A commonly used type of pesticide may be wielding hefty damage to honey bees and bumble bees, say a pair of European studies published, urging a hunt for safer alternatives.
Both teams of researchers focused on neonicotinoid insecticides, which have become the most widely used in the world on crops since their introduction in the 1990s.
They aim to get rid of insect pests by interfering with their central nervous system, but the experiments detailed in the US journal, Science, showed they can also spread to the nectar and pollen of flowering crops and cause devastating harm to bees.
Pesticides have previously been raised as a potential contributor to the puzzling epidemic of colony collapse disorder that has mysteriously killed off massive numbers of bees in recent years.
But British and French scientists sought to test that hypothesis by exposing select groups of bees to pesticides and comparing how well they fared compared to bees that were not exposed.
The British team focused on bumble bees and exposed some to doses of a neonicotinoid called imidacloprid, similar to what they might encounter in the wild. A control group of the same type of bees was not exposed.
The bees were then placed in an enclosed field area where they could forage as they would normally for six weeks.
Over that span, scientists found that the pesticide-exposed bee colonies were on average eight to 12 per cent smaller than the bees that were not subjected to the pesticide, and they produced 85 per cent fewer queens.
The lack of queens could lead to a drastically reduced number of nests in the next season, and would likely make it difficult for the colony to survive the winter die-off, researchers said.
"Bumble bees pollinate many of our crops and wild flowers. The use of neonicotinoid pesticides on flowering crops clearly poses a threat to their health, and urgently needs to be re-evaluated," said co-author Dave Goulson of the University of Stirling.
A French team focused on honey bees. The researchers glued microchips to the bees' thoraxes and subjected them to the pesticide thiamethoxam, but not enough to kill them.
They tracked the bees and found that exposure interfered with their ability to find their way back to their hive, and made them two to three times more likely to die while out and about than bees that were not exposed.
When they projected what effect that sort of death rate might have on a typical colony, researchers found that honey bee populations would likely drop so low that it would be difficult for them to recover.
Pesticide makers are required to keep their doses below a lethal level for honey bees, but they may not be accounting for how the poison affects the creatures' homing devices and leads indirectly to their deaths.
"Our study raises important issues regarding pesticide authorisation procedures," said study author Mickael Henry of the French National Institute for Agricultural Research (INRA).
"So far, they mostly require manufacturers to ensure that doses encountered on the field do not kill bees, but they basically ignore the consequences of doses that do not kill them but may cause behavioral difficulties."