Forest Huval, Reagan, Thomas E.
Corn earworms are one of the most detrimental pests to corn and other high-value crops in the U.S., including emerging crops, such as hemp. The species can be found throughout North America, excluding northern Canada and Alaska, and adults remain active year-round in tropical and subtropical regions. The adult corn earworm moth has a wing expanse of 1 to 1.5 inches (25 to 38 mm). The forewings are yellowish brown with irregular markings and possess a small, dark spot located near the middle. The forewing may have a dark band located distally near the wing’s edge. Larvae reach 1 to 1.5 inches (25 to 38 mm) in length when mature. The body varies greatly in color, ranging from light green to pink, brown, yellow or black. A pair of narrow black stripes is located dorsally running lengthwise across the body. The head capsule is commonly light brown and the thoracic plates are black. The presence and arrangement of black microspines helps to differentiate corn earworm larvae from those of the fall army worm (Spodoptera frugiperda), European corn borer (Ostrinia nubilalis) and other similar species of caterpillars. The brown pupa is 0.7 to 0.8 of an inch (17 to 22 mm) in length and 0.25 of an inch (5.5 mm) in width and similar in appearance to that of many other members of the family Noctuidae. The eggs are light green when first deposited and turn yellow and then gray as they develop.
Due to the broad host range on economically valuable crops, the species goes under several other common names, including “cotton bollworm” and “tomato fruitworm.” It is often treated under the name Heliothis zea in older literature. Members of the genera Helicoverpa and Heliothis are similar in appearance and habits, and both contain numerous economically important pests in various parts of the world.
Corn earworm moths lay single eggs on fresh corn silks or leaf hairs. Females lay 500 to 3,000 eggs during their lifetimes and may lay up to 35 eggs in one day. The eggs hatch and enter into their larval stage in three to four days. Upon hatching, the larvae move about the plant in search of food, most commonly the reproductive structure of the plant. When larvae are young, they do not compete for resources and may be found feeding on the same plant. However, as they mature they begin to cannibalize other larvae, often leaving only one per reproductive structure, such as an ear of corn or tomato fruit. Once larvae reach maturity, they leave the feeding site, burrow into the ground and pupate. In the following 10 to 14 days, newly emerged adults become active. Corn earworm adults are nocturnal and remain active throughout the night. During the day, most of their time is spent hiding in vegetation, but sometimes they emerge to feed on nectar. In warmer climates such as Louisiana, there can be four to five generations per year.
Corn earworms are the most economically important pest of corn in Louisiana. They feed on the silks, tassels, whorls and kernels of field and sweet corn. The larvae begin with feeding on corn silks, which inhibits pollination. They then move on to the kernels, burrowing into the ear and causing damage to up to half the ear of corn. This creates an unmarketable product as a fresh market crop. Furthermore, feeding increases susceptibility to plant pathogenetic fungi and facilitates invasion by secondary pests, such as sap beetles (Nitidulidae). This is problematic in seed production when molds in seed corn inhibit viability, causing yield loss. Corn earworms also pose an economic threat to hemp (Cannabis sativa), an emerging, high value crop in Louisiana. After the corn harvest, corn earworms migrate to hemp fields as a secondary host plant. The larvae burrow into the buds, often going unseen until harvest. Damage is significant in crops grown for CBD (cannabidiol) and other essential oils. There has been no such damage reported for fiber and seed varieties. Additionally, it burrows into the head of lettuce and the fruit of other crops,, such as tomatoes, cantaloupe, cucumbers, squash and beans. Damage to these crops is more likely to occur when corn is not silking or in season.
Monitoring. Scouting fields for eggs and larvae is not an effective means of determining pest pressure. They burrow into the corn’s ear, hemp’s bud or vegetable’s fruit soon after hatching, making them hard to detect. Adult moths can be monitored using blacklights and pheromone traps, which give valuable estimations on corn earworm populations. The presence of five to 10 moths is a sufficient threshold to begin a pest management protocol.
Cultural control. Resistant hybrids offer silks that are resistant to larvae feeding and husks that are tight around the ear. This combination offers the most effective type of host plant resistance. Due to the tightness of the husk around the ear, larval feeding is limited to the ear tip, reducing damage and yield loss. Neither crop rotation nor tillage significantly alters the insect pest populations. Controlling weeds by mowing or applying herbicides can reduce the development on insects in surrounding areas before moving to the cash crop.
Chemical control. Insecticides provide a management strategy with a narrow window of efficacy. The timing of insecticide application is critical, as larvae begin to move down the silk channels soon after they hatch. As larvae continue moving down the ear and reach the husks, spraying becomes ineffective. In order for chemical control to be successful, insecticides must reach the tops of the ear where the eggs hatch. See the Louisiana Insect Pest Management Guide for currently approved insecticides for control of the corn earworm.
Microbial insecticides. The use of formulations based on Bacillus thuringiensis as a bacterial insecticide provide suppression of larvae upon ingestion. However, due to larvae moving inside the ear to begin feeding, this does not provide complete control of pests. Other biological control methods include nematodes and parasitic wasps (Hymenoptera). Contact your parish agent for the availability of biological options in your area.
Cabanillas, H. E., and J. R. Raulston. 1995. Impact of Steinernema riobravis (Rhabditida: Steinernematidae) on the control of Helicoverpa zea (Lepidoptera: Noctuidae) in Corn. Journal of Economic Entomology 88: 58–64.
Capinera, J. L. 2000. Corn Earworm - Helicoverpa Zea. University of Florida. entnemdept.ufl.edu/creatures/veg/corn_earworm.htm (accessed 28 Nov. 2020).
Cranshaw, W.S. 2020. Hemp Insect Fact Sheets. Colorado State University College of Agricultural Sciences Hemp Resource Center. hempinsects.agsci.colostate.edu/hemp-insects-text/ (accessed 28 Nov. 2020).
Ditman, L.P., and E. N. Cory. 1931. The corn earworm: biology and control. Bulletin of the Maryland Agricultural Experiment Station, 1931(328): 443-482.
Adult moth of corn earworm. (John C. French Sr., Auburn University, Bugwood.org).
Corn earworm damage on hemp. (Whitney Cranshaw, Colorado State University, Bugwood.org).
Larval damage to ear of corn. (Erick R. Day, Bugwood.org).