Variable gut pH as a potential mechanism of tolerance to Bti in the biting midge Culicoides sonorensis

Authors: OSBORNE, CAMERON - Kansas State University; Cohnstaedt, Lee; SU, TIANYUN - Ecozone International, Llc; SILVER, KRISTOPHER - Kansas State University

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2024
Publication Date: 3/25/2024
Citation: Osborne, C.J., Cohnstaedt, L.W., Su, T., Silver, K.S. 2024. Variable gut pH as a potential mechanism of tolerance to Bti in the biting midge Culicoides sonorensis. Pest Management Science. https://doi.org/10.1002/ps.8104.
DOI: https://doi.org/10.1002/ps.8104

Interpretive Summary: Natural insecticides derived from living organisms such as the toxin from the bacteria Bacillus thuringiensis subsp. israelensis (Bti) is known to be an excellent pesticide for mosquitoes. It is environmentally safe because it does not kill non-target organisms when compared to traditional chemical insecticides for larvae or the immature stages of mosquito development. However the efficacy of this bacterial product on the biting midge Culicoides sonorensis (Diptera: Ceratopogonidae) is poorly understood. The product sometimes works and sometimes does not and biting midges are a pest of livestock in much of the United States so finding a reason why the product does not consistently work is important to protect animal agriculture. Here, we investigate the toxicity of a Bti-based insecticide that is commercially available on immature biting midges. A suspected mechanism of Bti tolerance is an acidic larval gut, and we used a pH indicator dye to examine larval C. sonorensis gut pH after exposure to Bti. The LC90 of VectoBac® WDG (386 mg/L) was determined to be almost 10,000 times more than that of some mosquito species, and no concentration of active ingredient tested achieved 50% larval mortality. The larval gut was found to be more acidic after exposure to Bti which inhibits Bti toxin activity. By comparison, 100% mortality was achieved in larval mosquitoes at the product’s label rate for this species and mosquito larvae had alkaline guts regardless of treatment. Altering the larval rearing water to alkaline conditions enhanced Bti efficacy when using the active ingredient. We conclude that Bti is not practical for larval biting midge control at the same rates as mosquitos but show that alterations or additives to the environment could make the products more effective.

Technical Abstract: Bio-rational insecticides derived from toxins of Bacillus thuringiensis subsp. israelensis (Bti) are safer alternatives for non-target organisms when compared to traditional chemical insecticides for controlling dipteran pests such as black flies and mosquitoes. The biting midge Culicoides sonorensis (Diptera: Ceratopogonidae) is an important pest of livestock in much of the United States and larval midges utilize semi-aquatic habitats which are permissive for Bti product application. Reports suggest that Bti and Bti products are ineffective at killing biting midges despite their close taxonomic relation to black flies and mosquitoes. Here, we investigate the toxicity of the Bti-based insecticide VectoBac® WDG and its active ingredient (Bti strain AM65-52) in larval C. sonorensis. A suspected mechanism of Bti tolerance is an acidic larval gut, and we used a pH indicator dye to examine larval C. sonorensis gut pH after exposure to Bti. The LC90 of VectoBac® WDG (386 mg/L) was determined to be almost 10,000 times more than that of some mosquito species, and no concentration of active ingredient tested achieved 50% larval mortality. The larval gut was found to be more acidic after exposure to Bti which inhibits Bti toxin activity. By comparison, 100% mortality was achieved in larval Aedes aegypti at the product’s label rate for this species and mosquito larvae had alkaline guts regardless of treatment. Altering the larval rearing water to alkaline conditions enhanced Bti efficacy when using the active ingredient. We conclude that Bti is not practical for larval C. sonorensis control at the same rates as mosquitos but show that alterations or additives to the environment could make the products more effective.