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Sweet Corn Newsletter Corn Earworm Description and Life Cycle The corn earworm (CEW) moth has a wingspan of approximately 1.5 inches (see Figure 7). The body of the moth is buff to grayish brown in color, with a dark spot in the center of each front wing and dark bands near the back of the wings. The hind wings are lighter tan with a dark band along the outer margins. The larvae vary tremendously in color, ranging from yellow to green to brown or even purple (see Figure 8). Alternating dark and light colored stripes run the length of the body. The head of the newly hatched CEW is black, but it turns yellowish brown as the larvae mature. The larvae are also covered with many small bumps, each with a small, sharp spine. These bumps and spines give the body a rough appearance, unlike the fall armyworm, which has a smooth body. Fully grown, the CEW larva is slightly more than one inch in length.
Corn earworm is distributed worldwide but cannot overwinter in the northeastern United States. The moths migrate to Maine from southern states around mid-July. The night-flying female moths search for fresh (green) corn silk on which to lay their eggs (see Figure 9). The eggs are laid singly on the silk, although several may be laid on one silk mass. Each female can lay several hundred eggs. If fresh silk is not available, they may lay eggs on other hosts such as tomatoes or peppers. The eggs hatch in two to 10 days. The newly hatched larvae feed on the corn silk, working their way down the silk channel to the tip of the ear. Once there, the larvae feed on the silk and developing kernels. The feeding area becomes filled with moist waste. This damage causes severe economic loss, and may go unnoticed until harvest.
Scouting and Control Thresholds Because corn earworm moths lay their eggs on the silks and the larvae crawl directly into the ears, field scouting is not an effective way to monitor for this insect. Therefore, control recommendations are made based on the number of corn earworm moths caught in pheromone traps on a nightly or weekly average. Scentry Heliothis® traps or Harstack 50:33 or 50:25 traps are currently recommended. The traps should be baited with Hercon® Helicoverpa zea or an equally effective pheromone. Place traps in corn fields just as the fresh green silks begin to appear. Place traps so that the base is at the same height as the silks. Traps should be checked at least once a week until the first CEW moth is caught, and checked two to three times a week thereafter. A treatment is recommended on any fresh silking corn when the first corn earworm is caught. Spray intervals after the initial application should be based on the number of moths caught on a nightly basis.
The table above provides the spray thresholds you can use to determine if control measures are necessary based on the number of CEW moths caught in pheromone traps. Corn that is not yet in silk need not be sprayed. Once the silks have dried, the corn no longer needs to be sprayed, and the pheromone traps should be moved to corn with fresh silks. |
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