Slug. Photo by Ohio State University Extension.

Get a jump on slug issues

By Matt Reese

The increase in cool, wet spring conditions in recent years has created a suite of challenges for planting and establishing early corn and soybean stands, including damage from slugs.

David Brandt has been dealing with slug issues for decades in his long-term no-till and cover crop fields in Fairfield County.

“In the 80s we were the slug capital of the world here on this farm and we’ve learned to delay planting a little bit to get the soil warmed up, get the cover crops growing really well and then plant the corn or the beans. We maybe wait a little bit to terminate the cover crop too. We found that the slugs do not like corn and beans all that much — they’ll eat the rye or the hairy vetch or the crimson clover way before they eat the corn and beans. The only reason they do is we kill everything else,” Brandt said. “Our increased insect population has actually worked really well to control the slug population. One of those is called a carabid beetle and we’ve seen them coming back on our farm with the cover crops, especially since we quit using neonics on our seed corn. We still might want to use seed treatment on corn if we are planting really early, but since we wait until the soil warms up to about 55 or 56 degrees, we plant naked seed and have got along really well the last 5 or 6 years.”

Brandt also emphasizes the importance of improving soil health in addressing many issues, including slugs.

 “We’re building the microbial herd underneath the soil. We’re also building a lot more mycorrhizal fungi and other fungi that help us. I think that’s been the reason we’ve not seen any problems with insects working on our seed corn since we’ve taken the seed treatment off,” Brandt said. “The radishes we have in the field with our cover crops tend to attract slugs. If we find a lot of them on a radish plant, we’ll really try to monitor things and we may just wait until it warms up. We’re planting a warm season grass with corn. Everybody wants to get it done in April, but that’s when it’s cool. If you can plant that corn when the soil is warm, it comes up in 3 or 4 days and the plants will outgrow the slugs.”

More extended cool, wet periods in the spring planting season, though, have set the stage for significant slug damage on a growing number of farms, especially when combined with more cover from increasing amount of heavily incentivized cover crops, said Kelley Tilmon, an associate professor in the Department of Entomology at The Ohio State University.

“I ask people what is the number one thing that pops into their head when you describe a slug. Hands go up and always every time I’ve ever given this talk, I get the same response: slimy. Yes, that is the first thing just about anybody thinks of when you want to describe a slug. Slugs need moist habitats and that is a very central fact about their biology that drives all kinds of aspects of them being a pest. Slugs thrive in habitats that are not only moist but protected, undisturbed and kind of moderate in temperature. They don’t like very high temperatures. They’re soft bodied and they need that protected environment and temperatures that aren’t too hot. They don’t want to be baked,” Tilmon said. “The type of agriculture where these conditions are particularly prominent is no-till and fields with a lot of a cover crop adoption, which is a practice that is increasingly common. I think that’s why we’re getting more and more slug problems. These types of habitats provide many benefits agronomically. They are good for weed control, erosion and soil moisture and it’s good for soil health, but then the downside is when you get this moisture and when you get this crop residue that provides that protected environment and you are opening the door for slug problems.”

As a result, more farms are seeing more slug damage in corn, soybeans and even small grains like wheat.

“We don’t always have firm estimates of damage, but a ballpark estimate in the middle Atlantic Region is a 20% yield loss in corn and soybeans from slug damage. Soybeans can actually have worse damage at the end of the day than corn. The reason for this is that the differences in growth style. Soybeans have an exposed growing point as it’s coming up out of the ground. If the slugs eat that, it’s game over for the plant and so you have a greater chance of stand loss. Whereas, if slugs are feeding on the leaves of corn, they’re not necessarily getting down to the growing point and so the plant has a better chance of recovery,” Tilmon said. “Slugs are much more prominent as a problem now than let’s say 20 years ago. The longer a farm or a field has been in no-till, the more favorable it gets for slugs. Slugs have a chance to accumulate their populations and there’s lots of soil residue to provide nice habitat. Even though slug problems are increasing, we really don’t have a lot of research on slugs as pests in field crops because it was historically less of a problem. And, because it’s only a problem in certain parts of the country, it’s not really a focus of research and development by big companies that tend to want to develop products that are good throughout the growing region for a given crop.” 

Ohio is home to a number of slug species.

“There are several species of slugs that you may find in field crops, but really the most common is the gray garden slug. Slugs are very broad in their diet. They can eat many different types of plants — they’re not picky eaters. They can also feed on weeds, cover crops and organic matter,” Tilmon said. “The slug life cycle is fairly simple. The adults can live more than one year and they lay eggs down in the soil. Grey garden slug females can lay 500 eggs per year and these eggs hatch into little baby slugs that just keep getting bigger and bigger until they are adults. The adults can live longer than a year so you can have some overlap of life stages, but generally speaking, the most damage is from juveniles that have hatched and are growing and eating, which happens early in the growing season in April, May and maybe a little bit of June. That is the typical time of real damage to soybeans and corn. 

“This time period corresponds pretty exactly with the planting for corn and soybeans, which is part of why we have this problem. In the spring you have all these juveniles. They’ve hatched, they need to put on body weight so they’re hungry at just this time you have crop planting and small plants. Slug feeding happens in the field throughout the year but you just don’t notice the damage in the middle of the summer when the plants are big and it’s trivial. But when those plants are small, that is when you see the slug problems because they’re really vulnerable to the feeding damage.”

Slugs are nocturnal, so scouting efforts should be done after dark. 

“You’re not really going to find them out and about and doing their thing during the day. You can see the damage, but it can be easy to confuse the damage with other things. If you want to really verify that it’s slugs, go out at dusk with a flashlight,” Tilmon said. “The easiest way to monitor for slugs is very low tech. You can take a small piece of plywood, maybe a few, and put them out in the field and then go out in the morning before it gets hot and lift it up. You can also do this with shingle — that’s what we do in our research. We have square foot shingles that we spray paint white so they don’t get really hot and we pin those down with a with a garden stake so they don’t blow away.”

Tilmon said slugs can be managed with tillage and there are some rescue treatments available. 

“It’s important to realize insecticides are not effective against slugs. There are other chemical options formulated as pellets that the slug has to eat. This is not a contact poison. They’re formulated to be tasty to slugs and the slugs have to eat them for them to be effective,” Tilmon said. “The most effective compound is metaldehyde — that is the active ingredient in probably most the most effective.  One example is Deadline M-Ps. There are also iron phosphate formulations — Sluggo and Ferroxx are two examples of iron phosphate. This is an organic certified material, whereas metaldehyde is not.”

Slug treatments are expensive ($20+ per acre) and will dissolve in water so they should not be applied before a rain. 

“When you’re dealing with these pellets, it’s a little bit tricky to spread them. It can be done, but you have to be careful how you do it. There are spreader devices that you can use to help get better coverage. You don’t want to clump everything in one place. You want to get a good broadcast through the field,” Tilmon said. “Spraying a nitrogen solution on the field to kill the slugs can work, but it’s a commitment. You’ve got to follow the rule of 3 with 30% nitrogen, which is a one-to-one ratio in water, at 3:00 a.m. when the slugs are out and about, for 3 nights in a row. It has to be at night because you have to spray it on the slug’s body. It’s not going to do any good if you just get it on the ground and the plants. One good study that I found showing that it can be effective had a good result at 10 gallons of urea per 10 gallons of water. Higher than that, and you’re really flirting with some nitrogen toxicity for the plants. Soybean is more sensitive to nitrogen toxicity so maybe avoid trying it in that crop, but it could work in corn.”

For more on slugs from Tilmon, go to: youtube.com/watch?v=FQ-XB5HMCJ0.

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