Image copyright Dara Stanley Image caption Bumblebees are frequent pollinators of apple crops

Bees exposed to nicotine-like pesticides are not as good at pollinating crops, research suggests.

A study found bumblebees collected pollen from apple trees less often when exposed to the chemical, reducing the success of the crop.

Scientists say policymakers should consider the potential impact on agriculture in the debate over the use of neonicotinoid pesticides.

The company that makes the pesticide said the findings were "premature".

Bees are essential for the pollination of many crops, including fruit, seeds and oils.

The global worth of the pollination services bees and other insects provide is estimated at between 152 billion and 379 billion pounds per year.

Dr Dara Stanley from Royal Holloway University of London said the research, published in the journal Nature, found for the first time that pesticide exposure reduces the pollination services bumblebees deliver.

"I think what's important about our work is that it shows that pollination services should also be included in the debate on neonicotinoids," she told the BBC.

"Pollination services are clearly important because about 30% of the food that we eat comes from crops that are pollinated by bees and other insects and these could include crops such as fruit crops, nut crops, seed crops and oil crops."

Image copyright Dara Stanley Image caption Bumblebees are frequent pollinators of apple crops

Image copyright Victoria Wickens Image caption Apples require pollination by insects, primarily bees, to set fruit

There is growing evidence that exposure to neonicotinoid pesticides - the most widely used group of insecticides worldwide - affects bee behaviour and reproduction.

But until now, the impact on pollination has not been studied.

Scientists in the UK and Canada looked at three groups of bumblebees exposed to varying levels of neonicotinoid pesticides in nectar, then released to forage among apple trees.

Co-researcher Dr Mike Garratt, from the University of Reading, said they found that bees exposed to pesticides returned from apple flowers with less pollen than bees in the control group.

"This suggests that bumblebees exposed to pesticides must somehow behave differently on flowers," he said.

Commenting on the study, Prof Felix Wäckers of Lancaster University said it demonstrated for the first time that "field realistic levels of neonicotinoid exposure actually compromises crop pollination".

And Prof David Goulson of the University of Sussex said farmers using these chemicals could potentially experience reduced yields.

But Syngenta, which manufactures the pesticide used in the study, thiamethoxam, described the findings as "premature".

Dr Peter Campbell said the conclusion that thiamethoxam impairs pollination services "is not conclusive, it is premature and only representative of a single experiment conducted under artificial conditions for the apple trees being pollinated and using unrealistically exposed bumble bees".

Habitat loss

A separate study this week by French scientists found wild honeybees exposed to the nicotine-like pesticides had shorter lifespans.

However, whole colonies were able to recover by breeding more worker bees, potentially disguising the effect.

Bees are in decline in Europe and North America due to a number of factors, including pesticides, habitat loss and diseases.

A Europe-wide ban on neonicotinoid use on flowering crops introduced two years ago is due to be reviewed at the end of 2015.