Three-quarters of the honey produced around the world contains nerve agent pesticides that can harm bees and pose a potential health hazard to humans, a study has shown.
Scientists who tested 198 honey samples from every continent except Antarctica discovered that 75 per cent were laced with at least one of the neonicotinoid chemicals.
More than two-fifths contained two or more varieties of the pesticides and 10 per cent held residues from four or five.
Environmental campaigners responded by demanding a "complete and permanent" ban preventing any further use of neonicotinoids on farm crops in Europe.
Experts called the findings "alarming", "sobering" and a "serious environmental concern" while stressing that the pesticide residue levels found in honey generally fell well below the safe limits for human consumption.
However, one leading British scientist warned that it was impossible to predict what the long term effects of consuming honey containing tiny amounts of the chemicals might be.
Dave Goulson, Professor of Biology at the University of Sussex, said: "Beyond doubt ... anyone regularly eating honey is likely to be getting a small dose of mixed neurotoxins.
"In terms of acute toxicity, this certainly won't kill them and is unlikely to do measurable harm. What we don't know is whether there are long-term, chronic effects from life-time exposure to a cocktail of these and other pesticides in our honey and most other foods."
For practical reasons it was "impossible to do a proper experiment to test this", he added.
otinoids are neuro-active chemicals similar to nicotine that have proved to be highly effective at protecting crops from pests, especially aphids and root-eating grubs.
They can either be sprayed on leaves or coated on seeds, in which case they infiltrate every part of the growing plant.
Years of research have shown that under controlled conditions the chemicals are toxic to honey bees and bumblebees, causing brain damage that can affect learning and memory and impair their ability to forage for nectar and pollen.
Controversy still surrounds claims that the chemicals may be harming bee colonies in the "real world" and sabotaging the insects' vital role as pollinators.
In June results from the world's largest bee colony field trial in three European countries found that neonicotinoid exposure reduced the survival of both honey and wild bees in the UK and Hungary, but not Germany.
The scientists were given a special dispensation to conduct the trial on 33 large farmland sites despite a temporary ban on three key neonicotinoids imposed by the European Commission (EC) in 2013.
Currently it is illegal to use the pesticides thiamethoxam, clothianidin and imidacloprid, on mass-flowering crops that attract bees. The chemicals can still be used to treat other kinds of crops including wheat and other cereals and sugar beat.
The new research published in the journal Science could not have come at a more sensitive time in Europe. EC policymakers are right now discussing whether to make the ban permanent and more wide ranging.
A total ban would have a huge impact on cereal growers in the UK.
For the study, an international team of European researchers tested almost 200 honey samples from around the world for residues left by five different neonicotinoids.
Three-quarters of all the samples contained at least one of the pesticides. A total of 45% contained two or more and a tenth contained four or five.
Concentrations were highest in European, North American and Asian samples.
While in most cases the levels were well below the EU safety limits for human consumption, there were exceptions.
Honey from both Germany and Poland exceeded maximum residue levels (MRLs) for combined neonicotinoids while samples from Japan reached 45% of the limits.
Samples from England had neonicotinoid levels that were no more than 1.36% of the amount thought to be safe for human consumption.
The researchers, led by Dr Edward Mitchell, who carried out the work as director of the Laboratory of Soil Biodiversity at the University of Neuchatel, Switzerland, wrote: "The fact that 45% of our samples showed multiple contaminations is worrying and indicates that bee populations throughout the world are exposed to a cocktail of neonicotinoids.
"The effects of exposure to multiple pesticides, which have only recently started to be explored, are suspected to be stronger than the sum of individual effects."
Dr Chris Connolly, reader in neurobiology at the University of Dundee, who wrote a commentary article accompanying the research, said the findings were "timely" and "alarming".
He added: "The levels detected are sufficient to affect bee brain function and may hinder their ability to forage on, and pollinate, our crops and our native plants.
"Clearly, the use of neonicotinoids need to be controlled. Their widespread use on crops is due to their prophylactic use, as insurance against the possibility of future pest attack. The neonicotinoids are highly effective insecticides with low toxicity to humans, but this unnecessary overuse is also driving the development of pest resistance against them. It is time that these chemicals are heavily restricted for use."
Sandra Bell, nature campaigner at the environmental group Friends of the Earth, called on environment secretary Michael Gove to back greater restrictions on neonicotinoids and pledge to ban them from the UK post Brexit.
She said: "The discovery of bee-harming pesticides in honey samples across Europe reinforces the need for a complete and permanent ban on these chemicals.
"Honey is a vital source of food to bees, not just a sweet treat for humans, so finding that so many of the honey samples contained a cocktail of these pesticides is a real concern - especially as scientists warn that exposure to a mixture of chemicals can be more harmful.
"Nothing short of a full ban will protect our bees."
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