed that good control of Culex tarsalis* Coquillett (Culicidae) could be achieved at stocking rates of about 480 or more females per ha., and Stewart and Miura (1985) reported excellent control with similar stocking rates against this mosquito in the San Joaquin Valley.
Although Cx. tarsalis appears to be controlled effectively by G. affinis*, the control of Anopheles freeborni* Aitken (Culicidae) in northern California rice fields is less apparent. Hoy et al. (1972) showed a reduction of An. freeborni* populations at various stocking rates of about 120 to 720 fish per ha., but the reduction was not nearly as striking as for Cx. tarsalis. It was suggested that improved control could be achieved by earlier season stocking, involving multiple release points in fields and a reliable source of healthy fish for stocking. Despite an ample research effort in mass culture, management and storage for G. affinis* by the State of California (Hoy and Reed 1971), a mass production procedure has never provided adequate numbers (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. 1987). Wild rice is a more vigorous and taller plant than white rice, requiring only 90 instead of 150 days to mature (Garcia and Legner 1999). Commercial production has been increasing in the 1980's in California (Kramer et al. 1988). Kramer et al. (1987) stocked at rates of 1.7 kg./ha. (ca. 2400 fish/kg.) released in 1/10 ha. wild rice plots, but 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). The abundance of fish in these experimental plots, based on recovery after drainage, reached about 100,000 individuals per ha. (ca. 32 kg./ha) or a density of about 10 fish per square meter, which did not produce significant control.
This study was repeated a year later 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 second study surpassed those of the first by about two fold at the 1.7 kg./ha. rate and three fold at the 3.4 kg./ha. rate, and these greater 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 the fields were drained (Kramer et al. 1988).
Davey et al. (1974) and Davey and Meisch (1977) showed that at inundative release rates of 4,800 fish per ha., G. affinis* was effective for control of Psorophora columbiae* (Dyar and Knab) in Arkansas rice fields. Fish released at the water flow inlets scattered quickly throughout the fields. This is an important attribute for controlling Psorophora spp. and Aedes spp., whose hatch and larval development are completed within a few days. A combination of 1,200 G. affinis* and about 300 green sunfish [Lepomis cyanellus* Rafinesque (Perciformes: Centrarchidae)] 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. Addition of the latter forced mosquitofish to remain longer in protected areas where mosquitoes were more abundant in order to elude the green sunfish. The lack of available large numbers of fish for stocking fields either by inundation, such as in Arkansas or for control later in the season as practiced in California, is the main reason why fish have not been used more extensively in rice fields (Garcia and Legner 1999).
An unusual use of the mosquito fish by inundative release was reported by Farley and Caton (1982). The fish were released in subterranean urban storm drains to control Culex quinquefasciatus* Say (Culicidae) 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, but no subsequent mating occurred, and after the initial increase in numbers fish populations declined 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 (Garcia and Legner 1999).
Although G. affinis* has been useful for control of mosquitoes in a number of situations, there are definitely some environmental drawbacks to its use. This fish probably never would have been intentionally introduced into foreign areas if today's environmental concerns existed in the early 1900's (Pelzman 1975, Lloyd 1987). A major objection to mosquitofish has been their direct impact on native fishes through predation, or their 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* spp. (Schoenherr 1981, Lloyd 1987). Introductions of Gambusia* spp. have also reduced numbers of other aquatic organisms coinhabiting the same waters (Hoy et al. 1972, Farley and Younce 1977a,b; Rees 1979, Walters and Legner 1980, Hurlbert and Mulla 1981). However, there are no reports of this species, through its feeding on zooplankton (Hurlbert and Mulla 1981, Hurlbert et al. 1972) causing algal blooms outside of the experimental aquarium environment (Walters and Legner 1980).
Another widely used fish for mosquito control is the common guppy, Poecilia reticulata* (Peters) (Microcyprini: Cyprinodontidae), which has been deployed successfully in Asia for the control of waste water mosquitoes, especially Cx. quinquefasciatus. Like their poeciliid relatives, Gambusia* spp., they are native to tropical South America. But, rather than being intentionally introduced to control mosquitoes, this fish was spread to other parts of the world through the tropical fish trade. Sasa et al. (1965) observed feral 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 they do not tolerate low temperate-zone water temperatures (Sasa and 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 (Sjogren 1972). In Sri Lanka, wild populations have been harvested and used for the control of mosquitoes in abandoned wells, coconut husks and other sources rich in organic rubbish (Sasa and Kurihara 1981, Sabatinelli 1990). This fish also now occurs in India, Indonesia and China and has been intentionally introduced for filariasis control into Burma (Sasa and Kurihara 1981). Mian et al. (1986) 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.
