Authors: Pamela Opfer and Dan McGrath, OSU Dept. of Horticulture
Corn Earworm (Helicoverpa zea)
Corn Earworm larvae Corn Earworm Adult Moth (Photos by Ken Gray)
False Corn Earworm Adult Moth (Photo from the University of California)
How to ID Pest
Adult corn earworm moths measure to about 19 mm long, with a wing span of 25 - 38 mm, and adults range from olive green, to tan, to dark reddish brown in color. Egg laying occurs throughout the sweet corn growing season. The tiny, white eggs are laid singly on the foliage and fresh corn silk, which is the favorite site for egg deposition. Eggs are spherical with 12 or more ridges radiating from the top. Young larvae are greenish with black heads and conspicuous black hairs on the body. Fully developed worms are about 38 mm long and range in color from pale green or pinkish to brown.
The Corn Earworm is sometimes confused with the False Corn Earworm (Heliothis phloxiphaga). They both are attracted to the same pheromone, so you will encounter a mixture of moths in your traps. Based on these pictures, it is hard to imagine that the two moths could be confused; they look very different. But, after the two moth types have been beat up and shed some of the wing scales, they look similar in the trap. A lot of confusion has occurred over the years because of this mix up. The False Corn Earworm has very similar coloration, but it is generally a bit smaller bodied than the Corn Earworm. But the best way to tell the species apart is by the dark markings on the hind wings of the False Corn Earworm. This spot on the hind wing is not present on the Corn Earworm. Occasionally, we have seen outbreaks of Heliothis phloxiphaga in the Willamette Basin; it is a pest of several species of trees commonly propagated in the nursery industry here in the valley. It is generally not a vegetable pest.
Female moths lay single eggs on corn, generally on the silk. Each female lays from 500 to 3,000 eggs. Larvae hatch in 2 to 10 days and start feeding immediately. They pass through several stages of development (1 to 5 instars) during growth, and when they have completed feeding, crawl or fall to the ground. Larvae burrow into the soil, construct protective cells, and change to the pupal stage. Adults emerge from the pupae after 10 to 25 days, depending upon soil temperature. Pupae of the last summer generation spend the winter in the soil, and adults emerge in the spring. There are typically 1 -3 generations of this pest each year.
Crops Affected & Damage
The corn earworm may be present throughout the season but is most abundant during late summer. Larvae feed on leaves, tassels, the whorl, and within ears, but the ears are the preferred sites for corn earworm attack. Ear damage is characterized by extensive excrement at the ear tip. Young larvae feed on corn silks, clipping them off. Shortly thereafter, they feed their way into the ear where they remain, feeding in the tip area until they exit to pupate in the soil.
Corn Earworm damage (Photos by Ken Gray)
Scouting & Monitoring
Corn Earworm moth populations can be monitored by using pheromones placed in Texas style wire mesh haystack traps. The traps need to be placed very close to the corn when it is forming it’s silk. In fact, it is best to place the traps out among the corn plants and configured in a way that the height of the trap can be adjusted upward as the corn grows. Keep the bottom of the trap about a foot above the top of the corn. Check the trap on a regular basis to make sure that the corn has not grown up into the trap where it can interfere with moth capture. In cases where the corn is mechanically topped, it may be necessary to set the trap up outside the corn.
Wire cone trap in corn field (Photo by Pamela Opfer)
Interpreting Flight Data
The action threshold for Corn Earworm insecticide applications depends on whether the corn is grown for fresh market or for processing. The tip of the corn is removed during processing. Unless the earworm has penetrated deep down the corn ear, processing generally removes the insect and the damaged tip. During a normal year, this is all that is required for processing corn. Occasionally, during an outbreak year when the egg laying is early and intense, an insecticide application at first silk may be justified. Insecticide applications for earworm control is rare in sweet corn grown for processing. Sweet corn for processing rarely is sprayed unless outbreaks are early and intense (20 to 30 moths per trap per day) at first silk.
The exact opposite is true for fresh market sweet corn, especially if the corn is packed and shipped out of the area. Fresh-market corn has very little tolerance of earworms. Generally, an insecticide application is applied to fresh market sweet corn regardless of moth count. Subsequent insecticide applications are made if there are five or more moths per night per trap.
Once larvae enter the corn ears, control is very difficult. Begin scouting for this pest at the first visible silk. Begin treatments during silking stage, at the start of egg hatch. Direct insecticidal control towards young larvae that are feeding on the exposed ear tips.
Biological Control: Many predators and parasites attack corn earworm eggs, including several species of Trichogramma. Generalist predators such as lacewings, minute pirate bugs, and damsel bugs feed on corn earworm eggs and small larvae.
Cultural Control: In sweet corn, very early plantings often escape significant damage compared with late-season corn, because earworm population densities increase as the season progresses. Plant corn resistant to earworm. Any corn variety with long, tight husks is physically safer from earworms. Resistant varieties include Country Gentlemen, Staygold, Golden Security, and Silvergent. Plow or dig up plants in the fall to prevent overwintering.
Chemical Control: Please consult the PNW Insect Management Handbook for pesticide recommendations.
References & Citations
Calvin, Dennis. 2000. "Corn Earworm." Pennsylvania State University College of Agricultural Sciences Entomology. Accessed December 7, 2011. (http://ento.psu.edu/extension/factsheets/corn-earworm).
"Corn Earworm Factsheet". Oregon State University. Accessed December 9, 2011. (http://insects.ippc.orst.edu/pdf/reb36.pdf).
Godfrey, L.D., S.D. Wright, C.G. Summers, C.A. Frate. 2008. "UC IPM Pest Management Guidelines: Corn." UC ANR Publication 3443. University of California Agriculture & Natural Resources. Accessed December 7, 2011. (http://www.ipm.ucdavis.edu/PMG/r113300911.html).
Hollingsworth, Craig S. (Ed.). 2011. Pacific Northwest Insect Management Handbook. Corvallis: Oregon State University.
Mayer, Daniel F., Antonelli, Arthur L., & VanDenburgh, Roy. 2003. "Insect Answers: Corn Earworm. Washington State University Cooperative Extension, EB 1455E. Accessed December 7, 2011. (http://cru.cahe.wsu.edu/cepublications/eb1455e/eb1455e.pdf).