OREGON STATE UNIVERSITY

OSU SLUG RESEARCH SHOWS TIMING IS EVERYTHING

06/21/1996

CORVALLIS - It's slimy work, but someone's got to do it.

Oregon State University entomologist Glenn Fisher conducts research on slugs. He is investigating the gray garden slug and its effects on the grass seed industry in the Willamette Valley.

These voracious molluscs cause significant damage to newly planted grass seed and wheat fields in western Oregon, costing growers millions of dollars in lost yields, replanting and pest control. Gray garden slugs are also a common garden pest west of the Cascades.

The gray garden slug is native to Europe and Asia. It spends most of its time underground in the soil, especially in the heat of the summer. Slugs come to the surface at night to feed on grass seeds, rootlets and young seedlings.

"When you take a flashlight out at night, you see lots of slugs, right?" Fisher said. "We find up to 40 slugs in one crown of perennial ryegrass.

"And that's just the tip of the iceberg," he added. "Studies have shown that in the summer, only 5 percent of the slug population is above ground at any one time. The rest are underground digesting food, laying eggs and feeding on rootlets and endosperm of seeds.

"Grass and wheat farmers lose acres of seedlings to these slugs. One mature slug can take out five to 10 wheat seeds in a night. And slugs have been found as deep as three or four feet down in cracked clay, crevices and night crawler tracks."

Growers facing slug damage often have to reseed, costing them precious growing days and money.

With funding from the Oregon Seed Council and the Oregon Department of Agriculture, Fisher and his colleagues at OSU are testing different varieties of grass seed to find those least appealing to the gray garden slug. They are also investigating various cultural management practices, including rotating grass seed crops with clover, and different methods of working the soil -plowing, disking, conservation tilling and "no-till."

They are also studying chemical controls - different types of commercial baits - and applying them at different rates at various times of year to see what most effectively keeps slugs at bay.

Fisher's findings include:

-The deeper and the more often you work the soil, the fewer the slugs.

-There are fewer slugs in annual crops than perennial crops. "The more plant residue, the greater the slug populations," explained Fisher.

-Metaldehyde or metaldehyde-carbaryl on wheat bran pellet slug baits works best for grass seed growers, applied at about seven pounds per acre. Metaldehyde kills slugs by drying them out. Small, compact pellets consistently reduced slug populations the most.

-The time of year slug bait is applied can be crucial. Bait is most effective in early October, when slugs come to the soil surface to feed, mate and begin laying eggs. Summer is not the time to treat slugs on unirrigated cropland, as they are hiding most of the time. Winter and spring are too wet as slugs recover from bait under saturated soil conditions. "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."

-Bait with carbaryl alone did not control of the gray garden slug as effectively as baits with metaldehyde. "Carbaryl also greatly reduces populations of earthworms and predator beetles, which are beneficial," he said.

-In general, field burning is not that important in keeping slug populations down unless fields are burned late in the year after it has rained. "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 beneficial predators such as the predacious ground beetle, spiders and harvestmen were controlled 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 are small and resulted from eggs that survived flooding. Eggs are the stage least affected by cold temperature and standing water."