A chemical used in mosquito repellents may disorient bumblebees, stopping them from finding their way back to their nests, a recent study found.
Researchers in Finland exposed 123 buff-tailed bumblebees (Bombus terrestris), one of the most abundant bumblebee species in Europe, to a standard consumer mosquito repellent containing prallethrin, a type of pyrethroid insecticide.
One group of 44 bees was exposed to the repellant for 1 minute; 35 were exposed for 10 minutes; while 44 were exposed for 20 minutes. A control group of 43 bees was exposed to an identical device that did not release the insecticide. After exposure, the researchers released the bees 1 kilometer (0.6 miles) away from their colonies.
They found 16 bees from the control group made it home. However, only six bees exposed to the repellant for 10 minutes and just two bees exposed for 20 minutes returned.
“Bumblebee colonies depend on workers collecting food,” lead author Kimmo Kaakinen, a biologist at the University of Turku in Finland, wrote in a statement. “So if they cannot find their way back to the nest, the colony’s ability to obtain nutrition deteriorates.”
Usually, the buff-tailed bumblebee forages around 2 kilometers (1.2 miles) from its colony and has been found to return home from distances reaching 9.8 km (6 miles), the study noted.
Researchers suggested the reduction in homing success, or even increased travel time, could be due to a disruption to the bees’ spatial navigation and memory, compromised flight capacity or a combination.
The study’s results challenge the assumption that mosquito repellents used commonly by people are safe for pollinators.
In 2024, the European Commission approved the use of prallethrin for a 10-year period, from March 1, 2026 to Feb. 29, 2036. Laboratory tests showed exposure to prallethrin did not increase bumblebee mortality, but the researchers said sublethal effects still “remain poorly understood.”
A 2023 study in the U.S. found prallethrin did not negatively impact honeybees’ recruitment dances or their foraging ability at a feeder.
Co-author of the honeybee (Apis mellifera) study Roger Schürch, a behavioral ecologist at Virginia Tech in the U.S., told Mongabay he was surprised by the magnitude of the effects the latest study uncovered.
“The untreated controls were 8 times better at completing the homing task. Given that we did not find effect at all, further studies may be warranted that try to elucidate the causes of this difference,” he wrote in an email.
He also said the intensity of the exposure, in concentrated 10- and 20-minute sittings, may not be extreme.
“Bees will hop from flower to flower, which may be at variable distances from a repeller… I find it unlikely that an individual bee is getting 20 minutes of full exposure in one sitting,” Schürch added. “But given the magnitude of the effect… I think we ought to find out when such huge effects are seen, and when not.”
Banner image: Bumblebees tracked during the experiment. Image courtesy of Kimmo Kaakinen.
