Scientists from Royal Holloway University, London have discovered that when bees are exposed to low levels of neonicotinoid pesticides their behaviour changes and they stop working properly for their colonies. The results showed that exposure to pesticides at levels bees encounter in the field has subtle impacts on individual bees, and can eventually make colonies fail.

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Although they said exposure to the pesticides did not kill bees outright, the scientists' findings add to previous research from Royal Holloway published last October which showed that exposure to a combination of pesticides is more harmful to bees than exposure to any chemical on its own. As the combinatory effects of chemicals are not studied under current EU risk assessments, the scientists concluded that current assessments for agricultural chemicals are inadequate.

Bee populations in the northern hemisphere are in decline; although the reasons for declines are clearly complex and multi-faceted, evidence of the role certain agricultural chemicals play in weakening colonies is growing ever-stronger. Manufacturers of neonicotinoid pesticides, amongst the most widely used agricultural chemicals in Europe, maintain that their products do not harm insect pollinators when used correctly, and have obstinately assured that bee decline is caused by other factors including disease, climate change and habitat loss.  

Nevertheless, a series of influential studies, supported by EU health watchdog EFSA, led the Commission to pass a partial ban on the controversial pesticides earlier in the year. 

Commenting on the Royal Holloway study on Monday, lead author Dr John Bryden said, "By understanding the complex way in which colonies fail and die, we've made a crucial step in being able to link bee declines to pesticides and other factors, such as habitat loss and disease which can all contribute to colony failure."

"Exposing bees to pesticides is a bit like adding more and more weight on someone's shoulders. A person can keep walking normally under a bit of weight, but when it gets too much – they collapse. Similarly, bee colonies can keep growing when bees aren't too stressed, but if stress levels get too high the colony will eventually fail," Dr Bryden added.

The research was part of the £10 million government-funded 'Insect Pollinators Initiative', set-up to understand the causes of pollinator declines and bring them to a halt. It is estimated that bees pollinate a third of the world's agricultural crops. The insects are thought to be responsible for 80 per cent of insect pollination around the world.

Dr Nigel Raine, whose earlier research on bees at Royal Holloway revealed "chronic exposure to low levels of pesticide is affecting the behaviour of individual bees and therefore the performance of colonies," said on Monday, "Pesticides can have a detrimental effect on bees at levels used in the field; Our research will provide important evidence for policymakers. The way we test pesticides, the way we assess their impact on bees, and the way we manage pesticides can all be improved."

Reacting to the findings, Emma Hockridge, head of policy at the Soil Association said, "This study provides further evidence on the effects of very low pesticide doses on pollinating insects like bumblebees. The European ban on neonicotinoid pesticides is due to start in December, and although the UK will be taking part, the Government continues to ignore the strong scientific evidence pointing to the damaging impact of neonicotinoid pesticides on pollinators.

"There are a range of methods which farmers can use which do not require the use of neonicotinoid pesticides—in Italy government research showed banning neonicotinoid use on maize did not affect farmers' profits."