Variable gut pH as a potential mechanism of tolerance to Bacillus thuringiensis subsp. israelensis toxins in the biting midge Culicoides sonorensis.

BACKGROUND: Toxins of Bacillus thuringiensis subsp. israelensis (Bti) are safer alternatives 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 products are ineffective at killing biting midges despite their taxonomic relation to black flies and mosquitoes. Here, we investigate the toxicity of a Bti-based commercial insecticide and its active ingredient in larval Culicoides sonorensis. A suspected mechanism of Bti tolerance is an acidic larval gut, and we used a pH indicator dye to examine larval Culicoides sonorensis gut pH after exposure to Bti. RESULTS: The lethal concentration to kill 90% (LC90) of larvae of the commercial product (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. CONCLUSION: We conclude that Bti is not practical for larval Culicoides sonorensis control at the same rates as mosquitos but show that alterations or additives to the environment could make the products more effective. © 2024 Society of Chemical Industry.

Laboratory and Semi-Field Evaluation on S-Methoprene Formulations Against Anopheles sinensis (Diptera: Culicidae) - Yuxi City, Yunnan Province, China.

What is already known about this topic?: Anopheles sinensis (An. sinensis) is the predominant malaria vector in China. The impact of S-methoprene on the emergence process of mosquito larvae suggests its potential as a control method for vector mosquitoes. However, the efficacy of S-methoprene in controlling An. sinensis has not yet been demonstrated. What is added by this report?: The effectiveness of S-methoprene against An. sinensis was assessed in laboratory and semi-field conditions in Yunnan Province. What are the implications for public health practice?: These results offer valuable options and guidance for utilizing S-methoprene products in malaria reimportation prevention areas within Yunnan Province.

Preparation of a long-lasting tablet of spinosad microspheres and its residual insecticidal efficacy against the yellow fever mosquito, Aedes aegypti (Diptera: Culicidae) larvae.

BACKGROUNDS: In order to provide a long-lasting formulation for spinosad (SP) targeting larval stages of Aedes aegypti (Linnaeus) and others alike, a SP tablet was developed based on microspheres, using polylactic acid as inside coating material. The microspheres were encapsulated using polyethylene glycol and 1-hexadecanol to form a sustained-release SP tablet. Micromorphology, active ingredient loading, structure identification, photolysis resistance and biological activity were evaluated in this report. RESULTS: (i) The SP microspheres had an average particle size of 6.16 ± 2.28 µm, low adhesion and good dispersion as evaluated by scanning electron microscopy and morphology. (ii) The average active ingredient loading and encapsulation of SP microspheres were 32.80 ± 0.74% and 78.41 ± 2.22%, respectively. (iii) The chemical structure of encapsulated SP was confirmed by Fourier transform infrared and 1H-nuclear magnetic resonance. (iv) The photostability of the microspheres and the tablets were evaluated. The results showed that DT50 (time required to dissipate 50% of the mass originally present) of SP was 0.95 days in microspheres and 6.94 days in tablets. (v) The long-term insecticidal activity of SP tablets was investigated, and the tablet had a long-lasting activity against the mosquito larvae, showing 100% larval mortality for 63 days. CONCLUSIONS: The study provided a new long-lasting formulation of SP, which displayed good efficacy in the control of Ae. aegypti larvae. © 2024 Society of Chemical Industry.