Using Pheromone Traps to Predict Egg-Laying and Damage by Corn Earworm
Pofessor and Extension Entomologist
Department of Crop Sciences
The corn earworm, Helicoverpa zea, is a perennial pest of corn, tomatoes, peppers, and other crops. In corn, moths lay eggs primarily on silks. In 2 to 5 days, larvae hatch from eggs, crawl down the silk channel, and feed on silks and kernels.
For most areas in Illinois, infestations of corn earworm result almost entirely from the migration of moths from southern states – primarily Louisiana and Texas – on weather fronts (much as black cutworm moths, potato leafhoppers, and other insects ride high-level winds to move into the region each year). The timing of significant immigrations of corn earworm moths varies. In many years, heavy flights do not occur until August, but in some years moths arrive in high numbers in early June (as they did in portions of Illinois in 2010). Corn grown for grain escapes serious damage if silks have browned and dried before moths arrive, but in years of early migration or late planting, infestations can result in meaningful losses. Damage is much greater in sweet corn and seed corn. Successive plantings of sweet corn result in a vulnerable crop whenever moths arrive, and infested sweet corn ears are mostly unmarketable. Damage to relatively few kernels per ear in seed corn results in significant losses because of the high value of seed. Where insecticides are needed for earworm control, sprays must be applied to silks so that residues kill larvae before they enter the ear, and repeated applications are needed as silks grow out from the ear tip.
Because seasonal population dynamics depend so much on the timing of one or more waves of immigration, monitoring moth flight with pheromone traps is an essential step in corn earworm management. Cone-shaped traps constructed of wire screening (“Hartstack” traps) are the most effective traps for monitoring corn earworm moth flight. Entomologists, vegetable growers, food processors, and seed companies have been using these traps since the late 1970s. For over 30 years we have used observations of moth counts from traps and damage to sweet corn and seed corn to “guestimate” thresholds for control programs, but we have never studied thoroughly the relationship between counts of moths in traps, numbers of eggs on silks, and infestations and damage caused by larvae.
This project, funded by a USDA North Central Region Integrated Pest Management grant and conducted in cooperation with Drs. Rick Foster and Christian Krupke of Purdue University, is quantifying the relationship between counts of moths in traps and subsequent egg-laying and damage in sweet corn. Successive plantings of two varieties of sweet corn at 4 locations (central and southern Illinois and central and southern Indiana) provide silking sweet corn from late May or early June through September. Traps at each location provide daily measures of moth flight. From first silk to harvest, ears are exposed to possible egg-laying for 1 night only, with 50 ears exposed for each of 10 successive nights. For the 50 ears exposed each night, we clip the tips of 25 the following morning and count eggs on silks. We re-cover the remaining 25 until harvest and count the number of larvae and the number of damaged kernels on each ear. To manage this process we used colored flagging (10 colors) and lots of lots of bags on ears, with “unbagging” and “rebagging” daily for a portion of each trial to allow pollination during the day and exclude egg-laying moths at night. Our findings should help to refine the use of traps to determine best management practices for this insect.