Bryan Jensen, UW Extension, UW-Madison Integrated Pest Management Program
Just the name “armyworms” conjures up images of defoliated corn fields. Well, this summer they are living up to their names in isolated fields. To be sure, there has been a range of damage from the curiosity, to economic damage to near complete defoliation.
Armyworm do not overwinter in Wisconsin. Migrating adults usually arrive early spring and this migration is usually not a single event but rather a sequence of arrivals over a period of time. Armyworm larvae have tan head yellow belly with faint orange stripes on their sides and darker striping on the sides and back. Overall color intensity may range from individuals which may have faint coloration to those which are extremely dark colored.
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 I 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 along 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% of the plants or one per plant on 75% 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 consult A3646, Pest Management in Wisconsin Field Crops.