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Bacillus thuringiensis israelensis (Bti) is a naturally occurring insect pathogen widely used as a microbial larvicide for mosquito control. The toxicity of Bti is specific to mosquitoes and a few other dipterans. It is applied to aquatic habitats colonized by mosquitoes and produces proteins that paralyze the digestive system of larvae that consume it. While Bti is an effective larvicide, it can also deter oviposition by female mosquitoes. Thus, reduced production of adult mosquitoes at the local (i.e., pond) scale may reflect both reduced colonization and reduced larval survival. It is important to distinguish between these mechanisms. While deterring oviposition can reduce local mosquito production, these eggs may be redirected to other suitable habitats and contribute to adult recruitment at the landscape scale. In contrast, larval mortality reduces both local and regional recruitment. Thus, to maximize mosquito control across spatial scales, we should attract rather than deter oviposition to Bti treated habitats, creating mosquito “sinks”. As mosquitoes often preferentially deposit eggs in aquatic habitats rich in organic matter, we hypothesize that increasing leaf litter to attract oviposition will increase the efficacy of Bti treatment. We tested this hypothesis using a 2 x 2 factorial experiment in which we manipulated litter abundance and Bti presence and quantified Culex spp. mosquito oviposition, larval abundance, and adult emergence. Each treatment was replicated 7 times in aquatic mesocosms arrayed in an old field at the VCU Rice Center. Bti had no effect on mosquito oviposition or larval abundance. In contrast, increasing leaf litter 50% resulted in a fivefold increase in egg rafts and mosquito larvae. The effect of Bti on adult emergence depended upon leaf litter. Bti reduced emergence by 75% in high litter treatments, but had no effect in low litter treatments. In summary, high litter combined with Bti application increased mosquito colonization fivefold but produced no more adults than low litter treatments. Thus, even though we found no evidence that Bti deterred oviposition, attraction to litter resulted in increased efficacy of Bti application. Our results suggest a potential cost effective, chemical insecticide free approach to enhanced mosquito control.
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