Imported fish have also been used to clear aquatic vegetation from waterways which concurrently produced excellent mosquito control. In the irrigation canals and drains of southeastern California, which extend to over 8,000 km., three species of subtropical cichlids <PHOTO>, Tilapia zillii (Gervais) (Percomorphi: Cichlidae), Oreochromis mossambica* (Peters) (Percomorphi: Cichlidae) and Oreochromis hornorum* (Trewazas) (Percomorphi: Cichlidae) were introduced and became established over some 2,000 ha. of Cx. tarsalis breeding habitat (Legner and 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 and Medved 1973, Legner 11978a, 1978b, 1983; Legner and Fisher 1980, Legner and Murray 1981 , Legner and Pelsue 1980, 1983). This is a unique example of persistent biological control and probably only apropos for relatively sophisticated irrigation systems where a permanent water supply is assured, and water conditions are suitable to support the fish (Legner et al. 1980). Advantages in the use of these fish are the clearing of vegetation to keep waterways open, mosquito control, and the fish are large enough to be captured for human consumption. Some sophistication is necessary when stocking these cichlids for aquatic weed control, which is often not understood by irrigation management personnel (Hauser et al. 1976 , 1977; Legner 1979b). Otherwise competitive displacement may eliminate T. zillii, the most efficient weed eating species (Legner 1986). The numerous crater nests of these cichlids found in irrigation drains attests to their firm establishment and aquatic weed cleansing action <PHOTO>.
Storage of water in open containers has frequently been the cause for outbreaks of human disease transmitted by Aedes aegypti (Linnaeus) (Culicidae) in less developed parts of the world. While conducting Ae. aegypti surveys in Malaysia during the mid 1960s, Dr. Richard Garcia of the University of California, Berkeley (pers. commun.) observed 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 of Ae. aegypti in their vicinity.
Neng (1986) reported that catfish, Claris* sp., controlled Ae. aegypti in water storage tanks in coastal villages of southern China. This indigenous, edible fish consumed large numbers of mosquito larvae, had a tolerance for a wide range of environmental extremes, and could be acquired in the local markets. One fish was placed in each water source with survey teams monitoring for its presence about every 10-15 days. If fish were not found on inspection the occupant was persuaded to replace the fish. The study was conducted from 1981 to 1985, during which mosquito-breeding surveys showed a great initial reduction in Ae. aegypti followed by a sustained control of mosquitoes 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 1/15th that of indoor house spraying (Neng 1986).
Alio et al. (1985) described the use of a local species of fish for the control of a malaria vector similar to that 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 were the only sources of water during the dry season for the large pastoral human population. Anopheles arabiensis Patton, a local vector of malaria, was essentially restricted to these sites, and introduction of fish into the "barkits" dramatically reduced both the vector and nonvector populations of mosquitoes. 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) suggested that the added benefits of reduced vegetation and insects in the water sources was also recognized by the local population, resulting in community cooperation. This 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 rather than imported fish in vector control programs. There are other examples where native fishes have been used in specialized circumstances (Kligler 1930, Legner et al. 1974, Menon and Rajagopalan 1977, 1978, Walters and Legner 1980, Ataur-Rahim 1981 and Luh 1980, 1981). Lloyd (1987) reasoned that only indigenous fish should be employed for mosquito control because of the environmental disruption affected by imports such as G. affinis*. However, he urged careful examinations for prey selectivity, reproductive potential and competence in suppression of mosquitoes before attempting their use. Lloyd (1987) also encouraged a multidisciplinary approach involving entomologists and fisheries biologists when utilizing indigenous fish for mosquito control. Paradoxically, in California where native pup fishes in the genus Cyprinodon* may afford a greater potential for mosquito control under a wider range of environmental extremes than Gambusia* spp. (Walters and Legner 1980), the California Department of Fish and Game discourages their use on the basis that unknown harmful effects might occur to other indigenous fishes, and that certain rare species of Cyprinodon <PHOTO> might be lost through hybridization.
