Field tests show sustained neonicotinoid exposure negatively affects bees

A honeybee worker has an RFID attached to its back that allows York University researchers to monitor when it leaves and returns to the colony, as well as when it is no longer active and presumed dead. Photo: Amro Zayed, York University.

American Association for the Advancement of Science – In two of the first large-scale, field-realistic studies to date, researchers in Europe and Canada have affirmed what’s been hypothesized about neonicotinoids — that prolonged exposure to these insecticides negatively affects bees. Their results reveal that local environment and species influence the impact of these widely used insecticides, but ultimately, their effects are largely negative — including reducing reproductive success and survival. In the 1990s, research into bee population health began suggesting neonicotinoids — applied broadly to major crops — were harming these pollinator species. However, determining just how, and by how much, has yielded mixed results — in part because evaluating the contributing impacts of other real-world threats to bees, such as climate change, has been difficult. Also, some have suggested that bees in previous studies have been exposed to higher doses of pesticides than realistically found in the field. Ultimately, notes Jeremy Kerr in a related Perspective, this has resulted in “uncertainties in the timing, magnitude, and species-specificity of neonicotinoid impacts on bees … [which has] forced policymakers to rely substantially on the precautionary principle in the regulatory process.”

Here, in the first of two studies designed to provide a clearer picture of neonicotinoid-specific effects, Ben Woodcock and colleagues performed “perhaps the most ambitious field experiment of neonicotinoid effects yet conducted,” says Kerr. Working near oilseed rape crops treated with neonicotinoids in Germany, Hungary, and the U.K., they gathered data on impacts to three bee species, finding the chemicals contributed negatively to local declines in each case — though uniquely, depending on environmental context. For example, exposure to treated crops reduced overwintering success of honeybee colonies in Hungary and the U.K., but not in Germany. In all three countries, however, increasing neonicotinoid residue in the bee nests was associated with lower reproductive success.

In a second study, Nadejda Tsvetkov and team, working in a commercial corn-growing area of Canada, sought to isolate neonicotinoid-specific impacts from other high-intensity agriculture threats. They found that worker bees exposed to neonicotinoids (which often came from neonicotinoid-contaminated pollen from nearby plants, not the treated crop) exhibited lower life expectancies, and their colonies were more likely to permanently lose queens. Tsvetkov et al. also observed that neonicotinoids were particularly potent when mixed with a common fungicide — suggesting honey bees near cornfields treated with both neonicotinoids and other agrochemicals face greater risks. Taken together, the two studies show that “neonicotinoid impacts on bee species include complex combinations of lethal, sublethal, and interactive effects…”Kerr said. “[They] add important new evidence to the neonicotinoid debate that policymakers will need to consider.”