Neonicotinoid pesticides, already blamed for short-circuiting honeybee brains, also diminish their sperm, possibly contributing to the pollinators' worrying global decline, researchers said Wednesday.
Widespread neonicotinoid use may have "inadvertent contraceptive effects" on the insects which provide fertilisation worth billions of dollars every year, said a study in the British journal Proceedings of the Royal Society B.
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In their experiment, researchers divided bees into two groups.
One group was fed pollen containing field-realistic concentrations of two neonicotinoids -- thiamethoxam and clothianidin.
The other group was given untainted food.
After 38 days, the male drones -- whose key role in life is to mate with the egg-laying queen -- had their semen extracted and tested.
The data "clearly showed... reduced sperm viability" -- which is the percentage of living versus dead sperm in a sample, said the study.
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Honeybee queens mate for just a single short period, but with many males in a sort of bee orgy, before storing the sperm for the rest of their fertile lifetime.
Bees have been hit in Europe, North America and elsewhere by a mysterious phenomenon called "colony collapse disorder", which has alternatively been blamed on mites, a virus or fungus, pesticides, or a combination.
The new study adds reduced sperm quality to the list of possible causes.
"As the primary egg layer and an important source of colony cohesion, the queen is intimately connected to colony performance," the paper said.
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Previous studies have found neonicotinoids can cause bees to become disorientated to the extent that they cannot find their way back to the hive, and can lower their resistance to disease.
The European Union has placed a moratorium on the sale of neonicotinoids.
Last year, a study found that wild bees provided crop pollination services worth more than $3,250 (2,950 euros) per hectare every year.
Bees account for an estimated 80 percent of plant pollination by insects.
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