Location, location, location: Optimizing surveillance efforts for Culicoides sonorensis and bluetongue virus through trap placement and attractant choices

Culicoides sonorensis (Diptera: Ceratopogonidae) is the primary vector of bluetongue virus (BTV) in the United States. Worldwide, the most common method for trapping Culicoides to estimate midge abundance and BTV risk is using UV light baited suction traps, placed near animals or larval development sites, despite evidence that trap type and placement may affect collections. Furthermore, work by Mayo et al. (2012) suggests that trap type may also affect infection rates of collected Culicoides. We evaluated the effect of trap type and placement on collections of C. sonorensis, particularly on the ratio of parous to nulliparous flies collected, and on BTV prevalence in pars.

We conducted a 6 week trapping study on 3 dairies in southern California. CDC suction traps baited with CO₂, UV, or CO₂+UV were positioned near either larval development sites, near animals, or in open fields adjacent to the dairy, away from all known attractants. Culicoides were sorted by sex and parity to look for differences between trap and site. Pars were tested for BTV using qRT-PCR. On average, CO₂+UV traps collected more pars than the other traps, though not always significantly. Traps placed in field sites, away from the most intuitive locations for collecting Culicoides, collected the highest numbers of pars. The proportion of parity was affected not only by location of the traps, but also by the bait used. Our findings have implications for estimation of BTV prevalence in midges, especially when pools are not sorted by parity status.