UC Riverside

Culicoides biting midges are the primary vectors of some of the most economically damaging pathogens of livestock worldwide, including bluetongue virus (BTV). Bluetongue disease affects mainly domesticated ruminant livestock, especially cattle and sheep. Morbidity and mortality can be very high in susceptible animals. In the United States, production losses and trade restrictions make BTV an economically important pathogen for the cattle industry. Both BTV and the primary North American vector species, C. sonorensis Wirth and Jones are endemic to California. Although temperatures in southern California are theoretically high enough to allow year-round transmission, adult vector activity and cattle seroconversions cease or exist at very low levels during the winter. Culicoides overwintering has been of considerable interest since BTV expanded into northern Europe in 2006, successfully overwintering in a temperate climate and causing consecutive epizootics that resulted losses of millions of animals and billions of dollars.

Little is known about the immature stages of Culicoides. I examined the desiccation tolerance of C. sonorensis eggs, and the low temperature tolerance of eggs, larvae, and pupae. Although previous literature had suggested that Culicoides eggs would be susceptible to environmental stress, I found that C. sonorensis eggs can withstand nearly complete desiccation and exposure to temperatures as low as -20°C without suffering complete mortality. Conversely, larvae succumbed to temperatures less than -4°C. Culicoides eggs are likely to be more important in seasonal persistence of vector populations than previously thought.

Our understanding of adult vector activity stems from trap collection data. However, trapping strategies can bias insect collections. I collected C. sonorensis on dairies in California using traps baited with either CO2, UV light, or CO2+UV, and placed either near cattle, oviposition sites, or in open fields. I found that traps placed in fields were more efficient at collecting adult midges. I also found that traps baited with UV collected insects with significantly lower BTV infection rates than traps baited with only CO2, suggesting that BTV+ midges are repelled by UV light. The use of UV traps runs the risk of missing active, infected vectors during periods of low activity, like winter, potentially resulting in a misunderstanding of true seasonal disease dynamics.