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Slug Wars

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Some gastropods do more harm to crops than we realized, an OSU scientist is finding.

Western Oregon is a paradise for slugs. With volcanic soils, lush vegetation and six or more months of dampness each year, what more could a naked terrestrial gastropod wish for?

Take a flashlight out on a warm night into a west side garden and you'll most likely see dozens of slimy slugs, silently consuming your growing vegetables. It's not pretty.

Those few slugs you see glistening in the beam of your hand-held light are just the tip of the iceberg, explained Glenn Fisher, an Oregon State University entomologist who is studying slugs that are agricultural pests in Oregon's Willamette Valley.

"In the summer, only 5 percent of the slug population is above ground at any one time," said Fisher. "The rest are underground digesting food, laying eggs and feeding on rootlets and endosperm of seeds."

Not all our slugs are pests. Many, including the infamous quarter-pound banana slug, are native species that play important roles in our ecosystems, serving as seed and spore dispersers, nutrient recyclers and soil builders. Slugs are food for many other creatures, including garter snakes, spiders, ground beetles and birds.


Though some slugs torment farmers and home gardeners, the creatures have become cultural icons of the Pacific Northwest. Here, seven years of "Slug Queens" line up for festivities at the annual Eugene Celebration in Oregon's second-largest city.

Cultural icons of the Pacific Northwest, slugs are part of our regional identity--we obviously think of ourselves as a little bit different. A revered "Slug Queen" is crowned each year in Eugene, Oregon's second-largest city.

It's the non-native species of slugs that spell trouble for home gardeners and farmers. There are 10 species of pest slugs in Oregon. All but one of these, the tiny marsh slug, were accidentally introduced from Europe or Asia. Hitching a ride on imported plant materials, these foreign slugs left many of their natural predators back in the old country. They have flourished in their New World habitats.

Close-up photo of a banana slug.

The banana slug, native to the Pacific Northwest, is the largest slug in North America. It breathes through the big hole in its side. Photo: Tom and Pat Leeson

To what extent these "new" slugs have wreaked havoc on native Pacific Northwest ecosystems is largely unknown. But we do know that they have significantly affected western Oregon agricultural systems and home gardens.

The non-native gray garden slug (scientific name: Derocerus reticulatum) is the number one pest of dryland crops in the Willamette Valley, according to Fisher. Hailing from northern Europe and Asia, these small, gray to brown slugs feed on most grass and cereal crops, from seeding through maturity, as well as legumes, nursery crops and vegetables. They reduce yields and increase production costs in many ways, including creating a need to reseed and apply costly baits for control. Slug-damaged plants suffer increased competition from weeds, increased disease outbreaks and more nematode infestations. Fruit and vegetable processors reject slug-damaged produce.

Historically, grass seed growers have burned many of their fields after harvest to rid fields of pests, crop residues and diseases. But field burning has been a source of controversy for Willamette Valley residents with concerns about reduced air quality. In 1988, smoke from a fire near Interstate 5 caused a fatal, multiple-car accident, intensifying the debate over field burning. The state restricts the number of acres farmers can burn.

"Grass seed growers have been looking for alternatives to field burning," explained Fisher. "Farmers, fieldmen and OSU Extension workers have observed greater slug populations and problems with increased straw left after harvest--They are now very interested in learning more about these slugs, since slug baits are among the 'top ten' most commonly used products for pest control in general in Oregon, including those used for insect control."

Man looking through glass panel filled with slugs.

OSU researcher Joe DeFrancesco sizes up his study subjects. Gray garden slugs like these do millions of dollars of damage to western Oregon crops and gardens. Photo: Bob Rost

Fisher and research assistants Joe DeFrancesco and Rene Horton have been working with growers and the U.S. Department of Agriculture to:

  • evaluate different on-farm techniques farmers can use to cope with hungry slugs.
  • help the growers deal with after-harvest grass seed residue that previously they would have burned.
  • test different varieties of grass seed to find those least appealing to the gray garden slug.
  • investigate farming practices that discourage slug populations, such as rotating grass seed crops with the oil seed crop meadowfoam, plowing or disking and "no-till" farming (planting without much plowing).
Woman kneeling in a field at night looking at a slug bait station.

Researcher Rene Horton counts the number of gray garden slugs coming to a bait station at a study site in a Linn County perennial ryegrass field. Photo: Bob Rost

  • determine which types of commercial baits, applied at which rates and times of the year, attract and kill the most slugs per unit area.

Fisher and his colleagues sometimes find more than 40 slugs in just one crown of perennial ryegrass. (The crown is the area where the plant's stem and root merge.)

"Grass and wheat farmers have literally seen acres of seedlings lost almost overnight when slugs reach these numbers," he said. "One mature slug may destroy or destructively weaken as many as five newly planted cereal seeds or grass seedlings in a night. And slugs have been found as deep as three or four feet in cracked clay, crevices and night crawler tracks."

Quite a lot of what we know about slugs comes from Europe, where biological and applied controls have been studied and reported for decades.

