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Neonicotinoid Pesticides Impair Bees’ Social Behavior

Bee with QR code
A bumblebee worker forages outdoors, outfitted with a unique tracking tag (BEEtag). | James Crall

Using an automated robotic platform to spy on bumblebees wearing tiny barcoded backpacks, researchers have discovered yet another detrimental effect of neonicotinoid pesticides on these insects: neonicotinoid exposure disrupts the behavior and social interactions of worker bees inside the hives, according to a study published in the November 9 issue of Science.

The study found that bees exposed to the pesticide became lethargic, antisocial and unable to keep up with key maintenance of the nest-all factors that may impair colony growth and lead to collapse.

As perhaps the most important pollinator species, bees are critical to agricultural crop production and to the survival of most flowering plants on Earth. However, bees' numbers around the world are in decline. Previous research suggests that neonicotinoids — the most widely used class of insecticide — are a likely contributor to the shrinking populations of wild and commercial pollinator species. Neonicotinoids are applied to a diverse variety of fruit, vegetable and cereal crops worldwide

"While indeed there is now substantial evidence that neonicotinoids can impair colony growth and performance, our team's research was aimed at expanding our understanding of how these effects occur," said Harvard University's James Crall, the study's lead author.

Neonicotinoid insecticides are neurotoxins and work by disrupting specific protein receptors in an insect's central nervous system, which has the ultimate effect of paralysis and death of crop-destroying pests. However, the pesticide also has been shown to impair a bee's ability to forage and collect resources outside the nest, even at low levels of exposure.

Crall and his colleagues say little is known about how neonicotinoid compounds affect bee tasks that occur within the nest and are vital to colony success, such as caring for young and regulating the colony's temperature.

To identify these effects, the researchers developed an innovative, robotic platform that monitored the behavior of individual workers in multiple bumblebee colonies. Each bee was identified by a tiny unique QR-type two-dimensional barcode attached to its back and tracked continuously over a period of 12 days using an autonomous camera system.

The authors then exposed some of the colonies to environmentally realistic concentrations of imidacloprid, a common neonicotinoid pesticide, and observed any behavioral changes.

"To be really confident in any behavioral changes we saw in response to neonicotinoid exposure, we wanted to replicate across not only many individual workers, but many entire colonies," said Crall. The robotic bee-observing platform allowed the researchers to monitor colony behavior in many more colonies, over a longer period and in greater detail, than could be done manually.

Imidacloprid exposure altered many aspects of within-nest bee behavior, they found. Exposed bees became lethargic, less likely to feed and care for young, and seemed to become more antisocial, staying in the nests' periphery and interacting with other workers less often. In addition, Crall and colleagues found that exposure to imidacloprid impaired the colony's ability to regulate nest temperature, through activities such as restructuring their nests by building insulating "wax canopies" over developing larvae.

Interestingly, these behavioral effects differed greatly depending on the time of day; worker inactivity was greater overnight than during the day.

"Why exactly this is happening is somewhat of a mystery and suggests there could be some complicated interactions between insecticide exposure and the natural behavioral rhythms of bee colonies," said Crall. Since many important nest care and maintenance activities occur overnight, disrupting nighttime behavior may have important consequences for the colony, he added.

The results offer an explanation for the declining numbers and increased mortality of neonicotinoid-exposed colonies, the authors noted. "We think it is likely that these effects on nest behavior are an important factor driving the declines in colony growth observed in other studies," said Crall.

Although their experiments were too short to directly investigate colony growth or decline, the computerized platform they developed offers a better tool for studying and assessing the impacts of pesticides and other agrochemicals on insects around the world.

"I find bees just endlessly fascinating, and I love that even though they can seem simple and well-studied, there are so many new mysteries being uncovered all the time," said Crall, "and at the same time, they're incredibly important for food systems and human well-being across the globe."