More plant diversity, less pesticides

Increasing plant diversity improves the natural control of herbivorous insects in grasslands. Species-rich plant communities support natural predators and at the same time provide less valuable food for herbivores. This was discovered by a team of researchers led by the German Center for Integrative Research on Biodiversity (iDiv), which conducted two similar experiments in Germany and the U.S. The results were published in Advances in science and demonstrate that increasing plant biodiversity could help reduce pesticide inputs into agricultural systems by improving natural biological control.

Biodiversity, the biological diversity of all species on Earth, their interactions and the different ecosystems they form, is critical to providing and maintaining ecosystem functions and services in planted and natural grasslands. With a growing demand to feed the world’s growing population by stepping up agriculture, these grasslands are also under pressure. Insect herbivores are causing an estimated 18-26% loss in global agricultural production, which has resulted in significant growth in the use of environmentally expensive pesticides.

To investigate whether and how increasing plant diversity can naturally reduce the impacts of herbivores on plants, an international team of researchers led by the German Center for Integrative Biodiversity Research (iDiv), the University of Leipzig (UL) and the Friedrich Schiller University of Jena (FSU) has use of two long-running grassland biodiversity experiments in Europe and North America: the Jena Experiment in Germany and the Cedar Creek Biodiversity Experiment in Minnesota (USA). Over the course of two years, scientists collected data from these two similar experiments, providing an in-depth insight into the structure of the food web of monocultures and species-rich grasslands. “These two long-term experiments generated invaluable insights, both for fundamental and applied research, which brought to light the importance of maintaining biodiversity,” said senior author Nico Eisenhauer, a professor at UL and also head of a research group at iDiv as speaker of the Jena experiment.

Plant communities rich in less attractive species for herbivores

Researchers found that plants in more diverse communities lose significantly less energy to herbivorous insects. In high diversity blends, the feeding rate of herbivores per gram of plant biomass was 44% lower than in monocultures. Therefore, for every gram of plant biomass produced, plants lose just under half the energy to herbivorous arthropods when planted in species-rich communities. “This ultimately means that where more species are planted together, this will produce more plant biomass per square meter and each individual plant in different blends will receive less damage from herbivores,” said first author and iDiv alumnus Andrew Barnes, now a senior lecturer. at the University of Waikato in New Zealand.

In areas with greater plant diversity, herbivorous arthropods have less chance of encountering their favorite plant species, making it less likely that they will remain in these high diversity areas. Furthermore, previous research had shown lower levels of tissue protein (nitrogen) in plant communities with a high species richness, making these plants less nutritious for herbivores.

Predators benefit from greater plant diversity

Although the total biomass of herbivores and predators increased in the species-rich grasslands, predators benefited the most from the different plant communities: compared to monocultures, they increased significantly in both total biomass and feeding rates. One possible explanation could be that arthropod predators such as spiders, some beetles or wasps significantly benefit from the more complex habitat of highly diverse plant communities, which reduces the risk of being detected and eaten by vertebrate predators such as birds and mammals.

The increase in plant diversity, therefore, has several positive side effects: compared to monocultures, plant communities with high diversity produce more total biomass. Furthermore, both natural enemies and resource concentration act in concert to limit the negative effects of herbivores on plant performance. Andrew Barnes said: ‘In other words, more diverse plant communities represent a double-edged problem for herbivores – more predators and less preferred food, which could help naturally reduce herbivore impacts.’

Plant biodiversity can limit epidemics of herbivorous parasites

Conversely, pest control that relies heavily on insecticides can lead to harmful rebounds of herbivorous pests as the application of pesticides can destabilize communities of natural enemies. “Our experiments show that preserving plant diversity offers multiple benefits for the control of herbivorous pests, which could play a key role in reducing agricultural chemical inputs and increasing plant productivity,” he said. Andrew Barnes said. Nico Eisenhauer added: “Ultimately, this study shows that support for biodiversity can harness sustainable management of ecosystems and benefits for people.”

Provided by the German Center for Integrative Biodiversity Research