From neutral neighbor to inner saboteur: Host range expansion of chalkbrood-causing fungal species harms native mason bee species

While introduced, managed bees confer significant pollination benefits to agricultural systems, introducing non-native species into new ecosystems also poses a risk of co-introducing pathogens alongside them. Pathogen spillover to wild, native bees may then drive native bee species declines. We examined a case of co-introduced mason bee species (Hymenoptera: family Megachilidae, genus Osmia) and fungal pathogens (Onygenales, genus Ascosphaera) from Asia into North America. We have documented declines of six native mason bee species in Eastern North America with the concurrent introduction, establishment, and proliferation of congeners Osmia cornifrons and O. taurus. These non-native mason bees brought along multiple fungal species to North America, including Ascosphaera fusiformis and A. naganensis, which have been documented in the nests of native mason bees, including the declining Osmia georgica. Using a controlled larval bioassay experiment, we tested the pathogenicity of these two introduced Ascosphaera species on native Osmia georgica and non-native O. cornifrons by feeding them fungal spores. We found that native O. georgica develops chalkbrood infections when consuming the spores of A. fusiformis, but O. cornifrons did not develop any infection to either of these fungal species with which it shares a native range in Asia. Our findings demonstrate harmful outcomes of introduced fungi to which native mason bees may not have experienced any prior exposure until arrival of the non-native mason bee congeners, and may help explain why there are declines of native mason bees with concurrent proliferation or stability of non-native mason bee species.