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With pesticides, bees can't find their way home

This is a moss carder bumblebee, Bombus muscorum. This image relates to the paper by Dr. Whitehorn and colleagues. [Image © Science/AAAS]

Scientists have discovered some of the ways that a widely used insecticide harms bumblebees and honeybees.

Bumblebees and honeybees are important pollinators of flowering plants, including many major fruit and vegetable crops. Each year, honeybee hives are trucked in on farms to help pollinate almond, apple and blueberry crops, among others.

In recent years, honeybee populations have rapidly declined, in part due to a phenomenon known as Colony Collapse Disorder. Bumblebee populations have been suffering as well. Researchers have proposed multiple causes for these declines, including pesticides, but it's been unclear exactly how pesticides are inflicting their damage.

Two studies published online by the journal Science this week looked at the effects of neonicotinoid insecticides, which were introduced in the early 1990s and have now become one of the most widely used crop pesticides in the world. These compounds act on the insect's central nervous system.

In one study, Penelope Whitehorn of the University of Stirling in Stirling, U.K. and colleagues exposed developing colonies of bumblebees, Bombus terrestris, to low levels of a neonicotinid called imidacloprid.

The researchers then placed the colonies in an enclosed field site where the bees could fly around, collecting pollen, under natural conditions for six weeks. At the beginning and end of the experiment, the researchers weighed each of the bumblebee nests Ė which included the bees, wax, honey, bee grubs and pollen Ė to determine how much the colony had grown.

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 eight to 12 percent smaller than the control colonies at the end of the experiment. The treated colonies also produced about 85 percent fewer queens. This last finding is particularly important because it's the queens that produce the next generation of bees.

In the other study, MickaŽl Henry of the French National Institute for Agricultural Research (INRA) in Avignon, France and colleagues tagged free-ranging honeybees with tiny radio-frequency identification, or "RFID," microchips that were glued to each bee's back. 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 low dose of the pesticide thiamethoxam.

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, so the bees couldn't find their way home, the researchers propose.

Both sets of scientists say we need to reconsider whether it's a good idea to use these pesticides. Their studies will appear at the Science Express website on 29 March, 2012.