Range expansion of and hybrids across mid-latitudes of North America.

We investigated recent reports of a northward expansion of and hybrids of the species complex, important vectors of West Nile virus in mid-latitudes of North America. Because more readily feeds on both birds and mammals, its movement into higher latitudes may increase the risk of WNV spillover from avian reservoirs to humans. Using an Ace2 PCR assay, we identified species and detected hybridization in mosquito specimens from 26 sites across the continental U.S. Our results reveal a strong latitudinal gradient in hybrid index values, consistent with climatic filtering of overwintering traits such as diapause. We detected both northward expansion of alleles and southward introgression of , with admixture occurring beyond previously defined hybrid zone boundaries. Hybrid zone structure varied regionally: the East Coast exhibited sharp latitudinal structuring of hybridization patterns; the Central U.S. showed broader corridors of admixture; and the Mountain/Southwest and West Coast zones of secondary contact displayed patchy distributions consistent with a mosaic hybrid zone. These patterns suggest incomplete reproductive isolation, with limits to interbreeding likely shaped by ecological barriers, such as winter survival constraints, and region-specific colonization histories. As climate change relaxes overwintering barriers and urbanization alters host and habitat availability, this hybrid zone may become increasingly dynamic and spatially complex. By updating the distribution of and hybrids, this study provides critical data for tracking range shifts, improving vector surveillance, and refining our understanding of WNV risk. More broadly, it advances integrated approaches to public health by linking mosquito ecology and evolution to emerging disease risk in both human and wildlife populations.