Bee populations have been declining for some time, often attributed to the use of pesticides. A new study now suggests that these are not only bad for bee health, but also for their success in finding a mate: the synthetic fabrics change the buzz and smell of male bees, thereby lowering their chance of being chosen as a mate by a female bee will.
In honey bees, the queen bee is solely responsible for laying eggs. When mating, she can choose her partner from up to 20,000 male bees – the drones. Male wild bees, on the other hand, have to compete for the attention of the female bees in front of their solitary nests. But whether wild or honey bee – as a male it is important to attract attention with certain physiological characteristics in order to be chosen by a female bee as a mating partner.
A basket for fungicide-infested bees
In mason bees (Osmia cornuta), when choosing a mating partner, the females evaluate male quality signals – above all their smell and vibrations of the breast. If these factors are influenced by pesticides, for example, in the early stages of insect reproduction, this could be another explanation for the decline in bee populations, say Samuel Boff from the University of Würzburg and his colleagues. To test this, the researchers exposed mason bees to a low, non-lethal dose of the fungicide fenbuconazole. “If the fungicide has an effect on male quality signals, it should increase the likelihood that pesticide-exposed males will be rejected by females,” explains Boff.
And indeed: male bees exposed to the fungicide were rejected by the females more often, although they approached the female bees just as often as the control group. According to the researchers, this is partly due to the change in the chest vibrations of the male bees affected by the fungicide: Although there were no differences in the length and persistence of the sonorous tremor, the variability in the frequency of the vibrations was reduced by about half. The untreated male bees, on the other hand, showed a much broader repertoire of high and low vibration frequencies, which the female bees apparently liked much better.
Bees need to be able to smell each other
But the researchers observed another physiological change. Male bees’ skin is covered with 47 different hydrocarbon compounds that give them a distinctive smell, signal readiness to mate to females, and provide information about their own fitness. However, the composition of this hydrocarbon profile, which is important for mating, changed significantly under the influence of the fungicide – and in turn influenced the choice of mate by the female bees, as the experiments showed. Boff and his team suspect that the fungicide triggers detoxification processes in the bees’ metabolism, which consequently affects the odor composition.
However, the study says nothing about how long the effect of the fungicides on the quality signals of the male bees lasts. But if pesticide exposure does permanently change male quality and females avoid mating, that could be a direct reason for population depletion, Boff and his colleagues explain. “Our study shows that the early phases of bee reproduction must be included in the risk assessment of pesticides,” says co-author Thomas Schmitt from the University of Würzburg. The team is now hoping for a broader testing of different classes of pesticides on the behavior and chemical signals of bees: “So that effective bee protection can really take place,” says Boff.
Source: Julius Maximilian University of Würzburg; Specialist article: Journal of Applied Ecology, doi: 10.1111/1365-2664.14169