By Kate Kelland, Health and Science Correspondent
LONDON (Reuters) - Bee populations have been declining steadily in recent decades but there is scientific disagreement over the contribution of pesticides called neonicotinoids to falling bee numbers.
Europe is expected to impose a temporary ban on the pesticides after EU governments failed on Monday to agree whether or not their use should be halted.
Some recent studies have shown neonicotinoids can have damaging effects on bee health by interfering with their homing abilities and making them lose their way.
Other scientific studies point to a virus spread by a parasitic mite called the Varroa as a prime suspect in fuelling so-called "colony collapse disorder" which has seen bee numbers drop rapidly in Europe, Asia, the Americas and the Middle East.
Bees are important pollinators of flowering plants, including many fruit and vegetable crops. A 2011 United Nations report estimated that bees and other pollinators such as butterflies, beetles or birds do work worth 153 billion euros ($203 billion) a year to the human economy.
Neonicotinoid pesticides are new nicotine-like chemicals and act on the nervous systems of insects. They pose a lower threat to mammals and the environment than many older pesticide sprays.
Because they are water soluble, they can be applied to the soil and taken up by the whole plant, making them "systemic" - meaning they render the whole plant toxic to insects. Neonicotinoids are often applied as "seed treatments", which means coating the seeds before planting.
A report from the European Food Safety Authority (EFSA) in January said three widely-used neonicotinoids, made mainly by Switzerland's Syngenta and Germany's Bayer, posed an acute risk to honeybees.
But Britain, whose department for environment, food and rural affairs (DEFRA) recommended abstaining in a previous EU vote in March, argues the science is inconclusive and advises caution in extrapolating results from lab studies to real-life field conditions.
Lynn Dicks, a bee expert at Cambridge University said it should come as little surprise that insecticides kill insects.
"They are designed to," she said, adding it is the extent to which they can be blamed for bee decline that is in doubt.
"They are unlikely to be the sole cause of falling insect numbers and diversity, but they represent one of a set of multiple interacting threats that seems to be driving declines."
Experts note that one of the key difficulties in establishing the potential danger lies in how to find out how much of the pesticides the bees come into contact with as they forage, and the degree to which this might lead to fewer bees.
Britain's DEFRA published a report in January in which it said its research "did not show conclusively that exposure to neonicotinoids used within a normal agricultural setting had major effects on bumble bee colonies".
James Cresswell, an ecotoxicologist at the University of Exeter, says the science has yet to produce unequivocal answers.
"While recent research based on artificial dosing shows that neonicotinoids can harm bees, uncertainty remains over the severity of environmentally realistic conditions," he said.
Lin Field, Head of Biological Chemistry and Crop Protection at Rothamsted Research, says there is not enough evidence to support a total ban on neonicotinoids and questions whether the "precautionary principle" should apply and a ban should be imposed just in case the threat turns out to be real.
"On the face of it that might be the best solution but it takes no account of the risk of the ban on our ability to control insect pests and secure crop yields," she said.
(Editing by Philippa Fletcher)