Northern Plains Integrated Pest Management GuideIPM

Authors: Frank Peairs, Buyung Hadi, Robert Wright, R. Jeff Whitworth, Holly Davis, and J.P. Michaud

Identification

The corn earworm, Helicoverpa zea (Boddie), also known as the cotton bollworm, is variable in color, ranging from yellow, pink, green, and sometimes almost black. Alternating light and dark stripes usually mark the body. Small spines (microspines) can usually be spotted on the surface of the larva body. The larvae always have yellow-brown head capsules, except when newly hatched. A fully developed larva is 1.6-2 inches (4-5 cm) in length.


The different colors of corn earworm, note the presence of microspines and brown head capsules on all the larvae

Adult moths are buff to grayish-brown with dark forewings markings and a wingspread of about 1.6 inches (4 cm). The hindwings usually have lighter color with dark marginal markings. The eggs are small (about half the size of a pinhead), off white in color, laid singly on the host plant's leaf .

Adult moth of Helicoverpa zea on peanut

Helicoverpa zea's egg on cotton leaf

Life Cycle and Seasonal History

Most adult moths are considered to migrate north from the southern states in the spring. Female moths lay single off-white colored eggs. Eggs hatch within 10 days and the first generation earworms often feed in corn whorls. After 5-6 larval stages, the larvae drop to the ground and pupate. In a few weeks adult moth emerge, mate and lay eggs for the second generation. These moths together with other incoming moths from the southern latitudes often seek out green corn silks on which their eggs are deposited. Several eggs may be laid on the green silks. The eggs hatch into young larvae within 10 days and begin feeding on the corn silk, sometimes clipping it off. Later, the larvae bore through the silk channel to the ear tip and begin feeding on the kernels. Larger larvae are cannibalistic, so usually only one larva reaches maturity in an ear of corn. Mature earworm larvae bore their way out of the husk, crawl down the stalk, burrow into the ground and pupate in an earthen cell. Adult moths of the second generation emerge 2-3 weeks afterward. In northern plains, corn earworm produces 1-2 generations per year.

Plant Injury and Damage

Infested young plants later show 'shot hole' injuries on the unfurling leaves. Developing tassels inside the whorl can also be damaged by feeding of the first generation larvae. Larvae of the second generation earworm feed on silks. Silk clipping by corn earworm may interfere with pollination and cause missing kernels on the developed ear. Later, corn earworms tunnel into corn ears where they feed on developing kernels, usually starting at the ear tip. Fecal pellets (frass) accumulate along feeding channels. Not only do larvae cause direct loss by feeding on kernels but also provide openings in the husks for entry of disease organisms and feeding by birds.

Typical shot hole injuries due to feeding by first generation corn earworm

Second generation corn earworm feed on the kernels within the ear, injuries usually start at the ear tips

Management Approaches

Scouting and Threshold

Chemical control is difficult, especially after corn earworm tunnels into the ear. Eggs are deposited on emerging silk and but silks that emerge post-treatment are unprotected. Newly hatched larvae can easily tunnel into the ear without being in contact with insecticide, unless sprays are applied every two or three days. Chemical control is not always economically feasible on field corn.

Cultural Methods

Late planted corn seems to be exposed to greater corn earworm injury due to the concurrence of H. zea egg laying period and the timing of silking. Corn planted early may escape the peak season of egg laying period and decreased risk of corn earworm infestation.

Host Plant Resistance

Corn hybrids with silk containing antibiotic chemical against H. zea larvae and with husks fitting tightly around the ear provide some protection against corn earworm. On hybrids with tight fitting husk, larval feeding is limited on the ear tip; larvae may leave the corn ear before completing their development.