They impair a bee’s memory and its ability to navigate, reducing the foraging skills of workers, and creating a vicious cycle that hobbles the colony’s workforce. Ineffective workers means a shortfall of food, which means that workers are sent on more foraging trips at the expense of caring for youngsters, which means that the colony produces fewer new workers, which means even less food, and so on. This slows the growth of existing colonies and, by reducing the production of queens, prevents the creation of new ones.

“Much of this previous work looked at things from the bees' perspectives,” says Stanley. But she wanted to know if the plants are also affected.

For two weeks, her team fed bumblebees on nectar spiked with extremely low levels of a common neonicotinoid called thiamethoxam. They then released the bees in outdoor cages containing virgin apple trees, which had never been visited by pollinators before.

The team found that bees that drank from the spiked nectar actually spent more time foraging, and visited more flowers. That ought to make them better pollinators, but not in this case. They didn’t succeed in collecting any more pollen and the flowers they visited didn’t produce more apples. Perhaps they were behaving differently on the actual flowers, or failing to learn from experience as bees normally can. Either way, the neonicotinoid-sipping bees became busier, but no more effective.

And even though each individual was more active, the insecticide-treated colonies deployed fewer foragers in total. As a result, the apple trees received fewer visits and produced fruit with 36 percent fewer seeds—a sign of poorer quality.

These changes were obvious when the bees’ nectar contained 10 parts per billion of thiamethoxam, but not when it contained 2.4 parts per billion. This suggests that dose matters, and that there could be a level where bees provide the same level of pollination services. But that’s academic, says Stanley, because “these are levels that have been actually measured in the nectar and pollen of treated crops.”

Apples aren’t just pollinated by bumblebees, but by many species of solitary bees that seem to be even more sensitive to neonicotinoids. It's possible, then, that Stanley’s results underestimate the costs that these insecticides exert upon apple orchards, especially since one study estimated that poor pollination already costs U.K. apple growers around £5.7 million a year.

“The obvious conclusion is that farmers using these chemicals could potentially experience reduced yields, as could their neighbors who may not be using the chemicals,” says Dave Goulson from the University of Sussex, who was not involved in the study. “There may also be knock-on effects for pollination of wildflowers growing on or near farms.” Indeed, if bumblebees are providing poorer pollination services to apples, they could also be short-changing others crops like beans, berries, tomatoes, and oilseed rape, not to mention legions of wild species.