An effective tactic was used in China where native fish serve both for mosquito control and as a protein source (Petr 1987, Garcia and Legner 1999). However, this approach for mosquito control is not novel, as Kligler (1930) used a tilapine fish to control Anopheles spp. in citrus irrigation systems in Palestine, where farmers cared for the fish, consuming the larger ones. According to Luh (1980, 1981), rearing of edible fish for the purpose of mosquito control and human food has been widely encouraged in China. The common carp, Cyprinus carpio Linnaeus (Cypriniformes: Cyprinidae), and the grass carp, Ctenopharygodon idella* Valenciennes (Cypriniformes: Cyprinidae), are generally used. Fish are liberated as fry when 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 give benefits of a significant reduction in culicine larvae, a lesser extent anopheline larvae, the fish are harvested as food, and rice yields are increased probably by a reduction of aquatic weeds and by fertilization of the plants through fish excreta (Luh 1981).
Annual or "instant" fishes, (Cyprinodontidae), native to South America and Africa, have been considered as possible biological control agents for mosquitoes (Vanderplank 1941, 1967; Hildemann and Wolford 1963, Bay 1976, 1972; Markofsky and Matias 1979). The desiccation resistant eggs of these cyprinodontids enable them to persist in temporary water habitats. They may also impact mosquito populations in native areas (Vanderplank 1941, Hildemann and Wolford 1963, Markofsky and Matias 1979). In California the South American Cynolebias nigripinnis* Regan (Cyprinoformes: Cyprinodontidae) and Cynolebias bellottii * (Steindachner) (Cyprinoformes: Cyprinodontidae), survived one summer in rice fields, but no reproduction was observed over a three-year period (Coykendall 1980). It was speculated that further research may enable their establishment in temporary pools and possibly rice fields. Cynolebias bellottii <PHOTO>, reproduced repeatedly and persisted in small intermittently dried ponds in Riverside, California for 11 consecutive years, 1968-1979 (Legner and Walters unpubl.). Four drying/flooding operations over two months were required to eliminate this species from ponds that were being used for native fish studies (Walters and Legner 1980). Because they survive an annual dry period, these fish might 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 and Laird 1985).
The practical use of fish species other than Gambusia* spp. in mosquito control often has been restricted by inadequate supplies, as the cost of tropical and semitropical species obtainable from commercial sources has been prohibitive for stocking large mosquito habitats. Low water temperatures during spring months are unfavorable for tropical species and frequently predispose them to fungal pathogens or predation by cold water fish species (Legner 1979b, 1983).
Predacious Arthropods.--Numerous species of predatory arthropods have been observed preying on mosquitoes, and in some cases are considered important in control (James 1967, Service 1977, Collins and Washino 1985, McDonald and Buchanan 1981). However, among the several hundred predatory species observed, only a few have been deployed to control mosquitoes. Dragonflies, or "mosquito hawks", were one of the first arthropods to be examined; but difficulties in colonization, production and handling have limited their use to only a few areas (Urabe et al. 1986, Sebastian et al. 1990). Thus, they probably never will be used extensively other than in a conservation sense.
Aquatic Coleoptera have been extensively studied in the field, with research facilitated by their habits of consuming solid prey. Although their value in effective mosquito predation has been minimized (Kühlhorn 1961), techniques in serology and radioactive labeling have established the importance of several species in mosquito predation (Baldwin et al. 1955, Bay 1974). The Dytiscidae appeared valuable to a number of workers, with common dytiscid genera including Dytiscus, Laccophilus, Agabus, and Rhantus. Laccophilus terminalus* Sharp (Coleoptera: Dytiscidae) was extensively studied (Borland 1971), but Washino (1969) and Kühlhorn (1961) found this predator to be of limited value in California and Germany, respectively.
Sometimes difficulties associated with the manipulative use of arthropods may be partially overcome. For example, the mosquito genus Toxorhynchites, whose larvae are predators of other mosquitoes, was liberated on several Pacific Islands in an effort to control natural and artificial container breeding mosquitoes such as Ae. aegypti and Aedes albopictus (Skuse) (Culicidae) (Paine 1934, Bonnet and Hu 1951, Peterson 1956). The introductions were not considered successful, even though predatory mosquitoes did establish in some areas (Steffan 1975). Follow-up studies showed 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, Trpiš 1973, Bay 1974, Rivičre and Pichon 1978, Rivičre 1985).
