Bumble bees exploit known sources but return with partial pollen loads when foraging under low evening light

Abstract

Diel activity of diurnal pollinating insects declines well before the end of day, but a few species, such as bumble bees, can continue foraging in the period before sunset, when light levels decrease. Foraging during this time could be costly for individuals, and key decisions regarding the daily cessation of foraging behaviour have to be made during this period of rapid abiotic change. However, little is known about the behaviour of diurnal bees under these light conditions. We conducted a field experiment in a mixed agricultural landscape to investigate pollen foraging in the hour before sunset in the buff-tailed bumble bee, Bombus terrestris , comparing it to an hour during the afternoon. We monitored activity at the colony entrance and collected pollen sacs from returning foragers. Overall, there were fewer exits and returns in the evening period than in the afternoon period. Evening foragers returned to the colony with partial pollen loads when compared to daytime foragers, and those returning in lower light levels had smaller loads. Additionally, we found that the same flower taxa were being exploited in the evenings as in the daytime. We found no effect of individual size on the likelihood of evening foraging or the size of pollen loads. Our results suggest that B. terrestris are facultative evening foragers that opportunistically exploit profitable flowers they have encountered during the day. Evening foragers decide to return to the colony with partial pollen loads in this period of the foraging day, which results in a largely reduced pollen influx to the colony at the end of the day. Our findings indicate that the gradual decline in light is likely to be an important factor guiding the decisions of bumble bees and other diurnal pollinators of whether to extend or end foraging, and when to return to the colony. • Bumble bees are facultative evening foragers. • Pollen foraging in the evening comes at an acquisition cost. • Light intensity is a key limiting factor for pollen collection. • Evening foragers have the same floral preferences as daytime foragers. • Individual size does not affect pollen foraging in the late evening.