Bumblebees spend an average of 7.5 hours a day collecting and transporting nectar and pollen from flower patches back to their hives. They might make up to 15 round trips a day. But when flying with a load, there’s a trade-off between stability and maneuverability, and bumblebees seem to choose the powder or the juice depending on how windy it is. The findings were published in Proceedings of the National Academy of Sciences this week.
Carrying substantial loads during flight is critical for the survival and reproduction for many insects, though few studies have looked at the impacts of cargo on flight performance. For example, Bombus impatiens bumblebees can carry loads of half the weight of their little bodies. The bees store nectar in a special pouch in their abdomen near their center of mass, and pollen is carried on their hind legs, farther from their center of mass. “Bumblebees are basically aerial tankers,” Harvard’s Andrew Mountcastle tells NPR.
To examine how load position affects their flight performance dynamics under various wind conditions, Mountcastle and colleagues tried to simulate nectar and pollen loads with 11.5-milligram steel ball bearings that are 1.4 millimeters in diameter. They attached two balls to the bees’ abdomen, near what’s called their nectar crop, or one ball to each of their hind legs at their pollen basket (pictured above). The team then placed the bees with their simulated loads in a wind tunnel and presented them with an artificial flower under three conditions: a stationary flower in unsteady airflow, a moving flower in steady airflow, and a moving flower in unsteady airflow. They recorded these flight trials using multiple high-speed cameras to reconstruct 3D flight trajectories and changes in body altitudes. You can watch some of these videos here.
Bees with pollen on their legs experienced increased flight stability, they found, but at the expense of maneuverability. While the pollen on their legs might have helped to stabilize the flight, these bees were less agile than nectar-laden bees when there wasn’t much wind.
These findings suggest that wind conditions likely influence what resources these helpful insects choose to gather. It seems like they may prefer pollen on breezy days, and this has important consequences for pollination dynamics as well as colony fitness.