Sep. 18, 2001 COLUMBUS, Ohio -- Researchers here are working on a natural way to kill slugs that is just as effective as poison, but safer to use around plants and animals. This work may give Americans a new way to combat slugs, which destroy corn, soybeans, and other key food crops every spring and fall across the Midwest.
Parwinder Grewal, professor of entomology at Ohio State University, and his colleagues are studying tiny parasitic worms native to Europe and parts of South America. The worms feed on snails and slugs, and laboratory-grown worms are sold commercially for slug control in Europe today.
In a recent issue of the journal Biological Control, the Ohio State researchers reported that the European worms kill American slugs just as well as the most popular slug poison, metaldehyde.
"Sure, poison kills slugs very well," Grewal said, "but it can harm pets and other animals, or contaminate crops and ground water if not used correctly," Grewal said. "We want to find something safer, something useful to farmers and gardeners alike."
In a greenhouse, the researchers exposed one population of typical American slugs to metaldehyde, and another to the worms. The worms were supplied by the UK biotech company MicroBio Group Ltd., which also partially funded this study.
Each population of slugs had access to its own set of leafy Hosta plants for food.
Half the slugs in the metaldehyde group died by the end of the second day. In contrast, the same number of slugs in the worm group didn't turn up dead until day 10.
But by day 20, almost all slugs in both groups were dead, indicating that the worms were just as effective as the metaldehyde at killing slugs over that extended period of time.
In terms of controlling damage to crops, the poison and the worms were similarly effective. Over the length of the study, slugs treated with metaldehyde were able to consume 18 percent of the leaves on the Hostas in their environment. Slugs treated with the worms consumed only a little more -- 25 percent -- of their Hosta leaves.
Poison kills slugs faster, Grewal said, but it must be reapplied every 10 days or so. In contrast, one application of worms will keep killing for weeks, until all slugs are gone.
Grewal envisions that American farmers and homeowners will someday be able to order live worms by overnight mail from commercial pest-control companies, as Europeans do today. A batch of several hundred tiny worms resembles a small pile of white cottony threads, he said.
Instead of spreading poison throughout the farm or garden, people could apply a tiny scoop of worms to wherever the nocturnal slugs sleep during the day - under a piece of mulch, for instance.
Once all the slugs were dead, the worms would die off as well, Grewal said.
Larval worms of this species infect slugs and snails by burrowing under their shells and into their skin. Infected slugs stop eating immediately, and die about five days later, as the adult worms emerge. Adults also lay eggs in the host's corpse, producing another generation of larvae that will infect other slugs.
The worm of interest is known as a nematode, Phasmarhabditis hermaphrodita. Before it can be used to control slugs in the United States, researchers will have to prove to the U.S. Department of Agriculture that the worm won't harm North American plants and animals, aside from the pests.
"P. hermaphrodita seems very well tuned to snails and slugs. So far, we've seen no evidence that it can infect other animals, but we will have to determine that for sure," Grewal said.
Cost is another factor that will determine whether the worms are used in the U.S., he said. One application's worth of worms, enough to control slugs over a quarter of an acre, costs about $50.
One dose of metaldehyde for the same size area costs about $10, Grewal said, but several doses have to be applied over the course of one growing season.
Grewal's research was supported in part by the Ohio Agriculture Research and Development Center, the Horticultural Research Institute, and MicroBio Group Ltd.
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