Compelling new evidence from the US government's top bee expert that modern pesticides may be a major cause of collapsing bee populations led to calls yesterday for the chemicals to be banned.

A study published in the current issue of the German science journal Naturwissenschaften, reveals how bees given minute doses of the widely used pesticide imidacloprid became more vulnerable to infections from a deadly parasite, nosema.

Bee experts described this as clear evidence of the role pesticides play in the plight of bees. Although research into the furry insects may seem like a very academic exercise, bees are vital to human survival. More than 70 of the 100 crops that provide 90 per cent of the world's food are pollinated by bees, and Albert Einstein once predicted that if bees died out, "man would have no more than four years to live."

The study, led by Dr Jeffrey Pettis, the head of the US Department of Agriculture's Bee Research Laboratory, says: "We believe that subtle interactions between pesticides and pathogens, such as demonstrated here, could be a major contributor to increased mortality of honey bee colonies worldwide."

Researchers found that bees deliberately exposed to minute amounts of the pesticide were, on average, three times as likely to become infected when exposed to a parasite called nosema as those that had not. The findings, which have taken more than three years to be published, add weight to concern that a new group of insecticides called neonicotinoids are behind a worldwide decline in honey bees, along with habitat and food loss, by making them more susceptible to disease.

Buglife, the invertebrate conservation charity, is calling for a ban on the controversial pesticides. Its director, Matt Shardlow, said yesterday: "The science is now clear, bees poisoned by neonicotinoid pesticides are much more likely to die from disease, gather less food and produce fewer new bees." He added: "Buglife's 2009 review of the science of environmental impacts from neonicotinoid pesticides showed that there was serious cause for concern. We called for a ban then, and as subsequent research has only added to concerns, including the revelation that neonicotinoids make bees prone to a diseased death, we are repeating our call for these toxins to be banned."

The Government needs to take urgent action, said Tim Lovett, of the British Beekeepers Association. He backs the findings of the new research: "Their conclusions are right ... here is some data that would appear to suggest links between widely used pesticides and pathogens."

Imidacloprid is the bestselling neonicotinoid made by Bayer CropScience, earning the company hundreds of millions of pounds a year. Neonicotinoids are "systemic" pesticides. Instead of spraying plants they are used to treat seeds – effectively becoming part of the plant, including the pollen and nectar that bees and other pollinating insects carry away. Concern over their effects on bees has led to restrictions on their use in Germany, Italy, France, and Slovenia.

Dr Julian Little, a spokesman for Bayer CropScience, sought to dismiss the new findings yesterday: "The key issue here is that Jeff Pettis's studies were carried out in the laboratory and not the open air." He added: "Bee health is really important, but focusing on pesticides diverts attention away from the very real issues of bee parasites and diseases – that is where Bayer is focusing its effort."