There is still considerable interest in the use of various Toxorhynchites spp. for inundative liberations (Gerberg and Visser 1978, Chadee et al. 1987, Lane 1992). Trpiš (1981) studying Toxorhynchites brevipalpis* (Theobald) showed a high daily consumption rate and long survival of larvae without prey, making this species a prime candidate for biological control. Observations on adult females showed a 50% survivorship over a 10-week period with a relatively high oviposition rate per female. The above attributes suggest that this species would be useful for inundative liberations against container breeding mosquitoes. Studies by Focks et al. (1979, 1980, 1982, 1983) with Toxorhynchites rutilis rutilis* Coquillett in Florida, showed that this species had a high success rate in artificial breeding containers. In a 12.6 ha. residential area, about 70% of the available oviposition sites were located over a 14-day period by two inoculations of 175 females. Mass culturing techniques have been developed for this species and Toxorhynchites amboinensis* (Doleschall) (Focks and Boston 1979, Rivičre et al. 1987b).
Focks et al. (1986) reported that inoculations of 100 T. amboinensis* 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 T. amboinensis* and not the insecticide treatment apparently accounted for most of the reduction. Reducing both the number of predators and malathion applications without lowering efficacy could further refine the procedure. Mosquitoes such as Ae. aegypti and Ae. albopictus, which breed in and whose eggs are dispersed by means of artificial containers, pose major health hazards as vectors of human pathogens throughout the warmer latitudes. Containerized products and rubber tires, which are discarded or stockpiled, give these mosquito species a considerable ecological advantage. The incapacity of governments to control disposal of these containers and difficulties in location once they are discarded makes inundative liberations of Toxorhynchites spp, either alone or in combination with other controls, a logical approach (Focks et al. 1986, Rivičre et al. 1987b).
Other mosquito genera that are predatory on mosquitoes breeding in temporary restricted habitats, such as containers include species of Megarhinus, Anopheles, Lutzia, Armigeres, Eretmapodites and Psorophora. Other Diptera that are predacious on mosquito larvae include Chaoboridae, Dolichopodidae and Empidae. However, manipulation of species in these genera and families has not been attempted directly, although their importance in natural predation of pestiferous mosquitoes is recognized.
Among the Hemiptera, the Notonectidae are voracious predators of mosquito larvae under experimental conditions and in waterfowl refuges in California's Central Valley (Garcia and Legner 1999, Legner and Sjogren, unpub. data). Notonecta undulata* Say (Hemiptera: Notonectidae) and Notonecta unifasciata* Guerin (Hemiptera: Notonectidae) 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 (Garcia and Legner 1999and Sjogren and Legner 1974). Studies on storage of eggs at low temperatures show a rapid decrease of viability with time (Sjogren and Legner 1989). The most workable use of these predators appears to be 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 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). High costs of colonization and mass production, coupled with the logistics of distribution, handling and timing of release at the appropriate breeding site, thwart the use of notonectids in mosquito control.
Other hemipterous genera that have been given some attention as useful mosquito predators are Belostoma, Abedus (Washino 1969) and species of Corixidae (Sailer and Lienk 1954). Immature dragonflies also are predatory on mosquitoes, but they do not possess the searching ability demonstrated by certain Hemiptera and Coleoptera. Spiders (Araneae) also have been shown to be effective predators of adult mosquitoes (Dabrowska-Prot et al. 1968, Garcia and Schlinger 1972, Service 1973).
Parasitic aquatic mites frequently occur on mosquitoes but their biological control importance has not been evaluated (Mullen 1975).
Predacious Crustaceans.--In addition to insect predators, several crustaceans feed on mosquito larvae, among which are the tadpole shrimp, Triops longicaudatus (LeConte) (Notostraca: Triopsidae)., and several copepods. Scott and Grigarick (1979) and Mulla et al. (1986), investigating the tadpole shrimp, showed that it was an effective predator under laboratory conditions and considered that it may play an important role in the field against flood water Aedes spp. and Psorophora spp in southern California. Drought resistance in predator eggs is an appealing attribute for egg production, storage and manipulation in field situations against these mosquitoes (Fry and Mulla 1992). 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 spp. and Psorophora spp. Tadpole shrimp are considered important pests in commercial rice fields.
