Common pesticides could be wiping out bee colonies by causing pollen-gathering insects to lose their way home, research suggests.
Two studies provide some of the strongest evidence yet that pesticides sprayed on farmers’ fields threaten bumblebees and honeybees.
One team of British scientists showed that bumblebee colony growth slowed after exposure to one of the chemicals.
Another group of French researchers tracked foraging honeybees and found that another pesticide tripled their chances of dying away from the hive. The chemical was thought to disrupt the bees’ homing systems, causing them to get lost and perish.
Insecticides called neonicotinoids may be partly to blame for "Colony Collapse Disorder", the research suggests.
The phenomenon, marked by the abrupt disappearance of honeybee colonies, is a growing problem in northern hemisphere countries. Bumblebees are also at risk.
Professor Dave Goulson, from the University of Stirling in Scotland, who led the British study, said: “Some bumblebee species have declined hugely. For example in North America, several bumblebee species which used to be common have more or less disappeared from the entire continent. In the UK, three species have gone extinct.”
A number of theories have been put forward to explain the declines, including the use of pesticides.
Both research groups focused on neonicotinoids which were introduced in the early 1990s and are now widely used around the world.
The chemicals are nerve agents that spread to the nectar and pollen of flowering plants.
Doses of the pesticides used on crops are not allowed to be lethal to bees. But the evidence suggests there may be significant indirect effects, such as interfering with an insect’s ability to navigate.
The British study exposed developing colonies of bumblebees, Bombus terrestris, to low levels of a neonicotinoid pesticide called imidacloprid.
Colonies were then placed in an enclosed site where the bees could forage naturally for six weeks.
At the start and end of the experiment, researchers weighed the bumblebee nests, including the bees, wax, honey, grubs and pollen.
Exposed colonies were found to gain less weight than untreated colonies, indicating that foraging bees were bringing less food back to the hive.
On average, the treated colonies were 8 per cent to 12 per cent smaller at the end of the study. They also produced 85 per cent fewer queens, the vital founders of future new colonies.
After the winter die-off, the lack of queens could mean 85 per cent fewer nests in the coming year.
“Bumblebees pollinate many of our crops and wild flowers,” said Prof Goulson.
“The use of neonicotinoid pesticides on flowering crops clearly poses a threat to their health, and urgently needs to be re-evaluated.”
The French team tagged free-ranging honeybees with tiny radio tracking devices glued to their thoraxes.
Some of the bees were then exposed to sub-lethal doses of the pesticide thiamethoxam. They proved to be two to three times more likely to die while away from their nests than untreated bees.
Between 10 per cent and 32 per cent of bees failed to return to their colonies after being released up to a kilometre away and foraging in treated crops.
Bees that were unfamiliar with the foraging site were most likely not to return home. The findings suggested that the pesticide upset their navigation skills.
Data from the tracking experiment was used to predict what might happen to colonies with “lost” worker bees.
The scientists found it was possible for bee populations to decline to a point where they might not recover.
Mickael Henry, from the INRA agricultural research institute in Avignon, said: “Our study raises important issues regarding pesticide authorisation procedures. 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 behavioural difficulties." Environmental group Friends of the Earth called the findings, published in the journal Science, "very significant".
Paul de Zylva, head of the group’s Nature and Ecosystems Programme, said: “The bee is a cherished icon of the British countryside and our gardens and is the farmer’s friend that helps pollinate our food crops so we cannot afford further decline.
“We now need the Government to look seriously at the emerging evidence from here and other countries and consider whether neonicotinoid pesticides should continue to be used freely in the UK.”
PA