Biological Control of Larvae Using Copepods
Biological control is an attractive alternative to chemical pesticides for the suppression of mosquito pests and disease vectors (Lounibos and Frank 1994). Copepods for biological control have not yet been field tested in south Florida. We are maintaining cultures of several copepod species at FMEL and are conducting experimental trials with Macrocyclops albidus, one of the more promising predatory species.
We are testing the cyclopoid copepod Macrocyclops albidus (Jurine) for biological control of mosquitoes in laboratory microcosms, in controlled field conditions, and in a long-term field experiment using discarded tires.
Preliminary results indicate that this predator is highly efficient in controlling mosquitoes in all three settings, reaching close to 90% reduction in larval survival under field conditions and exceeding the recommended predation rates for effective mosquito control in laboratory experiments. The predator is most effective on 1-4 day old larvae. Alternate food and habitat structure significantly influenced the predation rates on mosquito larvae.
Once established, the copepod has been able to maintain long-term reproducing populations in the field. This copepod species is a promising candidate for control of mosquito larvae because it is a widespread and highly effective predator that is capable of establishing and maintaining populations under a wide variety of field conditions. Additionally, M. albidus is relatively easy to culture, maintain, and deliver to the target areas.
M. albidus is an effective predator of mosquito larvae in artificial containers and capable of drastically reducing larval populations. Marten, considers that other species, such as Mesocyclops longisetus and Mesocyclops aspericornis may be better suited for biological control in tropical areas because they maintain larger population sizes than M. albidus (Marten 1990b and pers. comm.). However, in this study, population sizes of M. albidus in field tires were much higher than previously reported from more temperate (Marten 1990b) and tropical (Marten et al. 1994) areas.
Also, because of its cold-hardiness, M. albidus should be better able to maintain year-round populations in subtropical areas than species recommended for the tropics, and may be able to survive even under extreme conditions (e.g. record cold weather) which in the sub-tropics are usually of short duration For example, Schreiber et al. (1996) reported sharp declines in the populations of Mesocyclops longisetus in field tires when water temperatures dropped below 5 C, but, as previously stated, M. albidus is known to survive for months at 0 C. This species demonstrated kill rates that are more than appropriate for mosquito control and is able to survive on alternate prey when larval mosquito populations are low.
In the practical sense, this species is also a good candidate for use as a mosquito control agent. The species has a worldwide distribution so seed stocks should be easy to obtain, and practical and legal problems associated with exotic species introductions would not apply. Large numbers of copepods can be produced in a small space as they can be grown in small plastic pools, plastic garbage cans, and similar inexpensive containers; and cultures do not require heavy maintenance and are relatively inexpensive to maintain. Large numbers of copepods can be kept in water in a refrigerator for months, they can survive in soil and detritus that is only slightly damp (Marten 1989), and they are not killed by many pesticides commonly used for mosquito control (Marten 1989, Tietze et al. 1994).
The lag (if any) between the introduction of predator populations and effective mosquito control can be eliminated by initial treatment with larvicides or Bti (Tietze et al. 1994) or simultaneous introduction of other predators such as Toxorhynchites spp. (Schreiber et al. 1996). Another attractive strategy to eliminate the lag is to introduce the copepods in early spring when mosquito populations have not built up yet and mosquito control personnel are not as busy as during the "heavy" part of the mosquito season.
Work still needs to be done on effective strategies for large-scale deployment of the copepods. However, standard spray equipment can be easily modified and calibrated to dispense intact copepods (Marten 1989), and the fact that they can withstand almost-dry conditions means that storage and transport can be accomplished without having to also store and transport large quantities of water (Marten 1989).
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