Armyworm Heads Up

Bryan Jensen
UW Extension

It is about that time that we could be seeing problems from the summer generation of true armyworms. I bring this up because of the elevated number of corn fields which had significant feeding this spring and because of a particularly high late-June black light trap count reported by the WI DATCP in Rock County.

Damage from the summer generation of armyworm is often more difficult to predict and/or find. The migrating (spring) generation is somewhat easier because damage is often associated corn planted after a rye cover crop or no-tilled in to alfalfa. The summer generation is not typically attracted to a specific field or cropping history and the height of corn makes them difficult to find.

What I would suggest is to do some spot-checking in corn fields over the next few weeks. Try to catch armyworm feeding when the larvae are still relatively small and before damage is wide spread. Field populations of the summer generation can be quite high, and in extreme situations, can nearly defoliate corn.

Wheat and other small grains are at risk as well. Especially over the next few weeks and prior to harvest when larvae can switch from leaf feeding to head clipping.

Economic thresholds and guidelines for corn is to treat when either 75% of the plants have one armyworm/plant or 25% of the plants have two or more larvae AND the larvae are ¾ -1 inch or less in length. Treating in small grains is suggested if there are 3 or more armyworm/sq ft. But be careful of head clipping.

To be clear there have been no reports of damage from the summer generation to date. It is just the time to start spot-checking.