The authoritative, peer-reviewed research undermines the pesticide industry's long-repeated arguments that bees are not being harmed, and piles pressure on UK and US authorities to follow other countries by introducing bans on the chemicals.
Pesticide companies have been trying to protect their multi-billion pound businesses by lobbying internationally against bans on neonicotinoids, a group of toxic chemicals designed to paralyse insects by attacking their nervous systems.
Agricultural crops in Scotland, England and around the world are dosed with the chemicals to prevent insects from damaging them. But evidence has been mounting that they could be to blame for the "colony collapse disorder" that has been decimating bee populations.
The US has been losing one-third of its honeybee hives every year, while beekeepers in Europe say that more than one million bee colonies have been wiped out in France, Germany, Italy and the UK since 1994.
Although neonicotinoids have faced bans or restrictions in Germany, France, Italy and Slovenia, regulators in the UK and the US have so far accepted the industry's contention that the toxins were not poisoning bees.
But that view has now been seriously challenged by a new study from scientists at Purdue University in West Lafayette, Indiana. They found neonicotinoids in bees, in pollen, in soil and in dandelions, suggesting that bees could be contaminated in several different ways.
"We know that these insecticides are highly toxic to bees; we found them in each sample of dead and dying bees," said Christian Krupke, associate professor of entomology at Purdue and a co-author of the study. Bees also suffered from tremors, unco-ordinated movement and convulsions, which are all signs of insecticide poisoning.
"This material is so concentrated that even small amounts landing on flowering plants around a field can kill foragers or be transported to the hive in contaminated pollen," Krupke said.
"It stands out as being an enormous source of potential environmental contamination, not just for honeybees, but for any insects living in or near these fields. The fact that these compounds can persist for months or years means that plants growing in these soils can take up these compounds in leaf tissue or pollen."
Krupke's study, conducted with four colleagues was reviewed for errors by fellow scientists before it was published.
The study has been seized on by beekeepers and environmental groups in Scotland campaigning for a neonicotinoid ban. "We are facing a global ecological catastrophe in which honeybees, bumblebees and butterflies are being wiped from the face of the landscape in every country where neonicotinoids have been introduced," said Graham White, a beekeeper from the Scottish Borders.
"The appalling truth is that we no longer have a credible regulatory system for pesticides in Scotland or the UK. All of the so-called regulators are so symbiotically and financially dependent on the pesticide industry that they have no independent freedom of action."
Buglife, which campaigns to protect insects, described neonicotinoids as "massively toxic" to wildlife. "All the evidence indicates that this pollution kills bees, moths, hoverflies and other essential pollinator species," said Craig Macadam, the group's Scottish officer. "The government must ban neonicotinoids now before further damage is done to our fragile ecosystems."
The pesticide industry, however, blamed parasites and diseases for killing bees, and maintained that the levels of neonicotinoids in pollen were too low to damage their health. Restrictions in France, now withdrawn, had made no difference to bee health, it argued.
"Although poorly-targeted insecticides would certainly harm bees, farmers value bees and strict practices are prescribed and followed to ensure that exposure does not occur," said Dominic Dyer, the chief executive of the Crop Protection Association, which represents pesticide companies.
"The use of seed treatments reduces the need for a farmer to spray broad-spectrum insecticides on his or her crop for much of the season and is therefore seen as being more environmentally friendly."