Many bumble bee species (Bombus spp.) are in decline despite efforts to provide abundant floral resources to growing colonies. It is possible, however, that other life stages are also important since there is insufficient information about populations dynamics during overwinter survival and colony establishment. Queen body size varies across colonies and may be an important determinant of these early life stages. Larger queens may benefit during diapause because they can store more fat, and during establishment because of higher foraging efficiency, thermoregulation, and fecundity. Here, we analyze the relationship between queen body size (intertegular span) and overwinter survival and colony establishment. To measure overwinter survival, we evaluated changes in the distribution of body size between fall and spring. For two consecutive years, wild queens were collected and measured in the fall, then resampled in the spring. If larger queens were more likely to survive the winter, then we would observe a higher mean and less variation around that mean in the spring. We tested this by using the shift in size distribution to obtain a minimum estimate of overwinter mortality. To measure colony establishment, we hand-reared queens in the lab under two food treatments—high and low—and recorded success.
Results/Conclusions
In a wild population of bumble bees (Bombus impatiens) monitored for two consecutive years, spring queens were significantly larger than fall queens and more narrowly distributed about the mean. From this, we estimated the minimum mortality at 0.28, which is the first field estimate of overwinter survival for this taxon. In 2014, under the low-food treatment, 22% of queens established and small body sizes performed best. In 2015, 36% percent of queens established colonies under high food, and body size was not correlated with success. The loss of a relationship under a high food regime suggests that smaller spring queens are better able to cope with low food. These results provide evidence that selection on queen body size varies with life stage and environmental context and corroborate the maintenance of observed variation in queen body size across generations. In addition, by providing a framework for estimating overwinter survival, we allow for management efforts to be integrated throughout the bumble bee life cycle. Our work highlights the importance of understanding the drivers of these early life stages, and warrants future exploration of the impacts of queen body size selection for population viability.