Insecticides can help protect crops against troublesome pests, but they also pose a risk for beneficial insects such as pollinators. A new study led by researchers at Penn State provided insight into how even sublethal doses of insecticides can negatively affect pollinators by disrupting the mating process.
The study, published in the journal Science of The Total Environment, looked at the effects of imidacloprid, a neonicotinoid that is among the most widely used insecticides globally.
The researchers found that exposure to the insecticide, even at sublethal levels, reduced successful mating in bumble bees and altered the chemical signaling of both males and gynes - female bees capable of reproduction. It also negatively impacted both sperm viability in males and lipid storage in gynes.
Nathan Derstine, a postdoctoral scholar in the College of Agricultural Sciences, said the findings could help drive future research as well as help evaluate the costs and benefits of pesticide use in agroecosystems.
"Because pest control is such an important aspect of food production, finding novel pesticides or utilizing integrated pest management approaches that preserve pollinator populations without compromising crops will be crucial," he said.
Pollinators such as bees are facing global declines, Derstine said, and pesticides - including neonicotinoids - are among the key suspects. After pesticides are applied to seeds or plants, these chemicals persist in the soil, pollen and nectar, giving pollinators multiple routes of exposure.
Etya Amsalem, associate professor of entomology and a co-author on the paper, said that while pesticide effects on pollinators have been studied extensively, the focus usually has been on obvious short-term impacts.
"In contrast, our interest for this study was in examining fitness outcomes that are not immediately apparent and take time to influence population health and size, like impaired mating and chemical communication," Amsalem said.
To examine the effects of imidacloprid on bee mating behavior, Derstine said he and the other researchers exposed bumble bees to very low concentrations of the insecticide in a lab. The team gave bees access to sugar water that contained either six or 60 parts per billion of imidacloprid for three days, concentrations that mimicked what they might encounter in the wild.
Next, the researchers took gynes that either had been exposed to the insecticide or not exposed and placed them with male bees to examine whether the exposure affected their mating process. The researchers then flipped the experiment - placing males that either were exposed or not exposed with female bees.
The team also wanted to know if exposure to pesticides could alter chemical signaling, so the researchers measured potential sex pheromone compounds from the body surface of both males and gynes and from glands that produce pheromones.
"Finally, we wanted to see if pesticide exposure altered male and female physiology, so we measured sperm viability in males and the fat reserves of gynes," Derstine said. "Low sperm viability can affect the performance of the queens' colony, and gynes rely on fat reserves to make it through winter and start a colony in the spring."
After analyzing the data, the researchers found that exposure to the insecticide seemed to have a stronger effect on female bees. Gynes had similar pesticide levels in their bodies as males did despite being more than three times larger on average, and while males avoided the insecticide-treated gynes, the gynes didn't avoid treated males.
Still, males were affected by exposure to imidacloprid, which reduced the total amount of sperm and sperm viability in the 60 parts per billion group by 41% and 7%, respectively.
Amsalem said that, taken together, the findings add to growing concerns about the effects of neonicotinoids on pollinator health. She added that while metrics such as survival and reproduction provide a useful starting point for evaluating the ecological damage caused by pesticides, they likely represent only the tip of the iceberg.
"Many pesticide effects are subtle, not readily observable and often difficult to quantify," she said. "Comprehensive risk assessments should incorporate more long-term and mechanistic studies to reveal the full extent of pesticide impacts."
Cameron Murray and Freddy Purnell - who were both undergraduate research assistants at Penn State at the time of study - also co-authored this paper.
The U.S. Department of Agriculture's National Institute of Food and Agriculture helped support this research.
