The controversy over possible links between massive bee die-offs and agricultural pesticides has overshadowed another threat: the use of those same pesticides in backyards and gardens.

Neonicotinoid pesticides are ubiquitous in everday consumer plant treatments, and may expose bees to far higher doses than those found on farms, where neonicotinoids used in seed coatings are already considered a major problem by many scientists.

"It's amazing how much research is out there on seed treatments, and in a way that's distracted everyone from what may be a bigger problem," said Mace Vaughan, pollinator program director at the Xerces society, an invertebrate conservation group.

The vast majority of attention paid to neonicotinoids, the world's most popular class of pesticides, has focused on their agricultural uses and possible effects. A growing body of research suggests that, even at non-lethal doses, the pesticides can disrupt bee navigation and make them vulnerable to disease and stress.

Neonicotinoids are now a leading suspect in colony collapse disorder, a mysterious condition that's decimating domestic and wild bee colonies across much of North America and Europe. The emergence of colony collapse disorder coincided with a dramatic increase in agricultural neonicotinoid use.

Several European countries, including France, Germany and Italy, have banned agricultural neonicotinoids, though some researchers and pesticide-manufacturing companies say evidence of low-dose harm is still incomplete and methodologically unsound.

A garden center shelf. To determine if a pesticide contains a neonicotinoid, look at the ingredients: Imidacloprid, acetamiprid, dinotedfuran, clothianidin, thiacloprid and thiamethoxam are all neonicotinoids. Photo: Matthew Shepherd/The Xerces Society

Few researchers, however, doubt that high doses of neonicotinoids are harmful to bees – and though research on neonicotinoid use by gardeners, nurseries and urban landscapers has proceeded slowly, a troubling picture has emerged of products found on the shelves of most any garden center.

"For homeowner use products, for backyard plants, the amount of neonicotinoids used is like 40 times greater than anything allowable in agricultural systems," said entomologist James Frazier of Penn State University.

The Environmental Protection Agency sets its LD50 – the dose at which 50 percent of exposed honeybees will die – for imidacloprid, a common neonicotinoid, at a range of 40 to 400 parts per billion. In a recent study on the effects of imidacloprid, a food dose of just 20 ppb destroyed honeybee colonies. Critics said that bees in the wild wouldn't be exposed to such a high dose.

Even higher doses, however, have been measured in neonicotinoid-treated gardens. According to toxicologist Vera Krischik of the University of Minnesota, using a standard Bayer plant care product produced imidacloprid levels of 501 ppb in milkweed nectar and 682 ppb in the nectar of agastache, a bee foraging favorite.

In an official company statement from Bayer CropScience, the company said that its "neonicotinoid-based insecticides – both for lawn and garden and crop applications – are safe for honey bees and other pollinators when used according to label directions."

Yet Krischik's results came from by-the-label use. "It's not an artificially high dose," said Krischik, who doesn't consider agricultural neonicotinoids to be a threat to bees. "It's a much higher rate in landscapes than in agriculture. It's 4 milligrams per square foot in agriculture, but you can put up to 250 milligrams in a three-gallon pot."

Not everyone may follow instructions, either. "Gardeners have no training in their use, and will often overdose," said bee biologist Dave Goulson of Scotland's University of Stirling, co-author of a recent paper on neonicotinoids and hive health. Goulson said in an e-mail that "gardens and fruit-growing areas are potentially interesting/bad."

A review of neonicotinoids published by the Xerces Society cited several other findings of extremely high non-agricultural imidacloprid levels: up to 850 ppb in rhododendron blossoms measured nearly six months after being treated, and roughly 2,000 ppb in cherry trees tested more than one year after dosing.

According to Krischik, several other studies have found extremely high neonicotinoid levels in eucalyptus, maple and linden trees. "Linden trees are the best bee plants out there," she said. The Xerces Society also calculated that non-commercial apple trees receive neonicotinoid doses an order of magnitude higher than their commercial counterparts.

An open research question is whether neonicotinoids, which spread through a plant's vascular system and remain active for extended periods of time, accumulate from year to year, especially in perennial plants. "Maybe if we treat once, it stays below lethal levels. But if you treat it two or three or four times, we have no idea," said Vaughan.

Vaughan said that neonicotinoids are also commonly used in nurseries. People may purchase plants with the intent of providing habitat for bees, but end up poisoning them.

However, Vaughan stressed that an outright ban on neonicotinoids would be a mistake. They're popular in large part because they're far less toxic to people than earlier pesticides. In certain situations, such as in-home termite control, they may be appropriate. The key, he said, is determining what those situations are.

More than 1.25 million people have petitioned the Environmental Protection Agency to review its stance on neonicotinoids, which were approved on the basis of limited, largely industry-run safety studies.

"The EPA didn't ask for the data. They didn't realize systemic insecticides last a long time in pollen and nectar. They didn't give money to researchers to look at this. It was an oversight," said Krischik. "It's not anybody's fault. Things happen. And we need to fix it before we lose the bees."

"I don't think anybody should be using these things in their backyards. I think they don't understand that they're having such a negative impact," said Vaughan, who wants all neonicotinoid-containing consumer pesticides to be labeled. "Maybe a big butterfly with an X over it and a sign that says, 'May Kill Pollinators.'"