No. 9 Article 10/May 21, 2004

Armyworms Are Making Their Presence Known in Some Areas

Just this week, we began receiving reports of armyworms feeding in wheat fields and cornfields in the area of Bond, Clinton, and Montgomery counties. All reports of significant armyworm activity thus far have come only from those counties, so it is possible that the problem is localized. However, it is very important that everyone be on the alert for armyworms and their feeding injury.

The first report we received (May 24) was from Bill Tarter with Alvey Labs who reported armyworms moving from wheat fields into adjacent cornfields. Robert Bellm, Extension educator in Crop Systems in Edwardsville, and Matt Montgomery, Extension educator in Crop Systems for Sangamon and Menard counties, also reported armyworm infestations in Bond and Montgomery counties on May 24 and 25. Both of them had received initial reports about armyworm activity from others and followed up these reports with their own scouting activities. Robert Bellm learned from Lynn Weis, Bond County Extension Unit Leader, that armyworms were causing some problems in northern Bond County and southern Montgomery County. Some insecticide applications in that area were either being planned or were underway. Robert also learned from Jody Fouch, Syngenta Seeds, that in the same area, as much as 95% of the flag leaves had been devoured in some fields, with 10 to 12 armyworm larvae per foot of row.

On May 24, Matt Montgomery received a report of armyworms feeding on popcorn that had been planted into rye in a field in Mason County. He scouted some fields on May 24 and 25, and although he did not find large numbers of armyworms, he noted that the armyworm larvae were at various instars. Obviously, the smaller instars may still be difficult to find, but when they are large enough, they will begin causing noticeable injury. Matt also learned from Kelli Bassett, Academic Extension Hourly for Crops and Horticulture in Montgomery County, that armyworms were feeding in wheat fields in the area.

Injury to corn (popcorn planted into rye) caused by armyworm larvae (photo provided by Matt Montgomery, University of Illinois Extension).

Severe armyworm damage in popcorn planted into rye (photo provided by Matt Montgomery, University of Illinois Extension). All leaves have been devoured, leaving only "corn sticks."

Our first "warning" article about armyworms was published in issue No. 5 (April 23, 2004) of the Bulletin. Our information then was sketchy because we were aware only of moth flights at that time. In some areas, obviously, armyworm moths found sites suitable for oviposition, and the environment continued to promote survival of the larvae. The seemingly sudden onset of armyworms is simply because the larvae now are large enough to cause noticeable leaf-feeding injury to wheat or corn plants.

Once again we refer to the armyworm outbreak of 2001. The outbreak in 2001 was extremely widespread in North America, and the economic losses were significant, especially in grass hay fields for which few insecticides are labeled. We are not suggesting that armyworms will wreak similar havoc in 2004, but we strongly encourage people not to take early warnings for granted. If you have not begun to scout for armyworms in your area, start now. Be especially alert that different instars may be present. Young larvae are pale green in color, although longitudinal stripes are apparent, and the head is yellow-brown. They move in a looping motion. Older larvae are green-brown and more prominently striped. You can usually see a narrow, broken stripe along the center of the back and three stripes along each side of the body, at least one of which appears pale orange. The tan head is mottled with dark brown. Each proleg (the false, peglike legs on the abdomen of a caterpillar) has a dark band.

Different instars of armyworm larvae (photo provided by Matt Montgomery, University of Illinois Extension). Note the dark bands on the prolegs of the largest larva.

In wheat fields, control of armyworms may be justified if you find 6 or more non-parasitized larvae (3/4 to 1 1/4 inches long) per linear foot of row and before extensive head cutting occurs. In cornfields, the threshold is little bit more iffy. Control may be justified when 25% of the plants are being injured. Corn planted into rye or next to wheat is the most susceptible to infestations of armyworms. Insecticides for control of armyworms in corn and wheat are shown in Table 1. Please follow all directions and precautions on the insecticide labels. Be particularly cognizant of pre-harvest intervals for the products labeled for use in wheat.

For more detailed information about armyworm biology, etc., refer to articles in issue Nos. 8 and 9 (May 18 and 25, respectively) of the 2001 Pest Management & Crop Development Bulletin.--Kevin Steffey and Mike Gray

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