Corn and soybean plants may be injured by insects and noninsects alike. Slugs, although sporadic, are noninsect pests of both corn and soybeans. Research conducted by Ron Hammond, Ohio State University, has shown that primarily four species of slugs are numerous enough to be considered of potential economic importance. The gray garden slug is the predominant slug species found in fields, followed by, in order of abundance, the marsh slug, the dusky slug, and the banded slug. Slugs are more commonly associated with conservation tillage practices such as no-till, especially when growing conditions are cool and wet.
Slugs overwinter as both adults and eggs. Egg hatch occurs in late May to early June; feeding by juveniles soon follows. Slugs feed mainly at night, hiding in cool, moist places (under debris) during the day. The telltale sign of slug feeding is the presence of slime trails from their movement across the foliage. Slugs rasp the surface of the seed or plant foliage when they feed. This causes the streaks or holes on both the seed and leaves. Injury to plants may occur before emergence, reducing stand establishment. Slugs may hollow out germinating seeds, causing injury similar to wireworms. Stunted, lower-yielding plants may result from slug feeding, and heavy defoliation may result in plant death and stand losses. Planting into wet soil often causes the failure of seed furrows to close completely and increases the potential for slug damage.
Slug activity is generally greatest in late spring and early summer. As conditions become hot and dry, slugs enter an inactive state and then become active again in the fall before hibernating in the winter. Overall, slugs will likely cause little injury in most fields. Both corn and soybeans can tolerate some injury from slugs; however, it is important to note that there are very few economically viable rescue treatment options for slugs. Limited spot treatments with a molluscicide (metaldehyde) may be an option in severely infested areas of fields.
Switching to a reduced or conventional tillage program the following year will help decrease slug populations. Overall, it is important to correctly identify any problems that might be occurring in cornfields and soybean fields this spring, saving yourself from treating the wrong problem, or a problem that isn't there.--Kelly Cook