Unproductive Workers

Scientists have suspected for some time that pesticides are partly responsible for recent declines in bees. Now a study in Nature has shown that these chemicals can reduce a colony’s chances of success by making worker bees less efficient at collecting pollen.

The team monitored 40 bumblebee colonies to determine how the insects responded to two pesticides commonly used on flowering crops. Ten colonies were exposed to the pesticide imidacloprid, another 10 to the pesticide λ-cyhalothrin, and 10 to a mixture of the two.

Two colonies exposed to the mixture of pesticides failed within about a week, the team reports. In the λ-cyhalothrin and mixed-pesticide colonies’ nest boxes, the researchers found more than a third of the worker bees dead, compared to only 9 percent of worker bees in the unexposed colonies.

The researchers also used radio frequency identification (RFID) tags to track 259 bees on 8,751 foraging trips. Bees from the imidacloprid and mixed-pesticide colonies collected less pollen per trip and took longer to gather pollen than unexposed bees.

Because each bee was less efficient at foraging, the imidacloprid and mixed-pesticide colonies also appeared to be sending out more bees to find pollen. And many of them didn’t come back: 50 to 55 percent more worker bees failed to return to the colony than those from unexposed colonies.

Overall, the colonies exposed to the mixture of pesticides “suffered the highest overall worker losses,” the authors conclude. The study suggests that environmental agencies need to consider the combined effects of these chemicals on bees, not just effects of individual pesticides. Roberta Kwok | 23 October 2012

Source: Gill, R.J., O. Ramos-Rodriguez, and N.E. Raine. 2012. Combined pesticide exposure severely affects individual- and colony-level traits in bees. Nature doi: 10.1038/nature11585.

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