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Yellowstriped Armyworms in Soybeans
Author: Glenn Studebaker, Extension Entomologist/IPM Coordinator

I’m getting a lot of calls about yellowstriped armyworms in soybeans in NE Arkansas. They seem to be worse in fields with small plants, particularly double-crop soybeans planted behind wheat. In most cases, the caterpillars are still pretty small and not causing much defoliation yet. Some fields also have a few corn earworms along with the armyworms. Treatment level for armyworms or earworms before bloom is based on defoliation levels. Before bloom, this level is 40% defoliation. The majority of fields are not at, or near this level yet. However, many growers are getting ready to go across the field with a herbicide application and are asking if it would be a good idea to add an insecticide for the armyworms. With the high cost of fuel, it is understandable why a grower would want to add an insecticide and make one trip across the field to avoid possibly having to make another trip for the worms next week. The real question is, “Will the level of defoliation reach treatment level next week as the caterpillars get larger?” Obviously, none of us know the answer to that question. However, there are several things to consider other than just economics before adding that insecticide to an application for insects that are not yet at treatment level.

1) Is the field approaching treatment level? A field with a defoliation level of 30% or more is much more likely to reach 40% in a few days than one that is only at 10%. If the field is not near the 40% threshold level, recheck the field again in 3 or 4 days.

2) Are there predatory insects present in the field? Predatory insects can be very effective at lowering levels of these smaller caterpillars. Making an unnecessary insecticide application at this time is likely to sterilize the field, making it more likely to be infested with the more damaging levels of corn earworm later in the season. In other words, adding an insecticide to control a pest that “might” reach threshold to save a few dollars on making an extra trip across the field, may result in making several expensive trips across that field later in the season.

There are reports of growers adding an insecticide as they go across everything on their farm to clean up what is in the field, regardless of the presence of pests. This is NOT good IPM. Blanket, across-the-board spraying of an insecticide will take out the natural enemy complex and set us up for damaging levels of other pests, such as earworms or loopers later in the season. All fields should be scouted and then treated, IF they are at treatment level for the pests that are present.

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