Several species of slugs (gastropods) are common in fields and gardens throughout the United States. Although slugs are present in every corn field in Pennsylvania, damage to the leaves of young corn plants will most likely occur in no-tilled fields during periods of cool, wet weather in May and June.

Slugs feed on practically all crops grown in the Commonwealth. They are frequently serious pests in home flower and vegetable gardens and often cause damage to ripening strawberries. Extensive feeding and shredding of small corn plant leaves during early growth stages are often severe. However, young corn plants seldom die because the growing point usually escapes damage.

Mature slugs are mostly gray to brownish-gray, and are legless. They have two pairs of retractable front tentacles with eye spots on the tips of the larger pair. They are soft bodied, rather plump, and vary considerably in length from 1/2 inch up to several inches; most are 3/4 to 1 inch. All slugs are covered with a slimy, milky film of mucus and are extremely difficult to hold between the thumb and finger. The eggs lack color; have a watery, gelatinous appearance; are round to oval in shape; and between 1/8 to 1/4 inch in diameter. Newly hatched slugs closely resemble adult slugs, except for size. They are not much lager than alfalfa seeds and start feeding on plants immediately after hatching. Both young and adult slugs prefer environments with high humidity and relatively cool temperatures. They feed mainly at night and hide in the soil or under trash and debris during the day.

There is one generation and possibly a partial second generation per year of the several species. Slugs overwinter in either the egg or adult stage, depending upon the species. Juvenile slugs hatch from eggs in the spring soon after the soil warms. They reach sexual maturity by mid- to late summer and start depositing eggs when climatic conditions are favorable. Each individual slug possesses both male and female organs. It is capable of producing clumps of 10 to 20 eggs, which are laid in crevices in the soil, under debris, and in other protected locations. The majority of eggs are deposited during September and October.

Slugs move on their bellies, slowly and not very far. They feed by rasping the leaf tissue, causing the leaves to have a ragged, shredded appearance. All slugs leave a trail of slime wherever they move. The trail has a silvery, shiny appearance when dried: a telltale sign of the presence of slugs nearby.

The lower leaves of corn plants are the ones most severely damaged. In late summer, slugs occasionally will feed on brace roots of corn plants. What effect this feeding has on the plants is not known. Personal observations indicate that slugs feeding on brace roots have not generally affected plant performance. However, extensive feeding can prevent brace roots from developing and cause plants to become lodged.

Slug damage can make young corn plants sickly looking, but they usually recover. We do not have data to substantiate what effect slug damage has on field. At this time, we know of no practical or economical method of slug control that is registered for use in field corn. Refer to the current issue of the Penn State Agronomy Guide for registered pesticides, dosages, and methods of application. You also may wish to consult your local county agricultural Extension agent or farm supply dealer about seed treatment materials.

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Authored by: Stanley Gesell, Extension Entomologist, 1983

Dennis Calvin, Professor

Last updated April 20, 2000

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