By leaving the colony earlier to find and exploit flowers in low light, larger-sized foragers of the buff-tailed bumblebee (Bombus terrestris) are aided by their more sensitive eyes and can effectively increase their food contributions, according to a study published in the journal Ecology and Evolution.
The buff-tailed bumblebees (Bombus terrestris). Image credit: Leona B.
“Foraging on flowers early in the morning is a possibility for diurnal pollinators to reduce predation and competition for floral resources,” said University of Exeter Ph.D. student Katie Hall and colleagues.
“Some flowers open during dawn or secrete nectar before the majority of diurnal insect pollinators becomes active.”
“Accessing these floral resources early in the day can be particularly beneficial for bees because it can increase the influx of nectar and pollen to provision their nest or colony.”
“However, at this time of day light levels are low, limiting the performance of the apposition compound eyes of bees and most other diurnal insects.”
To explore how early and under which light levels colonies of buff-tailed bumblebees initiate their foraging activity, the researchers tracked foragers of different body sizes using RFID — similar technology to contactless card payments — over 5 consecutive days during warm periods of the flowering season.
They found that most foragers left at higher light levels when they could see well and fly faster.
Nevertheless, a small proportion of foragers left the colony shortly after the onset of dawn when light levels were below 10 lux.
According to the team, bumblebee vision is poor in low light, so flying at dawn or dusk raises the risk of getting lost or being eaten by a predator.
However, the bees benefit from extra foraging time and fewer competitors for pollen in the early morning.
“Larger bumblebees have bigger eyes than their smaller-sized nest mates and many other bees, and can therefore see better in dim light,” Hall said.
“We might expect all bumblebee foragers to leave the colony to forage as soon as there is enough light to allow them to fly.”
“In fact, colonies seem to regulate the start of foraging.”
“There is a balance of risks and rewards in low light — and most bees wait for higher light levels when they can see better and fly faster, with less risk from predators or getting lost and running out of energy.”
“Our finding that more experienced bees are more likely to fly in lower light suggests that knowledge of food locations helps them navigate safely.”
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Katie Hall et al. Onset of morning activity in bumblebee foragers under natural low light conditions. Ecology and Evolution, published online May 1, 2021; doi: 10.1002/ece3.7506
