Spillover of chalkbrood fungi to native solitary bee species from non‐native congeners

Abstract

Abstract Introduced, managed bees such as mason bees (genus Osmia) can confer significant pollination benefits to agricultural systems, but a risk of introducing non‐native species into new ecosystems is the co‐introduction of pathogens along with them. Pathogen spillover to wild, native bees may then drive native bee species declines. This study examined prevalence of the chalkbrood‐causing fungal genus Ascosphaera in the nests of both non‐native and native mason bee species. We conducted large‐scale trap‐nesting and pan‐trapping efforts across the Mid‐Atlantic United States with community scientists. Using molecular methods, nests were screened for all known Ascosphaera species in which genetic sequences have been published. After finding Ascosphaera species first described in Asia, we compared their local prevalence with the local abundance of mason bees from Asia. Lastly, we compared the prevalence of co‐introduced Ascosphaera species across sites with a variety of landcover profiles. Results indicate species originally described in Japan, Ascosphaera naganensis and Ascosphaera fusiformis, are now present in native Virginia mason bees, Osmia lignaria and Osmia georgica, with high prevalence of A. naganensis found in O. georgica. We also found that the declining native mason bee O. georgica experienced higher prevalence of non‐native Ascosphaera spp. at sites with larger numbers of non‐native O. cornifrons and O. taurus, perhaps indicating greater likelihood of spillover of these Ascosphaera species with greater sources of transmission. Lastly, when the proportion of agricultural landcover surrounding bee nests was high, there was greater prevalence of non‐native Ascosphaera in O. georgica compared to more natural landcover types. Synthesis and applications. Through community science programming, we documented species of Japanese chalkbrood fungi inside native mason bee nests in North America. Native mason bees encounter non‐native fungi more frequently with increasing abundance of non‐native mason bees. Agricultural landscapes may exacerbate spillover of non‐native fungi for native mason bees. Any use of non‐native bee species in agriculture should involve monitoring native bees for pathogens in the surrounding area for detection of spillover and species declines.