Camping in the flowers: Why do so many bumble bees stay out at night?

Background/Question/Methods

Numerous observations have been made of bumble bee workers spending the night exposed on flowers rather than returning to the colony, but the relative frequency of this behavior has never been characterized and little is known regarding its causes or consequences. For social insect species, there are clear advantages to remaining in the colony, such as colony defense and thermoregulation, but staying outside of the colony could be adaptive if cooler outside temperatures suppress parasitic infections, as has been shown for Bombus terrestris when infected with conopid parasitoids in Europe. Using commercial colonies of Bombus impatiens outfitted with RFID tags, we monitored worker foraging behavior for 8-10 days on seven occasions over two years and recorded whether workers remained in the colony or outside of the colony each night and whether they contained a conopid parasite at the end of the foraging period. Using a model of conopid development, we estimated the date of infection for each bee and examined whether parasite development was related to staying out behavior.

Results/Conclusions

Across all trials, complete day foraging trips were recorded for 707 individual workers. Of the 481 workers foraging for at least four days, 82% spent at least one night away from the colony. The likelihood of staying out of the colony increased steadily with foraging experience, and individual workers tended to spend successive nights away from the colony after they began staying out. Parasitism of foragers by conopids varied seasonally and ranged from 0% to 60% during the short foraging period observed. Seasonal variation in staying out behavior was not related to seasonal parasite risk, but in two individual trials the likelihood of workers staying out increased significantly with both foraging experience and infection with a conopid parasitoid. Air temperature during the trials was not an overall predictor of staying out behavior. While we partially confirm the influence of parasitoids on staying out behavior, we find that the biggest driver is foraging experience. Why foraging experience leads to increased staying out behavior is currently unclear.