In the Pacific Northwest, OSU entomology professor H. H. Crowell, now retired, contributed most of what is known about our pest slugs. But much about their habits and ecology remains elusive, said Fisher.

Hand holding many tiny slugs.

For every gray garden slug we see above ground, there are about 20 more below the soil, say entomologists. Photo: Bob Rost

"I can't over emphasize that an understanding of the biology and behavior of an organism is an absolute must, before economical and sustainable controls can be developed," he said. "For instance, we are trying to learn more about the natural predators of slugs, such as ground beetles and harvestmen ('Daddy Long Legs'). We feel that they may play a significant role in regulating slug populations. There may even be some possibility of biological control of slugs with nematodes some day."

Fisher explained that the Achilles' heel in a slug's life history is that it needs high relative humidity and undisturbed soil to develop and reproduce. Because of these needs, the gray garden slug spends most of its time underground, especially in the heat of the summer. It comes to the surface at night to feed.

"We've learned that if you can coordinate baiting with its life cycle, you get much more effective results," he said. "The best time to bait in unirrigated cropping systems of western Oregon is in the fall, as the days shorten and cool and our trademark rains begin. This weather brings the lazy summer forms to life to the soil surface for mating and subsequent egg laying. Those cool, damp nights followed by sunny, warm days greatly enhance the action of the baits, which largely kill slugs via dehydration."

The sex life of a slug might make even controversial magazine publisher Larry Flynt blush. Every slug is equipped with male and female reproductive organs, a property that biologists term "hermaphroditic."

Tractor in a grass seed field.

Grass and legume seed contribute about $250 million dollars a year to Oregon's economy. The gray garden slug is the number one pest of dryland crops in western Oregon. Photo: OSU Extension and Experiment Station Communications

Rather than mate with themselves, slugs seek out another individual of the same species, often coupling doubly. Sperm exchange is often preceded by lengthy and athletic courtship rituals. A single slug lays up to 500 pearly white or golden eggs under organic debris or in the soil. These eggs hatch after three to eight weeks and grow to maturity in a few months.

So far, Fisher, DeFrancesco and Horton have learned the following about the gray garden slug:

  • The deeper and more often the soil is worked, the fewer the slugs.
  • Annual crops host fewer slugs than perennial crops.
  • There are fewer slugs in grass seed fields cleaned of crop residue than those with straw remaining.
  • A substance called metaldehyde, alone or formulated with wheat bran into pellets, can be extremely effective in killing slugs. Pelleted baits of varying sizes and densities are used in grass seed. One or two applications of five to seven pounds per acre can give good control. Small, compact pellets are probably better than the larger and loosely compacted pellets after rains start in earnest in the fall.
  • As mentioned earlier, the time of year you apply slug bait is crucial.

"Slugs lay eggs in the fall after the rains start," explained Fisher. "If you can kill the slugs before they lay eggs, you have won half the battle."

For growers who do not irrigate in the summer, bait is most effective when broadcast in late September or early October, when slugs come to the soil surface to feed, mate and begin laying eggs. However, in regularly watered gardens and row crops, slug bait is helpful anytime, as the slugs don't hide underground as much as in unirrigated sites.

Winter and spring rains can make an otherwise effective bait perform miserably.

Man looking at piece of sod in a laboratory.

OSU entomologist Glenn Fisher examines soil plugs taken from grass seed fields. He's looking for slugs in the samples. Photo: Bob Rost

"Some researchers feel that slugs may actually rehydrate and recover from bait poisoning during rainy, wet periods," Fisher said. "Bait shyness has also been seen. Slugs avoid feeding on baits if they consumed a sublethal dose the first time."

Bait with a poison called carbaryl alone does not control the gray garden slug as effectively as bait with metaldehyde alone, or bait with metaldehyde and carbaryl. Apple pomace-based baits are not very effective on slugs in Oregon. And carbaryl in a bait greatly reduces populations of earthworms and predator beetles, beneficial creatures in farming and gardening.

In general, field burning may not be as important in keeping slug populations in check as previously thought.

"Open field burns simulated in grower field plots often had more slugs than when the residue was removed in other ways," said Fisher. "We think that natural predators of slugs such as ground beetles and harvestmen--which tend to be active on the dry soil surfaces, unlike slugs--may have been reduced or pushed from the plots during burning. Field burning often occurs in the summer, when the slugs are quite deep and insulated in the soil."

Flooding greatly reduces slug populations, at least temporarily.

"Last winter's floods caused tremendous slug population crashes," said Fisher. "Only this spring did they begin to reappear. Most of these slugs came from unhatched eggs that can withstand long periods under water or populations of mobile slugs that made it to a high point in the field."

It's not always glamorous being a slug researcher, but according to Fisher, DeFrancesco and Horton, those early mornings slogging through the mud, counting dead slugs and digging plugs of grass and soil to take back to the lab do have their charms.

"It's always interesting," Fisher said with a laugh. "Slugs are really interesting creatures biologically. It's slimy work, but someone's got to do it."

Published in: Ecosystems