Causal Factors in Bumble Bee Decline: Testing the Role of Nosema bombi

Emerging infectious diseases, often introduced from foreign sources, pose major threats to wildlife conservation, thus to ecosystem services important to the welfare of the planet. We have shown recently that multiple species of N Am bumble bees, a group of important wild pollinators, have undergone drastic declines in range and abundance over recent decades. This decline appears to correlate with the fungal pathogen Nosema bombi since N. bombi prevalence is higher in populations of declining species. Here, we tested the hypothesis that N. bombi found in N Am bumble bees was introduced from Europe in early 1990’s through commercial activities. We used a novel approach to extract DNA from historical bumble bee specimens (museum specimens) nondestructively and tested for the presence of Nosema. We showed that Nosema prevalence increased after 1992 in declining bumble bee species, supporting the notion that they experienced an invasion event. In addition, using sequence data from SSU rRNA and anonymous genomic loci, we conducted phylogenetic and population genomic analyses of Nosema derived from N Am, Europe and Asia. Our results indicated that Nosema in N Am has low phylogenetic diversity compared to those in Asia. Further, Nosema found in N Am bees were closely related to those obtained in Europe, suggesting that the latter was the source of invasion. Our findings have important implications for the movement of commercial insect species and the conservation of bumble bees here in N Am