The world's most widely used insecticide is an inadvertent contraceptive for bees, cutting live sperm in males by almost 40%, according to research. The study also showed the neonicotinoid pesticides cut the lifespan of the drones by a third.
The scientists say the discovery provides one possible explanation for the increasing deaths of honeybees in recent years, as well as for the general decline of wild insect pollinators throughout the northern hemisphere.
Bees and other insects are vital for pollinating three-quarters of the world's food crops but have been in significant decline, due to the loss of flower-rich habitats, disease and pests and the use of pesticides.
Neonicotinoids were banned from use on flowering crops in the EU in 2013. The UK opposed the ban and allowed a limited "emergency" lifting of the ban in 2015, but has refused further suspensions this year. There is clear scientific evidence that neonicotinoids harm bees, but there is only a little research showing the pesticides harm the overall performance of colonies.
Previous work has shown that neonicotinoids reduce the number of bumblebee queens produced and severely cuts the survival and reproduction of honeybee queens. But the new research, led by Lars Straub at the University of Bern, Switzerland and published in the journal Proceedings of the Royal Society B, is the first to test how neonicotinoids affect male bee fertility.
They exposed drones to the levels of two neonicotinoids, thiamethoxam and clothianidin, seen in fields, and found that they had on average 39% less living sperm compared with unexposed bees. "Any influence on sperm quality may have profound consequences for the fitness of the queen, as well as the entire colony," said the researchers.
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