The Effect of a Solar Eclipse on Honeybee Foraging

by Ilse Gebhard, KAWO member

The recent eclipse provided many scientists an opportunity to study how animal behavior is affected by sudden changes in light. One such study was done at Monarch Watch at the University of Kansas with honeybees. While Lawrence, KS, was not in the path of totality, 89% of the sun was going to be in the shadow of the moon. Honeybees are known to use the Ultraviolet (UV) component of sunlight for orientation. Would their foraging behavior change with the decrease in UV light intensity during the eclipse?

This report is based on observations provided by Chip Taylor, Founding Director of Monarch Watch. Chip reports that Monarch Watch had set up a honeybee hive to watch bee activity.  Mid-day the day of the eclipse the skies were clear, with temperatures in the 60s and light winds, conditions suitable for bees to forage. Photographs and videos recorded the activity at the hive and a Tempest Weather Station recorded the UV light. Pollen and nectar sources were available and the bees were actively going in and out of the hive before the eclipse. The hive had a 30-inch clear plastic entrance tube, and bees were observed running upside down on the top of the tube to exit while the returning bees rapidly ran along the bottom of the tube. While there were some collisions among those exiting and those returning, this two-way traffic seemed to be working well.

At the peak of the eclipse (1:52-1:53 PM CDT), there was lots of visible light, but the UV light had dropped to a low level and foraging had stopped. Even before the peak, the entrance tube was packed with bees; the traffic flow had broken down, and no bees were entering or exiting. It was a massive traffic jam with bees actively milling around in the tube but unable to either enter the body of the hive or progress to the exit. Seeing this, Chip says “I went outside to check the activity at the entrance of the tube. What I found was a patch of about a hundred bees that were clinging to the wall. Since this sight doesn’t occur except on extremely hot days, it seemed likely that these were returning bees that had not been able to enter the tube. A number of them had pollen.”

Photographs and videos of the entrance itself showed that while bees were returning, none of them were able to enter and run the tube to the colony. There was congestion at the entrance. Most interesting was the behavior of bees that apparently worked their way from the hive through the mass of bees to the exit. A number were observed to reach the takeoff point at the exit only to hesitate and then turn around and go back into the tube. Chip says “I’ve occasionally seen a bee hesitate and turn around under normal conditions, but none of the bees were taking flight at this point. It was as if they looked at the sky and decided that conditions were unfavorable.” This behavior was observed, even though it was brighter than on many cloudy days on which bees forage. It took most of an hour after the peak of the eclipse for the congestion in the entrance tube to lessen and for normal outgoing and incoming traffic to resume.

Chip explains that UV light is measured in watts per meter squared.  It varies with altitude and seasons, being less at lower elevations and in winter months, and across latitudes where the sun’s angle is lower at locations farther from the equator. This likely means that the response of the bees to declining UV light varies with latitude and would vary from one eclipse to another. UV light also varies with particulate matter in the atmosphere like smoke, which is also found to reduce flight. 

But what is the bottom line for bees? How much UV light in watts per square meter do they need to orient, and is it a constant?  In this case, with 89% coverage of the sun, visibility seemingly unimpaired, exit flights stopped at some point after the UV component dropped to less than 100 watts per square meter.

This is the UV record for the eclipse from the Tempest Weather Station. At the lowest point, the UV was 77 watts/meter squared. This value was close to the low at 8:00 AM CDT and 7:00 PM CDT. The temperature was too cold (44F) for flight at 8:00AM, and all flight had stopped by 7:00 PM.