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Salute Soil Health: Slugs were a major issue for some this year, but it depended on location and cropping system.

November 8, 2021

4 Min Read
tiny soybean seedlings in no-till feed with slug-feeding damage
SLUG TROUBLES: Slugs attacked soybeans in this field in 2021, thinning the stand to unacceptable levels.Tom J. Bechman

For some farmers, 2021 was a bad slug year. Phone calls from across the state indicated that this was a localized issue. Some farmers were so frustrated they wanted to till up their no-till fields or stop planting cover crops. Farmers have invested time, energy and economics into rebuilding healthy, functioning soils. Losing it all to tillage because of a slug is the last thing I want to see happen!

To get a handle on this issue for next year, I talked to farmers who have been in soil health systems for 10 or more years. I asked them if they had a slug problem this year and their responses were always similar: “What slugs?” or “Yeah, I saw them, but they were never a problem.”

So, why are slugs an issue in one cropping system and not in another, and what factors could be useful in understanding how to manage for these pests? The focus is to talk about proactive measures to control slugs. Being proactive starts with understanding slug life cycles, food and shelter preferences, predators, and scouting.

Cyclical population. Slug population numbers ebb and flow over time, and when conditions are right, their numbers can explode. The most effective slug control method is a very cold winter with very little snow cover. This is the perfect slug-killing weather. The last really cold winter in Indiana was the winter of 201-18.

The spring of 2018 was cool and wet, and I was sure slugs would flare up. Nope, not a peep about slugs. Cold weather knocked their numbers down to a point where they weren’t much of a problem.

There hasn’t been a winter that cold since. Slug populations slowly increased until this year, when we heard of slug damage and replant. Yet, it wasn’t consistently an issue. If we have another mild winter with few long, hard freezes, slugs could be an even bigger problem next year.

Food and shelter. Slugs can cause damage in many different systems, but they have a greater impact in no-till and cover crop systems because there are greater amounts of food and shelter available. This does not mean your first solution is to till your field and stop growing cover crops. Instead, when you confirm that there is a serious slug population in a field, manage these numbers by reducing food and shelter.

This could mean modifying your cover crop mix to have a balance of winter-kill cover crops with cereal rye so that biomass doesn’t get as thick. Or consider terminating your cover crop 30 days prior to planting. Deeper planting depths make it more difficult for slugs to access these seeds.

The role of predators. Lots of creatures love to eat slugs, including ground beetles, firefly larvae, centipedes, spiders, frogs, birds and mammals such as moles and shrews. Any damage tillage does to slug populations will also damage predator populations. Slugs will recover much more quickly than the predators. So, if you disk to kill slugs, know that slugs lay more eggs than ground beetles, and you will never really reestablish the predator population.

Also, insecticides have a negative impact on our predator populations. A slug is not an insect. Insecticides, especially seed treatments, can unintentionally reduce predator populations and leave the door wide open for slugs to slime through.

Slug scouting. Ironically, there are no economic thresholds for slugs in the Corn Belt. This is probably because there are no effective treatment options that can be used reactively. Scouting focuses on being proactive by assessing population density when you can still adapt management practices that could impact the slug population. The best time to scout for slugs is in the fall and 30 to 45 days before planting.

Proactive or reactive? Being proactive with slugs means you are aware of the environments that slugs prefer, actively monitor slug populations and use management practices to affect those populations. Unfortunately, once you find slugs damaging crops, the immediate, reactive options are not the most effective at controlling them in the short term and usually have a high economic and environmental cost.

If you had slug problems in a field this year, learn as much as you can about slugs from farmers in soil health systems. Monitor slug populations in those fields now and again in the spring. These scouting tactics will help you get a handle on population densities and help guide your management decisions early in the spring.

Cover crop management is not one-size-fits-all. Consider managing your fields with high slug densities differently than fields without slug problems. Resilient systems with plenty of slug-eating predators take years to build.

McLain is a state soil health specialist with the Natural Resources Conservation Service in Indiana.

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