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Armyworms Marching Throughout Southern and Central Illinois

May 18, 2001
Reports of serious damage to corn, wheat, and grass hay fields poured into our offices during the latter part of the week of May 7 and the first half of the week of May 14. We first called your attention to armyworms in a brief article in issue no. 5 (April 27, 2001) of the Bulletin, when we reported that Ron Hines, senior research specialist, Dixon Springs Agricultural Center, had captured some adults in traps in Pope and Massac counties. The following week in issue no. 6 (May 4, 2001) of the Bulletin, Mike Gray wrote a more extensive article to discuss armyworm biology, scouting procedures, thresholds, and insecticides suggested for control of armyworms in wheat and corn. And the fun had only just begun.

Wheat stems stripped of leaves by armyworms in Clinton County. (Photograph courtesy of Ike Leeper, Clinton County Unit Leader.)

Reports of grass hay fields and some wheat fields being completely stripped (all leaves devoured, only stems remaining) have been numerous. We have received so much information from so many people that naming the individuals would be cumbersome. However, some notable quotes have been:

· "Lots of sad stories of grass pastures, wheat, and corn being completely lost."

· "I've never seen armyworm damage worse."

· "I have 60 acres wiped out by armyworms."

· "Grass/hay losses have just about been Biblical in proportion."

Observers in some counties have reported that in some fields "the ground appears to be moving." They also have observed huge numbers of armyworms crossing roads to get from one food source to another. (I'm reminded of a heavy, although localized, outbreak of woollybear caterpillars a few years ago. They, too, were crossing county roads in such large numbers that vehicles were slipping and sliding on the crushed bodies [Blech!]. County officials put up "Slippery When Wet" signs to warn motorists of the potential danger of roads slippery with woollybear guts.)

Unfortunately, a lot of people are learning how the name "armyworm" was derived. "Armies" of armyworms will "march" from a depleted food source (for example, a stripped wheat field) to another source of food (for example, an adjacent cornfield), seemingly devouring everything in their paths. They will continue this behavior until they complete their development and pupate.

The burning question right now is, How much longer will these armyworms continue to cause this much damage? As some of you might have guessed already, the answer is, "It depends." If the armyworms are fully grown (1-1/2 to 1-3/5 inches long) or nearly so, their feeding will cease within a matter of hours to a couple of days. If the average length of the armyworms is less than 1 inch, they will continue to feed for a few days. Excerpts from the section about armyworms in the Entomological Society of America's Handbook of Corn Insects provide an overview of their life history: "The armyworm completes 2­3 generations per year. The first generation occurs in May and June, and the second and third generations occur in July and August, respectively. Armyworms overwinter as pupae in the soil in warmer climates. Adults migrate in April and May from southern parts of their range into northern corn-producing states. Pupation takes place in the soil or ground litter in a city or cell. Adults emerge in about 1 week, except for the generation that overwinters."

Leaf-feeding injury by armyworms in a wheat field in Madison County. (Photograph courtesy of Robert Bellm, Crop Systems Educator, Edwardsville Extension Center.)

Armyworm larva on the soil in a wheat field in Madison County. (Photograph courtesy of Robert Bellm, Crop Systems Educator, Edwardsville Extension Center.)

Several armyworm larvae on the soil in a wheat field in Madison County. (Photograph courtesy of Robert Bellm, Crop Systems Educator, Edwardsville Extension Center.)

Despite the widespread nature of the armyworm problem, we need to keep some perspective. Many of us receive reports of damage and large numbers of armyworms, but few of us receive reports about no damage and no armyworms. Nevertheless, not all fields are being devoured. On May 14, Robert Bellm, crop systems educator, Edwardsville Extension Center, conducted a quick survey of wheat fields in a transect across Madison County. He found armyworms in every field, but their densities exceeded economic levels (6 or more per foot of row) in only two fields, where densities were 12 to 16 larvae per foot of row. The sizes of the larvae ranged from 1/2 to 1 inch long. By now our advice sound clichéd, but here it is anyway: scout all fieldsdon't assume that armyworms are causing economic damage in all fields.

So, what to do? Insecticides suggested for control of armyworms in wheat and corn were listed in issue no. 6 (May 4, 2001) of the Bulletin. Unfortunately, options for control of armyworms in grass hay fields are limited to malathion (questionable control) and formulations of Sevin. However, the label for Sevin applied to pasture and grasses grown for seed states: "Do not apply within 14 days of harvest or grazing."

An interesting note regarding armyworm control is that Bt corn seems to be holding up well against the armyworm onslaught. Tim Marriott, information manager (Precision Farming) with Wabash Valley Service Company, sent a photograph of what was left in a cornfield in Lawrence County after armyworms had had their way. The producer had planted alternating 12-row strips of a Bt-corn hybrid (YieldGard) with a non-Bt hybrid. In the photo, you can see for yourself that only the Bt corn remains.

12-row strip of non-Bt corn devoured by armyworms adjacent to a 12-row strip of Bt corn in Lawrence County. (Photograph courtesy of Tim Marriott, Information Manager [Precision Agriculture], Wabash Valley Service Company.)

Kathy Flanders, an extension entomologist at Auburn University, spoke about the Status of Transgenic Crops for Control of Insects other than European Corn Borer at the 2000 Illinois Crop Protection Technology Conference. In her article for the Proceedings of the conference, she wrote: "Excellent control of armyworm, Pseudaletia unipuncta (Haworth), has been observed with the Mon810 event [the event responsible for most YieldGard hybrids]. These results were from no-till corn planted into small grain that had been burned down with a herbicide."

So we'll have to ride this one out, at least for a few more days. People in southern Illinois are more aware of the situation than people in central Illinois, so this article is a heads-up for people who might not have begun to look for armyworms. Because armyworms usually are inactive during the day and feed at night, their presence often is not detected until considerable damage has been done. So don't wait any longer; look for armyworms wherever they might be present, and be prepared to take action if the situation warrants it.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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