Check Corn & Hay for Armyworms in Late June

No one can reliably predict now if your corn or hay will be seriously attacked, but you can ensure that your fields are not stripped bare by checking them. It is easy. The time to look is late June. At that time armyworm caterpillars will be roughly ½ inch long, and very dark, almost black. You might be able to see lengthwise stripes. Note in the photos that there are distinct light and dark stripes. Some larvae may show white eggs of tachinid flies (parasites), as one of these photos does. The three photos here are from different sources. The one caterpillar on a stem is from the "Corn Insects of the Midwest" slide set, from Ohio State University. The photo showing a parasite egg is from a Purdue University slide set "Alfalfa pests of the midwest." The photo of two armyworms was taken by Ray Kriner many years ago.

How many is enough to worry about? We don't exactly know, but Cornell Univ. proposed treating corn if most plants are damaged, and 3 or more larvae are found per plant. Also, larvae should be small;1 inch or less. Spraying larger armyworms does little good, since they are hard to kill and have almost finished feeding. For hay land, Sid Bosworth at UVM has suggested treating if there are 4 to 5 larvae (less than 3/4 inch long) per square foot. To check hay land, you could make a wood frame that measures 1 square foot inside. I made mine from 5 dowels taped together (the fifth set diagonally). To use it, randomly throw it down, get down on hands and knees, and count caterpillars, especially close to the ground. Repeat in several spots in each field.. If you find few or none in late June, check again about July 2nd.

Click on thumbnail photos below for larger view:

    Armyworm overwinters here as tiny caterpillars (eggs were laid last fall). In some years, conditions are perfect to allow high survival of the caterpillars. In other years, few survive. We don't yet know all of those factors, so we can't reliably predict what will happen. Steve Turaj has already trapped adult armyworms (mid June) in Coos county. These may have flown in from farther south. If they contribute to a problem, it will be in July.

If you do have a problem, here are pesticide choices. For corn, registered pesticides include Sevin, Pounce, Ambush, Asana, Warrior, Dipel and Ketch. For hay land, there are several forms of Sevin (XLR plus, 80S, 80WSP, 4F...) and Bacillus thuringiensis (Ketch, Dipel) that are labeled for this use in NH.

Cutworms
I saw the worst case of black cutworm injury (to sweet corn) ever, on June 13th. Apparently this wet spring has been favorable for them. Usually cutworms hit one plant here, and another over there... This time every plant was severely chewed. You may run into cutworms, in which case I'd apply the same general guidelines. Cutworms are mostly nocturnal in their activity, so if an insecticide is warranted, it might be most effective if applied about dusk. That way it will still be wet when the cutworms become most active. We have several species of cutworms, so I'll just give a general description: they are caterpillars that are not hairy, and they usually bury themselves in the soil during the day, right below where they fed that evening. Some are striped, some not.

Alan Eaton, UNH Cooperative Extension Specialist, Entomology