Colony declines are a major threat to the world’s honey bees, as well as the many wild plants and crops the bees pollinate. Among the lineup of possible culprits, many have pointed the finger squarely at insecticides as a prime suspect. However, a new study from the University of Maryland shows that the world’s most common insecticide does not significantly harm honey bee colonies at real-world dosage levels.
The study, which was published March 18 in the journal PLOS ONE, looked at the effects of the insecticide imidacloprid on honey bee colonies over a three-year period. To see significant negative effects, including a sharp decrease in winter survival rates, the researchers had to expose the colonies to at least four times as much insecticide encountered under normal circumstances.
The study does not totally absolve imidacloprid of a causative role in honey bee colony declines. Rather, the results indicate that insecticides are but one of many factors causing trouble for the world’s honey bee populations.
Imidacloprid is one of a broad class of insecticides called neonicotinoids, so named because they are chemically derived from nicotine. While nicotine itself was once used as an insecticide, it has fallen out of favor because it is highly toxic to humans and breaks down rapidly in sunlight. Neonicotinoids have been engineered specifically to address these shortcomings.
“Imidacloprid is not restricted because it is very safe—an order of magnitude safer than organophosphates,” said UMD’s Galen Dively, lead author of the study, drawing a comparison with a class of chemicals known to be highly toxic to nearly all living things.
For the study, Dively and his colleagues fed pollen dosed with imidacloprid to honey bee colonies. The team purposely constructed a worst-case scenario, even at lower exposure levels. Even at these longer exposure periods of up to 12 weeks, much longer than colonies would be exposed in a real-world scenario, realistic dosage levels of imidacloprid did not cause significant effects in the honey bee colonies. Only at higher levels did the colonies start to have trouble producing healthy offspring and surviving through the winter.
“A lot of attention has been paid to neonicotinoids, but there isn’t a lot of field data. This study is among the first to address that gap,” said Dennis vanEngelsdorp, an assistant professor of entomology at UMD who was not involved in the study. “This study is saying that neonicotinoids probably aren’t the sole culprit at lower, real-world doses.”
Dively and vanEngelsdorp both agree that a synergistic combination of many factors, such as climate stress and malnutrtition, is most likely to blame for colony declines. “Except for the imidacloprid exposure, our test colonies were treated well,” said coauthor David Hawthorne, associate professor of entomology at UMD and director of education at the National Socio-Environmental Synthesis Center (SESYNC). “They weren’t exposed to additional real-world stressors such as malnourishment or multiple pesticides. Colonies coping with these additional pressures may be more sensitive to imidacloprid.”
“It’s a multifactorial issue, with lots of stress factors,” Dively said. “Honey bees have a lot of pests and diseases to deal with. Insecticide exposure is one factor among many. It’s not the lone villain.”