Corn Earworm in Sweet Corn
The corn earworm, Helicoverpa zea , is a very important pest of sweet corn. It can also be a pest in tomatoes, cotton, sorghum, vetch, and other hosts. Thus, it also goes by the name of cotton bollworm and tomato fruitworm. Here, it is primarily a pest in sweet corn, and we’ll use the name of corn earworm, and the acronym CEW. We typically see CEW later in the season because it has not overwintered well in Pennsylvania. It overwinters as a pupae in the soil, at a depth of about 2 to 4 inches. However, it does survive the winter in southern New Jersey, the Delmarva, and southern Illinois. Overwintering is more successful as you travel south, and winter temperatures are warmer. In recent years, we have seen low densities of CEW earlier in the field season, which may reflect some overwintering in southeastern Pennsylvania.
The corn earworm is a moth, in the insect family Noctuidae. As with all the noctuids (such as fall armyworm), it is a night-flying moth. It is a good flyer, and able to move long distances. Adults emerge in the spring. They are about 0.75 to 1 inch in length, tan to buff-colored, sometimes with some olive shading, with a wavy darker band near the edge of the wings of younger specimens. The eyes have a distinctive serpentine green reflection when held up to sunlight in live specimens. A darker brown spot is located about midway along the outer edge of the front wings. They fly when evening temperatures exceed 55 0 F, with increasing activity at higher temperatures. They can be caught up in winds and storms, and deposited with the weather patterns.
Females are strongly attracted to fresh silks, where they lay the eggs individually directly on the silk. A female can lay from 500 to 3,000 eggs, and average about 1,000 eggs per female. Eggs will be laid on other plant tissue or hosts when corn silk is not available. Eggs hatch in 2 to 10 days, depending on temperature, and probably hatch within 2 to 4 days during the summer in Pennsylvania.
Hatching larvae crawl away from light, and towards moist, shaded areas. When on silks, hatching larvae feed on the silk and burrow directly down into the ear. They feed on each other as well, which tends to limit the number of larvae to one per ear. They feed on the corn kernels at the tip of the ear, rendering the product unmarketable unless it is possible to cull and cut off the tips of the ear. In field corn, this tip-feeding is often not important. Thus, the corn earworm is a pest to vegetable growers, but not to field corn growers.
As they mature through 6 instars, they will leave large amounts of frass where they are feeding. Larval coloration will vary from greenish to yellow to reddish, with longitudinal stripes which are actually microspines along the body. The microspines give the larvae a rougher feel then the other worms in the ear. The head is tan to yellow, which helps distinquish it from the fall armyworm or European corn borer, which have darker head capsules.
Monitoring and Management
Of all our corn pests, the one that is most easily monitored is the corn earworm. Blacklight traps capture males and females, and traps baited with the proper sex pheromone capture only males. Traps are made from heavy wire hardware cloth, or purchased from Gempler’s. A cloth net trap sold by Scentry will also work, and are used in some states (including Massachusetts and New York), but they have not performed as well as the wire traps in the mid-Atlantic. Pheromone lures can be purchased from Great Lakes IPM or Gempler’s. For corn earworm, the lures made by Hercon have worked well in our area. Traps need to be placed near (or in) a corn field, and the area immediately around the trap kept free of tall weeds or debris. Place traps away from trees or wooded areas. Handle the lures to minimize contamination by using forceps or surgical gloves, or at least preventing the same hand from handling lures for different insect species. Also, keep the lures cool, in the refrigerator or freezer, while in storage. Contact your Extension agent for details on setting up the traps at your farm.
Since much of the CEW population builds as populations move from the south, it makes sense to look at the densities from a regional viewpoint. Penn State Extension, in with support from the Pennsylvania Vegetable Grower’s Association (PVGA) is monitoring CEW with pheromone traps from 15 to 25 sites in Pennsylvania. The data are reported approximately weekly at the 1-800-PENN-IPM telephone line. It is easier to view these data as a map, and we are developing that capability as a web site. We are cooperating with neighboring states. The data are fed to a computer in Entomology, where they are processed into maps for the region, and the maps then turned into web pages. You can look at the most current CEW map, or the most current map for the other pests of sweet corn ears. You can also look at previous maps. When looking at any map, you can also click on a given site, and get the trap capture over time at that site .
Please note, however, that the maps are showing average catch per day . We are showing the maps as catch per day, and not per week, because we need to report data from more than one state. The other states capture data more frequently than we do in Pennsylvania. (The neighboring states have State Department of Agriculture personnel, or people paid from state IPM funds, servicing the traps, whereas in Pennsylvania we rely on volunteer help). So you need to multiply the trap catch per day in the maps by 7 to estimate a trap catch per week.
This year, 1999, is a test year as we develop this interface for viewing these pheromone trap catch data as maps on the web. Bear with us, and send us comments to improve it for next year.
Since the lavae feed on silks and then burrow into the ear, they are hard to reach with insecticides. Insecticides should be applied in sufficient water to ensure thorough coverage. High water volumes (30 gal/A) is recommended. Timing insecticides to when the eggs are hatching and the larvae are feeding on silks is critical. This is best done by applying insecticides when the moths are flying, which is within a day or so of when they are laying eggs. A useful guide is to watch the change in trap capture over time, and tighten spray frequency as trap catch is increasing. A starting point for spray timing is:
|CEW almost absent (< 14 / week)||4 days to no spray, depending on ECB|
|CEW very low (14 to 35 / week)||5 to 6 day|
|CEW low (36 to 70 / week)||4 to 5 day|
|CEW moderate (71 to 350 / week)||3 to 4 day|
|CEW high (> 350 / week)||2 to 3 day|
It is very important to realize that trap capture can be influenced by all the factors that influence insect flight behavior at night, such as temperature, wind, humidity, etc. Therefore, the capture should be used as a guide.
Insecticide resistance has been a problem with the corn earworm. This is not well documented in the northeast, where we are often dealing with few generations. But in the southeast, this cotton bollworm is well known to show resistance to a range of pesticides. The current vegetable production guide lists carbamates (Lannate, Larvin) and pyrethroids (Asana, Baythroid, permethrin, and Warrior) as useful materials.
Systemics activity would help, and the transgenic sweet corn varieties, which express the proteins from Bacillus thuringiensis , have proven very effective in field tests. Our tests when spraying B. thurgingiensis has met with variable success, and is difficult to make work consistently, probably because it is hard to get the larvae to ingest a toxic dose. Younger larvae will die from a much smaller dose than will older larvae. By making the plant create the protein, the very young larvae end up ingesting a toxic dose very quickly. Processing varieties of the transgenic Bt sweet corn are now being grown commercially, and sh 2 varieties are being tested.
Prostak, D. J. Corn earworm. Pp 75-79 In Adams, R. G. and J. C. Clark (eds). Northeast Sweet Corn Production and IPM Manual. University of Connecticut, Resource Center U-35, 1376 Storrs Rd.,Storrs, CT 06269-4035.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Shelby Fleischer, Associate Professor of Entomology
Last updated January 2001
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