Native to North America, the armyworm can be found east of the Rocky Mountains, reaching northward into southern Canada. Armyworms get their name from their behavior of moving across fields in an army-like fashion.
As larvae consume available food sources, they migrate as an army to new host plants.
Though they feed primarily on grasses (oats, wheat, fall rye, corn, barley, and forage grasses), they can be a pest of some vegetables (bean, cabbage, carrot, onion, pea, pepper, radish and sweet potato).
There are usually two to three generations each year in Illinois.
Description:
Armyworm larvae vary in color from dark greenish-brown to black. On each side, there are long, pale white, orange, and dark brown stripes along the length of the abdomen. Mature larvae are approximately 1 ½ inches long.
The head capsule is yellowish brown with a brown network of veins, giving it a mottled appearance. Armyworm larvae may also be distinguished by a dark band on the outer side of each proleg. Armyworm pupa stay in a brown earthen shell just below the soil surface.
The armyworm moth, approximately an inch long with a 1 ½ inch wingspan, is tan to light brown, with a tiny white spot centered on each forewing. Eggs, which resemble small white globules, are laid in rows or groups on leaves of host plants. After oviposition, the moth rolls the leaf blade of the plant around each egg mass.
Life Cycle:
Few armyworms overwinter in Illinois. Most armyworm moths migrate into the Illinois from the southern states in April and May. During the day, moths remain hidden in grassy vegetation. Armyworm moths are active during the evening, feeding on nectar, mating, and searching for oviposition sites.
Eggs are deposited in rows or clusters on the lower leaves of grasses or at the base of plants. Eggs hatch in 1 to 2 weeks. Newly hatched larvae are pale green and move in a looping motion. Larvae are also active at night, feeding on host plants.
During the day, they can be found under plant debris or in the top few inches of the soil. After completing six instars, larvae pupate just below the soil surface. Adults emerge in 1 to 2 weeks. A second generation occurs in late June or early July and a third in late August or early September.
Injury:
Since moths prefer dense vegetation for oviposition sites, armyworm infestations generally develop in areas such as grass pastures, roadsides, and along fence rows. Oviposition sites often include weeds and grassy in weedy or reduced tillage fields. Consequently, these fields armyworms are often a problem in these fields.
If a herbicide is used to control the weeds, larvae move from the dead grasses to the corn. As larvae run out of available food sources, they move to other host plants such as small grains and corn. This usually occurs during May and early June.
Armyworms consume leaf tissue of corn plants. Feeding is usually confined to leaf margins, but in some instances, larvae may strip the plants entirely of leaf tissue.
Corn generally recovers from damage caused by moderate infestations if the growing point has not been injured. Ordinarily armyworms that attack young corn migrate from small grains or grass fields.
Armyworm larvae chew the leaves of small grains and grasses. They may strip the leaf margins and move up the plant to feed on the panicles or flowers. Larvae will feed on the flag leaves, kernels, and clip the stems just below the heads.
Injury caused by armyworms in forages is sometimes confused with that caused by other insects. Armyworms do not attack pure stands of alfalfa and other legumes, but cutworms do feed on these crops. If the field is a grass legume mixture, cutworms and armyworms may both be present.
If it is a pure grass stand, the insects are probably armyworms. The damage is the same, however, as both eat the foliage. Oftentimes groundhogs, rabbits, mice, and other small animals will cut stems of plants into small sections and pile them up. All of the plants in a small area may be cut up into tiny sections. Many people mistake this for armyworm damage.
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