New research shows low levels of nicotine-based pesticides in agriculture, equivalent to just one teaspoon in an Olympic swimming pool, are sufficient to impair bumblebees' brain cells.

As a result colonies of bumblebees perform poorly as natural pollinators. This is a matter of concern as up to a third of all our food depends on the pollination activities of the likes of bees and bumblebees, contributing an estimated £142 ($215) billion to worldwide economies.

The UK's wildflower-rich grasslands, prime habitat for pollinators, have been reduced by 97% since the 1930s; and of the 26 bumblebee species recorded in Britain 80 years ago, two have disappeared and six are found in a much reduced areas.

So not everyone is convinced that these pesticides or neonicotinoids contribute significantly to the global decline of insect pollinators. Indeed it is contested by many in the agriculture industry and government.

However, the new research at Dundee and St Andrews universities, published in the Journal of the Federation of American Societies for Experimental Biology, claims to demonstrate for the first time that the low levels found in the nectar and pollen of plants is sufficient to deliver 'neuroactive levels', impairing the bee brain.

Recent years have seen up to 30 per cent annual honeybee colony losses, while the population of butterflies and other insects is also down and similar declines in insect-pollinated wild plants.

Dr Chris Connolly, a Reader in the Division of Neuroscience at Dundee's School of Medicine, has spent several years examining the risk from neonicotinoids and other commonly used classes of pesticides on both honeybees and bumblebees.

He and his colleagues carried out combined laboratory and field studies and the data was analysed at St Andrews. The results showed very low levels of neonicotinoids caused bumblebee colonies to have an estimated 55 per cent reduction in live bee numbers, a 71 per cent reduction in healthy brood cells, and a 57 per cent reduction in the total bee mass of a nest.

Dr Connolly says the paper represents the best scientific evidence to date connecting neonicotinoid consumption to poor performance of bees and that the effects of the pesticide should be considered by policy makers seeking to protect the abundance and diversity of insect pollinators.

"Our research demonstrates beyond doubt that the level of neonicotinoids generally accepted as theaverage level present in the wild causes brain dysfunction and colonies to perform poorly when consumed by bumblebees," he said.

He said this wasn't surprising as pesticides were designed to affect brains of insects. The bumblebees didn't die due to exposure to neonicotinoids but their brains cells wouldn't perform well.

However he stressed this was not proof that neonicotinoids were solely responsible for the decline in insect pollinators such as bumblebees, but a clear linear relationship was established.

"We can now be confident that at these levels, neonicotinoids disrupt brain function, bee learning and the ability to forage for food and so limit colony growth."

But he said it might be be possible to help if more bee-friendly plants were available to bees in the countryside and in gardens. He suggested that the neonicotinoids were no longer used on any bee-friendly garden plants, or on land with crops visited by bees or other insect pollinators.

Neonicotinoid contamination of the nectar and pollen consumed by bees is around 2.5 parts per billion or one teaspoon in an Olympic swimming pool.

Two years ago the European Commission banned three neonicotinoids, but the UK Government opposed the measure as it was not convinced by the evidence.