Research by European scientists raised fresh questions on Wednesday about the impact on bees of neonicotinoids, a ubiquitous and controversial class of pesticides whose future use was restricted this month by the Environmental Protection Agency.

In a study published in the journal Nature, Swedish researchers reported that wild bees and bumblebees foraging in crops treated with that type of pesticide were less likely to reproduce than bees in untreated fields, and that bumblebee colonies in treated fields gained less weight. In addition, fewer wild bees and bumblebees were found in treated fields than in untreated ones.

Ordinary honeybees appeared unaffected, perhaps, the researchers said, because they were better able to rid themselves of neonicotinoid contamination than were their wild cousins. Honeybees make up the bulk of bees used to pollinate commercial crops.

A second study by British and Irish researchers concluded that honeybees and bumblebees actually preferred to drink sugar solutions containing two kinds of neonicotinoid pesticides over plain sugar solutions, even though they ate less — and even when they were more likely to die.