Fall armyworm feeding noted in cover crops in SE/SC Iowa

Fall is usually not a time I expect to get questions about armyworms, but last week I 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 this is the first time that I’ve seen them as a pest 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 pupate below the soil surface. Usually in Iowa we only see one generation of fall armyworm per year. 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. Windowpane 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.

What do you do if you have fall armyworms in your cover crops? Do you need to spray? Will the stand come back? Those are all great questions.

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 I’ve looked at, most larvae are at least an inch long or longer. Larvae typically reach a length of 1 ¼ to 1 ½ inches long. Once the larvae get much bigger than ¾ inch long it is 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 pre-harvest intervals listed in the insecticide label. 

For cover crop fields with extensive damage and that have been literally 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 still present in the field they will continue to eat off the new growth. Stand loss may be affected by how much regrowth there is before winter dormancy is induced.