Compared to control colonies that had not been exposed to imidacloprid, the treated colonies gained less weight, suggesting less food was coming in. The treated colonies were on average 8% to 12% smaller than the control colonies at the end of the experiment. The treated colonies also produced about 85% fewer queens. This last finding is particularly important because queen production translates directly to the establishment of new nests following the winter die-off. Thus, 85% fewer queens could mean 85% fewer nests in the coming year.

“Bumblebees pollinate many of our crops and wild flowers,” Goulson said. “The use of neonicotinoid pesticides on flowering crops clearly poses a threat to their health, and urgently needs to be re-evaluated.”

In the other Science report, a French team found that exposure to another neonicotinoid pesticide impairs honeybees' homing abilities, causing many of the bees to die.

Mickaël Henry of the French National Institute for Agricultural Research  in Avignon, France, and colleagues tagged free-ranging honeybees with tiny radio-frequency identification or “RFID” microchips that were glued to each bee's thorax. These devices allowed the researchers to track the bees as they came and went from their hives. The researchers then gave some of the bees a sublethal dose of the pesticide thiamethoxam, which has been sold under the brand names Cruiser and Platinum.

Compared to control bees that were not exposed to the pesticide, the treated bees were about two to three times more likely to die while away from their nests. These deaths probably occurred because the pesticide interfered with the bees' homing systems, the researchers propose.

In the second part of their study, the researchers used data from the tracking experiment to develop a mathematical model that simulated honeybee population dynamics. When the mortality caused by the homing failure was incorporated into the simulations, the model predicted that honeybee populations exposed to this pesticide should drop to a point from which it would be difficult to recover.

Henry said this simulation showed the bee populations could suffer “a marked decline in a matter of weeks,” leaving them more susceptible to other stresses such as parasites and climate change.

The authors note that even though manufacturers are required to ensure their pesticide doses remain below lethal levels for honeybees, the studies used to determine this lethality level have probably underestimated the ways that pesticides can kill bees indirectly, for example by interfering with their homing systems.

“Our study raises important issues regarding pesticide authorization procedures,” said Henry. “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.”

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