More than half the flowering plants sold at Home Depot stores in three Canadian cities contain the neonicotinoid pesticide linked to deaths of honey bees and other pollinators, according to a new study.

The findings are part of a report by environmental group Friends of the Earth that tested plants purchased at garden centres in 15 cities in the United States. The studies – which were conducted at an accredited laboratory – found 51 per cent of the plants, including daisies and salvia, contained the pesticide that has been restricted in Europe but is widely used in North America.

Since their widespread adoption by Canadian crop growers in 2008, neonicotinoids have been blamed by several studies and Health Canada for the collapse in bee populations. Honey producers say bees are exposed to the chemicals during the dusty crop planting process, as well as by ingesting the flower pollen that is their protein source.

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Farmers say the pesticide is essential to protecting against such insects as white grubs and wire worm, which can badly damage their crops, and is less harmful to humans than chemicals used years ago. They point to studies that say viruses, mites, loss of habitat and long winters are responsible for bee deaths.

But those who want to limit or ban the use of neonicotinoids say the pesticide – which is made by Bayer AG, Dow Chemical Co. and Syngenta – weakens the insects and should only be applied where it is needed.

"It is intended to make the plants easier to raise," said Beatrice Olivastri, chief executive officer of Friends of the Earth Canada. "They're probably using it prophylactically, just like they do with corn … Maybe in some cases there's an infestation and they need it, but 100 per cent of the plants don't need it."

Friends of the Earth sampled plants from Home Depot stores in Vancouver, Montreal and London, Ont., and found a range of pesticide residues.

Even though Home Depot was the only retailer included in the study, Ms. Olivastri says gardeners should assume the plants they buy from most garden centres contain the pesticide, given the prevalence of neonicotinoids. Other major flower retailers were not included in the study because they did not have plants for sale at the time it was conducted, in late spring.

She said people who purchased flowers from Home Depot should return them, or cut off the flowers to limit pollinators' exposure.

Home Depot, BJ's Wholesale Club and other companies in the United States have said they will require flower suppliers to begin labelling plants that contain the pesticide later this year, with an eye on phasing out their sale altogether.

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A spokeswoman for Home Depot Canada said the U.S. labelling rules did not apply in Canada, but the company is planning to address the issue.

"I know that we're working on something, and it's definitely an important issue for us," she said.

In response to a request for an interview, Rona Inc. issued a statement.

"We have been paying close attention to the neonicotinoid pesticides issue over the past several months and we are currently in communication with our suppliers to assess the situation," the company said.

Loblaw Cos. Ltd., Canadian Tire Corp. Ltd. and Canada Safeway Ltd. did not grant interview requests to the Globe and Mail. An official with Flowers Canada Growers, a group that represents companies that raise and import flowers, was not available for an interview.