Now is the time to check wheat fields for armyworm

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Check wheat fields for armyworm. Reports the week of June 9 range from none to a bunch, with Sanilac County reporting the most consistent populations. It is important to walk into fields to assess populations, because the populations are not consistent from field to field. The caterpillars are still small (1/2 inch), which is ideal for control if populations are over threshold. Older caterpillars may be harder to kill plus they will already have done considerable damage (Photo 1). Armyworms are often active after dusk and on overcast days, so they may not be apparent up on a plant in the middle of the day. Look for feeding damage on leaves, then check at the base of the plants and under crop residue (Photo 2) for caterpillars or even caterpillar poo (Photo 3). Before heading, the threshold is four or more caterpillars per square foot. At heading, the threshold drops to two or more per square foot. If damage is heavy and leaves are eaten, larvae sometimes clip heads (Photo 4). Obviously, we want to control infestations now, and not get to the point where grain ends up on the ground. If you need to treat, spray later in the day, larvae will crawl across the fresh product as they move up to feed on the plant. Labeled products with pre-harvest intervals include Baythroid (30 days), Entrust (21 days), Lannate (7 days), Mustang (14 days), Proaxis (30 days), Sevin (21 days), Tracer (21 days), and Warrior (30 days). Be sure to check labels as rates may differ among formulations.

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Photo 1,
Heavy damage from armyworm. This picture was taken near Sanilac, MI in 2004.
Photo 2, Armyworms litter the ground during the day in this heavily infested field.
At least 13 caterpillars are in the picture, well over threshold.

Armyworm can attack corn and soybeans as well, and I had a few reports of both. Fields with cover crops and weeds are at risk for armyworm infestation although even clean fields can be infested. As with wheat, caterpillars hide out at the base of plants during the day, and also in the whorl of corn, feeding leaves behind tattered leaves (Photo 5). With warm temperatures insects can grow quickly, so damage can increase in just a matter of days. Recognizing an over-threshold infestation early and treating smaller larvae, which are more susceptible to insecticides and eat less, protects yield. In soybean, treat at 25 percent defoliation (this may be difficult to judge). For corn, treat when 25 percent of plants have two or more larvae per whorl, or when 75 percent of plants have one larvae. For both corn and soybean, you can choose from the same product list as for wheat. Check labels for rates and precautions; of course pre-harvest intervals are not an issue in either crop. Note that armyworms sometimes originate in one field and move into another. In that case, you may be able to limit your spraying to an edge or barrier treatment near the infested field.

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Photo 3,
A sign of infestation - armyworm frass, or poo, in the form of little green pellets on the ground.
Photo 4, Wheat plants from Sanilac, 2004.  The leaves have been completely eaten, and many of the
heads on the left have been clipped.
Photo 5, Armyworm damage to corn - note the tattered leaves and intact midrib.

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