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.
Mickael Henry, from the INRA agricultural research institute in Avignon, France, 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".