True armyworm damage to corn plants in Centre County. Note the feeding damage along the leaf margins and the moist frass (insect feces) evident in the whorl. Image Credit: John Tooker, Dept of Entomlogy, Penn State
Just yesterday at our Penn State’s Centre County research farm, we found an outbreak of true armyworm in V5 corn that had been planted following cereal rye. This is the perfect time to remind you to keep this pest in mind because it can quickly cause damage to corn, hay, and wheat fields. For corn, very few transgenic, insect-resistant corn hybrids protect against true armyworm so I encourage growers to scout their corn and hay fields looking for armyworm populations and damage. Some growers will elect to add insecticides to post-emergence herbicide applications to help control armyworm, but these treatments typically do not protected fields against infestation because they are not timed to coincide with armyworm populations.
Here are a few details to satisfy your curiosity about armyworms. Adult moths fly into Pennsylvania after overwintering in the soil in states to our south. Females lay their eggs on weeds/grasses along field margins or on small grains and move to corn when weeds killed or small grains are harvested. Armyworm can occasionally cause problems feeding on small grains (often clipping heads of small grains), but tend to be problematic more often in corn when small grains are harvested because armyworms move to young corn plants. Armyworms tend to feed at night along the margins of corn leaves, avoiding midribs. During the day, larvae hide in leaf sheaths or in the soil or leaf litter. Rescue treatments are usually the most efficient and economical tactic for managing true armyworm because populations are very spotty and preventative applications may not have sufficient residual activity to kill caterpillars that hatch out later. Armyworms can warrant treatment should infestations reach 25% of plants in a field. A recently revised Armyworm fact sheet provides more information on armyworm.