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Bees may need multiple generations to recover from the lingering impacts of pesticide exposure, according to a new study. 

Scientists at the University of California, Davis tracked how blue orchard bees that encountered chemical-laced nectar and pollen as larvae or adults fared over two years. The researchers found that exposure early in life could impair reproduction, as could exposure during adulthood. However, the effects were especially dramatic in bees that faced a double whammy of pesticide exposure as youngsters and adults; these unlucky insects produced 44 percent fewer offspring than bees that were never exposed to the chemical. 

These delayed, or “carryover,” effects should be taken into account for future conservation efforts, the team reported on November 22 in Proceedings of the National Academy of Sciences.

“We have a better understanding now of the way pesticide exposure affects bee populations over time,” says study co author Clara Stuligross, a PhD candidate in ecology at UC Davis. “This really shows that pesticide exposure to bees in agricultural areas is additive, and exposure to pesticides in multiple years has a greater effect than just a single exposure.”

Pesticides are one of many threats contributing to declining insect populations. “But mostly the studies have focused on the current effects of pesticide exposure, despite the fact that pesticides could affect organisms long after direct exposure,” Stuligross says. “That’s where we came in.”

She and her colleague Neal Williams decided to investigate the long-term impact of pesticides on blue orchard bees, a common species in North America that pollinates crops such as almonds and cherries. Unlike honeybees and bumblebees that live in large colonies, blue orchard bees are solitary, with each female responsible for collecting pollen and nectar to provision her own offspring.

In agricultural areas, pesticides are often applied several times a year. This means that bees in these areas will likely come into contact with the chemicals at multiple stages of their life cycles and over multiple years, Stuligross says. 

To recreate these conditions, she and Williams allowed groups of captive bees to forage from flowers with or without pesticide treatment. The following year, they divided up the bugs’ grown offspring; once again, some groups foraged on pesticide-treated flowers and some did not. The team then counted how many offspring the insects produced. 

[Related: 5 ways to keep bees buzzing that don’t require a hive]

They found that bees exposed to insecticide as adults were slightly less likely to produce offspring and constructed their nests more slowly than other bees. Overall, they raised 30 percent fewer offspring than bees that didn’t encounter the chemical as adults. 

For individuals that had only been exposed as larvae the previous year, the damage was more subtle. The bees’ nesting behavior was unaffected, but they had 20 percent fewer offspring compared with bees without past exposure. “It means that it can sometimes be hard to detect these carryover effects,” Stuligross says. “It may be easy to miss them if you don’t look all the way through the life cycle.”

Bees that had fed on tainted pollen and nectar as larvae, and were then exposed again as adults, had 44 percent fewer offspring than bees that had never faced the insecticide. Overall, their population growth rate was 72 percent lower than that of the unexposed bees, the researchers calculated.

The pesticide that Stuligross and Williams used, a common one in the US known as imidacloprid, affects the nervous system and has been shown to interfere with bees’ learning ability, behavior, and physiology, she says. It’s likely that the chemical harms bees in multiple ways that collectively hinder their reproduction, foraging, and ability to build nests.

“We just looked at one little slice of how this one pesticide exposure could affect bees,” Stuligross says, noting that the study focuses on just one bee species and a single type of pesticide—in the wild, bees are often exposed to many chemicals at once. In the future, she will investigate how pesticides and other stressors, like limited food and the emergence of parasites, work together to influence bee populations.

Understanding the delayed effects that pesticides can inflict on bees and other pollinators will help researchers plan better guidelines for how, when, and where to apply the chemicals in ways that harm the critters as little as possible, Stuligross says.

“We can enable practical actions to mitigate these risks,” she says. 

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