Armyworm damages corn and small grain fields

There has been a higher than usual incidence of armyworm problems in corn and small grain fields this year in Wisconsin. The incidents have been variable, and some of them have been severe enough to cause economic damage. They are not necessarily hard to control, but people do need to look at fields and see if they are there. Right now we appear to be in the middle to end of the second generation which is sometimes called the summer generation. Some references suggest the possibility of a third generation but many think that is doubtful for Wisconsin. Typically, armyworms are attracted to grassy areas to lay eggs, and this may explain some of the infestations; however, it doesn’t explain all of them. Wheat and other small grains are at risk until harvest. Before considering an insecticide application, look at the PHI to make sure that the insecticide fits with your harvest plans. Many, if not most, insecticides have a long PHI which may prevent timely harvest. Pastures should also be monitored. If larvae run out of a food source they can move to adjacent crops and/or lawns. They may also move from adjacent marshes into fields. Feeding in corn fields above the ear zone is particularly of economic concern. If you find signs of armyworm feeding, check five sets of 20 plants at random. Record the number of damaged plants and the number of worms per plant. Spot treat, if possible, when you find two or more armyworms (0.75-1.0 inch or smaller) per plant on 25 percent of the plants or one per plant on 75 percent of the plants. When making a treatment decision think about damage you can prevent. Don’t focus on how much damage is currently there. Large larvae will be feeding for a much shorter period of time. However, from the reports and pictures I have received many fields had a range of different sized larvae making control decisions more difficult. Do not assume corn planted with an above ground Bt trait(s) will not have damage. Traits packages vary in their insect control spectrum and may not provide adequate control under heavy infestations. For specific insecticide recommendations please contact your County Extension Office or consult A3646, Pest Management in Wisconsin Field Crops.