Mating frequency estimation and its importance for colony abundance analyses in eusocial pollinators: A case study of Bombus impatiens

Abstract

Bumble bees (genus Bombus) includes approximately 250 social species, many of which are in decline in North America and Europe. To estimate colony abundance of bumble bees in natural and agricultural habitats, sib-ship relationships are often reconstructed from genetic data with the assumption that colonies have one monogamous queen. However, some species such as the common Eastern North American bumble bee (Bombus impatiens) can display low levels of polyandry that can bias estimates of colony abundance based on sib-ship reconstructions. In order to accurately quantify rates of polyandry in this species, we empirically estimated mating frequencies of queens using a novel statistical model and genotypes from 730 bees. To genotype individuals, we used a highly polymorphic microsatellite set for colonies established from 20 wild caught and 10 commercial queens. We found multiple fathers in 15% of wild colonies and 30% of commercial colonies. This resulted in average effective mating frequencies of 1.07 for wild and 1.15 for commercial colonies. Paternity was also skewed, with the 2nd or 3rd father contributing less than 30% of the offspring. These findings agree with previous reports of polyandry for B. impatiens. Using a large empirical dataset, we demonstrate that assuming monogamy for colony abundance estimation in species that violate this assumption may result in a vast overestimation of the number of colonies. Our results emphasize the importance of studying mating frequencies in social species of conservation concern for the appropriate implementation of genetic approaches for colony abundance estimation.