Biological control is the deliberate effort by humans to control pests by manipulation of antagonistic organisms (called biological control agents). Typically it is a treatment highly specific to the target (pest) organism and thus it differs from chemical control, because chemicals tend to hit target and non-target organisms fairly indiscriminately. Biological control is based on an understanding of the population dynamics of the pest organism as regulated by antagonistic organisms and other factors in nature. Some forms of biological control against some targets can offer a permanent control solution and can offer a remarkably good cost to benefit ratio.
Aquatic Predators and Pathogens
Use of predators and pathogens as biological control agents is especially difficult where survival of the target pests is limited by food supply. Put simply, if there is only enough food in the water held by a bromeliad tank to allow the development of 10 (pest) mosquito larvae but 100 mosquito larvae are present, then 90 will die of starvation. If a predator (or pathogen) kills 70 mosquito larvae, only 20 will die of starvation and still 10 will develop to the pupal stage. Only if the predator (or pathogen) succeeds in killing more than 90 of the mosquito larvae will there be any reduction in the number of mosquito larvae developing.
Larvae of some mosquito species are predatory on other organisms, including other mosquito larvae. In Florida, two such are the larvae of Toxorhynchites rutilus and Corethrella appendiculata. Normally found in water-filled treeholes, they also have been found occasionally in tanks of bromeliads such as Billbergia pyramidalis (17). Adults of these mosquitoes do not bite warm-blooded animals (adults of Toxorhynchites rutilus do not take blood at all). The natural rarity of the larvae in tanks of Billbergia pyramidalis denies them the opportunity for any important effect on Wyeomyia larvae.
The reason for their rarity in bromeliad tanks is doubtless because they are adapted for existence in treeholes and so the adult females search for treeholes in which to lay their eggs (8, 16). Rarely, the adult females make "mistakes" and lay a few eggs in Billbergia or Tillandsia tanks (more frequently into tanks of darker, exotic bromeliads such as Neoregelia spectabilis (37); these have little or no effect on Wyeomyia larvae (13).
In feeding experiments, Toxorhynchites rutilus larvae left a few Wyeomyia prey larvae untouched, no matter how many they were offered (13). Toxorhynchites larvae may conserve their prey in the manner of stock-rearers (28), and may be unsuitable as biocontrol agents where survival of the prey is limited by food availability.
It has been suggested that eggs of Toxorhynchites rutilus or exotic relatives (Toxorhynchites brevipalpis from Africa and Toxorhynchites amboinensis from Asia) should be mass-produced and distributed in bromeliad tanks for control of pest mosquitoes. However, the high labor costs of such mass-production, the short "shelf life" of the eggs (they hatch in fewer than two days), and the high labor costs of distribution make this approach impracticable ln Florida. Females of these three species do not search for plant leaf axils in which to lay their eggs, so mass-production and releases of these mosquitoes as adults would do little or nothing to reduce the problem caused by mosquito production from bromeliads.
Collection records suggest that Toxorhynchites guadeloupensis from the West Indies and Toxorhynchites superbus from Mexico south to Colombia are primarily inhabitants of bromeliads (10). They might be considered as biocontrol agents for bromeliad-inhabiting mosquitoes, but a performance assay has been made of only one bromeliad-inhabiting Toxorhynchites species. Toxorhynchites haemorrhoidalis (which, despite the different name, is very possibly the same species as the Toxorhynchites superbus mentioned above) larvae inhabit flower bracts of Heliconia, leaf axils of aroids, and tanks of the bromeliads Aechmea nudicaulis and Aechmea aquilega in coastal Venezuela (23). Experiments performed produced no evidence that Toxorhynchites haemorrhoidalis larvae reduced the abundance of pest mosquito larvae in either of these bromeliad species (21). There is still no evidence that larvae of any Toxorhynchites species are useful biological control agents of pest mosquitoes in bromeliad tanks.
