The effects of pesticide exposure on pollinator health may be more complicated than originally thought, according to a team of researchers in Penn State's College of Agricultural Sciences who recently published an article on the topic in Biology Letters.
In the study, the researchers examined the effects of imidacloprid, a common insecticide, on bumble bees. They found that while exposure to the insecticide was associated with shorter lifespans and reduced reproduction, low doses also were associated with new queens surviving longer in diapause.
This phenomenon of hormesis - when low doses of a normally fatal toxin actually benefit an insect - is poorly understood in pollinators, and these short-term benefits often come at a cost, said Etya Amsalem, associate professor of entomology at Penn State and lead author on the study.
"If hormesis goes unrecognized, it could lead to the mistaken conclusion that certain pesticides benefit bees," Amsalem said. "Such a misconception is dangerous, given the well-documented negative effects of imidacloprid and the trade-offs associated with hormetic responses - where short-term benefits trade-off with long-term fitness."
Amsalem added that a useful analogy for hormesis is caffeine: In small amounts, it can be beneficial, but at high doses, it becomes toxic.
"Even low doses can have unintended effects, such as disrupting sleep," she said. "Just as coffee drinkers should be aware of its multiple effects, researchers and policymakers must account for hormesis when evaluating pesticide impacts on pollinators."
Pennsylvania's native bumble bee queens are particularly vulnerable to outside pressures, such as harsh winters, according to the researchers. A bumble bee queen must go through diapause - a dormant stage that enables an insect to withstand harsh environmental conditions - to survive to the spring, when she can start a new colony.
Over 75% of a queen bumblebee's life can be spent in diapause - a significant amount of time when the bee faces extreme temperatures, possible infections and starvation. And if she doesn't survive this period, she will not be able to establish a new colony of bumblebees, meaning the world can lose up to hundreds of potential pollinators with every dying queen.
"But diapause is poorly understood," Amsalem said. "And despite the fact that most pollinators undergo winter diapause, pesticide risk assessments typically focus on the bee's active seasons, overlooking a significant portion of the bee's life cycle."
During active seasons, pesticides are known to have detrimental effects on pollinators, Amsalem said. Many pesticides can dissolve in water, where plants can absorb and transport them into their pollen and nectar. When this happens, pesticide residues can be picked up by beneficial insects - such as bees and other pollinators. This pesticide exposure can cause immediate death or a decreased lifespan and inability to reproduce.
To assess the impacts of one of the most widely used neonicotinoid pesticides, imidacloprid, during both the active season and diapause, the researchers mixed imidacloprid with sugar water and fed the solution to bumble bees. They then assessed the total lifespan of males, workers and queens and measured reproductive output in the females.
"The results were straightforward: Higher concentrations led to shorter lifespans and fewer offspring, a typical and expected response of bees to neonicotinoids," Amsalem said, noting that while neonicotinoids are banned in Europe, they make up about a quarter of the global pesticide market.
Amsalem said what came next was a little more surprising.
"In the second experiment, we fed queens sublethal concentrations of imidacloprid, induced diapause by placing them in cold storage and monitored survival weekly," she said. "Surprisingly, we found that sublethal imidacloprid exposure actually improved the queens' survival."
The researchers found that these queens were unexpectedly more resilient against harsh winter conditions after pesticide exposure.
Amsalem recommended that these findings be integrated into risk assessment and conservation management strategies for bumblebees. She also argued that these hormetic responses need to be studied further to assess the reasons why these responses occurred. Knowing the mechanism, she said, may give scientists a broader understanding of insect responses to pesticides.
"Most importantly, we must strive to conduct nuanced science that provides a holistic understanding of how stressors affect pollinators - ensuring better-informed conservation efforts," Amsalem said.
Co-authors of the paper were Nathan Derstine, postdoctoral scholar, and Cameron Murray, undergraduate research assistant, both at Penn State.
The U.S. Department of Agriculture's National Institute of Food and Agriculture and the National Science Foundation Division of Integrative Organismal Systems helped support this research.
