E-57
DEPARTMENT OF ENTOMOLOGY

field crops insects

ARMYWORMS
C. Richard Edwards and Marlin K. Bergman, Extension Entomologists

Armyworms feed on the leaves and stems of small grains, grasses, and alfalfa. They also may eat the beards of wheat heads and even clip off the heads of wheat, rye, oats, and barley. On corn and sorghum, armyworms feed on the whorl leaves of young plants.
Infestations of true armyworm, fall armyworm, and yellow-striped armyworm are found in Indiana every year. However, only the true armyworm and fall armyworm cause any substantial economic loss. The extent of destructive populations depends upon moth migration patterns, weather favorable for moth flight and larval development, and the abundance of armyworm natural enemies such as parasites, predators, and diseases. Significant infestations causing economic damage and requiring treatment develop once every four or five years.

LIFE CYCLE AND DAMAGE
True Armyworm - The true armyworm moth is about 1 inch long, tan in color, and has a tiny white dot in the center of each forewing. The moth is attracted to lights and is commonly found at night around windows and doors.
Initial moth flights are usually detected in early May. A few armyworms may overwinter in Indiana as pupae or adults, but most spring moths are immigrants 
from the southern U.S. The moths lay eggs in rows or clusters on the lower leaves of grass plants.
Newly hatched armyworm larvae are greenish in color and move about with a looping motion. Mature larvae are hairless 1-1/2 inch long caterpillars; their skin is smooth and dull-colored, with greenish and dark stripes running the length of the body. The true armyworm's head is pale gray or greenish-brown and covered with a network of dark lines. The larval stage will last three to four weeks, depending on temperature. The first or spring generation of armyworm usually begins to cause damage about the time wheat heads emerge above the flag leaf. Later generations are generally held in check by natural enemies.
Armyworms are most active at night and during cloudy days. At other times, larvae hide beneath crop debris. Infestations usually develop in grass pastures, fence rows, and roadsides where vegetation is dense. Other sources include areas in small grain fields where the crop has lodged or is matted against the ground and grasses not controlled in corn fields. Once the worms have consumed the available food, they "march" in great numbers to find other plants to feed upon. This is why they are called armyworms.
Armyworm moths sometimes lay their eggs in grass cover crops or small grain fields that are later planted to corn. As a result, the true armyworm may already be present to attack the corn seedlings as they emerge.
Fall Armyworm - The fall armyworm does not overwinter in Indiana or other midwestern states. Moths migrate northward from southern states in the spring

COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA 47907