The presence of a healthy population of earthworms in a garden’s soil works very effectively to protect plant leaves from being eaten by slugs, new research from the University of Natural Resources and Life Sciences Vienna has found.

Even though the earthworms don’t have direct contact with the slugs or the plant growth above, they seem to very effectively protect the plants via increased nitrogen content in the plants, as a result of the earthworms being present. The researchers also found that with increased plant diversity the amount of damage that the slugs did to individual plants was greatly lessened.

Spanish slugs (Arion vulgaris) are considered to be one of the most damaging invasive species in Europe, and they are considered a pest more or less everywhere. So the researchers decided to investigate possible solutions and means to limit the damage that they cause, specifically: What effect would the presence of earthworms and plant diversity have on the amount of damage these slugs caused?

The research was done by using large incubators to simulate grassland environments where “the researchers could regulate the diversity of plant species and time the introduction of earthworms and slugs.” What was found was that the presence of the earthworms increased the nitrogen content of plants and reduced the number of leaves damaged due to slugs by 60%. And interestingly, even when the total leaf area damaged was compared, the slugs ate 40% less at high plant diversity than at low. That’s a significant improvement.

Lead researcher Dr Johann Zaller, explains: “Our results suggest that two processes might be going on. Firstly, earthworms improved the plant’s ability to protect itself against slugs perhaps through the build-up of nitrogen-containing toxic compounds. Secondly, even though these slugs are generalists they prefer widely available food and in high diverse ecosystems slugs eat less in total because they have to switch their diets more often since plants of the same species are less available. Therefore gardeners are to help protect earthworms by increasing plant diversity in the garden in order to keep slug damage low. In order to elucidate the mechanisms behind these complex interactions, all parts of an ecosystem need to be investigated.”

The new research was just published in BioMed Central’s open access journal BMC Ecology.

Source: University of Natural Resources and Life Sciences Vienna

Image Credit: Garden via Wikimedia Commons