The more extensive organically managed agricultural areas are, the better populations of a certain species of mason bee can thrive. This is the conclusion of a research team led by bee expert Dr. Samuel Boff from Ulm University, Germany. The study, now published in the journal Science of the Total Environment, not only confirms that fewer bees live in areas where chemical pesticides are used and fewer flowers are available as food sources. The research also demonstrates that conventional farming can disrupt the reproductive communication of wild bees.
For years researchers have observed that the number of wild bees, especially in agricultural areas, has been declining dramatically. Wild bees are indispensable pollinators for food production and very important for biodiversity maintenance. The decline of these bees thus endangers food security and ecosystems worldwide. To investigate the reasons for the decreasing number of wild bees on farmland, Dr. Samuel Boff from the Institute of Evolutionary Ecology and Conservation Genomics at Ulm University, together with other colleagues in Ulm and an international team examined how different farming practices – organic versus conventional – affect wild bee populations. The study focused on Osmia bicornis, a mason bee.
Monitoring with “bee hotels”
For their observations, the researchers installed bee hotels, artificial nesting aids, on eight conventional and seven organic farms in the German state Baden-Württemberg. To ensure the most accurate results, they selected areas that covered the bees' typical movement radius, which is limited to around 500 metres from their nesting sites. The team documented landscape features, including the extent of organic farming, the number of plants serving as food sources, and compared the number of bee nests and offspring as well as the body sizes of the offspring. They also conducted chemical analyses of the so-called cuticular hydrocarbons on the bees’ body surfaces. These compounds have a protective function but also serve in sex communication between mating partners, playing a key role in reproduction. After detecting that farming management influences chemical compound composition, the researchers tested biological reactions in a laboratory setting to determine how the respective farming systems influence the chemical compounds on the bees’ bodies and whether this affects mating behaviour.
More organic, more offspring
“We found that population size increases with the expansion of organic farming in the landscape. In regions with a higher proportion of organic farming and little or no pesticide use, there was greater floral diversity and more food resources were available to pollinators. Here, we observed more females building nests, and the number of brood cells and offspring was higher,” reports project coordinator Boff. The body size of the fewer offspring from bees in conventional areas was, however, larger – possibly to improve their foraging capabilities in response to poorer food availability in the surroundings of their nests in conventional sites.
Disrupted sexual communication
“The study also shows that conventional farming disrupts the bees' chemical communication. The cuticular hydrocarbons on the body surfaces of bees in conventionally managed areas differed from those in organic areas,” says Professor Manfred Ayasse, Deputy Director of the Institute of Evolutionary Ecology and Conservation Genomics and co-author of the study. Two compounds in particular, (Z)-11-heptacosene and (Z)-9-nonacosene, were more common in females from conventional sites. In the laboratory, the researchers observed that these bees exerted a stronger sexual attraction on males from a neutral area. „These males displayed stronger sexual signals towards these females in the form of more pronounced precopulatory behaviour, such as wing fanning and mating attempts – an indication of altered sexual communication as a result of the more freque”, he explains. One possible explanation is that bees from conventional farms face higher competition due to smaller populations, therefore producing more sex pheromene to increase their own attraction to males.
“We have proven that conventional farming practices not only affect the bees' food sources but can also have profound effects on reproduction and their mating behaviour,” Boff emphasises. “We still don’t know exactly what the consequences are, but our results highlight the influence of farming management on sex reproduction of wild bees. Investigating the effects of these chemical changes further, for instance, on the molecular mechanism of these changes and whether similar effects can be observed in other pollinator species, are important points for future research.”
The research project was funded by the Aurelia Foundation and the Software AG Foundation (SAGST).
Further information:
Dr. Samuel Boff, Institute of Evolutionary Ecology and Conservation Genomics, Ulm University, E-Mail: samuel.boff(at)uni-ulm.de
Publication details:
Boff, S., Olberz, S., Gülsoy, I.G., Preuß, M., Raizer, J., Ayasse, M. (2024). Conventional agriculture affects sex communication and impacts local population size in a wild bee. Science of the Total Environment, 954/2024. https://doi.org/10.1016/j.scitotenv.2024.176319
Text: Christoph Karcher