Many home-garden plants promoted as “bee-friendly” are in fact pretreated with a class of pesticides that can kill bees and other pollinating insects, says a report to be released Wednesday in Portland by the environmental advocacy group Friends of the Earth.

Test results show that more than half of the garden plants from major retailers in 18 cities in the U.S. and Canada, including Portland, contain neonicotinoids, commonly used pesticides that are toxic to bees and many other organisms, according to the report.

The results will be made public by representatives of Friends of the Earth and the Pesticide Research Institute, along with more than 20 consumer and environmental organizations and beekeeping and organic gardening associations, including the Maine Organic Farmers and Gardeners Association.

Of the plant samples that tested positive for the pesticides, 40 percent contained two or more neonicotinoids, according to the report, which concludes that widespread use of the pesticides means that many home gardens have likely become harmful for bees.

Test samples were gathered by environmental advocates from various organizations, beekeepers and researchers from various universities, said Tiffany Finck Haynes of Friends of the Earth. Testing was done by independent laboratories in the study cities, she said.

The effects of the pesticides may be far more widespread than first thought, causing lasting injury to birds, mammals, humans and the soil, said a separate report released Tuesday in England by the Bee Coalition, a collaboration of British environmental groups.

The study reviewed 800 studies worldwide to create a profile of the impact of the pesticides on a range of invertebrate species in soil, vegetation, aquatic and marine habitats.

The reports join a growing body of studies linking neonicotinoids with Colony Collapse Disorder, in which honeybees leave their hives and do not return.

A Harvard University study in May focused on the collapse of honeybee colonies. Its results supported the conclusion that even sub-lethal exposure to neonicotinoids – insecticides that also function as nerve poisons and simulate the effects of nicotine – is probably the main factor in Colony Collapse Disorder.

Neonicotinoid pesticides, also called neonics, work by affecting insects’ central nervous systems, causing paralysis and death, according to the U.S. Environmental Protection Agency.

The Friends of the Earth report calls on major retailers to stop selling plants pretreated with neonics and urges consumers to buy only organic plants.

The sponsoring organizations are expected to offer tips to homeowners and backyard gardeners for reducing exposure to the pesticides and protecting bees.

DECLINE OF HONEYBEE COLONIES

Neonics are systemic pesticides, meaning they affect not just the surface of leaves of treated plants but are absorbed through the entire system, penetrating even into the soil, said Erin Forbes, a master beekeeper from Portland.

They are persistent, lasting as long as 15 years in soil, she said, describing neonics as “the most common class of pesticide mixtures in the world (and) the most common household pesticide.”

“They are absolutely a contributing factor to the increased decline of honeybee colonies in recent years,” Forbes said.

She said most plants that are started in soil and transported from one state to another have neonics in the soil. “Here in Maine, that’s the most common route of transmission,” Forbes said.

Neonic pesticides have come under scrutiny in recent years because of the decline of honeybees, particularly from Colony Collapse Disorder, which has decimated bee populations worldwide. The problem has been particularly severe in Europe, where declines of more than 50 percent have been reported in some areas.

The mechanisms of the disorder and the factors in its steady spread remain unclear, but many possible causes have been suggested, including pesticides, particularly neonicotinoids; infections with certain mites; malnutrition; pathogens; genetic factors; immune deficiencies; habitat loss; changing climate conditions; and evolving beekeeping practices.

“There’s a lot of factors in the mix,” said Carol Cottrill of Rumford, president of the Maine State Beekeepers Association.

“Pesticides have always been part of the mix of things affecting pollinators,” Cottrill said. “Neonicotinoids may be part of the problem, but they aren’t the whole problem.”

Cottrill called the new report “sane and sensible” in its focus on educating homeowners to avoid unwitting use of neonics and the need to check on the nursery stock they purchase. “You can buy plants that have not been pretreated,” she said.

SOME GROWERS AVOIDING NEONICS

At the Highland Avenue Greenhouse in Scarborough, annual and perennial plants are free of neonicotinoids because they are not grown in pretreated soil, said Christine Viscone, who co-owns the business with her husband, Joe.

Consumers concerned about pesticides in plants or soil benefit from trying local garden centers and asking whether trees and shrubs, in particular – which often are imported from other states – are treated with neonicotinoids, she said.

“If we can educate people and let them make informed decisions, it will be helpful … better than a ban,” Cottrill said. “If we ban something, I want to know what they’re going to use to replace it. You’ve got to give (people) an alternative.”

A proposal for a two-year ban on the sale, distribution and use of neonicotinoid pesticides in Maine failed in the Legislature last year.

Concerns about neonicotinoids caused Wyman’s of Maine, the nation’s largest producer of wild blueberries, to avoid using pesticides on its 10,000 acres of berries, said Ed Flanagan, the company’s president and CEO.

“We have never used them,” Flanagan said.

Beekeepers for Wyman’s – one of whom, David Hackenberg, is credited as being among the early detectors of Colony Collapse Disorder, in 2006 – expressed suspicion about the pesticides. The decision was made early on to seek alternatives, Flanagan said.

OBAMA ADDS VOICE ON BEE PROBLEM

On Friday, the White House issued a statement calling the decline of honeybees, native bees and other pollinators – including birds, bats and butterflies – a serious problem posing “a significant challenge that needs to be addressed to ensure the sustainability of our food production systems, avoid additional economic impacts on the agricultural sector and protect the health of the environment.”

President Obama called for several measures, including a task force to develop a national strategy to improve pollinators’ health.

The White House stopped short of singling out neonicotinoid pesticides, but included “pesticide exposure” as one of several factors in bees’ decline.

Pollinators contribute more than $24 billion to the U.S. economy annually, with honeybees accounting for more than $15 billion, according to the White House statement.

“Honeybees enable the production of at least 90 percent of commercially grown crops in North America,” the statement said.

According to the Food and Agriculture Organization of the United Nations, shortages of bees in the U.S. have increased the cost to farmers who rent them for pollination by as much as 20 percent.