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The global decline in bee colony populations may be due in large part to the widespread use of a common class of pesticide called “neonicotinoids”, according to two recent studies. Exposure to these substances, while not directly lethal to bees, may result in disorientation, loss of vitality and failure to produce queens among resident bee populations.

Since 2006, beekeepers in the US have reported losses of 30 – 90 percent of their hives, and bee populations have been in decline worldwide. The failure of bees to return in sufficient numbers to support the hive, termed “colony collapse disorder”, has alarmed US farmers who rely on about 2.68 million managed bee colonies to pollinate crops, a $15 billion industry, according to the US. Department of Agriculture.

In 2010, an annual survey by the Apiary Inspectors of America found the number of managed honeybee colonies that survived the winter fell by 33.8%. That was the fourth year in a row where more than one-third of honeybee colonies died. US scientists have found 121 different pesticides in samples of bees, wax and pollen, lending credence to the notion that pesticides are a key problem, and spurring research into the possible effects of pesticides on bee populations.

Researchers in the UK and France, in two separate studies, examined the effects on bee populations of neonicotinoid-based pesticides, which were introduced in the 1990’s and are used today in more than 100 nations on farm crops and in gardens. The findings of the two studies were recently published in the journal Science.

In the UK study, University of Stirling entymologist Dave Goulson and colleagues studied the impact of the neonicotinoid ‘imidacloprid’ on bumblebees. In a side-by-side experiment, researchers exposed one hive of bees to pesticides at levels in common use on commercial farms, and left the second hive untreated. Both hives were released into separate enclosed fields where they could forage naturally.

After six weeks, colonies exposed to the pesticide were lighter than the others, suggesting that workers had brought back less food to the hive.

But the most dramatic effect was on queen production. The naturally-fed hives produced an average 14 queens each – those exposed to the pesticide produced only 2 queens, an 85% drop in production. And since queens survive winter to initiate the spring hive, the loss of the queens is particularly significant.

“What we are seeing are subtle effects of non-lethal doses of these agricultural chemicals,” Dr. Goulson says.

…those exposed to the pesticide produced only 2 queens, an 85% drop in production

Pesticides sprayed directly on crops are not always uniformly applied, due to wind and weather conditions at the time of spraying. And as plants grow, treatments may be washed off by rain or may be insufficient to cover new vegetative growth. This has led to the development of pesticides which can be applied to seeds before planting. As the plant grows, the pesticide is contained in every part of it, deterring common insect pests. But this method also results in pesticide within the pollen and nectar, where bees come into contact with it.

In the French study, researchers from the French National Institute for Agricultural Research under the direction of Dr. Mickael Henry also used a parallel test method to compare the impact of a different neonicotinoid, ‘thiamethoxam’, on the behavior of honey bees. Tiny radio transmitter tags were attached to the bees’ backs in two groups, with one group exposed to farm-level dosage of thiamethoxam and the second group left unexposed.

The number of bees from the treated group which failed to return to the hive was significantly greater than the control group. The pesticide, researchers concluded, impaired the homing ability of bees, and the exposed bees were two to three times more likely to die while away from the hive. That “high mortality … could put a colony at risk of collapse” within a few weeks of exposure, especially in combination with other stressors, they noted.

Certification testing for pesticides mostly requires that approved dosage levels do not kill bees, says Dr. Henry. But the testing doesn’t take into account the behavioral impact to bees exposed to pesticides. Impaired bees, the study showed, can lead to colony collapse.

Over the years of studying bee colony collapse, many causes have been suggested, including diseases, mites, poor nutrition, reduction in the range of wildflowers, pesticides, or a combination of them all.

Impaired bees, the study showed, can lead to colony collapse

In a 2010 study at Texas Tech, Professor Shan Bilimoria, a molecular virologist, concluded that bee populations were declining due to the combined impact of an insect virus and a fungus. Researchers studying bees which had succumbed to CCD discovered through spectroscopic analysis evidence of a moth virus called insect iridescent virus (IIV) 6 and a fungal parasite called Nosema. But while the study showed an association between exposure and bee deaths, it was unable to establish conclusive cause and effect.

Today’s research findings are significant because both studies found causality between specific pesticides and the diminished vitality and impaired behavior of bees.

The scientists behind the studies in Europe called for regulators to consider banning neonicotinoid insecticides. In the U.S., a coalition of environmental groups and beekeepers asked the EPA to suspend the use of the pesticide, which is widely used in flowering crops like corn, sunflower and cotton to combat insects. The EPA has replied that the new findings will be incorporated into a pesticide review currently in progress.

An easily overlooked factor which contributes to the widespread use of pesticides on US farms is consumer preference. Consumers expect to buy perfect fruit, for example, yet fruit does not consistently develop this way in nature. Organic fruit and vegetables may have spots or scale, and may be misshapen or undersize. As we relax our standards for perfection, farmers will be under less pressure to apply pesticides on such a wholesale scale.