Miura and Takahashi (1985) reported that Cyclops vernalis* Fisher (Copepoda) was an effective predator on early instar Cx. tarsalis larvae in the laboratory. It was 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 may be suited for more extensive application is the cyclopoid predator, Mesocyclops aspericornis* Daday (Copepoda)(Rivičre et al. 1987a,b). Studies have shown >90% reductions of Ae. aegypti and Aedes polynesiensis* Marks (Culicidae) after inoculation into artificial containers, wells, treeholes and land crab burrows. Although not able to survive desiccation, the small cyclopod predator has persisted almost 2.5 years in crabholes and up to five years in wells, tires and treeholes under subtropical conditions. It 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 (Rivičre et al. 1987a,b). The species is also very tolerant of salinities greater than 50 parts per thousand. The benthic feeding behavior of M. aspericornis* makes it an effective predator of the benthos foraging Aedes spp., but limits effectiveness against surface foraging mosquitoes. Rivičre et al. (1987a,b) reported that the effectiveness against Aedes spp. was due to a combination of predation and competition for food. Perhaps the greatest value of this Mesocyclops is in the control of crabhole breeding species, such as Ae. polynesiensis* in the South Pacific.
Other Invertebrate Predators.--The most important nonarthropod invertebrates to receive 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 (Yu and Legner 1976a,b; Collins and Washino 1978, Case and Washino 1979, Legner 1979a, Meyer and Learned 1981, Ali and Mulla 1983, George 1983, George et al. 1983, Perich et al. 1990, and Legner 1991 ). Several biological and ecological attributes of flatworms 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 (Legner and Tsai 1977 ,1978, Legner 1977, 1979a; Darby et al. 1988) and tolerance to environmental contaminants (Levy and Miller 1978, Nelson 1979).
Collins and Washino (1978) and Case and Washino (1979) suggested that flatworms, particularly Mesostoma spp.* (Microturbellaria), 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. An analysis using extensive sampling showed a significant negative correlations between the presence of flatworms and population levels of Cx. tarsalis and An. freeborni* (Case and Washino 1979). However, these workers cautioned that an alternative hypothesis related to the ecology of these species may have accounted for the correlations. Subsequent investigations by Palchick and Washino (1984), employing more restrictive sampling, were not able to confirm the correlations between Mesostoma spp.* and mosquito populations. However, problems associated with sampling in California rice fields, coupled with the complexity of the prey and predator interactions (Palchik and Washino 1986), indicate that further studies are necessary before the role of this group of flatworms in rice fields can be clearly established.
Considering all the attributes for manipulative use of flatworms, it is surprising that they have not been developed further for use in mosquito control. Undoubtedly the contemporary development of Bacillus thuringiensis var. israelensis DeBarjac (H-14), a highly selective easily applied and "marketable" microbial insecticide, has been partially responsible for slowing further work and development of these predators. Their mass culture must be continuous and demands skilled technical assistants (Legner and 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 (Qureshi and Bay 1969). Chlorohydra viridissima (Pallas) (Hydrazoa) is efficient in suppressing culicine larvae in ponds with dense vegetation and this species also can be mass-produced (Lenhoff and Brown 1970, Yu et al. 1974). However, like the flatworms, work on these predators has declined, probably for similar reasons as speculated for the flatworms. Microbial pesticides can be employed over an extensive range of different mosquito breeding habitats. Yet the relative seasonal permanence of control achieved with the flatworms and hydra should restore their importance as resistance to and costs of microbial pesticides accelerates.
