Can Earthworms Help Prevent Slug Damage?

Information provided by  Meta®/Lonza, a sponsor of the Yard, Garden & Patio Show and “Earthworms in Your Garden May Help Prevent Invasive Slugs from Devouring Plants,” a blog by Mohi Kumar for Smithsonian.com, May 16, 2013.

Photo: Robin Rosetta, Oregon State University

Scientists at the University of Natural Resources and Life Sciences Vienna found that earthworms can help protect plants from the voracious appetites of slugs (which are colorfully described in Mohi Kumar’s above referenced blog as “fleshy, squishy bodies [that] are basically one huge stomach on a foot”). The study was conducted using Spanish slugs (Arion vulgarisI), but it probably isn’t too much of a stretch to think that varieties in our neck of the woods would respond similarly.

The earthworms and slugs in the study didn’t actually come mouth-to-mouth…or whatever the correct anatomical analogy is to the human equivalent of face-to-face. “Instead, the mere presence of earthworms reduced the number of leaves damaged due to slugs by 60%,” according to Kumar and the study by Zaller et al titled “Herbivory of an invasive slug is affected by earthworms and the composition of plant communities.”

The study, published in the journal BMC Ecology, also found that higher plant diversity decreases the likelihood of leaf damage from voracious slugs. Research results noted that slugs ate even less—40% less—when earthworms were present in the higher plant diversity test.

Dr. Johann Zaller, who led the research, explained the outcome of the study this way: "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 [that slugs may find less palatable]. 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."

Encouraging earthworm populations to protect against slugs and planting a diverse mix of plants probably isn’t enough to keep slugs at bay or even under control in the wet Pacific Northwest. But it certainly gives us another good reason to nurture earthworm populations!

Other studies have shown that metaldehyde (Meta®) does not harm earthworm populations whereas other slug bait formulations may cause harm. Snail and slug baits that contain Meta® active ingredient include That’s It™, Bug-Getta® and One and Done®, which can be found at home improvement and home and garden centers throughout the Pacific Northwest.