Armyworms are plump, careless, striped caterpillars that feed on grasses and grain crops. They are so named because of their habit of moving in large numbers from field to field as they exhaust their food supply. It can be very disconcerting to a homeowner when they see their lawn being mowed down by an army of caterpillars that seem to appear overnight.
There are two main species of Armyworms:
• True Armyworms
• Fall Armyworms
True Armyworms can be found across the United States and in southern Canada. Fall Armyworms are mainly located east of the Rockies and can be a major insect pest for warm season grasses, especially Bermuda grass.
Armyworms are the larval stage of an adult, dull colored moth. The adults do not cause damage to any plant material. The females lay anywhere from 1,000 to several thousand eggs during their lifetime. Depending on the location and species, there can be anywhere from 2 to 10 generations per year.
The adult true Armyworm moth is pale brown to grayish brown with a wingspan of about an inch and a half. There is a small, distinct white spot in the center of each forewing. The larvae are grayish green to greenish brown in color and have two pale orange stripes along each side of the body. The head is covered with a net like pattern. When disturbed, the larva will roll into a tight ball.
The larvae cannot survive the cold winters in the northern part of the U.S. New populations arrive from adults that have flown up from warmer climates. In the southern states, the first generation can be seen as early as February. In the northern states, the first populations generally are not seen until May.
True Armyworms favorite food is corn and small grains, but they will occasionally feed on turf grasses. Therefore, they are not usually seen as a major pest problem. Their feeding habits include skeletonizing leaf blades or chewing leaf edges.
The adult fall armyworm moth is dull colored and has white blotches on the wings, which are also about an inch and a half in length. The larval will range in color from light green to olive green to nearly black and have longitudinal stripes along their sides. They look very similar to True Armyworms, but have an inverted Y on the head along with the net like pattern.
The favorite food of Fall Armyworms seems to be Bermuda grass. When populations are large, the larva will climb up onto the grass blades as if they were enjoying the sun. If disturbed, they too will drop to the ground and roll up into a tight ball. They are very gregarious in their feeding habits and will quickly move across the lawn, chewing grass blades all the way down to the crown.
Preventing Lawn Damage from Armyworms
If damage is seen by either of these two insects, an insect control application is recommended. The larva can go from the egg stage to mature larva in as little as three days. Timing of the application is critical to get adequate control before a lawn is devoured.