New research has begun to unravel the mystery of why bees are disappearing in alarming numbers worldwide: Some of the pesticides most commonly used by farmers appear to be changing bee behavior in small but fatal ways.
In Thursday's issue of the journal Science, two teams of researchers published studies suggesting that low levels of a common pesticide can have significant effects on bee colonies. One experiment, conducted by French researchers, indicates that the chemicals fog honeybee brains, making it harder for them to find their way home. The other study, by scientists in Britain, suggests that they keep bumblebees from supplying their hives with enough food to produce new queens.
Ironically, the relatively new pesticides have been welcomed as an environmental plus because they are, by almost all accounts, less harmful to other wildlife than previous generations of pesticides.
Although the authors do not conclude that the pesticides, called neonicotinoids, are the sole cause of the U.S. and international decline in bees or the more immediate and worrisome phenomenon known as colony collapse disorder, they say that the omnipresent chemicals have a clearly harmful effect on beehives.
Spraying of the older pesticides could be halted when plants were flowering so bees and other pollinators would not be harmed. With the neonicotinoids, which kill pest insects by attacking their central nervous systems and are derived from the same nicotine found in tobacco, that kind of timing is not possible since they are most often introduced directly into the seeds of crops and thus permeate the entire plant as it grows, including the pollen and nectar the bees feed on.
The initial changes in bee behavior found by the researchers may have been small, but the longer-term impact was large: Researchers found a sharp drop in the number of queen bumblebees produced, a decrease in the size and weight of beehives, and a demonstrated increase in the number of bees unable to find their way home. When the hives as a whole don't thrive, then the individuals become more susceptible to disease and other threats.
The research, one study of honeybees and the other of bumblebees, points to flaws in the way pesticides are evaluated by regulators, said the author of the study of a kind of honeybee widely used as a pollinator in agriculture.
"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," said Mickael Henry of the French National Institute for Agricultural Research in Avignon, France.
Environmental toxicologist David Fisher of Bayer CropScience, which makes some of the neonicotinoids, said the studies appear to be well done, but are inconsistent with earlier results.
Pesticides were an early suspect in the decline, but many other factors have been implicated as well -- including a relatively new invasive mite that kills bees in their hives, loss of open land for foraging, and the stresses on honeybee colonies caused by moving them from site to site for agricultural pollinating.
"We know that these agents can kill bees at high dosages, but previous studies did not show that effect at the low doses found in fields," Fisher said. "We have a definite problem with the health of our bee populations, but we don't believe the research has shown neonicotinoid pesticides to be the reason why."
He said the honeybee study in particular was problematic because it exposed the bees to concentrations of pesticide not found in farm environments.
The research leader for bee issues at the USDA's Agriculture Research Service in Beltsville, Md., however, found the bumblebee study to be convincing and possibly able to alter the debate. Jeffrey Pettis said that by showing that the hive's production of queens was significantly impaired, declining by 85 percent, by the kind of low dosages of neonicotinoids found in field environments, the authors provided important new information.
"The results were so dramatic that you just had to take notice," Pettis said. "I think that line of research will now drive a lot more of the discussion."Bee toxicity specialist May Berenbaum of the University of Illinois, who chaired a National Academy of Sciences study group on bee declines, said the studies were important in establishing that the neonicotinoids are affecting wild bees as well as those used in controlled pollinations.
"It's easy to see the decline in honeybees kept for pollination, but much harder to see it in the wild bees with their nests underground and in trees," she said. "Now we can say with more confidence that they are at risk as well."
The New York Times contributed to this report.