Effects of sublethal insecticide exposure on Culicoides sonorensis vector competence for bluetongue virus

Biting midges in the genus Culicoides are known to transmit dangerous and economically important pathogens like bluetongue virus (BTV), which can cause epizootic outbreaks of livestock abortion and illness, particularly in sheep. Commonly, we try to control insect-borne illnesses by combating their vectors with insecticides. Even with correct application, these interventions lose efficacy over time and can be reduced to sublethal concentrations that can affect insects in unexpected ways. Studies on mosquitoes have shown that exposure to sublethal concentrations of different insecticides can alter vector competence. To determine whether insecticide exposure changes Culicoides susceptibility to BTV, adult midges were exposed to permethrin and cuomaphos at predetermined LC₃₀, LC₂₀, and LC₁₀ concentrations, and then exposed to an infectious bloodmeal. Exposed midges were then held at 25C, 75-80% humidity, and 12:12 L:D for up to 14 days. Midges were subsampled on days 1, 3, 7, and 14, and tested for BTV infection and dissemination using qRT-PCR. We predict that in the earlier subsampling days (1-3), as the midges are expending metabolic resources on detoxification of the chemicals, exposure to insecticide will positively correlate with increased rates of infection and dissemination. This study helps shed light on the transferability of vector competence research between mosquitoes and midges, opens the door for additional vector competence work in midges, and may also guide future recommendations for Culicoides control in livestock.