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American farmers have been dousing their fields with pesticides for decades, meaning that honeybees--which can fly as far as 5 miles (8 km) in search of forage--have been exposed to toxins since well before the dawn of CCD. But neonicotinoids, which were introduced in the mid-1990s and became widespread in the years that followed, are different. The chemicals are known as systematics, which means that seeds are soaked in them before they're planted. Traces of the chemicals are eventually passed on to every part of the mature plant--including the pollen and nectar a bee might come into contact with--and can remain for much longer than other pesticides do. There's really no way to prevent bees from being exposed to some level of neonicotinoids if the pesticides have been used nearby. "We have growing evidence that neonicotinoids can have dangerous effects, especially in conjunction with other pathogens," says Peter Neumann, head of the Institute of Bee Health at the University of Bern in Switzerland.
Ironically, neonicotinoids are actually safer for farmworkers because they can be applied more precisely than older classes of pesticides, which disperse into the air. Bees, however, seem uniquely sensitive to the chemicals. Studies have shown that neonicotinoids attack their nervous system, interfering with their flying and navigation abilities without killing them immediately. "The scientific literature is exploding now with work on sublethal impacts on bees," says James Frazier, an entomologist at Penn State University. The delayed but cumulative effects of repeated exposure might explain why colonies keep dying off year after year despite beekeepers' best efforts. It's as if the bees were being poisoned very slowly.
It's undeniably attractive to blame the honeybee crisis on neonicotinoids. The widespread adoption of these pesticides roughly corresponds to the spike in colony loss, and neonicotinoids are, after all, meant to kill insects. Chemicals are ubiquitous--a recent study found that honeybee pollen was contaminated, on average, with nine different pesticides and fungicides. Best of all, if the problem is neonicotinoids, the solution is simple: ban them. That's what the European Commission decided to do this year, putting a two-year restriction on the use of some neonicotinoids. But while the EPA is planning to review neonicotinoids, a European-style ban is unlikely--in part because the evidence is still unclear. Beekeepers in Australia have been largely spared from CCD even though neonicotinoids are used there, while France has continued to suffer bee losses despite restricting the use of the pesticides since 1999. Pesticide makers argue that actual levels of neonicotinoid exposure in the field are too low to be the main culprit in colony loss. "We've dealt with insecticides for a long time," says Randy Oliver, a beekeeper who has done independent research on CCD. "I'm not thoroughly convinced this is a major issue."