Pesticides and adjuvants disrupt bees’ sense of smell

It has long been known that exposure to pesticide sprays is harmful to bees. In a new study, researchers discovered the effect of such sprays on bees’ sense of smell, which could disrupt their social signals.

Honey bees live in vibrant communities and constantly communicate with each other using chemicals that serve as social cues. For example, nurse bees, responsible for caring for the larvae that eventually become queens and workers, constantly monitor the larvae using pheromones in the dark. The larvae emit brood pheromones to indicate that they need food. There are also alarm pheromones that workers produce to warn other bees of danger. If these signals are attenuated or poorly perceived, the colony may not thrive.

Since 2007, scientists have known that honey bees are in trouble. One stressor that has raised concerns is insecticides, which affect the health of honey bees. As they are usually used in combination with other chemicals, the resulting mixture can become unexpectedly toxic to bees.

“For many years, it was assumed that fungicides did not negatively impact insects because they were designed to target fungi,” said May Berenbaum (GEGC/IGOH), professor of entomology. “Surprisingly, in addition to insecticides, fungicides also have a detrimental effect on bees and the combination of the two can disrupt colony functioning.”

For more than a decade, reports from almond orchards, where two-thirds of America’s honey bees are transported each year when the flowers are in bloom, have blamed mixtures of sprayed pesticides. Part of the problem is the use of so-called inactive chemicals called adjuvants, which increase the “stickiness” of the insecticide so that it stays on the plants.

Because adjuvants have long been considered biologically harmless, they are not subject to the same level of safety testing as other insecticidal agents. “Recently, researchers have shown that adjuvants alone or used in combination with fungicides and insecticides are toxic to bees,” Berenbaum said.

Nurse bees are particularly vulnerable to these combinations. “The health of the queens is paramount,” Berenbaum said. “If healthy queens are not produced, the colony can suffer.”

To understand how the combinations affect nurse bees, researchers tested their effect on the olfactory system of honey bees using the adjuvant Dyne-Amic, the fungicide Tilt and the insecticide Altacor.

The researchers divided the bees into four groups of ten bees and exposed them for a week to either untreated commercial pollen, pollen treated with Dyne-Amic, Tilt and Altacor, or all three together. The bees were then anesthetized on ice and one antenna was carefully removed from each bee. The researchers then exposed the antenna to chemical mimics of brood and alarm pheromones and recorded the antenna’s response using a technique called electroantennography.

With this method, Ling-Hsiu Liao, a research scientist, and Wen-Yen Wu, a graduate student in the Berenbaum lab, discovered that when nurse bees consumed pollen contaminated with the three chemicals, their antennae responses to certain brood pheromones and alarm pheromones have been altered. Their finding suggests that these commonly used pesticides may interfere with bee communication.

How these chemicals interact and influence bees is still unclear. “There are many possible explanations for how consuming these chemicals can affect bees’ sensory responses,” Liao said. “The antenna detects and triggers the response to olfactory signals. In this study, we did not examine what other changes are triggered, including behavioral changes.”

In addition to analyzing the underlying molecular pathways that are affected, researchers also want to test other commonly used pesticide mixtures as well as the response of bees in other populations. They hope their work can help beekeepers rethink how they manage and protect their colonies.

The study “Effects of pesticide-adjuvant combinations used in almond orchards on olfactory responses to social cues in honey bees (Apis mellifera)” was published in Scientific reports.