Fight Back Against Grain Crops’ Slimiest Pest

Del Voight, a Penn State Extension educator, speaks about slugs at the Keystone Crops and Soils Conference.

GRANTVILLE, Pa. — Slug damage shows up as shredded leaves and reduced stands every year in Pennsylvania grain fields.

Now that many farmers leave a lot of residue in the fields, “we’re going to see more and more of this,” said Del Voight, a Penn State Extension educator.

Voight spoke about slug control at the Keystone Crops and Soils Conference on Tuesday at the Holiday Inn Harrisburg-Hershey.

The worst thing a farmer can do in a slug-plagued field is plant when the fields are wet and smeary.

The planter will leave a narrow trench that the slugs can chug right along, chewing on plants and potentially causing a complete crop failure, Voight said.

Slugs will take out the soybeans’ cotyledons, or seed leaves, which can lead to stunting.

“That translates to yield (loss) when they’re feeding on those cotyledons,” Voight said.

If the corn can get out of the ground, slug damage is not generally a big deal, he said.

In one field where slugs had attacked, the young corn was a leaf shorter than the corn in an adjacent field where slugs were not a problem. At the end of the year, though, “there was no difference in yield,” Voight said.

Slugs love residue, and they infested the field where the farmer had left the corn fodder on the field. He had baled the fodder off the field that escaped injury, Voight said.

Slugs are active on wet, calm nights down to 33 degrees F. At other times, they sneak into the soil as deep as 4 feet. “They don’t move over dry soil,” Voight said.

Juvenile slugs are hard to control but are particularly destructive to plants.

Farmers may be able to avoid young slugs by planting before or after the hatching period — typically the end of April and beginning of May, he said.

Some researchers are working a model that would tie slug hatching to growing degree days. “I think that would be useful,” Voight said.

To get the crop out of the ground faster, farmers can use row cleaners, and apply a starter or pop-up fertilizer. Soybeans can also be planted shallowly, no more than an inch deep, he said.

Tillage controls slug populations by breaking up the mollusks’ soil cavities and exposing them to the elements. “On a windy day, you can desiccate them really quickly,” Voight said.

Tillage has fallen into disrepute, but farmers may be able to fight slugs by doing vertical tillage once a year, he said.

However the field is planted, farmers need to avoid sidewall compaction, he said.

If slugs become an issue during the season, farmers can spray UAN and Lannate.

The slugs need to be on the surface to be affected, so nighttime applications are likely to be most effective, Voight said.

Lannate usually provides only two to four days of control, he said.

Metaldehyde is another spray option. The product will dehydrate and kill slugs and snails, but is a low risk to beneficial organisms, Voight said.

Bait pellets are best applied after rain, irrigation or dew. They are ineffective in dry soil.

If the farmer finds new eggs after a bait treatment, a second treatment will be warranted in one to three weeks, Voight said.

For any slug treatment, it is best to treat the whole field, even if only part of it is infested.

Slugs can move 20-30 feet in a night, so they can easily flee to parts of the field that were not treated, he said.

In organic systems, biocontrol is the main strategy. Some companies supply beetles that prey on slugs, Voight said.

Salt, burnt lime and iron phosphate products are also OK for organic production, he said.

Cleaning machinery can reduce the chance of transporting slugs from field to field.

Slugs will cling to almost anything, and once they are in a field, “you’re not going to get rid of them,” Voight said.

Neonicotinoid seed treatments get into the mucus layer of slugs and can kill slug predators.

In fields where there is a severe slug issue, farmers may want to skip insecticide seed treatments until the predators come back, he said.

Planting into a living cover crop sounds like a good idea, as the slugs would theoretically focus on the cover crop.

Voight has found, though, that planting into lush vegetation can lead to hairpinning, where the trash is pushed into the furrow, reducing seed-soil contact.

Farmers will not be able to eradicate slugs from their fields, but they can keep the slimy menaces from causing economic damage, he said.

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