By Carl Zimmer, The New York Times, March 29, 2012
Scientists have
been alarmed and puzzled by declines in bee populations in the United States
and other parts of the world. They have suspected that pesticides are playing a
part, but to date their experiments have yielded conflicting, ambiguous
results.
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.
The authors of both studies contend that their results raise serious
questions about the use of the pesticides, known as neonicotinoids.
“I personally would like to see them not being used until more
research has been done,” said David Goulson, an author of the bumblebee paper
who teaches at the University of Stirling, in Scotland. “If it confirms what
we’ve found, then they certainly shouldn’t be used when they’re going to be fed
on by bees.”
But pesticides are only one of several likely factors that scientists
have linked to declining bee populations. There are simply fewer flowers, for
example, thanks to land development. Bees are increasingly succumbing to mites,
viruses, fungi and other pathogens.
Outside experts were divided about the importance of the two new
studies. Some favored the honeybee study over the bumblebee study, while others
felt the opposite was true. Environmentalists say that both studies support
their view that the insecticides should be banned. And a scientist for Bayer
CropScience, the leading maker of neonicotinoids, cast doubt on both
studies, for what other scientists said were legitimate reasons.
David Fischer, an ecotoxicologist at Bayer CropScience, said the new
experiments had design flaws and conflicting results. In the French study, he
said, the honeybees got far too much neonicotinoid. “I think they selected an
improper dose level,” Dr. Fischer said.
Dr. Goulson’s study on bumblebees might warrant a “closer look,” Dr.
Fischer said, but he argued that the weight of evidence still points to mites
and viruses as the most likely candidates for bee declines.
The research does not solve the mystery of the vanishing bees.
Although bumblebees have been on the decline in the United States and
elsewhere, they have not succumbed to a specific phenomenon known as colony
collapse disorder, which affects only honeybees.
Yet the research is coming out at a time when opposition to
neonicotinoids is gaining momentum. The insecticides, introduced in the early
1990s, have exploded in popularity; virtually all corn grown in the United
States is treated with them. Neonicotinoids are taken up by plants and moved to
all their tissues — including the nectar on which bees feed. The concentration
of neonicotinoids in nectar is not lethal, but some scientists have wondered if
it might still affect bees.
In the honeybee experiment, researchers at the National Institute for
Agricultural Research in France fed the bees a dose of neonicotinoid-laced
sugar water and then moved them more than half a mile from their hive. The bees
carried miniature radio tags that allowed the scientists to keep track of how
many returned to the hive.
In familiar territory, the scientists found, the bees exposed to the
pesticide were 10 percent less likely than healthy bees to make it home. In
unfamiliar places, that figure rose to 31 percent.
The French scientists used a computer model to estimate how the hive
would be affected by the loss of these bees. Under different conditions, they
concluded that the hive’s population might drop by two-thirds or more,
depending on how many worker bees were exposed.
“I thought it was very well designed,” said May Berenbaum, an
entomologist at the University of Illinois at Urbana-Champaign.
But James Cresswell, an ecotoxicologist at the University of Exeter in
England, was less impressed, because the scientists had to rely on a computer
model to determine changes in the hive. “I don’t think the paper is a trump
card,” he said.
In the British study, Dr. Goulson and his colleagues fed sugar water
laced with a neonicotinoid pesticide to 50 bumblebee colonies. The researchers
then moved the bee colonies to a farm, alongside 25 colonies that had been fed
ordinary sugar water.
At the end of each year, all the bumblebees in a hive die except for a
few new queens, which will go on to found new hives. Dr. Goulson and his
colleagues found that colonies exposed to neonicotinoids produced 85 percent
fewer queens. This reduction would translate into 85 percent fewer hives.
Jeffery Pettis, a bee expert at the United States Department of
Agriculture, called Dr. Goulson’s study “alarming.” He said he suspected that
other types of wild bees would be shown to suffer similar effects.
Dr. Pettis is also convinced that neonicotinoids in low doses make
bees more vulnerable to disease. He and other researchers have recently
published experiments showing that neonicotinoids make honeybees more
vulnerable to infections from parasitic fungi.
“Three or four years ago, I was much more cautious about how much
pesticides were contributing to the problem,” Dr. Pettis said. “Now more and
more evidence points to pesticides being a consistent part of the problem.”
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