Slug Damage in Soybean

What are slugs?

Slugs are slimy mollusks closely related to snails but without a shell. Their bodies are soft, legless, and have four front tentacles – of which two have eyes and two are essentially antennae (Figure 1). They secrete mucus that aides in movement and defense, and that also serves as an identifiable clue to their presence.¹

Of the many slug species, which ones are potentially the most economically injurious to crops?

The gray garden (Deroceras reticulatum), marsh (Deroceras leave), dusky (Arion subfuscus), and the orange-banded (Arion fasciatus) slugs are the most commonly found species in crop production. Of these, the dominant and most economically important in agricultural crops is the gray garden slug. The gray garden slug is light to dark gray, about two inches long as an adult, and has a mottled appearance.¹

What is the lifecycle of a slug?

A slug has three growth stages – egg, juvenile, and adult. The eggs are small, gelatinous, oval-shaped, and are usually found on crop residue or in the soil. Depending on the region, slugs may overwinter as eggs or adults, though extreme cold winters are likely to eliminate all overwintering stages unless there is a good snowpack.² The overwintering adults lay eggs in early spring after temperatures support their activity. This activity could begin in early April in southern areas and will likely occur later in northern areas. Juvenile slugs resemble small adults as soon as they hatch from overwintering or newly laid eggs. The juveniles generally start feeding on nearby plants within one to two weeks after hatching, reach maturity in late summer or early fall, mate, and – depending on location – may lay eggs. The lifespan of the gray garden slug is about twelve months.¹

The lifecycle of the slug species may vary, regarding egg laying time and the overwintering stage. Various stages of the gray garden slug can be found throughout the growing season as the stages of development are not well synchronized across a population.

Which fields are more likely to have slug populations?

Slugs prefer a moist, cool, and undisturbed environment which can be provided with the residue of conservation tillage systems. Reports from the mid-Atlantic states indicate that about 20% of no-till fields suffer economic losses and 47% of growers indicate that considerable slug damage occurs every two to three years.¹

How do slugs injure plants?

Along with soybean, slugs feed on a wide range of plants including cereals, corn, canola, alfalfa, and cover crops. Usually, the most injury to field crops – including soybean – occurs soon after planting, when the juvenile gray garden slugs are searching for food. Injury can appear as scarifying on the surface of plant tissues, holes in leaf tissues, and holes in soybean cotyledons (Figure 2). The growing point for an emerging soybean seedling is within the cotyledons where they connect to the hypocotyl (stem). Should slug feeding injure the growing point or if the cotyledons are chewed off at their attachment point, seedling death is likely.

When should scouting occur?

Prior to planting, look under and on field residue for eggs and adults. After planting, seedlings should be scouted for juvenile slug feeding. The soil may show signs of the slimy trails that slugs leave behind as they move around. If feeding is occurring on the seedlings, observe the cotyledons for chewed out depressions or holes (Figure 1) and leaves for holes or scarification. Observation for slugs after planting is best completed at dusk or later with a flashlight, as slugs avoid sunshine and heat. However, on cloudy, cool, wet days, they may be found feeding during daylight. Another way to check for slug presence is by placing a piece of cardboard, shingle, wood, or other object on the soil surface to create a cool, wet environment. After a few days, pick up the object and observe the soil surface for the adults and juveniles.

Are there any management techniques?

In-season economic thresholds are unavailable for slugs. Research has shown that soybean seedlings can withstand at least 50% defoliation of unifoliate and first trifoliate leaves without losing yield potential.³

In-season options to control slugs are limited. Molluscicide baits containing a concentration of 3.25% to 4% metaldehyde may be available for management; however, they are water soluble which can limit their longevity. Baits containing iron phosphate are another potential option.^3,5,6

Salt from fertilizers can desiccate slugs rapidly if applied at night when the slugs are active, as the slugs must encounter the salt for it to be effective. Two salty fertilizer options include dry potash (0-0-60) at a rate of about 30 to 40 lb/acre and liquid UAN (28% to 32% N) at a rate of about 10 to 15 gallons/acre when used as an herbicide carrier. However, leaf burn may occur and UAN is most often used as a carrier with preplant and preemergence applications. Foliar fertilizers are likely to contain too little salt to desiccate slugs.⁴

Tillage can disrupt and expose slugs to the elements and is a good way to reduce infestations. However, tillage is an unlikely solution for dedicated conservation tillage producers. Row cleaners on the planter may help move slug-protective residue away from the crop row and help warm the soil for quicker soybean seedling emergence. Seedlings that grow quickly can become too large for slug feeding to cause appreciable injury.

Seed row closure is very important, as seed rows that are open allow slugs to move freely from one plant to the next. Planter row closers should be adjusted to help assure soil is moved on to the seed row. Proper closure can also help keep seeds protected from birds and other wildlife. Seeds in an open seed furrow could also be fed upon.

Planting before slugs appear may help soybean seedlings become established and become too large for economic injury from slugs. Alternatively, planting later after soils have warmed and become drier may reduce the potential for feeding as dry soil is not conducive for slug development. Warm weather, sunshine, and dry soils are likely the best way to reduce the potential for slug feeding and potential yield loss.

If the soybean plant population is greatly reduced, replanting may be an option. However, replanting should be evaluated based on a measured population, extra inputs (seed, chemistry), labor, weather forecast, and other factors. For additional soybean replanting information, please see Soybean Replant Decisions.

Sources

¹Douglas, M.R. and Tooker, J.F. 2012. Slug (Mollusca: Agriolimacidae, Arionidae) ecology and management in no-till field crops, with an emphasis on the mid-Atlantic region. Journal of Integrated Pest Management. 3(1): C1–C9. https://doi.org/10.1603/IPM11023

²Douglas, M.R. and Tooker, J.F. 2010 (updated 2023). Slugs as pests of field crops. PennState Extension. https://extension.psu.edu/slugs-as-pests-of-field-crops

³Igwe, P.-G., Cramer, M., Owens, D., Dively, G., and Hamby, K. 2023. Managing slugs in field crops using IPM principles. University of Maryland Extension, Agriculture & Food Systems. FS-2022-0629. https://extension.umd.edu/extension.umd.edu/sites/extension.umd.edu/files/publications/ManagingSlugsInFieldCrops_FS-2022-0629_ada.pdf

⁴Lee, C. 2017. Slugs in soybeans. University of Kentucky College of Agriculture, Food and Environment Cooperative Extension Service, KYGrains.info. https://www.kygrains.info/blog/2017/5/31/slugs-in-soybeans

⁵Cochran, R. and Tilmon, K. 2023 revision. (original 2014 authors: Hammond, R.B., Michel, A., and Eisley, J.B.) Slugs on field crops. Ohio State University Extension, Ohioline. ENT-20.https://ohioline.osu.edu/factsheet/ENT-20

⁶DiFonzo, C. and Tilmon, K. MSU-OSU field crops insect pest management guide. Michigan State University and The Ohio State University. https://aginsects.osu.edu/sites/aginsects/files/imce/MSU%20-%20OSU%20Insect%20IPM%20Guide.pdf

Web sources verified 4/1/25. 1315_125601