No. 7 Article 3/May 9, 2008

Time to Crank Up the Vigilance for Armyworms

The potential for an armyworm outbreak has been simmering over the past couple of weeks. Although the numbers of adults captured in pheromone traps near Princeton, Kentucky, declined recently, the numbers of potential larvae from the record-setting adult captures near the end of April are what we need to watch. Extremely large numbers of adults were captured in early May near Lexington, much farther east. You can view the captures of armyworm adults in Kentucky graphically. The extension entomologists at the University of Kentucky wrote useful articles about armyworms in both corn and wheat in the April 28 issue of Kentucky Pest News.

The situation with armyworms in Illinois has not been as dramatic; the pheromone traps in southern Illinois have not captured record numbers of adult armyworms this year. The captures of note among the five traps in southern Illinois were in Franklin County (196 and 109 adults during the weeks ending April 29 and May 5, respectively). It is entirely possible that the armyworm threat will be more significant to our east. Nonetheless, vigilance for armyworm larvae right now and continuing for a couple of weeks will pay dividends if infestations are observed early rather than too late (i.e., after considerable defoliation). Based on one report from Kentucky, very small armyworm larvae (about 1/4 to 1/2 inch) were found in a cornfield in Carlisle County, just south of the southern tip of Illinois. The field had been planted no-till into burned- down weeds (May 5, 2008, Kentucky Pest News).

People in southern Illinois should be on the alert for small armyworm larvae feeding on the leaves of plants in wheat fields, grass pastures, and cornfields. The most recent serious outbreak of armyworms in Illinois (and in many other states as well) occurred in 2001. We received our first reports of armyworm activity in early May that year, and by mid-May we were in the middle of a full-blown outbreak. The articles we published in the Bulletin in 2001 contain a lot of information that might be worth reviewing now. You can find the articles by clicking on "Past Issues" on the main page of the Bulletin , then on "Check out issues published prior to 2004." In 2001, issues 6 (May 4), 7 (May 11), 8 (May 18), and 9 (May 25) have articles with a lot of biological, ecological, and management information. However, don't use the insecticides suggested for armyworm control in those 2001 issues. Up-to-date recommendations for 2008 can be found in the 2008 Illinois Agricultural Pest Management Handbook, Chapter 1, tables 1 (corn, page 4), 5 (small grains, page 19), and 6 (grass hay or pasture, page 20). Not included in Table 6, however, is Mustang Max, which is labeled for control of armyworms in "grass forage, fodder, and hay group and grass grown for seed." The rates of application for armyworm control are 2.8 to 4 oz per acre.


Armyworm larvae on wheat heads, 2001 (photo courtesy of Robert Bellm, University of Illinois Extension).

As I stated previously, we might miss the brunt of an outbreak in Illinois, but predicting what will happen with insect populations is always influenced by other circumstances. With continued wet weather, it is possible that fungal disease pathogens could suppress armyworm populations. Other disease pathogens (e.g., viruses) and parasitoids could also take their toll in armyworm populations. However, when it comes right down to it, you will know what's going on only if you scout regularly and frequently over the next couple of weeks.--Kevin Steffey

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