Researchers have found that a commonly used pesticide reduces the number of viable sperm produced by drone bees. The findings, just published in the journal Proceedings of the Royal Society B, provide a possible explanation for dwindling wild bee populations.
Drones are male bees that develop from unfertilized eggs. Their job is simply to mate with the queen, they don’t gather nectar and they lack stingers. Queen bees go on a single mating flight and store sperm from multiple drones, not mating again in their lifetime. Population crashes are generally due to problems with the queen as her survival and ability to reproduce are critical to the health of the colony.
A research team in Switzerland collected 20 honeybee colonies from the wild. Colonies were randomly assigned to one of two groups. Both groups were fed a standard pollen paste but the experimental group also received small doses of pesticide. The pesticide application, added to the paste, consisted of two neonicotinoid insecticides. The insecticides, thiamethoxam and clothianidin, are both commonly used to protect crops. The doses added to the pollen paste were based on concentrations the bees would realistically be exposed to in the field.
The researchers discovered that drones exposed to the pesticides had shorter lifespans and produced less viable sperm. They produced 39% less living sperm, though the overall quantity was the same as the control group. This difference is significant enough to explain dropping bee populations.
There has been some previous research on the effects of insecticides on wild bees but this was the first study to specifically investigate effects on drones. Since queen bees only mate once, a drop in viable drone sperm may seriously affect a queen’s productivity and therefore the survival of the colony. Bees and other pollinators are important to the ecosystem and human crops. The authors note that there may be other factors contributing to bee population crashes but the effects of insecticides are significant enough to warrant further investigations.
Lars Straub et al. Neonicotinoid insecticides serve as inadvertent insect contraceptives. Proceedings of the Royal Society B (2016).