Pathogenic Fungi.--Species of fungi such as Beauveria bassiana (Bolsano), Metarrhizium anisopliae (Metsch.), Entomophthora spp., Coelomomyces spp. and Lagenidium spp. have been used to control mosquitoes (Garcia and Legner 1999); but the most promising fungal pathogen is a highly selective and environmentally safe oomycete, Lagenidium giganteum* Couch (Oomycetes: Lagenidiales) which it is applied by aircraft to rice fields (Kerwin and Washino 1987). Lagenidium giganteum* develops asexually and sexually in mosquito larvae, and recycles in standing bodies of water. This creates the potential for prolonged infection in overlapping generations of mosquitoes. Lagenidium giganteum* 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. Nevertheless, problems with production and activation of the oospores remain (Garcia and Legner 1999). 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) reported 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) indicated that laboratory fermentation production of the asexual stage of Lagenidium for controlling mosquitoes in the field may approach the development requirements and costs for the production of Bacillus thuringiensis israelensis. A distinct advantage of this pathogen over the Bacillus is its ability to recycle through successive host generations. There are disadvantages in that the asexual stage is relatively fragile, cannot be dried and has a maximum storage life of only eight weeks, thus, the focus of attention for commercial production is on the oospore, which is resistant to desiccation and can be easily stored. Axtell and Guzman (1987) succeeded to encapsulate 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. Limitations on the use of this pathogen include intolerance to polluted water, salinity and other environmental factors (Garcia and Legner 1999). However, there are numerous mosquito-breeding sources where these limitations do not exist and, therefore, this selective and persistent pathogen may become available for routine mosquito control.
The fungus Culicinomyces clavosporus Couch, Romney and Rao, first isolated from laboratory mosquito colonies and later from field habitats, has been studied for biological control (Sweeney 1987). The fungus is active against a wide range of mosquito species and also causes infections in other aquatic Diptera. The relatively inexpensive media in fermentation tanks facilitates production. However, problems in storage must be overcome if this fungus is to be widely used (Sweeney 1987). Although the fungus has shown high infection rates in field trials with high dosage rates, appreciable persistence at the site has not been demonstrated (Sweeney 1987).
Various species of Coelomomyces have been studied for use in mosquito control, with epizootic infection rates in excess of 90% being recorded. Although these fungi persist in certain habitats for long periods, the factors responsible for triggering outbreaks are not well understood (Chapman 1974). Field-testing that has been done shows great variability (Federici 1981). Difficulties associated with the complex life cycle of these fungi have encumbered research. Federici (1981) and Lacey and Undeen (1986) reviewed the potential of these fungi for mosquito control. Nevertheless, infections of up to 100% have been reported on some populations of Anopheles gambiae* Giles (Culicidae) in Zambia (Muspratt 1963), but lower rates of 24-48% were reported in Anopheles quadrimaculatus Say (Culicidae) and Ae. crucians* Wiedemann (Culicidae) in the southeastern U.S. (Umphlett 1970, Chapman et al. 1972). Higher infections exceeding 95% were reported from Culiseta inornata (Williston) and Psorophora howardii Coquillett by Coelomomyces psorophorae* Couch and in Aedes triteriatus* (Say) (Culicidae) by Coelomomyces macleayae Laird and 85% in Culex peccator* Dyar et Knab (Culicidae) by Coelomomyces pentangulatus* Couch (Bay et al. 1976).
Although Coelomomyces species have been difficult to mass produce, new introductions of these fungi were made by Laird (1967) on a tiny Pacific Island against Ae. polynesiensis* Mark, a vector of filariasis. This represents one of the few attempts to establish new mosquito pathogens in an area where they did not exist. Further application of Coelomomyces spp. as a direct mosquito control is dependent on the development of easily cultured inoculum. Reports of research with B. bassiana on Culex tarsalis and Aedes nigromaculis* (Ludlow) (Culicidae) (Legner et al. 1974) substantiates that of Clark et al. (1968): Aedes nigromaculis* was more susceptible than Cx.. tarsalis with the third host passage resulting in 100% infection under laboratory conditions.
Parasitic Nematodes.--Among the various mermithid and rhabditoid nematodes pathogenic for mosquitoes, Romanomermis culicivorax* Ross and Smith (Mermithidae: Nematoda), has received the most attention (Poinar 1979, Platzer 1990, Kaya and Gaugler 1993). This mermithid is active against a wide range of mosquito species, and has been mass-produced and deployed in a number of field trials. The nematode was commercially produced and sold as Skeeter Doom TMR, but the eggs showed reduced viability in transport and the product currently is no longer sold (Service 1983). 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, favors further research and development for biological control (Petersen and Willis 1975, Brown-Westerdahl et al. 1982). Several field applications showing good results have included both the preparasitic stage and post parasitic stages with the former more applicable to a "rapid kill" and the latter for more long-term continuous control such as in rice fields (Levy et al. 1979, Brown-Westerdahl et al. 1982). Obstacles 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 (Brown-Westerdahl 1982). Although such environmental problems are not as important for anopheline control, the cost of in vivo mass production is a disadvantage for use of this pathogen. However, it may be adapted for use in specialized habitats integrated with other controls (Brown-Westerdahl et al. 1982).
