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Armyworms in wheat: When treatment is warranted
Author: Gus Lorenz, Extension Entomologist

By Gus Lorenz, Extension Entomologist

There have been sporadic reports of true armyworm feeding in wheat this past week.  The question is: “Are they really causing any damage?” The armyworm population is very early this year and we continue to receive reports of armyworms in wheat that is flowering to early grain fill.

Figure 1.  True armyworm

Figure 1. True armyworm

Everybody is asking about sampling or how to find out how many armyworms they have. The following is some information that may be useful. Armyworms largely hide on the ground under litter during daylight hours and feed on the plant at night.   They feed on the lowest leaves first and work their way up.   The first visible symptom is the lack of a lower wheat canopy where the leaves are completely consumed.  Look under debris at the base of plants or in soil cracks to confirm the presence of larvae.

The most important risk factor from armyworms is the developmental stage of the plant at the time of attack.  Wheat in the late stages of development (soft dough and later) can be completely defoliated with no measurable loss to yield.  Usually, armyworm populations build only late in the wheat production season, when wheat has already reached this stage.  However, this year is the exception, and armyworms are feeding on the flag leaf prior to soft dough stage. Severe defoliation in boot-stage and flowering wheat can reduce yields.   If defoliation is significant in fields still in the flowering stage, treatments may be warranted.    Refer to the University of Arkansas Insecticide Recommendations Guide (MP144) for the currently labeled insecticides. The best time to apply an insecticide would be late afternoon since the armyworm feeds primarily at night.

Arkansas thresholds do not recommend treating wheat once it has reached the soft dough stage even if plants are completely defoliated.   However, in rare situations armyworm densities may get so large that they may cut the stem just below the head.   Obviously, serious yield damage would result in these situations, so fields should be observed closely to determine if head cutting is occurring.  The Arkansas threshold states that growers should “Apply insecticides when larvae are present and head cutting is occurring after wheat has reached milk/soft dough stage.” This threshold is based on sound research conducted here in Arkansas and much of it under extremely high numbers of armyworms. This threshold has the potential to reduce insecticide costs for growers and save money for growers.


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