Even as they try to help the bees, people may inadvertently poison them by planting pesticide-laden plants purchased from big-box garden centers, suggests a new report.

More than half of ostensibly bee-friendly plants sampled at 18 Home Depot, Lowe’s and Walmart garden centers in the U.S. and Canada contained high levels of neonicotinoids, which are considered highly toxic to bees, butterflies and other insect pollinators.

Even when they don’t kill pollinators outright, neonicotinoids can impair their immune systems and sense of navigation, potentially turning gardens and backyards into flowery traps.

“That’s what we’re concerned about,” said Tim Brown, a chemist at the Pesticide Research Institute, a pesticide consulting company. “People are being encouraged to help the bees out, and unfortunately what we found is that sometimes these flowers are contaminated at pretty high levels.”

The report, released June 25 by the Pesticide Research Institute and Friends of the Earth, an environmental group, is one of the most comprehensive investigations yet of an often-overlooked source of neonicotinoids in the environment: gardens and the built landscape.

By contrast, most attention paid to neonicotinoids comes in farming contexts. First introduced in the 1990s, they’re now the world’s most widely-used agricultural insecticides, and have been linked to a troubling decline in honeybee colonies used to pollinate many crops.

Also implicated in honeybee declines are mites, viruses, fungal diseases and a loss of natural foraging habitat. It’s a complicated picture, and some uncertainty remains as to the precise role of agricultural neonicotinoids.

Scientists do agree, though that the chemicals can be tremendously harmful to pollinators at higher doses and indirectly harmful at lower doses—and the entire range was found in plants at the big-box garden stores.

The researchers purchased 71 bee-friendly plants—including daisies, lavender, marigolds, asters and primrose—at 18 Lowe’s, Walmart and Home Depot outlets across the United States and Canada. For more than half of the plants, the researchers measured neonicotinoid residues in the flowers at levels between 2 and 748 parts per billion.

According to ecotoxicologist Vera Krischik of the University of Minnesota, who was not involved in the study but reviewed it in detail, the data is solid and troubling. A dose of 192 parts per billion is enough to kill a honeybee, she says, and dozens of studies have found impairments in bee navigation, memory and foraging ability at between 4 and 30 parts per billion.

These exposures may pose an especially grave threat to wild pollinators. While problems with domesticated honeybee hives are well-publicized, many populations of wild bees and butterflies are also in precipitous but comparatively ignored declines. The life cycle of wild bees leaves them highly vulnerable, says Krischik: While domesticated honeybee hives contain thousands of workers, and can handle some losses, many wild bees live in small hives containing perhaps several dozen workers.

If even a few lose their way or gather contaminated food, the hive soon suffers. “You have these small annual colonies, maybe 30 workers with a queen. If you start losing those, there’s no nectar, no pollen being brought back. That native hive is going to decline, and when it declines, they don’t produce queens,” Krischik said.

The neonicotinoid levels measured in the report far exceed what’s typically found in agricultural settings, but are in keeping with manufacturer-recommended doses for garden and landscapes.

'If there's demand expressed in the marketplace, that's how changes can be made.'

As the report details, a single corn plant grown from a seed treated with imidacloprid, the most popular neonicotinoid, has access to 1.34 milligrams of the pesticide. The recommended application rate for a perennial plant in a three-gallon plot is a full 300 milligrams.

An open question is to what extent neonicotinoids added to soil spread to other plants. Reliable data doesn’t yet exist on this question, Krischik, but Brown warned that it’s certainly possible. Neonicotinoids are water-soluble and can take years to break down.

“If one potted plant is highly contaminated, then when it’s planted, it’s very likely that nearby plants will take up these insecticides following a heavy rain or watering,” said Brown.

Environmentalists are now pushing the U.S. Environmental Protection Agency to immediately review how neonicotinoids are used, and Congress is considering the Saving America’s Pollinators Act, which would halt both agricultural and landscape neonicotinoid use until the EPA’s review is completed.

The chances of either initiative succeeding appear to be slim, though, and citizens are also pushing forward on other fronts. Neonicotinoid restrictions have been proposed in nine states, including agricultural powerhouses California and Minnesota. Many retailers, including BJ’s Wholesale Club, have pledged to reduce or eliminate their neonicotinoid use.

“We need consumers to have this dialogue with local retailers,” said Brown. “If there’s demand expressed in the marketplace, that’s how changes can be made.”

Among the retailers to curtail its neonicotinoid use is Bachman’s, a Minnesota-based chain with 21 outlets. According to John Daniels, a vice president of nursery production at Bachman’s, it should be possible for larger garden centers like Home Depot and Lowe’s to do the same without substantially raising their prices.

“I don’t think that chemical application is the final difference-maker in how they can produce plants at low cost,” said Daniels. He did note, though, that cutting back on neonicotinoids could lead to increased use of other pesticides. While these should be less-toxic to pollinators, an even more pollinator-friendly option may simply be for gardeners to be more tolerant of pests.

In nature, said Daniels, there are a great many plant-infesting insects, and they only become pests when they enter our yards and interfere with visions of perfect-looking plants.

“I understand that perspective, and that’s okay. I respect that as a matter of personal opinion,” Daniels said. “But to have a perfect plant takes a great deal of effort.”