OSU researchers discover a worm that kills slugs

(NOVEMBER 21, 2020) A slug is "a small creature with a soft body and no legs that moves very slowly. Slugs are similar to snails but they have no shell," according to Macmillan American Dictionary. They are common in Oregon, but invasive slugs -- slugs that are not native to the area -- can also cause a lot of expensive damage to both agricultural crops and backyard gardens.

In Oregon, for example, at least half of the 20 top agricultural crops suffer from slug damage, according to Rory McDonnell. He is an associate professor in crop and soil science at Oregon State University (OSU) in Corvallis. He is also a "slug specialist" for OSU Extension Service.

However, Dee Denver says that a worm called Phasmarhabditis hermaphrodita could decrease those losses significantly. Denver is professor and head of the OSU Department of Integrated Biology. Denver is also a "nematode specialist" (a nematode is a kind of worm).

Phasmarhabditis hermaphrodita has been used in Europe for more than 25 years, but it is not registered in the U.S. by the Environmental Protection Agency. That's because the government didn't think the worm was native to the U.S. But McDonnell wants to see a change. If they can prove that the worm exists in the U.S., he says, then maybe the EPA will approve its use to control slugs. “If we can provide evidence it’s native, that makes a strong case for developing it as a bio-control. But we want to make sure there are no effects on native slugs or snails. We don’t want bio-control gone awry. That’s very, very important," says McDonnell.

Currently, growers rely on expensive chemicals called pesticides to control slugs. However, pesticides work only 10% to 60% of the time. Also, the chemicals can have unintended effects on other living things, and they can pollute waterways.

McDonnell studied the problem for three years. Then he found the worm nearby -- on the OSU campus in Corvallis. The discovery was the first in North America outside of California, where researchers at the University of California, Riverside, including McDonnell, found it in 2014.

Finding the nematode wasn’t easy. There are thousands, if not millions, of different kinds of nematodes. This one is about the size of a comma. It is almost invisible to people who don't know about it. In fact, Denver had to compare its DNA to national records in order to identify it.

The Phasmarhabditis hermaphrodita nematodes kill the invasive slugs near fields. The nematode enters the slug through a hole in its back. Then the nematode kills and feeds on the slug. The nematode then reproduces. “When a slug is infested with nematodes, it liquifies,” Denver says. “You end up with a swarming pile of worms. It’s pretty gruesome.”

The gray field slug, which bothers home gardeners as well as commercial agriculture companies, has invaded most of the world. It is the most important invasive slug species in agricultural production, McDonnell says. But in Europe, the nematode can reduce crop damage by slugs by up to 90%.

“Nematodes are abundant and diverse – there are millions of them in every aspect of the earth’s biosphere,” Denver said. “They are really understudied and, with Rory’s lab as one of a very few in North America devoted to slugs and snails, we’re in a good position to do this research.”