Many popular "bee-friendly" plants from big-box garden centers come already soaked with pesticides, meaning backyard gardeners may be contributing to the decline of the very insect they're trying to attract.

A new study by groups Friends of the Earth and the Pesticide Research Center found that many apparently bee-friendly plants widely available at Home Depot, Lowe's and Walmart garden centers contain high levels of neonicotinoids -- a class of pesticides research suggests are highly toxic to bees, butterflies and other pollinators.

Researchers bought a sampling of 71 different varietals -- from daisies and lavender to marigolds, asters and primrose -- at 18 different big-box retailers across the U.S. and Canada. More than half the purchased plants tested positive for the toxic pesticides, ranging in concentration from between 2 and 748 parts per billion. Previous studies have suggested just 192 parts per billion is enough to kill a honeybee.

"Most gardeners have no idea that their gardens may be a source of harm to bees," said Lisa Archer, director of the Food & Technology program at Friends of the Earth. "We're calling on retailers to get neonicotinoid pesticides out of their plants and off their shelves as soon as possible. Until then, gardeners should buy organic plants to ensure the safety of bees."

Makers of neonicotinoids continue to claim their products are safe and effective.

"There is very little credible evidence that these things are causing untoward damage because we would have seen them over 20 years of use," Julian Little, a spokesperson from Bayer, one of the manufacturers of neonicotinoids, recently told the BBC.

But many scientists argue otherwise.

"Overall, a compelling body of evidence has accumulated that clearly demonstrates that the wide scale use of these persistent, water-soluble chemicals is having widespread, chronic impacts upon global biodiversity and is likely to be having major negative effects on ecosystem services such as pollination that are vital to food security," the study's authors wrote.

"There is so much evidence, going far beyond bees," agreed Dave Goulson, a professor from the University of Sussex who recently shared his concerns about neonicotinoids with the BBC.

"They accumulate in soils, they are commonly turning up in waterways at levels that exceed the lethal dose for things that live in streams," he continued. "It is impossible to deny that these things are having major environmental impacts."