Previous studies have hypothesized bumblebee (Bombus) worker body size is predictive of colony success, as bumblebee colonies which produce larger workers tend to be more successful. However, there may be some advantage to producing smaller workers at early stages of the colony life cycle; for example, small workers are more resistant to starvation stress. Here, we examine the relationship between bumblebee colony age and body size of foraging workers. As large workers are better suited than small workers to performing most tasks, we also explore if colony success is correlated with the average body size of the first cohort of foraging workers.
We located 21 wild Bombus griseocollis nests prior to worker initiation. Of these, 15 colonies produced workers. We visited colonies once a week to monitor worker activity and to collect foraging workers entering and exiting nests. As a metric of body size, we measured the intertegular span of each captured worker. After colonies expired, we collected the remains of each brood comb (if possible) to determine the size of each colony and whether the colony successfully produced gynes.
Results/Conclusions
Average foraging worker body size increased significantly with colony age, suggesting that smaller workers are either less common or less likely to forage as the colony ages. The body size of the first cohort of workers varied considerably between the colonies (among colony mean = 4.22mm, among colony SD = 0.28mm). However, there was no relationship between the body size of the first worker cohort and colony performance, e.g. the size or reproductive success of colonies. Our research suggests that, in nature, colony success is unrelated to the initial body size of foraging workers. This result contrasts with experimental studies of other species, which show that larger workers of other Bombus species are better foragers and enable faster colony growth. There may be a tradeoff between worker body size and the number of workers in wild B. griseocollis colonies, a hypothesis that we are currently exploring in our data. Bombus griseocollis also has a shorter life cycle than previously-studied Bombus species, which may affect the importance of the initial cohort. Our study showcases the feasibility of studying the colony dynamics of bumble bees in natural populations, and the potential contrast between patterns observed for different bumble bee species in different settings.