A new study of male honeybees (drones) conducted by a doctoral candidate at the University of Bern, Switzerland, shows that two neonicotinoid insecticides, thiamethoxam and clothianidin, can significantly reduce the drones’ ability to reproduce. “We’ve been able to show for the first time that neonicotinoid pesticides are capable of having an effect on the male reproductive system,” the study’s lead author, Lars Straub, a doctoral student at the University of Bern in Switzerland, told a British biological sciences journal. 

Up to now, studies have shown that neonics harm the health of individual bees and the reproductive ability of female insects. The Swiss study took a closer look at the effect the pesticides have on males and found that bees subjected to the two active ingredients had 39% less living sperm on average than bees that had not come into contact with the pesticides.

As part of the field research conducted between April 2015 and October 2015 – the study was completed in April 2016 and published on Jul. 27 – Straub said that after reaching maturity the drones that had been subjected to insecticides were dissected, with their testes and mucous glands removed and analyzed for sperm viability.

In this first experiment, the male bees were raised to sexual maturity after being removed from the colonies in which they were exposed to the chemicals. According to Straub, the next step will be to investigate the effects of the pesticides on the drones that were allowed to stay in the colonies longer.

The research team plans to do what Straub called “further field-realistic studies,” which would also focus on the interaction of several stressors seen as acting together to decimate bee populations. In the US, 44% of the bee populations died off between April 2015 and April 2016, according to an annual survey conducted by the Bee Informed Partnership. This was an increase of 3.5% against the results of 2014 and 2015.

The two neonicotinoids implicated in Switzerland were subject to a two-year EU-wide moratorium in 2013 that was supposed to have been reviewed last year but is still in place. They are still used on an industrial scale in the US while the Environmental Protection Agency (EPA) continues studying them.

The agency has announced it will release risk assessments for the two chemicals, as well as another neonicotinoid, dinotefuran, in December of this year.