The Institute of Bee Health at the University of Bern, Switzerland has carried out a study to explore the effects of two popular neonicotinoids, thiamethoxam and clothianidin, on the sperm of drones, or male honey bees.
Lead researcher Lars Straub said the presence of the chemicals could be "one reason behind the struggling bee numbers we've seen over the past 15 years in the northern hemisphere."
The pesticides are widely used in the USA. The European Commission is currently reviewing a temporary ban it imposed in 2013.
There has been an alarming decline in the bee populations in North America and Europe over the past 15 years. Bees account for an estimated 80 percent of plant pollination by insects.
'Inadvertent contraceptive effects'
The researchers, led by Straub in Bern assigned batches of drones from 20 bee colonies to either insecticide exposure or control groups. They then took semen samples from males once they had reached sexual maturity.
The study published in the British journal Proceedings of the Royal Society B showed that widespread neonicotinoid use may have "inadvertent contraceptive effects" on the insects which provide fertilisation worth billions of dollars every year.
Male honey bees exposed to the neonicotinoids at levels found in fields showed, on average, around 39 per cent less living sperm. In addition, the mortality rate of the drones exposed to the pesticides during the study was almost doubled.
Honey bee queens make a single mating flight from the nest to collect sperm from as many as 20 different males, which they store within a dedicated organ over their entire lifespan. This is essential for the hive's survival as it provides genetic diversity which is essential for resistance to disease, parasites and environmental challenges.
"The process of the queen's mating flight is a one-time thing so it's really important that she collects plenty of quality sperm," Straub said. If she does not, then worker bees will sense the queen is ineffective and kill her. "As the primary egg layer and an important source of colony cohesion, the queen is intimately connected to colony performance," the paper said.
The new study adds reduced sperm quality to the list of possible causes for "colony collapse disorder" in Europe, North America and elsewhere, which has alternatively been blamed on mites, a virus or fungus, pesticides or a combination of all of them.
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.
jm/kl (AFP, Royal Institute)