Larval development habitats of Culicoides sonorensis midges in the western United States.

Culicoides midges are vectors of bluetongue virus (BTV), an arbovirus affecting wild and domestic ruminants. Bluetongue distribution generally overlaps with vector range, so understanding the vector's ecology is necessary for predicting BTV risk. Culicoides require moist substrate for development, and C. sonorensis, the primary recognized vector in the western United States, is classically associated with livestock wastewater ponds. However, it is well-known that BTV can be found outside of livestock areas, with transmission occurring in natural settings. To better classify development habitat, we conducted a broad biweekly survey of moist and wet habitats from June to September of 2022 in northern Colorado at ten sites, including large livestock operations, non-commercial domestic operations, and natural spaces. Samples were maintained in the insectary for 11 weeks and monitored for emergence of adult midges. Standing water substrates displayed higher presence and abundance of midges than running or transient habitats, though all microhabitats showed emergence. Additionally, livestock sites did not produce more midges than other site types, and in fact more midges were observed in natural spaces per sample and overall. Livestock spaces did, however, show significantly higher proportions of C. sonorensis midges, which are thought to be the most competent vectors of BTV in this region, as opposed to other Culicoides species. These results suggest that development sites in natural areas may play an important role in maintaining vector populations in the western U.S. outside of previously implicated livestock operations, and that differences in larval habitat lead to differences in vector species composition.