Scientists say they have conclusive evidence that two widely used pesticides are killing bees and other insects, and harming the environment.

The international panel of 50 scientists working as the Task Force on Systemic Pesticides says use of the pesticides should be phased out.

In the meantime, they're calling on regulatory bodies to tighten controls on their use.

But environmental groups say the chemicals should be banned now.

The panel's conclusions were released as a new study Wednesday by Friends of the Earth Canada showed that large numbers of supposedly "bee-friendly" plants sold at garden centres in 18 cities across Canada and the United States are contaminated with neonic pesticides.

Concerns over the use of neonicotinoids and neonics have been raised over the last two decades.

But the panel says a study of 800 research papers provides conclusive evidence that the pesticides are causing the mass deaths of insects that are essential to the process of pollinating most crops.

The study also showed the chemicals pose a significant risk to earthworms and birds, and are harming the planet's ecosystem.

The effects on the environment cannot be understated, said Dr. Jean-Marc Bonmatin of the National Centre for Research in France, one of the lead authors of the analysis findings.

"We are witnessing a threat to the productivity of our natural and farmed environment equivalent to that posed by organophosphates or DDT" in the 1960's, he told an Ottawa news conference.

"Far from protecting food production the use of neonics is threatening the very infrastructure which enables it, imperilling the pollinators, habitat engineers and natural pest controllers at the heart of a functioning ecosystem."

Bonmatin pointed to research conducted in Italy which he said showed that crop production actually increased after neonic pesticides were banned.

Neonics are a preventative pesticide sprayed over crops or coated onto seeds before they are planted.

They make up roughly 40 per cent of the insecticide market, with global sales nearing $3 billion in 2011.

The pesticides are picked up by insects such as bees and butterflies when plants absorb them into their leaves, flowers, pollen and nectar.

Health Canada has said it is monitoring crops this growing season and could impose restrictions after careful evaluation of the impacts of pesticides.

In the meantime, it has recommended steps to minimize bee exposure to neonicotinoids including reducing dust from coated corn and soybean seeds and offering safe seed-planting tips.

But Health Minister Rona Ambrose has also said that research done by her department has so far been "inconclusive."

The European Commission announced a moratorium on certain uses of neonics in 2013, and it's time for Canada to do the same, said Lisa Gue a researcher and analyst with the David Suzuki Foundation.

"The conclusive findings of this new study demand a rethink of Canada's lackadaisical approach to neonics," said Gue.

"We need to stop these chemicals from entering the environment."

Friends of the Earth warns it may be too late.

A new study conducted by the group found that 60 per cent of "bee-friendly" home garden plants sold at stores in London, Ont., were pre-treated with neonicotinoids pesticides.

The group tested Calibrachoa, Gerbera Daisies, Shasta Daisies and Zonal geraniums. All but the geraniums showed some level of neonic pesticides.

The same study found 50 per cent of similar garden plants sold at centres in Vancouver were pre-treated with neonics.

The findings were part of a larger study released by Friends of the Earth in Canada and Friends of the Earth U.S. in conjunction with the Pesticide Research Institute.

Plant samples were collected from Canadian garden retailers in London, Montreal and Vancouver.

CropLife Canada, a trade association representing agricultural pest control developers, is opposed to a ban on neonicotinoids.

It says the varroa mite is to blame for the recent decline in the health of bee populations, and that there is no conclusive link between pesticide use and bee health.

"One of the primary concerns of both farmers and the plant science industry is protecting the environment," CropLife says on its website.

"Technology has advanced a great deal in recent years and most of today's modern pesticides degrade to non-toxic substances in a matter of weeks, leaving little or no trace behind."