51����

Male bees may have inadvertently been taking contraceptives – a possible factor in the alarming decline in bee populations across North America and Europe over the past 15 years.

Neonicotinoid pesticides, a controversial class of neurotoxins used in agriculture for pest control, significantly impairs the fertility of male honeybees, according to a new study.

These pesticides are still widely used in the US, while the European Commission is currently reviewing a temporary ban it imposed in 2013.

and colleagues from the Institute of Bee Health at the University of Bern, Switzerland, explored the effects of two popular neonicotinoids, thiamethoxam and clothianidin, on the sperm of male honey bees, also known as ‘drones’.

The team randomly assigned batches of drones from 20 bee colonies to either insecticide exposure or control groups, and then took semen samples from males once they had reached sexual maturity.

Drones exposed to the neonicotinoids at levels found in fields showed around 39 per cent less living sperm on average. On top of this, the mortality rate of the drones during the study was almost doubled when exposed to the pesticides.

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 vital for the survival of the hive, as it equips the colony with the genetic diversity needed to resist disease, parasites and environmental challenges.

Game of drones

“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,” says Straub. “If not, then worker bees in the hive will quickly sense that the queen is ineffective and kill her. We like to call it ‘game of drones’.”

Replacement of the queen is costly as it hinders the growth of the colony. And as replacing a queen can only occur at certain periods in the year, colonies can sometimes go for long periods without one, putting them in danger of collapse.

The insecticides may irreparably damage sperm DNA and hinder sperm motility via a process known as reactive oxidative stress, has previously been shown to impede sperm production in birds and humans. The exact process behind this is still not known.

“The presence of these chemicals could be one reason behind the struggling bee numbers we’ve seen over the past 15 years in the northern hemisphere,” says Straub. “We can’t be sure though yet because of the sheer number of different factors that affect bee populations – it’s unlikely just the pesticides at work in the wild.”

Christopher Connolly, at the University of Dundee, UK, fears neonicotinoids could be reintroduced into UK farming when the country leaves the European Union.

“There is a lot of evidence for the negative effects that these chemicals have on our vital pollinators, particularly bumble bees. Britain was one of the loudest voices to oppose the ban when it came in, so I’m terrified of the ecological consequences if they’re brought back.”

Paul Hetherington at campaign group Buglife, which has long lobbied against neonicotinoid usage in the UK, is more optimistic: “While the consequences could be serious if we fully exit the EU, it’s more likely that we’ll at least partially remain a part of the European Commission and so still have to abide by their agricultural regulations, as with other non-EU countries like Switzerland and Norway.”

Journal reference: Royal Society B: Biological Sciences, DOI: