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Elevated numbers of true armyworm moths present in Missouri during the past four weeks have resulted in economic infestations of larvae in fescue seed field and grass pastures, wheat fields, and in seedling field corn. Low levels of larvae can be found in most fields in central and southern regions of the state, although a majority of heavy infestations are present in south central and southwestern Missouri counties. Infestations have followed the typical pattern of first attacking fescue seed fields and grass pastures, followed by infestation in wheat and seedling corn. Many acres of these crops have been treated over the past 2½ weeks with much effort going to protect seedling corn at this time. Remember that small true armyworm larvae are nocturnal feeders (feed at night) and feed from the soil surface upward on the host plants. As larvae grow in size, they will feed during daylight hours and spend time higher in the plant canopy. Young larvae are pale green to light tan in color, but change to dull tan-yellow to brown as they grow. Larvae have smooth bodies with three dark lines running the length of the worm (one on top and one down each side). Some larvae will have a lighter tan-orange line present on each side of the body running from front to back of the larvae. The main identifying characteristic for true armyworm larvae is the presence of a dark triangle or spot located on the outside of each leg of the four pair of prolegs (legs found near the center of the caterpillar's body). Larval head capsules may be yellow, tan or green in color and mottled with darker brown coloration. True armyworm moths have grayish-brown to tan forewings, each with a white spot in the center and grayish-white hind wings. Three or more populations of true armyworm occur in Missouri each year, although the first generation tends to attack field crops before moving to turf and other host in later generations.

In addition to feeding on plant foliage, some populations of true armyworm larvae will also cut fescue seed and wheat heads from plants as the larvae approach maturity. Larvae grow to about 1½ inches in length and often move from field to field as each food source is exhausted. Thus their name of Armyworm which suggests they move or march in large numbers to new food sources once a food source has been exhausted. Treatment is warranted when an average of 4 or more non-parasitized, half-grown or larger worms per square foot are present. The economic threshold for head cutting in fescue seed fields and wheat is to treat when 2-3 percent or more of the heads are cut from fescue or wheat seed stems. The threshold for field corn is to treat when 25 percent or more of seedling corn plants exhibit foliar damage and larvae are present. Because corn seedlings are often attacked by later instars of this pest, damage from feeding larvae can be very severe. It is common for all foliage of corn seedling to be consumed by true armyworm larvae. This pest has been known to attack much larger corn where they often strip all leaf tissue from these larger plants.

Wayne Bailey
573-864-9905 (cell)
BaileyW@missouri.edu