Could pesticides be limiting the ability of bees to reproduce?

Bees are responsible for pollinating 70 out of the top 100 human food crops. Eighty percent of all pollination around the world is done by the humble little honeybee. Yet despite bees' primacy in the world agricultural web, scientists are hard pressed to identify a single cause of their rapid and alarming decline.

A recent study by Swiss researchers may help demystify the causes of the decline in world honeybee populations. Scientists have long suspected that pesticides, specifically neonicotinoids, may play a role. Now, researchers say that neonicotinoids could be at the root of the problem, reducing bee sperm count and limiting reproduction.

“Most neonicotinoid studies that employ honey bees have focused on workers, which are typically the non-reproductive females of the colony,” said lead author Lars Straub of the University of Bern in a press release. “Male honey bees have really been neglected by honey bee health scientists.”

“While not surprising,” Mr. Straub added, “these results may turn a few heads.”

In January, a study found that honeybee populations had decreased by 25 percent in Europe since 1996, and 59 percent over the last 58 years in North America. A nationwide survey conducted in the United States between April 2015 and April 2016 found that beekeepers lost 44 percent of their colonies during that twelve-month span.

Research indicates that a number of factors, from climate change to to parasite problems to the reduction of biodiversity among the plants that bees visit for pollen, could be in play. Recently, scientists also identified that problems with colonies’ mothers, the famous queen bees of pollinator colonies, also contribute to the decline.

"Queen failure is a big problem and this helps explain it," said US Department of Agriculture bee scientist Jeff Pettis of the recent research on bee sperm counts, in an interview with the Associated Press. Dr. Pettis took part in the queen study earlier this year. "It's not the queens themselves, it's the drones. It's significant."

In the most recent study, researchers found that male drone sperm counts decreased 39 percent in the presence of neonicotinoid pesticides. Bees exposed to neonicotinoid treated pollen produced about 1.2 million living sperm, while bees exposed to pesticide free pollen produced about 1.98 million living sperm.

Thus, while neonicotinoids doesn’t kill the bees or make them completely sterile, it acts as an insect contraceptive at a time when bees couldn't need it any less.

Queen bees mate with male drone bees during a mating flight during in which the queens stockpile sperm to increase the colony. That single flight is the highlight of the drones’ short lives; they die after performing their mating duty.

Bayer Corporation spokesman Jeffrey Donald stated in response to the study that while Bayer, a neonicotinoid producer, would study the problem, laboratory studies are not necessarily indicative of real-world experience. But researchers say that the results of this study indicate that there is a need for environmental risk assessments of pesticides containing neonicotinoids.

Pesticides such as neonicotinoids have long been the subject of suspicion among the apiological community, many of whom urge farmers and homeowners to reduce their pesticide use in the face of declining pollinator populations.

While scientists are unwilling to attribute bee colony failure to a single factor, Pettis told the Associated Press that reduced sperm counts could account for as much as a third of the problem.