PCR reveals high prevalence of non-sporulating Nosema bombi(Microsporidia) infections in bumble bees (Bombus)in northern Arizona

Authors: BLAKER, ELIZABETH - Northern Arizona University; Strange, James; MONROY, FERNANDO - Northern Arizona University; James, Rosalind; COBB, NEIL - Northern Arizona University

Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2014
Publication Date: 11/30/2014
Citation: Blaker, E., Strange, J.P., Monroy, F., James, R.R., Cobb, N. 2014. PCR reveals high prevalence of non-sporulating Nosema bombi(Microsporidia) infections in bumble bees (Bombus)in northern Arizona. Journal of Invertebrate Pathology. 123:25-33.

Interpretive Summary: Some bumble bees have recently undergone significant declines in North America, and elsewhere; the bee disease called nosema may be an important factor in these declines. To determine how common the nosema pathogen is in wild bumble bee populations, we collected bees at three different elevations in Northern Arizona. Using both microscopy and DNA-based methods to determine whether the bees were infected, we found surprisingly high levels of infections in two common species of bumble bees, species thought to be relatively resistant to the disease. Interestingly, most of these infections did not progress to a state where spores were produced (the pathogen spreads by way of spores). Conversely, the Western bumble bee had high infection levels, with high numbers of spores. The Western bumble bee is highly susceptible to this disease, and was once common but now has recently declined steeply. We also determined that the DNA-based method was a more sensitive means for detecting infections than microscopy, and revealed that wild bumble bees have a much higher prevalence of nosema than has previously been recognized. We hypothesize that the non- (or low) sporulating infections found in the common bumble bee species reflected some resistance to this pathogen. Our use of both microscopy and DNA-based methods deepened our understanding of the dynamic interactions between this pathogen and its bumble bee hosts.

Technical Abstract: About 20% of bumble bee species are in decline in North America, and the microsporidian pathogen, Nosema bombi, may be a factor in these declines. We performed a comprehensive survey of N. bombi infections in the bumble bee communities throughout the flight season along an elevation gradient in Northern Arizona. Focusing on the two species, Bombus (Pyrobombus) huntii and Bombus (Pyrobombus) centralis, we used a combination of PCR and microscopy to distinguish between sporulating and non-, or low, sporulating N. bombi infections. Surprisingly high levels of non-sporulating infections were found in B. huntii (31-63%) and B. centralis (56.5-66.5%), while the prevalence of sporulating infections was low (3.0-11.8% and 0-12.9% respectively). We determined the prevalence of sporulating N. bombi infection in six other co-occurring but rarer bumble bee species (0%-62.5%,), but did not test them using PCR. The prevalence of sporulating N. bombi infections in B. (Bombias) nevadensis was significantly higher than in either B. huntii or B. centralis (29%). The declining bumble bee, Bombus sensu strico occidentalis, had the highest prevalence of sporulating N. bombi infections (62.5%), but we purposely captured very few B. occidentalis because of its declining status. PCR was a more sensitive measure of infection and revealed that wild bumble bees have a much higher prevalence of infections than has previously been recognized. However, we propose that the non- (or low) sporulating infections found in the common bumble bee species reflect a higher level of resistance to this pathogen, and that spore abundance could serve as a metric for degree of resistance. Microscopy and PCR could provide complementary, not redundant, information that deepens our understanding of the dynamic interactions between N. bombi and their bumble bee hosts.