Description

Adults of the corn earworm, also known as the cotton bollworm, are light tan in color and are about 1.25 in. 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 in., but at maturity can reach 1.75 in. in length. Three pair of true legs occur on the thorax and four pair plus an anal pair are found on the abdomen (Photo 5-15). Color of larvae varies greatly. Mature larvae that have developed on foliage are mostly green while those developing on ears 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 corn, damage to tassels and silks, and direct damage to kernels (Photo 5-16). The corn earworm also plays a role in occurrence of aflatoxin. Damage to corn ears may serve as an entrance for the fungi responsible for producing aflatoxin.

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 corn and larvae consume large amounts of foliage. When accurate counts are made, however, the percentage of seedling corn plants infested with corn earworm is generally very low and control is not feasible. Later generations deposit eggs on silks and emerging larvae chew down the silk channel to the developing ears. Again, use of foliar insecticides to reduce damage on ears is not economically effective on Arkansas field corn. 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

Foliar insecticides are not practical for corn earworm management on field corn and their use may ultimately increase the problem. Numerous beneficial organisms affect corn earworm including naturally occurring insect pathogens, parasites and predators and insecticide use may reduce their effectiveness. Soil insecticides may have some effect on larvae attacking seedling corn but are not practical if directed only at the corn earworm. Some corn hybrids possess some level of resistance to the corn earworm. Finally, the use of transgenic Bt corn likely has some impact of reducing damage to foliage and ears. Future transgenic lines may possess much stronger toxicity to the corn earworm.

Insecticide recommendations for corn earworm on field corn

See Table 5-2 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.