Are you killing bees with kindness?

Scientists and environmental activists have urged people to plant bee-friendly gardens to create safe havens for the pollinators that have experienced a mass die-off over the past decade because of suspected pesticide contamination, disease, and poor nutrition.

Now a new study finds that big-box retailers are selling plants and seeds contaminated with neonicotinoids (neonics), a class of agricultural pesticide implicated in the death of 10 million beehives in the United States since 2006. Friends of the Earth and the Pesticide Research Institute collected and tested plant samples from Walmart, Lowe’s, Home Depot, and other garden stores in 18 cities across the U.S. and Canada.

The results: 51 percent of 71 plant samples contained neonic residues in their stems, flowers, and leaves. Among the pretty but potentially deadly flowering plants were salvia, lavender, and primrose—all catnip for bees. Researchers found that one primrose plant purchased in Massachusetts was contaminated with five types of neonics.

“Gardeners may be unwittingly planting bee-attractive seedlings and plants purchased from major retailers for their bee-friendly gardens, only to poison pollinators in the process,” wrote the authors of the report, called Gardeners Beware.”

Neonic-treated nursery seeds and plants pose the biggest threat to native bees, such as bumblebees, that frequent suburban backyards rather than to commercial honeybees that pollinate big agricultural crops. Still, bumblebees play a crucial role, as about a third of crop pollination comes from such "feral" pollinators, according to scientists.

Neonics are so-called systemic pesticides, which distribute pest-killing nerve poison throughout a plant’s stems, leaves, flowers, pollen, and nectar. While farmers and nurseries can spray plants with neonics, most often the pesticide is applied to seeds, meaning a plant is infused with the pesticide as it grows.

In the U.S., some 90 million acres of corn and 74 million acres of soybeans are grown from neonic-coated seeds, according to the report. “Bees can be exposed through dust during planting, as well as pollen and nectar in mature plants.”

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But what’s happening in your backyard could be even worse for the bees that pollinate a third of the world’s food supply. That’s because nursery plants are dowsed with much higher concentrations of neonics than crops. One corn plant typically will take up 1.34 milligrams of the pesticide, while a three-gallon, perennial nursery plant is soaked in 300 milligrams of neonics, the report’s authors wrote.

It can take up to five years for the pesticide to degrade, and when it does it often breaks down into components even more toxic to bees.

Other studies have shown that exposure to neonics can affect bees’ ability to forage for food and navigate and can increase their susceptibility to disease and parasites.

“Neonicotinoids can also leach throughout soils and runoff to ground and surface waters, thereby increasing pollinator exposure to these bee-toxic pesticides through their sources of drinking water,” the report states. “Worker honey bees foraging on contaminated plants and drinking from contaminated water sources ultimately carry these harmful insecticide residues back to the hive.”

Neonics and another systemic insecticide called fipronil have become pervasive in the environment over the past 20 years and now account for 40 percent of the global pesticide market, according to another new study. Bayer CropScience and Syngenta are the two biggest makers of neonics.

The Gardeners Beware report, released Wednesday, is an expanded version of a 2013 pilot project.

The research has its limitations. Most bees ingest neonics through pollen and nectar, but the scientists did not test the nursery plant pollen for the pesticide.

Report coauthor Timothy Brown, a scientist with the Pesticide Research Institute in Berkeley, Calif., said in an email that collecting sufficiently large pollen samples from nursery plants to test for neonics was simply too expensive.

“It would definitely be interesting to compare the neonic concentrations in flowers to levels in the flowers' pollen and nectar,” said Brown. “Very little comparison data is available.”

So what can you do? The U.S. Environmental Protection Agency, which regulates pesticides, does not require nursery seeds or plants containing neonics to be labeled as harmful to bees. So that leaves it up to gardeners to scrutinize seed packages for neonics’ active ingredients, the most common of which are acetamiprid, clothianidin, imidacloprid, thiamethoxam, and dinotefuran.

The best bet, though, is to buy organic seeds and plants, such as those listed here.

Brown said consumer pressure has already led some retailers, albeit mostly small ones, to stop selling seeds and plants treated with neonics.

Getting a Walmart to jump on the neonic bandwagon would be a big boon for the bees.

“The collective action of many smaller, regional garden stores can definitely have an impact on the market,” said Brown. “However, the market would likely shift toward neonic-free much faster if larger retailers took actions to remove neonics from their supply chains.”