In Florida, small predatory fly larvae (Stenomicra: Aulacigastridae; Neodexiopsis: Muscidae) have been found in water held in tanks of Tillandsia utriculata, but their effect on populations of Wyeomyia larvae is thought to be negligible (7).
In Venezuela, larvae of a damsel fly, Leptagrion siquierai, occur in water held in tanks of Aechmea aquilega and to a lesser extent in Aechmea nudicaulis. They are predatory and amphibious. Unlike larvae of Toxorhynchites, these damsel fly larvae can crawl from one tank to another, so they may be better adapted to survive in Aechmea aquilega than in Aechmea nudicaulis because the former is more compartmentalized (has more water-filled axils). Their effectiveness as predators against populations of pest mosquito larvae has not yet been demonstrated (22).
Predatory water beetles have been recorded from bromeliad tanks in the neotropics (7), but their effects on mosquito larval populations have not been evaluated.
Wyeomyia larvae infected with the protozoan pathogen Pilosporella fishi or the fungal pathogen Coelomomyces sp. are rare in Florida (9, 19). Clearly these pathogens have a negligible effect on numbers of Wyeomyia larvae surviving. Their rarity has hindered research on their life cycles, and no explanation is available as to why they are not more common.
In Florida, spiders and ants are common among the leaves of native and imported bromeliads, but are not specialists to this habitat. They may contribute to limitation of Wyeomyia populations, by capturing some of the Wyeomyia adults emerging from the aquatic pupal stage, but no evaluation of this contribution has been made. Predation by such organisms on adult Wyeomyia may be useful because adult populations (unlike larvae) are not suspected of being limited by food resources.
Ants have been seen to prey on Wyeomyia eggs in the laboratory, but no evaluation has been made in nature. Some species of predatory beetles of the families Carabidae and Staphylinidae are known to be specialists to the habitat provided by bromeliads in the neotropics (11), but their life cycles are unknown or poorly known and no evaluation of their effect on mosquito populations has been made.
Mosquito eggs, larvae and pupae are not the only organisms inhabiting bromeliad tanks. However, just as Florida has few native species of bromeliads in contrast with neotropical countries such as Costa Rica (7), and just as Florida has few bromeliad-inhabiting mosquito species in contrast with neotropical countries such as Jamaica (10), so it seems to have relatively few other sorts of organisms (7). Tank bromeliads in a few acres of coastal Venezuela were found to have more mosquito species and more species of other organisms than in all of Florida (21-23), but each bromeliad contained far fewer larvae of any pest mosquito than is typical of bromeliads in a natural habitat in Florida (13). Nutrients in neotropical bromeliads are shared between a diversity of organisms. Thus, it can be surmised, fewer nutrients are available to mosquitoes and, consequently, smaller mosquito populations are produced.
Reduction of nutrients suitable for nutrition of mosquito larvae in bromeliad tanks in Florida could perhaps be accomplished by introduction of innocuous organisms which would consume these nutrients. These innocuous organisms would be from neotropical tank bromeliads, and they would act as competitors with the mosquito larvae. This seems a rational approach to biological control, when survival of the target organisms is limited by food. However, research toward this objective has not yet begun, and thorough characterization of the specialized diet of any of the organisms in bromeliad tanks has not been accomplished.
Damage caused to plants by pests can be quantified in terms of loss in value of the plants and in costs of control measures taken. Such loss figures are important in drawing up lists of priorities for publicly-funded research on biological control. Damage caused by diseases transmitted to humans by mosquitoes can be quantified in terms of medical costs and lost wages. Methods for quantifying damage caused by mosquitoes that are almost entirely a nuisance lack persuasion, and public funds for biological control research are much more likely to be awarded to projects of perceived higher priority (of which there is no shortage). The prospects are poor for adequate funding of research on biological control of bromeliad-inhabiting mosquitoes in general. However, the threat of disease transmission by Aedes albopictus is likely to make that mosquito a target of research projects, though not with special reference to bromeliads.