Armyworms: Scouting and control tips
The University of Missouri Extension releases information about armyworms, when scouting should begin and how you can control them. They released the following:
Scout Fescue Pasture for True Armyworm
Scout for true armyworm in fescue pastures. Migrating moth flights start in April and scouting should begin in May for Southwest Missouri. According to Kevin Rice, University of Missouri State Entomologist, there are overwintering moths in Missouri, but the moths that migrate from the south produce the larvae that causes the most damage in fescue.
According to Jill Scheidt, University of Missouri Extension agronomy field specialist, ¾ to one-inch larvae have been found at near threshold levels in Southwest Missouri fescue fields. Rice recommends utilizing control methods when larvae reach threshold level in forages of three or more ½” non-parasitized larvae per square foot. If conditions are conducive for hay harvest, look to harvest hay early. If a pesticide is needed, use a pyrethroid.
Wait to treat with an insecticide until larvae are half grown or at least ½” long. There are several natural viruses, bacteria and fungal pathogens and natural predators that can control armyworms during the early instars. Larvae that are 1 ½” long are too big to treat, as they are close to pupation, or the end of their life cycle.
Scout often, armyworms are one of the fastest growing animals on the planet, which means they eat a lot and fast. Armyworms have the potential to destroy a whole field in one day in the event of an outbreak. Populations are likely to increase with cool, wet weather. The best time to scout is dusk or dawn, when it is cool; activity is slowed with warmer temperatures. Start on field edges and target fields with lush growth. Beat fescue against itself to knock larvae to the ground; look on the ground and under debris for larvae. Ensure proper identification by locating four sets pro-legs in the middle of the body and looking for a dark longitudinal band on each pro-leg. Do not count parasitized larvae in population estimates. Parasitized armyworms may be white, or dark colored and dried out in appearance; or use a hand lens to inspect any slow-moving larvae for parasitoids. If there is a lot of bird activity in the field, armyworms are likely present, as armyworms are a food source.