By Rebecca Vittetoe

Fall is usually not a time you expect to get questions about armyworms. But last week in southeast and south-central Iowa, Iowa State University Extension received several questions about fall armyworms in cover crops, particularly in cereal rye, triticale or wheat cover crops.

In Iowa, fall armyworms can be pests in corn, hayfields and pastures, but has rarely been seen in cover crops.

Fall armyworms are unpredictable pests that do not overwinter in Iowa. While the larvae vary in color, most fall armyworms in the fall are black with three narrow, yellow lines from the head to the end of the abdomen. They also have a wider dark strip and a wavy yellow-red blotched stripe on each side of the body. Their head is dark brown with a prominent inverted “Y” on the face.

The adult moths migrate north from the Gulf Coast states. The adults will lay eggs. Those eggs hatch, the larvae feed, and then they pupate below the soil surface. Usually in Iowa, only one generation of fall armyworm per year is seen. Seeing fall armyworms in your cover crop fields this fall is no indication of whether you could potentially have issues with armyworms next spring.

Fall armyworm larvae feed on tender green tissues. A windowpane effect from feeding is the first sign of injury. Young larvae feed on the underside of the leaf, but leave the clear upper epidermis intact. This may give the field a “frosted” appearance. As the larvae grow, feeding is usually confined to leaf margins, but larvae can strip plants entirely of leaf tissue.

Spraying tactics
Where you may consider spraying is if you are planning on grazing or harvesting the cover crop for forage either this fall or next spring. With spraying, it’s important to consider the size of the larvae. In the fields Most larvae are at least an inch long or longer. Larvae typically reach a length of 1¼ to 1½ inches long. Once the larvae get much bigger than ¾ inches long, it’s harder to control them and their feeding is almost completed. Additionally, fall armyworms cannot survive freezing temperatures. The treatment threshold to spray is three armyworms per square foot. If you do spray, be sure to follow preharvest intervals listed in the insecticide label. 

Will the cover crop recover?
For cover crop fields with extensive damage by being chewed to the ground, the question becomes will the cover crop come back? The growing point of cereal grains would still be below the ground, especially if the field was drilled, as the growing point doesn’t move above the soil surface until it reaches the jointing stage, which occurs in the spring.

This is good news because it means the plants will continue to grow. However, as long as armyworms are present in the field, they will continue to eat the new growth. Stand loss may be affected by how much regrowth there is before winter dormancy is induced. 

Vittetoe is an Iowa State University Extension field agronomist covering southeast and south-central Iowa. Contact her at [email protected].