Early-season corn caterpillars

Seedling corn is attacked each spring by a variety of caterpillars. Identification is the first step in integrated pest management in determining if there is a potential problem from insects. This article discusses how to identify caterpillars most likely to be found in Iowa corn and the injury they cause to seedling plants.

Black cutworms less than 0.5 inch in length feed on leaves, whereas larger larvae can cut or drill plants. Almost all cutworm damage to corn is caused by this caterpillar. Cutting can occur below the surface when soil is dry, or above ground when soil is wet and tight around the plant. Cutting rarely occurs after the fifth true-leaf stage but cutworms may drill into the side of larger stalks. The black cutworm does not overwinter in Iowa. Eggs are laid in the spring when adults fly into Iowa from southern states.

Black cutworm.

Dingy cutworms eat leaves on young corn plants and injury is similar to that caused by black cutworms. This insect rarely cuts corn, therefore, it is important to determine if leaf feeding is from black or dingy cutworms. This insect hatches in the fall and overwinters in Iowa as partially grown larvae. Larvae found in corn during late April and early May are usually dingys and not black cutworms.

Dingy cutworm.

Black and dingy cutworms are identified by skin texture. Blacks have grainy skin like sandpaper, whereas dingys have smooth skin. Identification also is based on the size of the four tubercles (warts) along the top center of each body segment. On the dingy cutworm, these tubercles are about the same diameter. On the black cutworm, the inside pair of tubercles is about one-half to one-third the size of the outside pair.

On the black cutworm (top), the inside pair of tubercles is about 1/3 to 1/2 the size of the outside pair. On the dingy cutworm (bottom) these tubercles are about the same diameter..

Sandhill cutworms injure leaves and cut plants, causing damage similar to that of the black cutworm. However, this cutworm only occurs in areas of very sandy soil. Most of the cutting occurs below the soil surface. Injury first appears as wilted leaves, then as dead plants. This cutworm is light tan, semitranslucent, and has several pale, longitudinal stripes. It hatches in the fall then overwinters as a partially grown larva.

Sandhill cutworm.

Glassy cutworms feed on seedling plants and injury is similar to that caused by other cutworm species. Stand loss is more common in corn planted in grass pasture. Larvae spend most of their life below the soil surface. The head and neck region are reddish brown. The body is cream colored and semitransparent or "glassy" in appearance; the internal organs can be seen through the skin on small larvae. The insect overwinters as small larvae and there is 1 generation per year.

Glassy cutworm.

Bronzed cutworms cut seedling plants but feed primarily on grasses. It is most common in corn planted into a pasture or sod field, and it is not likely to be found in continuous corn or soybean-rotated fields. The larvae have seven stripes, four brown and three yellow, running the length of the body. The general body color is dark brown with a bronze sheen.

Bronzed cutworm.

Stalk borers tunnel into the aboveground portion of the corn plant. Newly emerging leaves from the whorl often have numerous large holes. Injury becomes obvious when the plant terminal begins to wilt and turn brown. Stalk borers are very common pests of corn, especially in rows next to grassy waterways and fence lines. These rows often are stunted in height.

Early stage stalk borer has a purple midsection and an orange head with a black stripe.

Young larvae have cream-colored stripes along the center of the back and along each side. A lateral pair of dark brown to purple stripes also extends the length of the body, but the stripes combine to form a "purple-heart" section directly behind the true legs. The dark stripes and purple heart fade in older larvae. All larvae have a dark stripe on each side of the head. Females lay eggs in grassy or weedy areas during late summer and the larvae hatch the following spring. There is 1 generation per year.

Hop vine borers are stem-tunneling caterpillars that kill young corn plants. They attack the underground portion of corn and tunnel up from underneath the plant to hollow out the base of the stalk. This injury is unique and should not be confused with that of black cutworm, which cuts plants, or stalk borer, which tunnels in the aboveground stalk. The aboveground injury appears as wilting or death of the central whorl leaves. Plants with fewer than eight leaves often die; older plants become stunted and have wilted leaves.

Hop vine borer.

The hop vine borer has an orange head; the body is cream colored with dark brown to purple spots. Eggs are laid on grass during late summer and infestations occur near these areas. There is 1 generation per year.

Sod webworm injury is similar to that caused by cutworms. Plants are cut off at or just below the soil surface and holes are chewed in the leaves. Larvae make webbed tunnels near the base of the plant and down into the soil. Damage is most severe in corn following sod or in very grassy fields. Larvae are dark brown to black and have a dark head. Along the body are numerous polished tubercles (warts) with long hairs. The insect overwinters as small larvae and there are 2 or 3 generations per year.

Sod webworm.

This article originally appeared on pages 82-83 of the IC-482(12) -- May 31, 1999 issue.

Updated 05/30/1999 - 1:00pm