Evaluating larvicide performance against Culicoides sonorensis in semi‐field conditions

Abstract BACKGROUND Culicoides biting midges are small flies that are considered a nuisance pest that also can serve as vectors for a number of human and veterinary‐important pathogens. This study evaluated the efficacy of larvicide products against C. sonorensis populations under controlled semi‐field conditions and investigated how environmental factors influence the degradation and persistence of the larvicide active ingredients (AIs) in an artificial larval habitat at three study locations (Lubbock, TX; Manhattan, KS; and Slidell, LA). RESULTS All tested larvicide products resulted in larval mortality in bioassays, but efficacy declined significantly with days post‐treatment across all products. Overall, larval mortality in bioassays post‐treatment was significantly lower at the TX site relative to the LA and KS sites. A general linear model revealed a significant main effect of larvicide products on larval mortality. Qualitative comparisons between site‐specific observed mortality and environmental conditions show variations in climate (precipitation but not photodegradation) possibly associated with product performance. Greater amounts of rainfall occurred in LA and KS than in TX during the study periods, which is likely to have reduced residual efficacy over time. CONCLUSION These findings indicate that environmental factors, particularly precipitation and larval habitat substrate disturbance may have played a role in limiting larvicide efficacy against Culicoides midges. The results are discussed in relation to providing information to guide appropriate larvicide AI choices and application frequencies to target Culicoides populations. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.