One of the most commonly used pesticides may be acting as an inadvertent contraceptive for male honey bees, known as "drones", by reducing the viability of their sperm. This means the chemical ordinarily used to kill other insect pests may be contributing to the decline of honey bee colonies, which has been observed across much of Europe and North America.
The use of neonicotinoids is highly controversial. There is evidence to suggest that sub-lethal levels of the pesticide do impact pollinating insects, such as bees, but the degree of this harm is hotly debated. This latest study, published in the Proceedings of the Royal Society B, looked at the impact that two of these neonicotinoids – thiamethoxam and clothianidin – have on drone reproduction, and found that males exposed to the chemicals produced close to 40 percent less live sperm than those that weren’t. Effectively, the chemicals have been acting as accidental contraceptives.
Normally, drones only have one role in life – the stingless males hatch and then leave the hive in search of a virginal queen on her maiden flight. She will mate with multiple drones, which will all subsequently die. The researchers took drones from colonies exposed to neonicotinoids and the controls just after they had hatched, and then kept them in the lab until they reached sexual maturity. They found that while both sets of drones produced the same volume of ejaculate, the level of live sperm in the control group was much higher than those that had been exposed to the chemicals.
The live sperm (blue) was reduced by 40 percent. Lars Straub/University of Bern
The results from this latest study dove-tail nicely with another that came out earlier in the year, which found that the incredibly high rate of US honey bee colonies dying – in some cases more than 50 percent of hives – coincides with the failure of queen bees. They found that half of all queens were replaced within six months, a dramatically high turnover when normally one queen will last anywhere between two and four years. This usually occurs when the worker bees decide that the queen is not fulfilling her duties, and can be incredibly costly for a colony if they are constantly replacing her.
In that study, published in PLOS One, the researchers found that the failure of the queens, and thus the health of the colony, was related to the quality of sperm she had collected from drones at the beginning of her life. While the study did not explore the cause for the low sperm viability that was measured, they did suggest that it could be the temperature at which the queens were shipped. This latest research, though, may have found another reason, in that the neonicotinoids commonly sprayed on fields are the real culprit.