A single pesticide application can affect multiple generations of bees, according to a new study published by researchers at the University of California, Davis.

“Our findings suggest we need to be doing more to help mitigate risks, or we limit critical pollination services,” said lead author Clara Stuligross, a doctoral candidate in ecology.

At a UC Davis bee research facility, the scientists exposed blue orchard bees as larvae and then again as adults to the insecticide imidacloprid, the most widely used neonicotinoid in California.

“Even then, we saw strong results that added up, each exposure reducing fertility,” said Stuligross.

They placed the bees in “flight cages,” where the insects could forage on wildflowers, some treated with imidacloprid. Females exposed just once as larvae had 20% fewer offspring. Exposing them again in the second year cut their number of offspring nearly in half. The overall population growth rate for the bees dropped by 72% following exposure in the first and second years. This signaled to the researchers that neonicotinoid exposure can have lasting effects.

“One could draw parallels to human health, where impacts early in development show up much later in life,” said co-author Neal Williams, a UC Davis entomology professor. “We just didn’t know the same was true for bees. Now we do, and we need to continue to manage risks appropriately.”

The results, published this week in the journal Proceedings of the National Academy of Sciences, indicate that it may take bees multiple generations to recover from a single neonicotinoid exposure. The carryover effects can have profound implications for risk assessment, conservation and pollinator management, according to the study.

Imidacloprid acts as an insect neurotoxin to control sucking insects, termites, soil insects and fleas on pets, according to the National Pesticide Information Center. It mimics nicotine, which is found in many plants, including tobacco. More than 400 products contain imidacloprid in the U.S. The blue orchard bee is native to North America and, being a highly efficient pollinator, often assists honeybees in pollinating almond, apple, plum, pear and peach trees.

The approach to the study drew skepticism from Dr. Henry Miller, a senior fellow at the Pacific Research Institute, former FDA medical reviewer and the founding director of the agency’s Office of Biotechnology. He raised several concerns with Agri-Pulse. Since the tests use solitary bees, for instance, the hygienic practices and protections of the hive have been left out.

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"Most wild bees are solitary bees, and although they're important to pollination, these tests tell us nothing about the effect on honeybees," he said in an emailed statement.

Miller added that this level of testing can easily exaggerate effects, since it confines bees to forage inside of a tent rather than on a variety of plants as they do in the wild. Allowing bees to forage without restrictions provides the only accurate sense of how pesticides impact bees, he explained.

"They appear to have structured the study in every way possible to come to a negative and dramatic conclusion," he added, noting that imidacloprid was the first and by far most toxic neonicotinoid for bees and is largely being phased out.

Miller was also dubious of the conclusion, which focused on a decline in the rate of growth, rather than an overall decline.

The two researchers focused on the blue orchard bee in a 2020 study that found pesticide exposure combined with food scarcity drastically affects reproduction. Exposing the bees to imidacloprid reduced reproduction nearly twice as much as limiting food.

The Department of Pesticide Regulation is currently investigating any potential impacts to native bees and other wildlife from crop seeds coated with neonicotinoids or other pesticides. Environmental groups have charged that neonic-treated seeds are also causing brain and heart damage in children and that the products may represent the largest insecticide use in the state.

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