Co-introductions of consequence: Associations of bee-specialist fungi (Ascosphaera spp.) from Asia with native and non-native mason bees (Osmia spp.) in North America

Bees are routinely transported for crop pollination services, and their associated parasites and pathogens are consequently co-introduced into novel environments. Agriculturally important pollinators including Apis mellifera, Megachile rotundata, and Osmia spp. can suffer from chalkbrood disease, caused by species of fungi within the genus Ascosphaera. Two mason bee species from Asia have been recently introduced to North America: Osmia cornifrons was successfully introduced to the United States for orchard pollination services in the 1970s and Osmia taurus, first found in the US in the early 2000s, has an unknown manner of introduction. Since its first detection, O. taurus has proliferated widely and abundantly across Eastern North America. There have been concurrent declines in native mason bee species in the same places as the non-native mason bee introductions of O. cornifrons and O. taurus. This study examined prevalence of Ascosphaera across Virginia in the nests of both non-native and native Osmia, with trap-nesting efforts made possible by community scientists at 100 locations. Results from molecular analyses indicate Japanese species Ascosphaera naganensis and Ascosphaera fusiformis are present in native Virginia mason bees, Osmia lignaria and Osmia georgica, with high prevalence of A. naganensis found in O. georgica. Our study is the first record Japanese Ascosphaera in native North American Osmia. Future work will involve assessing pathogenicity of these species of Ascosphaera for Osmia, especially in co-infection contexts, to better inform our understanding of these novel associations between native hosts and non-native fungi.