Bombus lepidus, an alpine bumblebee
by Xu et al.
A Day in the Life of an Alpine Bumblebee
Bumblebees are among the most important pollinators in the world. As they move from flower to flower, they help many wild plants produce seeds and reproduce. On a warm summer day in an alpine meadow covered with flowers, you can often see bumblebees becoming active in the morning, becoming more numerous as the day goes on, and then gradually disappearing in the late afternoon.
But have you ever wondered what happens at the bumblebees' home?
If you are lucky enough to find a bumblebee nest in the wild, you can watch workers leaving the nest to search for flowers and returning later with the food they have collected. This raises an interesting question: Do bumblebees follow the same daily schedule at their nest as they do when visiting flowers?
We became interested in this question while studying pollinators on Yulong Snow Mountain in southwestern China. At high elevations, we noticed that bumblebees and other pollinators were most active at two different times of day—once in the morning and again in the afternoon. Why did this happen? Perhaps the bees were responding to changes in sunlight and temperature. Or perhaps they simply took a break around lunchtime.
To find out, we needed to watch the bees at home.
In 2019, we were fortunate to discover a wild nest of Bombus lepidus, an alpine bumblebee species that has received relatively little scientific attention. Even better, the nest was located between two of our study areas where we were already recording flower visits. This gave us a rare opportunity to follow the bees from their nest to the flowers and back again.
For six days in August, we watched the nest throughout the day. We recorded when worker bees left the nest, when they returned, and whether they were carrying pollen. We then compared these observations with our records of bumblebees visiting flowers in the nearby meadows and forests.
The bees had a surprisingly regular daily routine. They did not simply come and go throughout the day at a steady rate. Instead, there were two clear busy periods: one in the morning and another in the afternoon. We saw a similar pattern among B. lepidus visiting flowers in the surrounding landscape.
However, the timing was not exactly the same. In the morning, flower visits reached their highest level a little later than activity at the nest. In the afternoon, flower visits increased around the same time that many bees were leaving the nest, before many returning bees arrived. One possible explanation is simply that bees need time to fly between their nest and their foraging sites. In other words, what we see at the nest may happen slightly earlier or later than what we see at the flowers.
Life on a high mountain can also change quickly during the day. Temperature, humidity, sunlight, and wind can all change within a few hours. We found that the bees' activity was closely related to conditions around the nest. They were particularly active when the temperature near the nest was around 24°C. Their activity increased as temperatures approached this point but declined when it became warmer. They were also especially active when humidity was around 50–55%.
Interestingly, the bees did not respond to their environment in exactly the same way throughout the day. Their morning activity seemed less affected by changes in weather, while their afternoon activity was more closely linked to changes in temperature, humidity, sunlight, and wind.
We also noticed something unexpected during our six days of observations. The total number of bees leaving and returning to the nest gradually decreased, but a larger proportion of returning bees were carrying pollen. This could mean that the colony was changing how it used its workers over time. Perhaps the flowers available to the bees were changing, or perhaps the colony's needs were changing. We do not yet know the answer because we did not measure these factors directly.
Our observations offer a small but fascinating glimpse into a day in the life of an alpine bumblebee. The bees' movements at their nest and their visits to flowers followed a similar pattern, with busy periods in the morning and afternoon. Weather conditions around the nest also appeared to influence their activity.
We were especially fortunate to find a natural nest of Bombus lepidus, because wild bumblebee nests are difficult to find and observe. Our study involved only one colony, so we cannot yet say whether all B. lepidus colonies behave in the same way. More observations of different nests, in different seasons and mountain environments, will help us understand how these remarkable bees organize their daily lives—and how their daily routines shape pollination in alpine ecosystems.
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