True Armyworm

Bryan Jensen, UW Department of Entomology and Division of Extension

It is time to start monitoring for armyworms. There have been several significant flights in WI and in states to our south. Will it be worse than last year? I don’t have that answer. First generation armyworm damage is hard to predict in terms of timing, location and severity.

Adult armyworm moths are attracted to grassy areas to lay eggs. Consider scouting all corn and wheat fields but especially those with significant grassy weed growth, corn no-tilled in to alfalfa and fields with a grass cover crop. Although larvae typically feed on grasses they do not feed exclusively on grasses. Soybean and alfalfa could have significant damage if there are no other foods sources.

Armyworm larvae have a tan head with numerous vein-like lines in the compound eyes. Body color and intensity can be quite diverse but alternating light to darker colored lines are usually noticeable. Typically, the “belly” is lighter colored than the rest of the body.

Armyworm Larvae

Armyworm larvae feed nocturnally and damage from small larvae may not be evident at first. In corn, look in the whorl for either damage or an accumulation of frass. In wheat and other small grains, look for early signs of feeding and/or larvae on the soil surface.

Seedling corn can be resilient to insect defoliation. If 50% of corn seedlings have injury, control maybe be warranted if larvae are still relatively small. Once larvae reach an inch to 1 ¼ inch in length they will soon pupate, and spraying is not advised. That may sound simple but many times the infestation has different size caterpillars.

Armyworm Feeding On Corn

In wheat and other small grains, damage may be concentrated in lodged areas or areas with dense growth. Check all fields closely by looking for both leaf defoliation and head-clipping. An economic threshold of 3 or more larvae/square foot has been established. However, crop stage and presence of head-clipping may influence your decision. If you do choose the option of treating with a foliar insecticide, choose one with a pre-harvest restriction that fits within your harvest schedule.