Caterpillars (Lepidopterous Pests)

Beet armyworms lay their eggs in distinctive cottony masses on leaf surfaces. Newly hatched beet armyworms are small, green worms that often feed in groups. Older beet armyworms vary in color, but usually have many fine, wavy, light-colored stripes down the back and a broader stripe down each side. The body appears hairless.

Loopers arch their backs as they crawl. Cabbage loopers usually have a narrow, white stripe along each side and several narrow lines down the back. Eggs are dome-shaped and laid on the undersurfaces of older leaves. Adult moths have brown, mottled forewings marked in the center with a small silver figure 8.

Corn earworm eggs are white when first laid, but soon develop a dark red or brown ring around the top. Before hatching they darken as the larvae develop inside. Deeper ridges and a more hemispherical shape distinguish corn earworm eggs from looper eggs. Larvae have discrete rows of tubercles with one or two protruding hairs along their backs. As larvae mature they develop distinct stripes, but the overall color is variable. Earworms often migrate into spinach from surrounding crops. Other common names for this pest are tomato fruitworm and cotton bollworm.

Cutworm larvae frequently roll into a C-shape when disturbed. Black cutworm larvae are greasy gray to brown with several black bumps or tubercles on each segment. They may tunnel beneath the soil dragging parts of plants with them. The variegated cutworm larva is from 1.5 to 2 inches long, yellow to brown, with a row of four to six dull, yellow or pink diamond-shaped spots down the back. The granulate cutworm varies in color, but is lighter than the black cutworm, and does not tunnel. Adult cutworm moths have dark gray or brown front wings with irregular spots or bands, and lighter hind wings.

Larvae of the western yellowstriped armyworm are almost black, with two prominent and many fine, bright yellow stripes on the side. At maturity, it is about 1.5 to 2 inches long. Eggs are laid in clusters and covered with a gray, cottony material. This insect is an occasional pest of spinach; however, because it usually develops higher population levels than beet armyworm when it does occur, it can cause greater crop damage in a shorter period of time.

Caterpillars feed in the crown of the spinach plant and can severely stunt or kill seedlings. The potential for damage and contamination continue right up until harvest. In the San Joaquin Valley, fall populations are much more damaging than spring populations. Cutworms feed at or below ground level.

Many natural enemies attack these caterpillars. Among the most common parasites are the wasps, Hyposoter exiguae and Chelonus insularis, and the tachinid fly, Lespesia archippivora. Viral diseases also kill significant numbers.

Disc fields immediately following harvest to kill larvae and pupae. Destroy weeds along field borders.

Cultural and biological controls and sprays of Bacillus thuringiensis or the Entrust formulation of spinosad are acceptable for use on organically grown produce.

Start monitoring before seedlings emerge by checking for eggs and young larvae in surrounding weeds. If populations are high on weeds, watch carefully for larvae on seedlings. Once seedlings emerge, check for egg masses and young larvae twice a week. Loopers, armyworms, corn earworms, cutworms, and other caterpillars can be assessed together. Most insecticides are more effective against young larvae than against eggs, so wait until the majority of eggs have hatched before treating. For beet armyworm control, the best time to apply insecticide is at dawn or dusk (twilight hours).