Public Enemy Number One: Slugs on Soybeans

Bill Cissel, Extension Agent – Integrated Pest Management,; Phillip Sylvester, Extension Agriculture Agent, Kent County,; David Owens, Extension Entomologist,

Last week, we were focused on slug injury on corn. While this is still a concern, the current weather is setting us up for another problem, possible slug injury on soybeans. Unfortunately, slugs in soybeans can be challenging to manage. The reason for this is that slugs will feed on soybeans before they emerge and kill the plants outright. In many cases, you don’t realize you have a slug problem until you return to a field, 7-14 days after planting, expecting to see a beautiful stand of soybeans, only to find that you have poor emergence or that you don’t have a stand at all.

The classic scenario where we tend to see the greatest problem is when soybeans are no-tilled into heavy crop residue and planting conditions were a little wet and the seed trench did not get closed all the way. Slugs will travel up and down the open seed furrow, using it like a “highway”, to feed on the germinating seeds. With the recent rains and stall in planting, we know many of you will be anxious to get back to planting and may be tempted to plant when conditions are a little wet. If you can relate and your fields have a history of slug problems, you might be setting yourself up for a slug problem.

Pay close attention to planter setup as there are several adjustments that may help. Row cleaners move residue away from the emerging seeds and help with uniform planting depth. Soil should cover the seed trench, not residue since it serves as a hiding place for the slugs. On the back end of the planting unit, be sure the closing wheels are completely closing the seed trench. The standard rubber-edged closing wheels that came with your planter work by down pressure to close the seed furrow. This might work in drier soil or under tilled conditions, but perform poorly under marginal conditions (think wet, clay soils) and will not properly close the seed trench. Several aftermarket closing wheels are available, commonly a variation of a spike-type wheel that collapses the seed trench from the side rather than from the top, which may provide a better close in sub-par conditions.

“Slug highway” left when the seed trench isn’t closed all the way.

Below ground slug feeding injury to cotyledons.

Once the soybean plants emerge, slugs will continue to feed on the cotyledon, unifoliate, and trifoliate leaves. Above ground slug feeding injury can be confused with bean leaf beetle damage so look for slugs and “slime trails” to make sure you accurately identify the culprit. Checking for slugs with a flashlight after dusk is another good option as they are most active during the night.

Above ground slug feeding injury. Check for slugs under residue close to the plant.

Above ground slug feeding injury. Notice heavy feeding on the cotyledons. Slugs will also feed on the unifoliates and trifoliate leaves.

Above ground feeding injury. Checking after dusk is a good time to catch the culprits in action.

Tillage is the most effective cultural control method if a field has a slug infestation, especially in re-plant situations. We have limited experience with how well “slug bait” can control slugs when they are feeding on germinating plants before they emerge and if slug bait is effective at reducing damage in a re-plant situation.

Here is a report from our 2013 Delaware Soybean Board funded project: Management of Slugs in Delaware Soybean Fields. This report has more information evaluating the effectiveness of chemical control management of slugs in soybeans:

Another tactic, aside from tillage or making sure the seed furrow is closed, is to adjust your planting date. Planting early, ahead of egg hatch allows the plants to emerge before heavy slug feeding injury occurs from juvenile slugs. Once the slugs hatch, delaying planting has been effective by allowing plants to rapidly emerge. Unfortunately, hitting the perfect planting date is difficult because it is a moving target. What is considered “early” this year isn’t what we considered “early” last year, and probably won’t be what we consider “early” next year. We have already seen healthy populations of newly hatched grey garden slugs in fields throughout the state. In general, at this point in time, anything that will promote rapid seed germination and emergence will help to get the plants out of the ground before slugs have an opportunity to kill the plants outright. In some cases, that is half the battle.

To help determine if your field has a slug infestation, you can actively sample your fields for slugs using shingle trapping methods, searching under crop residue, and sampling for slug eggs.

Click on the links below to watch several Youtube Videos about slug injury on soybeans and demonstrating each of these sampling methods:

When and Where Slugs Can Be a Problem in Soybeans:

How to Sample for Slugs in Soybeans Using Shingle Trapping and Residue Sampling Methods:

How to Sample for Slug Eggs in Soybeans:

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