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Reports of Armyworm Injury in Corn and Wheat Continue

June 1, 2001
Observations of armyworm injury have begun to slow down south of Interstate 70 in Illinois. However, reports of armyworm infestations in central and northern counties are becoming more common. Stan Eden, crop systems unit educator, Ogle County, reported on May 25 that armyworms were damaging cornfields in Ogle County that had been planted in sod. We've also heard that armyworms can be found easily in wheat fields throughout much of central and east-central Illinois. Marion Shier, crop systems unit educator, Livingston County, has observed armyworm infestations in several wheat fields that are approaching economic levels. Wheat fields in Iroquois County, especially near Cissna Park, also are infested with numerous armyworms. Producers are encouraged to continue scouting efforts in wheat and be ready to apply rescue treatments as needed. Attention should be given to any head-clipping activity that is observed, and fields should be monitored regularly for any signs of this armyworm behavior. An insecticide rescue treatment should be considered when there are six or more nonparasitized armyworms (3/4 to 1-1/4 inches long) per linear foot of row and before extensive head cutting occurs. Products labeled for armyworm control in wheat include *Penncap-M (2 to 3 pt product per acre), Sevin XLR Plus (1 to 1-1/2 qt), and *Warrior (2.56 to 3.84 oz). Those products preceded by an asterisk are restricted-use insecticides and may be applied only by certified applicators. Please read and follow product labels for more complete application instructions.

As cornfields mature beyond the seedling stage of development, they become less susceptible to economic losses caused by armyworm defoliation. Experiments conducted in the early 1980s by entomologists (Phillip Mulder and William B. Showers, Journal of Economic Entomology, 1986) at Iowa State University provided the following results concerning the effects of armyworm defoliation on corn yield: "Specifically, under normal Iowa environmental conditions (1982), 7-8 leaf growth stage corn can sustain nearly 21% defoliation by armyworm larvae and somehow compensate or recover, producing yields comparable with those of undefoliated corn. During hot dry years, 9-10 leaf growth stage corn sustaining very little (<6%) or heavy (>50%) defoliation also produces yields similar to those of undefoliated plants." These results point out the need not to panic when you observe armyworm injury in corn, especially when it reaches the 7-leaf stage of development.

As we've mentioned in many previous issues of the Bulletin, two to three generations of armyworms occur each growing season throughout much of Illinois. Although corn is becoming less susceptible to economic losses, pastures will remain at risk to damage for much of the summer and early fall because of subsequent generations of armyworm larvae. So, producers will need to remain vigilant in their monitoring efforts of armyworm activities in pastured areas for the remaining portion of the summer. Hopefully, diseases and parasitoids will reduce populations of armyworms to below economic levels in the following generations.--Mike Gray

Armyworm defoliation to corn planted in sod, Ogle County, May 25, 2001. (Photo courtesy of Stan Eden, Crop Systems Unit Educator, Ogle County, Illinois.)

Author: Mike Gray

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

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