Bee colonies could be in danger from a pesticide known as imidacloprid, the U.S. Environmental Protection Agency (EPA) warns in a "preliminary risk assessment" released this month. It comes 22 years after the EPA approved imidacloprid, one of five neonicotinoid insecticides increasingly linked to the collapse of bee colonies.

Imidacloprid is now widely used to kill crop pests, but it can also leave a toxic residue on plants pollinated by bees. The EPA offers a new threshold for that residue of 25 parts per billion (ppb), above which it says effects "are likely to be seen" in bees.

Bees have been dying in droves across North America and Europe for about a decade, a plague known as colony collapse disorder (CCD). Scientists have found several possible culprits, including invasive varroa mites and loss of natural habitat, but many also point to neonicotinoids and other pesticides as a likely factor.

honeybeesU.S. beekeepers reported losing 42 percent of their colonies in 2014. (Photo: U.S. Department of Agriculture)

Neonicotinoids were developed in the 1980s to mimic nicotine, a toxic alkaloid made by some plants in the nightshade family. They're popular partly because they have low toxicity to humans and other mammals, yet are powerful neurotoxins to a wide range of insects. After a patent was filed for imidacloprid in 1986, the EPA approved its use in 1994. Now marketed mainly by Bayer and Syngenta, it's sold in a variety of insect killers under brands like Admire, Advantage, Confidor and Provado.

Concerns grew during the 1990s and 2000s, especially after CCD broke out in 2006. The EPA began studying neonicotinoids individually in 2009, an ongoing process that includes the new imidacloprid report plus more updates due by 2017. The agency has tried to restrict some neonicotinoids in the meantime, with a proposal to not spray when crops are in bloom and a plan to stop approving new uses until risk reviews are complete. The European Union also temporarily banned the pesticides in 2013, as have some major cities like Montreal and Portland, Oregon.

bee pollinating lime flowerHives with high imidacloprid exposure are more likely to lose pollinators, the EPA reports. (Photo: Shutterstock)

"EPA is committed not only to protecting bees and reversing bee loss, but for the first time assessing the health of the colony for the neonicotinoid pesticides," says Jim Jones, assistant administrator of the Office of Chemical Safety and Pollution Prevention, in a press release. "Using science as our guide, this preliminary assessment reflects our collaboration with the State of California and Canada to assess the results of the most recent testing required by EPA."

Imidacloprid may exceed 25 ppb in the pollen and nectar of certain plants, according to the EPA report, such as citrus and cotton. Plants like corn and leafy greens, however, either have lower residues or don't produce nectar. (A report by Health Canada recently listed similar distinctions in other crops, with potential risk found on tomatoes and strawberries but not melon, pumpkin or blueberry plants.

"Additional data is being generated on these and other crops to help EPA evaluate whether imidacloprid poses a risk to hives," the agency says. The insecticide's top U.S. crop is soybeans, but while the EPA notes soybeans are "attractive to bees via pollen and nectar," it describes their residue risk as uncertain due to unavailable data.

imidacloprid graphSoybeans are a big reason for recent growth in U.S. imidacloprid use. (Image: U.S. Geological Survey)

In hives exposed to more than 25 ppb, the EPA reports a higher chance of "decreases in pollinators as well as less honey produced." Less honey is bad, but fewer pollinators is worse. Bees pollinate plants that produce a quarter of the food eaten by Americans, accounting for more than $15 billion in increased crop value per year.

CCD has been most apparent in commercially managed honeybees, whose U.S. numbers declined by 42 percent in 2014. But there are also signs of trouble in wild bees, including rare bumblebees and other unheralded native species. These pollinators are important parts of their ecosystems, helping plants reproduce and predators stay well-fed, so losing them could be even costlier than we realize.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.