Armyworm on wheat heads.
Armyworm on wheat heads.

This season, armyworm infestations have been severe in some fields and, currently, they are marching to adjacent fields. Many observers still report a large number of small and large larvae, probably because moth flights were extended over several weeks.

Michigan State University’s recommendation is to treat when there are two or more larvae per square foot in wheat that is headed. Usually at this time of the season, wheat harvest is a month or more away. However, this year much of Michigan’s wheat is only two or three weeks from harvest.

The question is whether or not to treat even though the wheat is nearing maturity or the field may already be mostly defoliated. In fields that are over threshold, the answer is probably “yes.” Although some larvae may be exiting these fields, there are usually plenty of larvae remaining, and few parasitoids have been observed killing these larvae.

Furthermore, there is a real risk of further feeding loss, as armyworms sometimes have a rather sudden and inexplicable drive to feed on the stem just below the head (also known as head clipping) as green tissue and plant moisture become scarce. Where this phenomenon occurs, many bushels of yield are lost in just a few evenings of feeding.

Finally, larvae from these defoliated fields may move into neighboring crops, causing further loss. In this case, a border treatment, rather than a whole-field spray, may take care of the problem.

If a decision is made to treat a field now, be very careful about pre-harvest intervals (PHI) on labels, which range from 7 to 30 days. Again, usually at this time of year we are four to five weeks from harvest, and most insecticides labeled for wheat would be legal. However, the short time to harvest now functionally limits the choices to Mustang Max (14-day PHI) and Lannate (seven-day PHI).