An alarming discovery by University of Guelph researchers raises concerns about the health, survival and reproduction of bumblebees. University of Georgia environmental science researchers Drs. Nigel Raine and Sabrina Rondeau discovered that bumblebee queens are more likely to hibernate in pesticide-contaminated soil than in clean soil, for reasons they don’t yet fully understand.
“Some bee behaviors are not well understood, and we wanted to see if these bumblebee queens exhibited avoidance behavior that could reduce their risk of harm from pesticide exposure in the field,” says Raine. “But these results were alarming.”
The team conducted field experiments in which newly emerged queens of the eastern common bumblebee (Bombus impatiens) were allowed to fly freely in outdoor enclosures, mate and then choose a site in which to hibernate for the winter.
The choice was between clean soil or soil contaminated with one of five common pesticides, including insecticides and fungicides, at different concentrations.
Researchers from the School of Environmental Sciences then carefully searched the soil for hibernating bumblebee queens. They found that queens avoided pesticide-free soil and, in fact, were about twice as likely to be attracted to pesticide-contaminated soil.
Most of the bees studied survived, but other consequences for the colony are highlighted in the study published in Total Environmental Science.
The results surprised Raine and Rondeau, who is now a postdoctoral researcher at the University of Ottawa. The study was part of Rondeau’s Ph.D. research at the University of Georgia’s Ontario Agricultural College and was recently featured in the New York Times.
“This raises serious concerns for bumblebee health,” says Raine, “especially since this group of important pollinating insects already faces many challenges.”
Bumblebee queens typically hibernate underground over the winter before emerging in the spring to start new colonies. The researchers wanted to study how bees respond to contaminants at this key but vulnerable stage of life.
Previous studies have shown that pesticides used on crops can attract or repel bees, depending on the type, environmental situation and concentration used. Raine and Rondeau initially hypothesized that bumblebee queens would simply choose to avoid pesticide residues in the soil.
“We didn’t expect this result,” says Rondeau. “This suggests that queens might actually prefer these contaminated soils, although we don’t yet fully understand why.”
Pesticides make soil more attractive, but more research needed
One possible explanation is that pesticides changed the properties of the soil and made it more attractive to queens.
For example, the fungicides used in the study could have killed soil fungi and nematodes, and queens could avoid soil containing fungi because they can be harmful during hibernation.
Another possibility is that the queens may have developed an “acquired taste” for pesticides, as the researchers put it, due to previous exposure in their environment.
They might also be looking for something new, as novelty-seeking behavior is common among bees and often leads the colony to discover new resources.
Further research is needed to fully understand the mechanisms behind this unexpected queen behavior. But the results suggest the need to reconsider how pesticides are used and managed in agricultural settings.
Environmental risk must take into account pesticides present in the soil
Exposure to pesticides contributes to the global decline of insects.
In addition to sometimes being fatal, exposure to pesticides can impair bees’ movements and their ability to collect and feed on nectar and pollen. These effects could have disastrous consequences for our global food system.
Questions remain about whether bumblebee queens would be attracted to contaminated soil in larger spaces than those used in the experiments, where actual odors and field characteristics could mask the presence of pesticides. Further research is also needed to determine whether these findings apply to other bee species.
Current pesticide risk assessments do not take into account soil as the main route of exposure for bees. The study calls for improving these assessments so that they take this newly discovered behavior into account.
“As we move forward, it is crucial to study how agricultural practices, such as the types of pesticides used, application methods and vegetation management, impact the accumulation of pesticide residues in soils where queens are likely to hibernate,” explains Rondeau.
“By better understanding how pesticide residues accumulate in soils and how they affect bees, we can find ways to protect these essential pollinators.”
More information:
Sabrina Rondeau et al., Bumblebee queens (Bombus impatiens) prefer pesticide-contaminated soil when selecting underground hibernation sites, Total Environmental Science (2024). DOI: 10.1016/j.scitotenv.2024.176534
Provided by the University of Guelph
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