Corn Earworm, Helicoverpa zea, Lepidoptera: Noctuidae

Description

Adults of the corn earworm, also known as the cotton bollworm, are light tan in color and are about 1.25 inch long. Moths generally have green eyes. Eggs are near white when laid but darken just prior to larvae emergence. Larvae initially are minute, about 1/16 inch, but at maturity can reach 1.75 inch in length. Three pair of true legs occur on the thorax and four pair of prolegs plus an anal pair are found on the abdomen (Photo 8). Color of larvae varies greatly. Mature larvae that have developed on foliage are mostly green while those developing on seed are reddish brown with longitudinal lines. The pupal stage occurs in soil and color ranges from light tan shortly after pupation to dark brown just prior to moth emergence.

Distribution, damage and impact

All stages of the corn earworm can be found throughout Arkansas and resulting damage may be severe. This damage occurs in several forms including foliar damage to young grain sorghum and direct damage to seed heads.

Life History

Adults that are active in late winter arise from two sources, i.e., overwintering pupae and flights of moths from southern areas. Adults are attracted to many host plants but flowering plants are favored. Eggs are deposited on foliage of seedling grain sorghum and larvae may consume large amounts of foliage. When accurate counts are made, however, the percentage of seedling grain sorghum plants infested with corn earworm is generally very low and control is not feasible. Later generations deposit eggs during flowering and head formation and emerging larvae feed directly on seed. Regardless of host plant structure on which the larvae develop, mature larvae move to the ground and pupate within the upper six inches of soil. In Arkansas three or four generations occur each year.

Management

Lepidopterous caterpillars often feed within the heads of grain sorghum and visual examination of the head my not detect all larvae. By shaking the seed head over a white bucket, the larvae will be dislodged and can be more easily counted. At least 30 plants per 80 acre field should be sampled. Thresholds for caterpillars on grain sorghum heads vary greatly due to the profit potential of the crop. In general when lepidopterous caterpillars (corn earworm, fall armyworm and sorghum webworm) number two or more per head in grain sorghum nearing maturity foliar insecticides may be warranted. Use of foliar insecticides to reduce damage may not produce economic benefits and should only be used against high insect populations. Numerous beneficial organisms affect corn earworm including naturally occurring insect pathogens, parasites and predators and insecticide use may reduce their effectiveness. Fall plowing to destroy and bury crop residue will reduce overwintering populations of corn earworm. Finally, “open headed”grain sorghum hybrids better expose caterpillars to foliar insecticide sprays and to beneficial organisms.

Insecticide recommendations for corn earworm on grain sorghum

See Table 2 for insecticide recommendations current at time of publishing. Current updated recommendations can be found in the Cooperative Extension Service publication MP-144 and on the world wide web at WWW.cdms.net/manuf/default.asp. Always follow instructions on pesticide labels.