A new study by Canadian researchers has found that honeybees have been exposed to a controversial pesticide for much longer than previously thought at levels that have led to higher mortality and overall weakened health.

The research, conducted in Ontario and Quebec, found virtually all pollen studied -- 99.5 per cent -- contained neonicotinoids that spilled over from corn and soy crops into plants and wildflowers such as maple trees, dandelions and clover.

"At the current level of agricultural practices, what we're doing now generates a byproduct of neonicotinoids in environments around corn fields that is most likely going to cause reduction of honeybee health and the health of other pollinators," said Amro Zayed, the lead author of the study published Thursday in Science.

Neonicotinoids are an insecticide that coats seeds for crops like corn and soy to protect it against insect pests, but since it is water soluble, it is mobile, said Zayed, a York University associate professor of biology. At large doses it's been shown in the lab to cause paralysis and death by blocking signal transmission between neurons in the brain, he explained.

Zayed said once his team established the real-world levels of neonicotinoids found in the field, he replicated that in a research apiary at York, treating some "pollen patties" with neonicotinoids and tracking the comings and goings of honeybees.

"Honeybees treated with neonicotinoids suffered a range of negative effects including a 23-per-cent shorter lifespan and differences in behaviour," he said.

"Those bees also took progressively longer foraging trips as they aged, suggesting they are either unhealthy, can't fly as fast or are having a hard time remembering how to fly back to their colony."

The treated colonies also tended to lose their queens and were unable to replace their queens, meaning certain death for the hives, unlike the untreated colonies in the study, he said.

A similar study in Europe found neonicotinoids dramatically weaken vulnerable honeybee hives.

Researchers in Britain, Hungary and Germany planted about 2,000 hectares of fields of rapeseed, which is made into cooking oil, called canola in America. Some of the fields were planted with seeds treated with the insecticide, others with untreated seeds. The researchers followed bees from the spring of 2015 when the seeds flowered to the following spring when new bees were born.

The bee hives in the Hungarian and British fields that used pesticide-treated seeds did worse surviving through the next winter, the researchers found. In Hungary, the honeybee colonies near treated fields had 24 per cent fewer worker bees the next spring when compared to those near untreated crops.

But in Germany, the bees didn't seem harmed. Hives there were generally healthier to start and when scientists analyzed the pollen brought back to the hives, they determined that the German bees ate a far broader diet with much less of their nutrition coming from the pesticide-treated rapeseed plants, said study director Richard Pywell. Only about 10 per cent of the German bee diet was from neonicotinoid-treated plants, compared to more than 50 per cent in Hungary and England, he said.

When hives are weakened by disease, parasites or bad diet -- as many hives are worldwide -- then the neonicotinoids "pushes them over the edge," said Pywell, a scientist at the Center for Ecology and Hydrology in England. So many of the British hives died, in both treated and untreated fields, that scientists couldn't calculate the specific effect of the insecticide, he said.

For more than a decade, the populations of honeybees and other key pollinators have been on the decline, and scientists have been trying to figure out what's behind the drop, mostly looking at a combination of factors that include disease, parasites, poor diet and pesticides.

Europe banned neonicotinoids, or neonics, in 2013 and researchers needed a special exemption to do their study.

The European and Canadian studies show that neonicotinoids harm bees, but still may not quite be the leading cause of bee losses, said University of Maryland entomologist Dennis vanEngelsdorp, who wasn't part of the study.

"The problem remains complex, like cancer," he said.

Neonicotinoids makers Bayer and Syngenta paid for the European study but had no control over the results or the published paper, Pywell said.

Company officials pointed to the results in Germany and the lack of harm to hives there.

"The study shows that when hives are healthy and relatively disease free and when bees have access to diverse forage, neonics do not pose a danger to colony health," Bayer spokesman Jeffrey Donald wrote in an email.

In a statement, Syngenta's Peter Campbell, head of research collaborations, said the study "strongly suggests the effects of neonicotinoids are a product of interacting factors."