Social insects receive numerous benefits from group living, including task differentiation, thermoregulation, and a vigorous nest defense. For diurnal species, nighttime clustering in the colony provides maximum defense against nocturnal predators, both for the colony as a whole and for individual survival. Despite the benefits of spending the night inside the nest, many bumble bee workers leave the colony and remain inactive and exposed on vegetation until daybreak. The only current adaptive hypothesis for this behavior is that workers infected with parasitoid flies seek cooler outside temperature to suppress their infection. Overnighting behavior, however, occurs even at times of year when parasitoid infections are rare and thus can only be a partial explanation. Using observational colonies of two bumble bee species (Bombus impatiens and B. griseocollis) outfitted with RFID, we tracked daily foraging patterns of individuals. For some colonies, we tracked conopid parasitism as well as pollen return, and for one B. impatiens colony we also tracked the weight of each bee entering and exiting the colony. We asked whether overnighting behavior was associated with conopid parasitism and improved foraging efficiency, and whether behavior in the nest predicted the likelihood of overnighting, as if the behavior were intentional.
Results/Conclusions
On a given day for an experienced colony, 20-65% of foragers stayed outside the colony at night. For individual foragers, the likelihood of staying out increased with foraging experience but there was no evidence that the behavior was driven by infection with parasitoid flies. There were several apparent costs to the behavior: overnighting individuals were less likely to return to the colony carrying pollen after the overnight trip, left the colony later in the morning for their first trip of the day, and consumed more than the usual amount of colony resources before leaving for the first trip following an overnight. Two lines of evidence suggest that overnighting trips are intentional rather than incidental: foragers spend more time in the colony between trips just before embarking on an overnight trip, and they leave heavier when they do so, as if tanking up before the trip. We discuss possible but yet untested advantages of overnighting behavior, given its ubiquity and apparent intentionality.