Neonicotinoid Pesticides - Sublethal Doses Studied

By Alan Harman  3/29/12

Two new studies reveal the multiple ways that a widely used insecticide harms bumblebees and honeybees.

The reports, one by a UK team and the other by a French team, appear online at the Science Express Web site of the journal Science, published by the American Association for the Advancement of Science.

Both of the Science studies 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, and they spread to the nectar and pollen of flowering crops.

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

The doses were comparable to what the bees are often exposed to in the wild.

The researchers then placed the colonies in an enclosed field site where the bees could forage under natural conditions for six weeks.

At the beginning and end of the experiment, the researchers weighed each of the bumblebee nests – which included the bees, wax, honey, bee grubs and pollen – 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 8% to 12% smaller than the control colonies at the end of the experiment.

The treated colonies also produced about 85% fewer queens.

Dave Goulson of the University of Stirling and co-author of the UK study, says this last finding is particularly important because queen production translates directly to the establishment of new nests following the winter die-off – 85% fewer queens could mean 85% fewer nests in the coming year.

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

In the second report, the French team found that exposure to another neonicotinoid pesticide impairs honey bees’ homing abilities, causing many of the bees to die.

Mickaël 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 thorax.

The 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 sub-lethal 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, the researchers propose.

In the second part of their study, the French team 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.

The researchers say 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,” Henry says.

“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.”

(The above brought to you by CATCH THE BUZZ (Kim Flottum) Bee Culture, The Magazine of American Beekeeping, published by A.I. Root Company.)