Neoaplectana carpocapsae* Weiser (Mermithidae: Nematoda) and other nematodes have shown a high level of infection in nature (Platzer 1990).
Pathogenic Bacteria.--Bacteria are not commonly associated with mosquitoes in nature, but one spore forming bicrystalliferous strain of Bacillus thuringiensis var. israelensis (H-14), was isolated by Goldberg and Margalit (1977) and the toxin it produces has been shown by numerous studies to be an effective and environmentally sound microbial insecticide against mosquitoes and blackflies. A high degree of specificity and toxicity, coupled with the relative ease of production, have made it the most widely used microbial product to date for mosquito and blackfly control. Several formulations have been available commercially throughout the world. Nevertheless, its efficacy varies under different environmental conditions and there are some problems associated with its use (Garcia 1987, Lacey and Undeen 1986, Garcia and Legner 1999). The bacterium as applied commercially cannot multiply in the environment, thus it acts essentially as a synthesized insecticide. Evolution of the bacterium to counteract developing resistance in the host is thus precluded, and there are limitations on the development of new strains in the laboratory (Smits 1987).
Another spore forming bacterium, Bacillus sphaericus* Neide, is larvicidal against certain mosquito species (Mulla 1986, Mulla et al. 1991, Singer 1990, Weiser 1984). Several strains of this pathogen show a high degree of toxic variability among species of mosquitoes. Culex spp. appear to be highly susceptible, whereas other species such as Ae. aegypti respond poorly to treatment. Unlike the transitory larvicidal activity of Bt. toxin (Cry IV), some strains of B. sphaericus persist and apparently recycle in certain aquatic habitats (DesRochers and Garcia 1984, Lacey 1990, Yap 1990, Yousten et al. 1992). Although evolution to counteract resistance in the insect is thus possible, real resistance has developed nonetheless (Rodcharoen and Mulla 1993).
Parasitic Protozoa.--Many species of protozoa have been isolated from mosquitoes and other medically important Diptera (Roberts et al. 1983, Lacey and Undeen 1986). These include flagellates (Blastocrithidia spp. and Crithidia spp.), eugregarines (Lankesteria spp.), ciliates (Vorticella spp. and Tetrahymena spp.), and schizogregarines (Caulleryella spp.) and microsporidians. Due to their complex life cycle and the in vivo production methods necessary for maintaining them, research on their practical utility has been limited. However, if more information is developed on their life cycle, it may be found that they could play a role in suppressing mosquitoes through inoculative and augmentive releases in certain habitats (Lacey and Undeen 1986). Infection of mosquitoes by most Microsporida is transovum and field transmission has yet to be shown. Only a few species including Nosema, Stegomyiae and Stempellia sp. possess the ability to infect their hosts per os (Chapman 1974).
Among other promising protozoa is the endoparasitic ciliate, Lambornella clarki Corliss and Coats (Ciliophora: Tetrahymenidae), a natural pathogen of the treehole mosquito, Aedes sierrensis* Ludlow (Culicidae), which has received considerable attention as a potential biological control agent for container breeding mosquitoes (Egerter and Anderson 1985, Egerter et al. 1986, Washburn and Anderson 1990). Cysts resistant to desiccation allow persistence of the ciliate from one year to the next. In vitro production methods have been sought and field trials initiated to determine its efficacy for biological control (Anderson and Washburn 1990).
