Essential oils from thyme and spearmint are proving lethal to crop-damaging slugs without the toxicity to humans, animals or the environment that chemical solutions can present.

An added advantage of these oils is the rapid mortality they cause in slugs, whereas one of the most common chemical molluscicides used by Oregon farmers, iron phosphate, simply causes them to stop feeding, said Rory McDonnell, Oregon State University’s slug specialist.

“The oils were essentially just as effective as metaldehyde,” another common molluscicide, McDonnell said during the Oregon Seed League’s annual meeting, held in Salem, Ore., on Dec. 10-11.

Thyme and spearmint oils achieved 100 percent mortality at a concentration of just 0.25 percent, most likely through direct contact with slugs — though it’s possible their volatile emissions could also serve as repellents for the pest, McDonnell said.

Because they’re natural compounds, these oils would be exempt from the U.S. Environmental Protection Agency’s registration and residue tolerance regulations for conventional pesticides, he said.

Before they could be commercialized as biological pesticides, data would need to be submitted to the Oregon Department of Agriculture proving they’re not toxic to humans or non-target organisms, though this should not be a big obstacle, McDonnell said.

“I’ll eat my hat if it’s toxic,” he said.

McDonnell was hired by OSU in 2016 after Oregon farmers told the university’s leaders that more research was needed to fight slugs, which have become increasingly destructive in recent years.

Another positive development from McDonnell’s research is the discovery of a nematode that’s naturally parasitic to grey field slugs — phasmarhabditis hermaphrodita — on OSU’s campus in Corvallis, Ore.

The location if the discovery was ironic given that McDonnell had traveled thousands of miles around the state searching for the species, which is native to Europe and used in slug control there.

“The darn thing was a stone’s throw from my office,” he said.

Since then, McDonnell has discovered two other nematode species in Oregon that show promise as biological control agents.

In the United Kingdom, the phasmarhabditis hermaphrodita nematode is sold as a commercial biopesticide that’s been shown to reduce slug damage in winter wheat by 85 percent, he said.

The nematode finds a hole in the back of a slug’s head, then vomits up a bacterial soup that’s toxic to the gastropod. As the slug’s body decomposes, the nematode’s offspring feed on its corpse.

The BASF chemical company also markets the nematode in Europe, producing it in enormous vats through a secret process, McDonnell said.

Before the nematode can be commercialized in the U.S., BASF or another pesticide manufacturer would need to demonstrate to USDA that it’s not harmful to other species, such as the native banana slug.

“I think that would be a major stumbling block,” he said.