Know what to look for when scouting for armyworms

True armyworms do not overwinter in Michigan. Armyworm outbreaks occur when adult moths are blown in from Southern states in the spring. The warm winter and early spring experienced in much of the Midwest this year may have contributed to the arrival of armyworm moths in the state by giving them a head start as they made their way north this season. Both Indiana and Ohio were reporting scattered outbreaks of armyworm feedings — some cases were quite severe — in late May.

Armyworms lay their eggs in grassy areas such as hay fields, in grass margins along the edge of fields, and in grass crops such as wheat and small grains. Armyworm feeding injury may be the first sign of a problem. Early feeding is generally along leaf edges of newer leaves

Key Points

• Armyworm feeding injury may be the first sign of a problem.

• Early feeding is generally along leaf edges of newer leaves.

• Always follow label directions if an insecticide application is warranted.

Armyworm larvae are the most active at night and on cloudy, overcast days. Larvae are most easily found and counted during the morning hours in the first few hours of daylight. In the afternoon during sunny days, larvae will seek shelter on the ground under plant debris, and in corn they may be found down in the whorl. Scouting in the afternoon will take patient observation, searching through the debris on the ground and shaking plants to find the larvae.

Lacking in uniformity

While armyworm outbreaks can be large, they tend not to be uniform. 2010 had many examples of fields at threshold and well below threshold, separated by a mile or less, and outbreaks did not occur in every part of the state. The only effective way to determine a need for control is to scout fields for larvae.

The threshold for armyworms in wheat is two or more larvae per square foot at heading. The threshold for corn is 25% of plants with two or more larvae per whorl, or 75% of plants with one larva per whorl.

Do not add an insecticide to fungicide sprays as an insurance measure. Many beneficial insects are at work, especially in wheat fields, keeping other pests such as cereal leaf beetles in check. There are also natural predators of armyworms that help keep low populations in check.

Always read and follow label directions if an insecticide application is warranted and note the preharvest interval.

The only tried-and-true way to identify fields in need of an insecticide treatment is to scout. Don’t panic, don’t act on a rumor of what the neighbors are doing, but do send out the scouts.

More information can be found at the following resources: “March of the Armyworms, Part I: Wheat” and “March of the Armyworms, Part II: Corn,” both by Chris DiFonzo, MSU, and “Small Grain Insect Pests,” MSU Extension Bulletin E-1582.

Springborn writes for MSU Extension.


This article published in the July, 2012 edition of MICHIGAN FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.