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Issue No. 18, Article 3/July 28, 2006

Western Bean Cutworm Larvae in Corn in Northwestern Illinois

With the discovery of the first western bean cutworm moth in Illinois in 2004, it seemed to be only a matter of time before the insect posed a threat to corn in our state. If you have stayed abreast of current activities related to western bean cutworms, you already know that the distribution of this pest is relatively widespread in Illinois. Western bean cutworm moths also have been captured in pheromone traps in Indiana, Michigan, Missouri, Ohio, and Wisconsin. Captures in Iowa have been relatively commonplace over the past few years, but in 2006 the numbers of moths being captured there are extraordinary. If you want to review 2006 moth capture data to date in the aforementioned states, go to this web site. Click on "Maps of Trap Captures" for a visualization of the distribution of western bean cutworms.

It's time to begin checking cornfields for western bean cutworm eggs and larvae. Fortunately, the ever-astute and skillful photographer Jim Donnelly, Ag View FS, has provided some photographic records that might help with your identification of the pest in corn. Jim took a series of photographs of a western bean cutworm egg mass from July 14 through July 18. The photographs show the progression of the eggs from pearly white to pink to purple until just before hatch. Jim then created a time-lapse series of these photographs that shows the change in color in just a few seconds. Very impressive stuff.

Time lapse series of western bean cutworm eggs, from July 14 through July 18 ("movie" courtesy of Jim Donnelly, Ag View FS, Inc.). Note: If you cannot see the video, you may need to update your Windows Media Player plug-in. Please visit the Windows Media Download Center and update to the latest version of Windows Media Player.

If you are still unable to view the video, click here to view the slides in a new window.

Jim also found western bean cutworm larvae feeding on corn ears in a cornfield in Bureau County on July 25. The larvae ranged in size from .5 inch to 1.5 inches long, with an average length of about 1 inch. He found six larvae in a very small areaabout 10 square feet. There was only one larva per ear on most plants, although one ear supported two larvae.

Western bean cutworm larva, Bureau County, July 2006 (photo courtesy of Jim Donnelly, Ag View FS, Inc.).

Western bean cutworm larva in ear tip, Bureau County, July 2006 (photo courtesy of Jim Donnelly, Ag View FS, Inc.).

Don't delay scouting for western bean cutworms in areas where moths have been captured. Treatment is warranted when 8% of the plants have egg masses on the leaves or young larvae feeding in tassels. This is a nominal threshold recommended by entomologists at the University of Nebraska. However, it points out the importance of looking for egg masses and/or young larvae rather than waiting to find larger larvae. It is very difficult to control western bean cutworm larvae when they begin feeding in the ear tips, so an insecticide, if needed, must be applied before the larvae attain significant size. Insecticides suggested for control of western bean cutworms in corn in Illinois are listed in Table 1.

Another approach for making a decision about controlling western bean cutworms in corn is to consider the cost of control and the value of the corn crop. Marlin Rice, extension entomologist at Iowa State University, referred to a table of such economic thresholds in an article he wrote for the Proceedings of the 2005 Illinois Crop Protection Technology Conference. We have learned, however, that the table was not included in the article (an oops on our part), so we are publishing it as Table 2 here. Note once again that the economic injury levels are numbers of eggs per plant, another demonstration that control of larger larvae in ears is not recommended. You can view Marlin's article (without the table) on pages 102-105 of the proceedings (Adobe PDF, 1.8 MB). The article includes information about the life history of the western bean cutworm, as well as its damage to corn and its movement through Iowa into Illinois. Scouting guidelines also are included.

The western bean cutworm is here to stay, and we are interested in learning about the occurrence of larvae and the injury they cause to ears in any area where they are found. Please keep us apprised of the situation in your neck of the woods.--Kevin Steffey

Kevin Steffey

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