Several species of sod webworms, Crambus spp., occasionally may be serious pests to young corn plants during May and early June. Sod webworms are widely distributed throughout the United States, with grasses being their primary hosts. They often cause severe damage to home lawns and other sod areas.
The corn root webworm, Crambus caliginosellus (Clemens), and the bluegrass webworm, Crambus teterrellus (Zincken), are the two most common species that damage corn in Pennsylvania.
Young corn plants in relatively small areas are occasionally destroyed by sod webworm caterpillars during late May and early June. Damage usually occurs in corn previously in sod and is seldom uniform throughout an entire field. Up to 75 percent of the plants from infested areas may be injured.
Moths are buff to light brown and 1/2- to 3/4-inch in length. While the moth is resting, the labial palps project forward from the head like a snout and the wings are folded around the body. When not in motion, the body is slanted at an angle to the surface, with the head lowest. If disturbed, moths flit about in a jerky, zig-zag fashion before landing a short distance away.
Eggs, deposited singly on the lower blades of grass plants, are tiny, oval, slightly flattened, and have a greenish tinge. Larvae are dirty white with numerous small black spots. The body is sparsely covered with numerous pale hairs. Webworms are very active. They are approximately 3/4-inch when fully grown. Pupae are dark brown but are enclosed in a thin white, flimsy silken cocoon.
Sod webworms overwinter as partially grown larvae enclosed in silken cases usually covered with soil particles. They are near ground level in grass crowns. They start feeding in April and complete their development in early June. They pupate in loosely woven silken cocoons and moths emerge by mid-June.
Moths drop their eggs in grassy areas and a new generation of larvae hatch in about 7 days. There is only one generation per year for the corn root webworm. The bluegrass webworm, however, has two generations per year with moths of the second generation appearing in August and September.
Young corn plants are damaged near ground level. Some plants are cut off and partially dragged into the silk-lined tunnels made by larvae. A webworm usually consumes some of the leaves and stem of the cut plant. This type of injury is very similar to that caused by cutworms; however, webworms usually eat more of the plant.
Webworms also may chew into plant stems slightly above or below ground level. This often results in deformed plants.
Webworm infestations usually are found in corn fields that were in grassy sods the previous year. Plowing and turning over sod before planting corn does not provide sufficient control where webworm populations exist. Thus, the cultural system used (conventional or reduced tillage) has little impact on control.
Birds, beneficial insects, and other biological and natural controls play an important role in reducing webworm populations. These natural control agents probably are responsible for the low incidence of webworm damage in Pennsylvania.
Currently, no chemical insecticides are registered for control of webworms on corn. Several insecticides commonly used on corn are registered for webworm control on turf areas.
Check the Agronomy Guide or consult with your pesticide supplier or county agent for details of pesticide use.
Authored by: Stanley Gesell, Extension Entomologist, 1983 and Dennis Calvin, Professor
Last updated April 24, 2000