Cutworm is a general term referring to the larval stage of many night-flying miller (Noctuid) moths. Nationally the most economically important ones for potato are the variegated cutworm (Peridroma saucia), black cutworm (Agrotis ipsilon) and spotted cutworm (Amathes c-nigrum). They all have similar habits and appearance; therefore, variegated cutworm is used as the model.
Adults are called miller moths and are usually drab gray or brown but also can be somber yellow and tan.
Larvae are the cutworm which is the damaging stage. Cutworms are caterpillars that when disturbed curl their body into a tight ‘C’ appearance. They have a smooth skin and a wet or greasy texture; their body is plump. The variegated cutworm is grayish brown and lightly speckled with darker brown; it has a single row of pale yellow dots along each side of its body. The black cutworm is greasy gray or brown with faint lighter stripes and granular appearance. The spotted cutworm has a dark stripe along each side of its body and several pairs of triangular-shaped black dashes at the rear of its back. Full grown cutworms are two inches long.
Eggs are small and hemispherical laid under debris, in the soil or on leaves and stem depending on geography.
Pupae are tiny and form in the soil.
Cutworm larvae -- variegated and black
Developing larvae, cutworms, and pupae overwinter in the soil especially from previously grassy areas. Cutworms emerge in the spring. Mature cutworms return into the soil where they will dig a small chamber in which they pupate. Adult moths emerge from overwintered pupae or early-season pupae. Causing no damage, they fly around at night (attracted to electric lights), mate and lay eggs late in the afternoon or at night. Some species lay a single egg or small groups of eggs while others like the variegated cutworm lay closely-packed rows of over 600 eggs. The incubation period ranges from two to 14 days depending on species and temperature. The eggs hatch as cutworms. All cutworms have the same general life cycle; the length of stages varies somewhat. All stages of the variegated cutworm develop rapidly and three or four generations per season are possible. Others may have only one generation per season.
Initially, spring-emerged cutworms do slight damage by cutting into young stems while eating only a little bit. Unlike, armyworms, cutworms are loners; they do not travel in hordes and are not as prolific. Most cutworms only attack the stems of a few small, often weak, plants. However, the variegated cutworm and a few others will climb up the plant and eat leaves. Feeding is only at night and cooler times of the day. During the day, they hide in soil cracks, or under debris and clods at the soil surface. Their leaf feeding appears as ragged holes or cut-outs in the leaflets. On rare occasions, cutworm feeding on an exposed tuber, leaving shallow holes, has been observed. Economic damage occurs only when there is a high population with intense feeding in the middle of the season during early to mid bulking when plants tolerate up to 10% defoliation. Most foliar damage usually occurs late in the season after bulking when there is little if any effect. Since cabbage and other loopers and armyworms are seen during the day, they may be blamed for cutworm damage.
Biological -- Grassland which will be rotated to potato, should be plowed in late summer of early fall thereby reducing the number of eggs deposited. Early fall plowing and clean cultivation will remove debris on which they feed. Cutworms will die of winter starvation or even cannibalism. Do not plant immediately after stubble, grass or sod. In general, cutworms are naturally controlled by parasitic wasps and tachnid flies, and are prone to various diseases.
Chemical -- Special chemical treatment for cutworms is discouraged. Soil-applied systemic insecticides used for other pests work well. Since their damage seldom appears until late in the season, it is not economical to treat.
Adult - night-flying miller moths, usually gray or brown
Larva - smooth-skinned caterpillars, cutworms, up to two inches
Overwinter as cutworms or pupa
Up to four generation per season depending on species and climate
Foliar feeding causing ragged holes usually late in season
Fall-plowing especially of grassland
Insecticides used against other, more important, pests