Appearance and Life History
Slugs are found throughout the eastern United States in almost every cropping system. Slug activity is favored by plant residue on the soil surface and moist conditions.
Slugs are soft-bodied, legless, slimy, white, gray, or black gastropods (not insects). Some species measure up to 4 inches (10 cm) in length but 1/2 to 1-1/2 inches (13 to 38 mm) is more common. Slugs are actually snails without shells.
In the spring, translucent round eggs are laid in masses in damp places. These eggs hatch in about a month. The juvenile slugs are very similar to mature slugs except for their size. They develop slowly and live a year or more. Slugs overwinter as both adults and eggs in soil in the Midwest.
Slugs are active at night and cloudy days. During the day they often hide under soil clods and crop residue or in unsealed seed furrows. They leave a slimy, silver-colored trail wherever they go. Slugs feed on decaying organic matter and plant foliage.
Slug populations tend to be greatest in conservation tillage systems and very weedy fields. This occurs, because the moisture required by these animals is preserved by high surface residue and the lack of soil disturbance.
Slugs usually feed on the lower part of the plant, eating partly or completely through the hypocotyl and cotyledons. Unifoliolate leaves may be damaged before unrolling, making them appear distorted and tattered. Stand losses by slugs occur when fields are generally too wet, and not ideal for planting, thus resulting in seed furrows that do not properly close during planting operations. In this situation, slugs are able to feed day or night on the seedlings, often destroying the growing point(s).
Sampling for slugs in soybean is not to be considered routine. If damage is suspected or seen, check 20 plants in each of 5 areas of the field. Record the number of plants showing damage and/or dying.
Slug counts can be taken day or night. To check for the presence of slugs during the day, look at 5 linear feet (1.5 m) of row in each of 5 areas of the field. Check on each side of the row until 5 feet (1.5 m) of row have been covered. Turn over and inspect clods and pieces of plant residue in each sampling area for slugs. Slugs in the soil can be difficult to find since they contract their bodies into small spheres resembling little balls of goo. Record the number of slugs found in each sampling area. Night sampling can be accomplished by counting slugs on plants or the soil surface. Use a flashlight to illuminate the sampling area. Keep a record of all samples taken and note if taken during the night or day.
Soybean Insect Control Recommendations: E-series 77-W (PDF)
Reducing crop residue in slug history fields by tillage discourages slug-buildup. Zone tillage or row sweepers at planting may reduce slug damage by encouraging soybean growth and the drying of the soil. Early planting and crop emergence, before spring egg hatch, will allow most seedlings to advance beyond the soybean’s vulnerable stage to slugs. However, this practice will only be successful if soils are suitable for planting. Poorly sealed seed furrows can result in severe slug damage and crop stand losses. Where replanting is necessary, the field or area should be tilled first to disrupt the slugs’ environment.
The following table may help in making replanting decisions.
There are no established economic thresholds or rescue treatments for slug control. However, if the stand is being threatened, control is warranted if the slugs are still present and feeding and weather conditions (rainfall) still favor slug activity. Insecticides do not normally control slugs because slugs secrete a protective layer of slime over their bodies. Pelleted chemical baits are available for slug control. However, these are expensive and difficult to evenly dispense [i.e., 5-8 pieces per square foot (0.09 square meter) for one product] over the field.
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.