The Behaviour of Bees in Dim Light

Abstract

Foraging bees rely on vision for most of their behaviours. They use landmark cues and optic flow to navigate in the environment, avoid obstacles and locate foraging patches. The ability to distinguish colours and patterns helps them to find and identify flowers. During dim light conditions, however, the little light available can compromise their ability to fly safely and forage effectively thus increasing the risks of foraging. Although previous work has explained the mechanistic and behavioural adaptations in bees for controlling flight at low light levels, very little is known how other behaviours are affected. The aim of this thesis was therefore to investigate the activity, foraging and decision-making of bumblebees, Bombus terrestris, under dim light conditions to determine how light limits their behaviour.

Using radio-frequency identification (RFID) tags, the activity of bumblebee foragers was continuously tracked over several days. Bumblebees did not always fly in low light conditions although light was above the threshold under which they would not be able to see and therefore fly. Instead foragers left the nest at light levels that were more favourable. When bees departed in the morning, body size was a factor determining their foraging activity in low light, with them being more active in lower light conditions compared to small bees. Larger-sized bees have larger and more sensitive eyes which could be advantageous under low light condition. Experience was another factor that did not correlate with size, as both smaller and larger-sized bees foraged regularly. Bees that accrued more flights, left the nest in dimmer light conditions than less experienced bees, suggesting that increased knowledge of the environment and location of food sources facilitated a bee’s decision to leave the colony in dim light. Experienced bees were also more likely to stay outside the hive at night, termed overnight foraging. Previously this has thought to have been an uncommon behaviour, but it was observed here that a third of all final foraging trips of a day ended in an overnight trip.

The effect of food depletion on morning activity was also investigated using RFID tracking at the nest exit. A pronounced effect was found when bees departed on their initial flight on their first foraging day. Foragers in the low-food colonies left the colony in lower light conditions compared to fed colonies. This suggests that starving bees can increase the duration of their foraging day to refill their stores.

The food deficit was effectively removed by the fifth day of the experiment, when both colonies showed similar foraging activity. Small-sized foragers were significantly more active in the low-food colonies compared to the fed colonies, which suggests that alloethism allows for temporary shifts in the division of labour.

Finally, I show that bumblebees are able to distinguish between rewarding and unrewarding artificial flowers even under low light conditions. However, the tested level of low light did not affect the foraging success or cause the bees to trade off speed for accuracy.

The findings from this thesis show that bumblebees are naturally active in low light conditions and are able to successfully forage. They show flexibility in their behaviour to take advantage of periods of the day when competition for resources is low, and facultatively expand their daily activity range to include dawn or dusk. Understanding how bumblebee foragers exploit foraging opportunities under low light conditions helps to inform predictive scenarios of environmental change that affect illumination conditions, such as under light pollution and climate change.