There are many potential culprits behind the plight of pollinators. As populations of bees and other insect pollinators decline in places around the world, people have pointed fingers at pesticides, invasive species, disease, climate change, genetically modified crops and habitat loss as potential causes.
What’s less clear is which of these many factors is taking the biggest toll and, therefore, deserves the most attention. But new research in Chinese rice fields suggests that the disappearance of wild habitat outranks pesticides.
“Although the impact of pesticides cannot be ignored in efforts to reduce the decline in pollinator diversity, our results suggest it would be better to concentrate on conservation of semi-natural habitat than on reduction of pesticide use in rice fields,” says Yi Zou, an ecologist and entomologist at Xi’an Jiaotong-Liverpool University, a joint program of the United Kingdom’s University of Liverpool and China’s Xi’an Jiaotong University.
A clue lies in the tiny balls of pollen that a mason bee native to China makes to feed its larvae. The bee, Osmia excavata, is widely used in China to pollinate commercial crops such as fruit orchards. But their populations have been in decline.
In the experiment by Zou and collaborators from China, the UK and the Netherlands, colonies of these mason bees were placed in 20 different rural spots near the city of Nanchang in southeastern China. The scientists chose a range of levels of farming intensity. Fields of rice – the primary crop there – covered between 11% and 85% of the surrounding land. Hives of European honeybees were also placed there, to see if effects differed between the insects.
To gauge the effects of pesticide exposure, they measured pesticide levels inside honeybee’s bodies, as well as on pollen that dusted the bees. They also checked for the chemicals in the pollen balls and soil the mason bees packed into the slender tubes where their larvae live.
To gauge the effects of the presence of nearby natural lands, they counted the number of pollen balls and the number of occupied tubes in the mason bee colonies, both indicators of whether a colony was thriving.
The survey revealed a strong connection between the health of mason bee colonies and the amount of nearby habitat not taken up by crops. As the percentage of uncultivated land increased, so did the total number of tubes in use, the number of total pollen balls and the amount of balls found in a single tube, the scientists reported in early July in the Journal of Applied Ecology.
“Our study shows that in regions with a lower percentage of rice fields and more semi-natural habitat, there was more food available for female mason bees to feed their larvae with,” said Xiaoyu Shi, the study’s first author and a postdoctoral researcher at the Chinese Academy of Sciences, China.
The importance of these uncultivated lands was echoed in data from another experiment by some of the same scientists. They placed insect traps in fields of flowering oilseed rape, capturing 85 different species of pollinators. The areas with more nearby uncultivated land had a greater variety of species. Their analysis showed the presence of natural habitat could, in nearly all cases, explain more than 60% of the differences in pollinator diversity.
The effects of pesticides, meanwhile, were less clear cut. The tests found pesticides in nearly all the pollen taken from honeybees bodies, but roughly half of the pollen balls and nest material of the mason bees. And pesticide use appeared to not have a big influence on the diversity of pollinators found in the traps.
That doesn’t mean, however, that pesticides don’t pose a danger. “The impact of pesticides is still an important consideration,” said Zou. The scientists found that honeybees had a higher risk of exposure when rice fields were more dense. And more than a third of the pollen samples from honeybee hives showed pesticide levels high enough to affect the health of the insects.
So while habitat loss might outrank pesticides, both are likely taking a toll on our winged helpers.
Shi, et. al. “Influence of agricultural intensification on pollinator pesticide exposure, food acquisition and diversity.” Journal of Applied Ecology. July 3, 2024.
Photo: Osmia excavata male face, by Chelsey Ritner
