Extension Ag Update
November/December 2003
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Western Bean Cutworm Adults Found in Northwestern Illinois Counties

Kelly Cook and Kevin Steffey, Extension Entomologist, University of Illinois
This article first appeared in “The Bulletin” No. 17 Article 1/July 15, 2005

As we described in the article "Preparing to Monitor for Western Bean Cutworms," published in the May 27, 2005, issue of the Bulletin, the western bean cutworm, a midsummer pest of corn, has been moving steadily eastward across Iowa. Professionals and graduate students at Iowa State University have been tracking its movement. In 2004, it was discovered in southeastern Iowa as well as in Warren County, Illinois, and Harrison and Putnam counties in Missouri. The findings of western bean cutworms in Illinois and Missouri in 2004 were new state records, and the moths were captured during only one night of trapping.

The discovery of western bean cutworm adults in Illinois in 2004 prompted us to establish a network of pheromone traps across Illinois to monitor for the presence and distribution of this pest in our state. The network is a cooperative effort between University of Illinois Extension and Pioneer Hi-Bred International. Traps have been placed throughout northern and much of central Illinois, and in some selected counties in southern Illinois. Operation of most of the traps began on July 1. The traps will be checked two or three times each week unless they begin to capture large numbers of moths, after which some of the traps will be checked daily.

Since July 1, 2005, western bean cutworm moths have been captured in pheromone traps in eight counties-Adams, Lee, Mercer, Rock Island, Warren, Whiteside, Will, and Winnebago. The moths captured were identical to those in the photographs that Dr. Marlin Rice, Extension entomologist at Iowa State University, supplied for our cooperators. If the capture in Will County is verified, the eastern edge of this insect's distribution now is Indiana. The traps will continue to operate through July and into August. All captures will be reported in a database developed and maintained at Iowa State University http://www.ent.iastate.edu/trap/westernbeancutworm/. If you visit the Web site, you will notice numerous western bean cutworm moth captures already made in Iowa and Missouri.

What does this mean for corn producers in Illinois? The confirmation of western bean cutworm moths in Illinois counties indicates that cornfields in those counties will need to be scouted. Future captures in other Illinois counties will create the same obligation. Female moths currently are laying egg masses on host plants, including field corn, sweet corn, tomatoes, nightshade, and ground cherry. In corn, females lay eggs primarily on the upper surface of leaves. Fields that are tasseling or near tasseling or fields that have upright leaf characteristics are attractive to western bean cutworm adults. Eggs within masses range from 5 to 200 in number, but the average number of eggs per egg mass is about 50. Eggs are white when first deposited, but they turn tan and then dark purple as they mature. Larvae hatch from eggs 5 to 7 days after oviposition. Larvae feed on corn plants for about a month before pupating. The larvae are very mobile and may infest plants in the same and adjacent rows in an area 6 to 10 feet in diameter. Young larvae can be found feeding on flag leaves and tassels. As they mature, larvae feed directly on the silks. As both corn and larvae mature, the larvae feed on ears and developing kernels.

When should scouting for western bean cutworms begin? Scouting should start now, if you are in a county in which western bean cutworm moths have been positively identified, or in the near future, if and when the first moths are found in your area. It is especially important to note that of the current available transgenic hybrids, only those that contain the Cry1F toxin (Herculex I Insect Protection) are labeled for control of the western bean cutworm. YieldGuard hybrids do not control this insect.

To look for western bean cutworms in cornfields, examine the upper surfaces of plant leaves for egg masses and/or small larvae. Also look at tassels before pollen shed. Look at 10 consecutive plants in at least 5 different areas of the field. Egg laying varies with plant growth stages, so sample hybrids with different maturities within a field separately.

When should I treat corn to control western bean cutworm larvae? Entomologists at the University of Nebraska indicate that an insecticide treatment may be warranted when 8% of plants have egg masses and/or small larvae. This is a nominal threshold based on experience, and it is the most widely used threshold. Timing of an insecticide application is critical. If larvae have hatched, apply an insecticide after 95% of tassels have emerged, but before larvae enter the silks. Control is more difficult after larvae have moved to the silks. Once larvae have moved inside the husk, insecticides are ineffective and larvae will feed until they mature. If larvae have not hatched and the corn plants have tasseled, time the insecticide application to coincide with the hatch of larvae. If the eggs are purple, hatch usually occurs in about 24 hours.

According to entomologists at the University of Nebraska, control decisions associated with western bean cutworms need to be made shortly after peak moth flight. The moth flight in Nebraska usually peaks between July 10 and July 24. If you want to read the experts' words, visit the most recent issue of Crop Watch, the University of Nebraska's weekly newsletter.

More information about the western bean cutworm can be found on the Western Bean Cutworm Factsheet at http://www.ipm.uiuc.edu/fieldcrops/insects/western_bean_cutworm/index.html. We'll continue to provide updates on the distribution of this insect in Illinois. Sign up to receive “The Bulletin” for weekly updates on this and other pests at http://www.ipm.uiuc.edu/bulletin/index.php.