Tom Turpin
Professor of
Purdue University







Insect Armies on the March

They don't have drill sergeants, barracks or boot camps. But they behave very much like armies. That's why they are called armyworms.

These caterpillars have gained their common name primarily because of their habit of marching, like an army, in great numbers from location to location.

In addition, these worms appear to use a military tactic of surprise strikes. Armyworms also sometimes use an army-like tactic of destroying everything in their path.

When armyworm populations are high, they move from place to place. Sometimes roadways are literally covered with crawling caterpillars. Large-scale movement is generally associated with the search for new plants on which to feed. Large populations can quickly devour the available leafy-green material in an area. The hungry caterpillars then march off in search of food.

Armyworm damage doesn't just appear overnight. Like all insects, the caterpillars begin life as an egg. In fact, each caterpillar can have as many as 500 brothers and sisters. That is one reason why populations and damage can show up very quickly.

Adult armyworms are brown-colored moths. These moths constitute the air corps of this insect army. The moths wing their way, sometimes riding weather fronts, into Midwestern states early each spring.

The female moth then fastens her eggs onto leaves of grass plants. The eggs hatch in a couple of weeks, and the young caterpillars begin feeding. But like little kids, they have a little appetite-at least compared to the teenage caterpillars that follow. In the last two weeks of their caterpillar life, the true meaning of "the hungry caterpillar" surfaces. They can at this time "eat themselves out of house and home!"

The damage to plants during this feeding frenzy can be substantial. Grassy pastures can be eaten down. Young corn plants can be chewed to a stub, and wheat can be totally defoliated. Sometimes, the hungry caterpillars can clip the heads of wheat so that they end up on the ground.

In general, it is grassy plants like wheat, oats, fescue and corn that are damaged. Armyworms only feed on non-grass plants, like soybeans, when they have eaten all the available grasses.

Once it has completed feeding, the caterpillar crawls into the ground and pupates. In about two weeks, a new armyworm moth appears, and the cycle of the armyworm continues. There are two or three generations in northern states, but it is the first one that causes most of the damage.

There are actually several insects that are called armyworms. They are all moths and behave in a similar way as caterpillars. One is called the true armyworm. These are the ones that cause damage to plants in May and June.

There is an insect known as the army cutworm, which also sometimes begins the marching habit. Fall armyworms, as the name suggests, develop later in the season than do true armyworms. They have been known to consume all the leaf surface of mature corn plants, leaving only the mid-veins of the leaves. To add insult to injury, they sometimes attack the ears of corn as well.

Armyworms can disappear as quickly as they appear, because they can be attacked by other insects or diseases. These opposing armies wage a battle against the invading forces of the armyworms. Mother Nature has a way of making sure things don't get too far out of hand-even with something as fearsome as an army of worms!


Writer: Tom Turpin
Editor: Olivia Maddox