D3018: Look elsewhere for larval Culicoides control: Bacillus thuringiensis subsp. israelensis works poorly in C. sonorensis

Efforts to control pest insects can come up against a desire to protect natural ecosystems from large-scale chemical insecticide applications. Bacillus thuringiensis subsp. israelensis (BTI) is a bio-rational insecticide which targets dipteran pests specifically and is touted as a safer alternative for non-target insects and the environment. While effective against mosquitoes, reports suggest that BTI is inefficient at killing midge larvae. Culicoides sonorensis is an economically important vector of animal viruses in the United States and effective control of the immature stages is necessary to reduce adult populations which will protect vulnerable livestock. Here, we examine the toxicity of VectoBac® WDG and separately its active ingredient (BTI strain AM65-52) against larvae of the biting midge C. sonorensis (Diptera: Ceratopogonidae) in a controlled lab environment. The determined LC₉₀ of VectoBac® WDG was 386 mg/L (95% CI: 319-503 mg/L), which is over 2,000 times more material than is required to kill most mosquito species. Additionally, treatment using the active ingredient alone did not achieve more than 50% larval mortality, even at the highest concentration tested (4,830 mg/L). This discrepancy between mosquitos and midges has previously been attributed to a larval gut environment impermissible for BTI toxin activity, and indeed, we show that the midge gut becomes unfavorably acidic when larvae were soaked in a pH indicator dye. We conclude that BTI is not practical for killing C. sonorensis larvae and are now investigating new larval control methods that seek to balance efficacy, specificity, and environmental safety.