No. 9/May 22, 1998
Non-Insect Creatures in Corn
As people begin to scout corn fields for assessment of stands and potential presence of various "bugs" that visit seedling corn plants, they are finding an assortment of problems. We've already indicated that wireworms and seedcorn maggots have been responsible for some stand reductions in certain areas of Illinois, and we are waiting to see whether black cutworms will become problematic. However, in the meantime, some folks have found a couple of noninsect creatures in a few cornfields: slugs and millipedes.
Slugs are sporadic pests of corn, most often associated with conservation tillage practices, especially no-till. Population densities of slugs increase when residues are abundant on the undisturbed soil surface. Crop injury is most severe in the spring when growing conditions are cool and wet, so it's not surprising that slugs have been present in corn fields, at least up to now. Seed furrows that do not close completely also increase the potential for slug injury to seeds and seedlings.
Slugs feed on germinating seeds, seedlings, and foliage. Severe stand losses may occur when slugs destroy seeds and seedlings in the furrow below the surface residue. Defoliation of young corn plants may retard plant development and also cause stand loss. As slugs consume foliage on corn seedlings, they leave feeding tracks between veins, and the bottom epidermis often is intact. The result is a "windowpane" effect. Also, slugs leave slime trails on the leaves, a very characteristic sign of the presence of slugs. However, you usually cannot find the slugs easily during the heat of the day.
Chances are that as temperatures warm up and corn seedlings begin to grow rapidly, slugs will cause no economic damage. However, it is important that you diagnose the injury correctly to avoid applying an insecticide for an insect that isn't causing the problem. Millipedes are primarily another diagnostic problem.
Most millipedes are scavengers and feed on decaying vegetation; some are predaceous. However, some species injure planted seeds or growing seedlings, especially in no-till corn where the conditions are moist and the seed slot remains open after planting. They may feed on any underground portion of corn seedlings, hollowing out seeds or tunneling in roots or stems. Nevertheless, millipedes rarely cause economic damage.
Unfortunately, millipedes frequently are mistaken for cutworms or wireworms. However, millipedes look quite different from their pestiferous cousins. Millipedes are wormlike, elongate, cylindrical, and hard shelled. Often called "thousand-leggers," these insect relatives have 30 or more pairs of legs, usually with two pairs of legs per body segment. They usually are light tan to gray to dark brown, and they often are found coiled up under debris. The large number of legs distinguishes millipedes from cutworms and wireworms.
In summary, slugs likely will cause little damage in most fields this spring, and millipedes rarely cause economic damage. Tune up your diagnostic skills this spring, and avoid making costly mistakes. Kevin Steffey, (email@example.com), Extension Entomology, (217)333-6652