The last several years have witnessed one of the most dramatic impacts on insect life in modern history.
At least a third of U.S. honeybee colonies have died out in the past six years.
Initially, the phenomena baffled scientists. But several new studies indicate that they may be honing in on a potential cause: overuse of pesticides. In particular, researchers now say that they suspect a class of pesticides known neonicotinoids, including the widely-used insecticide imidacloprid, which are used primarily in corn fields. Introduced in the early 1990s, these insecticides have sharply increased in popularity and virtually all corn grown in the United States is treated with them.
This class of nicotine-based chemicals are designed to become an intrinsic part of the plant and when insects ingest them, the chemicals target their nervous system, resulting in paralysis and death.
The wider use of the insecticide corresponds with the proliferation of genetically-modified corn and both coincided with the start of colony collapse disorder, according to a new paper that will be in the June issue of Bulletin of Insectology, by Chensheng Lu, an associate professor of environmental exposure biology at Harvard School of Public Health, along with members of the Worcester County Beekeepers Association in Massachusetts.
The Harvard researchers concluded that these pesticides are likely to blame for colony collapse disorder.
Commercial beekeepers first began reporting the mass collapse of bee colonies in the mid-2000s. The decline varied depending on region and some areas saw 90 percent of their bees die. At the outset, other culprits, including fungus, mites, viruses, bacteria, were suspected as the root problem.
But evidence against neonicotinoids such as imidacloprid has steadily accumulated.
“From the ecological and apicultural perspectives, the results from this study show a profound and devastating effect of low levels of imidacloprid in high fructose corn syrup on honey bee colonies,” said the soon-to-be-released paper in Bulletin of Insectology. The paper also found that 94 percent of hives whose bees had been fed corn syrup laced with the pesticide died off entirely within less than six months.
Other Studies Concur
One experiment conducted by French researchers found that these insecticides fog honeybee brains, which makes it tougher for them to find their way home. Another study by scientists in Britain concluded that the chemicals keep bees from stocking their hives with enough food to produce new queens.
Researchers 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. The bees exposed to the pesticide were 10 percent less likely than healthy bees to make it home when they were in familiar areas, the researchers found. In unfamiliar places, that figure rose to 31 percent. This is especially noteworthy because many modern commercial beekeepers truck their hives to pollinate crops across the US, so these honey bees are likely to find themselves in unfamiliar places.
Since they were first introduced in the 1990s, these types of pesticides have exploded in popularity among farmers and home gardeners and today, they are registered in over 120 countries for use on over 140 crops.
The approval of neonicotinoids in the U.S. has long been controversial. Leaked E.P.A. memos, published by Wikileaks in December 2010 showed (http://emagazine.com/daily-news/leaked-memo-reveals-danger-to-bees) that the agency had been asked to revoke the pesticide’s approval in part because it was suspected as the root of colony collapse disorder. But the E.P.A. failed to act.
Now, an emergency petition, filed by 30 beekeepers and national environmental groups that includes Beyond Pesticides and the Center for Food Safety, has been filed. That petition targets just one form of the pesticide, focusing on field studies that the petitioners argue were inadequate.
Presently, the chemicals are conditionally approved for use while the EPA conducts a routine review of the products. The EPA has said this conditional approval was based on hundreds of studies, and in 2010, it said that it had not seen any data showing that bees were harmed by neonicotinoids. Under pressure to take a closer look, the agency agreed to accelerate its review, which will now be finished in 2018.
Scientists say that pinning down the source of colony collapse disorder has been especially difficult because bees are exposed to so many toxic chemicals. One study, from 2010, was able to identify 121 different pesticides and related products that were found in bees, wax, pollen and beehives.
Pesticides are also not the only potential culprit. Scientists have investigated the possibility that fungus, mites, viruses, and bacteria are responsible. Bees become susceptible to a wide range of problems when they are weakened by other conditions, so researchers kept searching for the root problem even when mites or viruses were discovered in some collapsed colonies.
The nicotine-based pesticides now suspected in the die-offs are popular because they are much less toxic to humans and other mammals than previous pesticides. But they are a neurotoxin to insects, and for bees, they are one of the most toxic pesticides ever registered for use in the U.S.
Bees are exposed to neonictinoids in several ways. The pesticide is concentrated in droplets of sap that form on many plants, according to a 2009 study published in the Journal of Economic Entomology. That study found that the sap from corn grown from pesticide-coated seeds can be so contaminated that bees who ate the sap died within minutes.
The chemicals tend to accumulate in the soil where corn and other crops are grown because companies sell seed that is coated with the pesticide. They are moderately persistent in soil, and can take years to dissipate.
Unlike pesticides that are sprayed on plants at predictable times, allowing beekeepers to keep their hives away when levels are at their highest, the neonicotinoids become a part of plants that are grown from coated seeds, making it impossible for bees to avoid the chemicals.
Ninety percent of seed corn is coated with neonicotinoids. The pesticide is not only used for corn — it is commonly used for apples, pears, broccoli and cauliflower crops. The chemicals are also the active ingredient in hundreds of backyard products.
Modern beekeepers sometimes feed their colonies high fructose corn syrup to replace over-harvested honey. But pesticides are especially difficult to test for in the high fructose corn syrup that beekeepers feed to their hives over the winters because the syrupy goo gums up the laboratory equipment. Bayer, which sells imidacloprid under several different brand names, said that their tests of corn syrup from fields where the pesticide was used found no detectable levels of the chemical. A study by The Organic Center, an organic food research organization, however, found traces of the pesticide in corn syrup. Because of testing difficulties, they added, it was impossible to determine the levels present.
In parts of Europe like France in Italy, neonicotinoids have been banned for the past few years and early results seem to indicate that colony collapse disorder may have slowed. But the problem is not simple and much remains unknown about colony collapse disorder. In many places where neonicotinoids are used, there has been no colony loss. Researchers say that low-level exposure to the chemicals makes bees stressed, and susceptible to other problems like parasites.
One distinguishing characteristic of colony collapse disorder is that bees die away from the hive. In the Harvard study on neonicotinoids, the hives were empty except for food stores, some pollen, and young bees, with few dead bees nearby. When other conditions cause hive collapse—such as disease or pests—many dead bees are typically found inside and outside the affected hives.
“The significance of bees to agriculture cannot be underestimated,” said Chensheng Lu, lead author of the Harvard study. “And it apparently doesn’t take much of the pesticide to affect the bees. Our experiment included pesticide amounts below what is normally present in the environment.”