Exploring the switch from growth to reproduction: A bumble bee case study (Bombus spp)

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Sylvana Finn and Elizabeth Crone, Department of Biology, Tufts University, Medford, MA, Diane Thomson, W.M. Keck Science Department, Claremont, CA

Background/Question/Methods
The switch from growth to reproduction is a well-characterized trade off in many semelparous organisms. This tradeoff is unique in bumble bees (Bombus spp), where growth and reproduction is defined at the colony level. Bumble bees are social bees that produce annual colonies. In early spring, queens emerge and establish nests, the colony grows as workers are produced until at some point, the colony switches from growth to reproduction, when males and new queens are produced. This life cycle has been characterized by exponential growth followed by exponential decline in four Bombus species (B. flavifrons, B. vosnesenskii, B. impatiens, and B. grisecollis). In this study, I test whether a fifth species (B. occidentalis) demonstrates the same type of growth and decline. Bombus occidentalis (the western bumble bee) was once common throughout its range in the Western continental U.S. and has since dramatically declined. For three consecutive years, between 5 and 9 commercially propagated B. occidentalis colonies were placed on a reserve in Lucia, CA. Colonies were observed throughout the summer and forager entrances and exits were recorded. To identify the relative time that colonies switched from growth to reproduction, we evaluated worker foraging activity by fitting a change point model. We compared this to a variety of alternate models, including exponential and quadratic models.
Results/Conclusions
Out of 23 colonies examined, only 4 exhibited the characteristic growth and decline that we expected. These results warrant two possible explanations. 1) It is possible that Bombus occidentalis has a fundamentally different life history than other bumble bees. These data were collected right at the time the species began declining rapidly (1998-2000) and therefore may lend a possible window into the past of this now-threatened species. 2) Alternatively, these data may demonstrate the difference between commercially propagated colonies and wild bumble bee colonies. Further research is necessary to evaluate whether commercially propagated colonies have different colony dynamics than wild colonies in this species and other Bombus species. These data highlight that Bombus is a diverse genus with distinct variation in life history across species. We additionally demonstrate that Bombus are an interesting and informative model system to study the switch from growth to reproduction.