A Test of Risk‐Sensitive Foraging in Wild Bumble Bees

Abstract

Risk—sensitive foraging decision (i.e., those based on both reward mean and variance) have often been observed in laboratory contexts, but there are few observations of risk—sensitive foraging in the wild. Indeed, wild foragers need not show risk sensitivity, since they can adopt alternative tactics for minimizing their probabilities of energy shortfall. I tested for risk—sensitive foraging in a natural context in coastal southwest British Columbia, Canada. Seablush (Plectritis congesta) and dwarf huckleberry (Vaccinium caespitosum) are co—occurring plants that offer foraging bumble bees equivalent expected rates of net energy intake, but that differ in variability. The equalization of mean profitabilities of floral rewards results from the actions of nectar—collecting foragers. Risk—sensitive diet choice can therefore be based solely on the variances of the foraging alternatives. I manipulated the energy requirements of bumble bee colonies (Bombus melanopygus, B. mixtus, B. sitkensis) by draining or enhancing their honey pots. I then censused foragers from these colonies on the two flower species. If bees were risk sensitive in their diet choice, they should have increased their relative use of the more variable flower type (dwarf huckleberry) when their colonies were nectar depleted compared to when their colonies were nectar enhanced. This was indeed the case. Wild—foraging bumble bees thus appear to be sensitive to both the mean and the variance of the energy rewards offered by alternative flower species, and this sensitivity is affected by colony energy requirements. These results suggest that wild foragers may change their risk sensitivity.