Slugs are similar to snails in general structure, except that they have no external shell or visceral hump, the mantle being a smoother area occupying a forward fourth or third of the back. They vary in color from whitish yellow through various shades of gray to black, usually more or less mottled or marked with darker shades. At least one species has a period when it is rusty orange in color. Both snails and slugs have a definite mouth, which is equipped with a horny file, or radula, with which they rasp away the substance to be eaten. Slug damage to crops varies considerably with the season and year.
Slugs can cause serious farm, home garden, and greenhouse losses.
They damage plants by eating the leaves and tender shoots, attacking almost all field and garden crops, including grains, clovers, corn, young vegetables, ornamentals, and small fruits. They are especially attracted to ripe strawberries and tomatoes. Some slugs are also injurious in mushroom houses. Slugs leave a slime trail of mucus on the surfaces on which they crawl, and, on drying, silvery marks result which are objectionable, especially on floral or ornamental plants. Very humid environments favor the development of slugs and snails.