Viruses.--A number of pathogenic viruses have been isolated from mosquitoes and blackflies (Granados and Federici 1986). A natural population of Aedes sollicitans* Walker (Culicidae) in Louisiana sustained an epizootic by a cytoplasmic and a nuclear polyhedrosis virus where more than 71% infection occurred (Clark and Fukuda 1971). Bay et al. (1974) also reported that H. C. Chapman observed a similar epizootic infecting over 65% of the larvae of Ae. sollicitans*, but reflooding after drying of these habitats greatly reduced infection. Mosquito iridescent viruses have been reported from various mosquito species in Europe and the United States (Clark et al. 1965, Weiser 1965), but natural infection levels rarely exceed 1%. Therefore, viruses do not appear practical for use in control (Lacey and Undeen 1986).
Larvicidal Plants.--Certain plants and plant products are lethal to developing mosquitoes (Azmi et al.1998, Joshi et al 1998, Su and Mulla 1998a, 1998b, Sukumar et al. 1991). However, practical deployment has not been demonstrated, and in the absence of insect population interaction with the substance, insect resistance should rapidly develop. Nevertheless, the possibility of some plant extracts such as Neem, Azadirachta indica A. Juss, being innocuous to nontarget organisms (e.g., mosquito predators) makes such substances highly desirable for integrated control (Su and Mulla 1998, 1998b). Particularly interesting is the activity of ethanol extracts of fresh Neem showing antimalarial activity against chloroquine resistant Plasmodium falciparum strain K1. (Joshi et al. 1998).
Chironomid midges pose nuisances in metropolitan areas such as southwestern California wherever there is a great proximity of urban development to paved flood control river channels, sewage oxidation ponds and recreational lakes. Infestations in paved river channels characteristically become especially severe following winters with above average rainfall. Rapid recolonization of the scoured habitat occurs due to fertile urban runoff water which stimulates algal growth.
Fish have been used for chironomid midge abatement in lentic habitats as an adjunct to chemical pesticides. Such species as the common carp, Cyprinus carpio L. and goldfish, Carassius auratus (L.) and pupfish, Cyprinodon macularius Baird and Girard, have been effective in shallow California ponds (Anderson and Ingram 1960, Bay and Anderson 1965, Legner et al. 1975, Walters and Legner 1980, Legner and Warkentin 1990). However, other cichlid species in the genera Tilapia and Oreochromis are useful for the lotic situation in the paved storm drain habitats (Legner 1983). The addition of three species of tilapine fishes to drainages in the Los Angeles area in the 1970's resulted in widespread establishment of an apparent hybrid of Oreochromis mossambica* (Peters) (Cichlidae) and Oreochromis hornorum* Trewazas (Legner 1983). Densities of Chironomidae, principally Chironomus attenuatus* Johannsen larvae, declined significantly in the drainages and resulted in complete adult midge control. The foraging on Chironomidae in certain detritus substrates by very dense populations of the fish influenced the ability of such substrates to produce chironomids. The chironomid-sustained fish biomass in autumn may exceed 4 X 105 kg.. over a distance of 18 km. of one studied paved river channel. By 1990 the tilapine fish were regularly ranging in the neritic zone along the southwestern California coast, and their contribution to predatory marine fish biomass was considered significant (Legner and Pelsue 1980, Legner et al. 1980).
The Planaria and Hydra noted previously in mosquito control also significantly reduced chironomid population densities in experiments (Yu and Legner 1976a, Garcia and Legner 1999). However, they were never deployed specifically for chironomid control. Hilsenoff (1964), Hilsenoff and Lovett (1966) reported on leeches and a microsporidian as significant natural enemies of chironomids.
Tabanidae, or horseflies and deerflies, although widespread and on occasion serious pests and vectors of disease to livestock, have not received much attention. Only one successful inundative release of the egg parasitoid, Phanurus emersoni* Girault (Hymenoptera: Scelionidae), has been recorded (Parman 1928). Apparently, this effort was precipitated by a severe outbreak of anthrax at the time and since this disease diminished and other control tactics are available, interest in their biological control has not been continued. Other references to natural enemies of tabanids include James (1963) and Magnarelli and Anderson (1980).
The genera Simulium and Eusimulium are of special importance because adults emerge in great numbers to inflict vicious bites on humans. Moreover, some species are vectors of onchocerciasis. Attempts were made in 1931 to establish certain dragonflies and a predacious chironomid, Cardiocladius sp., in New Zealand on Simulium sp., but results were not positive (Clausen et al. 1978). This group apparently does not lend itself well to biological control, probably due to the rapidly flowing water habitat.
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