The scientists fed bumblebees neonicotinoid at levels commonly occurring in agricultural concerns and then measured how it accumulated in their brains. They found that the insecticide impaired brain cell function in the bees, causing them difficulty with such tasks as realizing that flower scents imply food and being able to find their way back home after foraging.
Such problems impacted whole colonies, the team found. By providing nests with the same amount of neonicotinoid in sugar water in a cup, the researchers determined that bumblebee colonies that had been exposed to the insecticide fared poorly in the number of bees in their nests as well as in the size and condition of the nests themselves.
"Our research demonstrates beyond doubt that the level of neonicotinoids generally accepted as the average level present in the wild causes brain dysfunction and colonies to perform poorly when consumed by bumblebees," said Dr. Chris Connolly, of the Dundee School of Medicine, in a statement.
Overall, the team documented that low levels of neonicotinoids caused a 55 percent drop in live bees; a 71 percent reduction in healthy brood cells; and a 57 percent drop in the total mass of a nest.
"This is not proof that neonicotinoids are solely responsible for the decline in insect pollinators," said Connolly, "but a clear linear relationship is now established. We can now be confident that at these levels, neonicotinoids disrupt brain function, bee learning and the ability to forage for food and so limit colony growth."