Armyworms in Wheat in NE Wisconsin

Bryan Jensen, IPM Program

Many thanks to Jeff Polenske and his colleagues of  Polenske Agronomic Consulting, for alerting us to the potential for armyworm injury in the northeast part of the state. Currently they are finding significant numbers of large armyworm larvae in wheat. Very little head clipping at this time but harvest is still a ways off. Let’s take some time to review armyworm habits. Hopefully this will help you with scouting and making control recommendations.

Armyworm larvae feed primarily on grasses, whether they are weeds or cultivated crops. Now is a good time to check corn, pastures and all small grains regardless of crop stage.   Adults seek areas of dense cover to lay eggs. For that reason, concentrating scouting efforts around weed escapes can be a good place to start. Also, in small grains armyworm damage may be more severe in lodged areas.  

How much longer will they be feeding? That is very difficult, if not impossible to answer on a statewide basis. Adults migrate to the state each year and time of arrival (and intensity) is variable. We have seen significant armyworm active from late June to early August. Locally, it is easier to answer. Although length is not a true indication of instar (stage between molts), length can be used to get a relative estimate of age, particularly the time left to feed. Armyworms have 6 instar stages and the final instar can measure between 15/16 and 1 5/16 inch.   Again, these are relative size estimate but it is helpful when formulating recommendations. For instance if most of your populations is less than ¾ inch you can be certain they will be feeding for a much longer period of time.

Armyworms are nocturnal feeders. Look for signs of leaf feeding and, especially in small grains, look at the soil surface for larvae. The economic threshold for small grains is a minimum of 3 per sq. ft. However, after physiological maturity leaf feeding is not the primary cause of concern; head clipping is. Armyworms may continue to feed on matured leaves or leaves of other grasses within the field, migrate out of the field to another hosts or to feed on the stem below the head causing it to fall to the ground. Head clipping is unpredictable and it is not a foregone conclusion that larvae will switch to clipping heads as the plant matures. When larvae continue to feed on mature leaves but show no signs of head clipping, field revisits are important and can help ease the anxiety.     

In corn, the threshold is to treat if larvae are less than ¾ inch and you have and average of 2 on 25% of the plants, or 1 larvae on 75% of plants. 

 

For more information, see page 198, Pest Management in Wisconsin Field Crops (A3646).

http://learningstore.uwex.edu/pdf/A3646.PDF