The cost of parasitism to bumble bee foraging range

Background/Question/Methods

Many species of bumble bees (Bombus spp.) are experiencing rapid population declines across North America. Two mechanisms are hypothesized to drive these declines: (1) loss of foraging habitat due to agricultural and urban development; and (2) parasite and pathogen spillover from commercially managed bumble bees. We hypothesize that parasite infections may impose energetic costs to bees which come at the expense of flight ability, and therefore interfere with pollen acquisition in landscapes where resources are sparsely distributed. Because bumble bees rely on pollen for larval provisions, a reduction in foraging range is a mechanism whereby parasites may indirectly affect bumble bee population dynamics. We predict that flight ranges are shorter in infected B. vosnesenskii individuals, relative to uninfected individuals. To test this prediction, we collected bees from 8 sites in central California and used Bayesian clustering analysis to reconstruct sibships and estimate foraging distances. We used PCR of bee gut tissue with broad range primers to identify incidence of the gut parasites Crithidia and Nosema.

Results/Conclusions

Crithidia-infected individuals of B. vosnesenskii were present at 6 of the 8 surveyed sites, and Nosema was not present at any sites. Crithidia incidence was moderate within sites (average incidence within sites 30%). These numbers for Crithidia are higher than have been reported in prior surveys of bumble bee parasites in California, and may result from the limited geographic region of our study and the proximity of our sites. On average, bees infected with Crithidia had a shorter foraging range (mean distances: 187m uninfected bees, 126m infected bees), but the difference in foraging range between infected and non-infected bees was not significant (P = 0.13, permutation test of mean difference). Because the movement paths of bumblebees are often circular, point estimates of foraging range may not be an accurate measure of foraging effort. We suggest that future work should attempt to quantify both foraging area and duration, to provide a more complete picture of the ecological costs associated with parasitism in Bombus.