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Watch for armyworm invasion

“Armyworms love cool, wet springs with lush grass growth,” said Wayne Bailey, University of Missouri Extension entomologist.

“We have the conditions for an outbreak — and loss of all grasses,” he said. “The only thing holding down an armyworm hatch has been the windy nights. True armyworm moths like calm nights to fly out to lay their eggs in dense forage. High winds have kept the moths in hiding.”

There are other reasons to expect armyworms this year. “The true armyworm outbreaks come about every four years, and it’s been four years since we’ve seen large numbers.”

A mass of armyworms moving across wheat fields and pastures can be devastating. “Huge populations eat every grass blade in front of them. If you go out one morning and all you see standing are the stems, you’ve had armyworms,” Bailey said.

Bailey recommends early scouting of grasses by getting down on hands and knees and digging into the debris on the ground. The small larvae hide at the base of the plants during the day and feed at night, so they are easier to find by scouting at night with a flashlight.

The tiny worms start eating the lower grass leaves and work upward. Early detection when larvae are small allows easier control.

When mature armyworms are moving across the landscape in severe outbreaks, aerial applicators often fly into the affected area to offer control.

Armyworms can be identified by colored bands on their sides. One stripe on each side will be orange or salmon colored. The worms have four pairs of stubby “abdominal legs,” which they use for climbing grass leaves. Each leg has a distinctive dark triangle.

The early larval stages might be less than a half-inch long, but when the worms become 2 inches long, they cause major damage. When they eat all of the forage, they move to new fields.

Armyworms receive their name from the way they march across fields, devouring all leaves in their paths.

“When small larvae start feeding, the damage appears as smooth holes shaved from the edge of the leaves,” Bailey said. “Grasshoppers leave rough edges where they eat. Armyworms cut neat holes.”

The pests do not eat legumes but they consume almost any grass, including corn. Their favorite foods in Missouri are tall fescue, especially in fields saved for seed harvest, and wheat.

Armyworms cause most damage when the plants’ flag leaves appear and seed heads start to form.

Armyworm outbreaks can be quite localized, depending on where the moths fly and lay eggs. “Scouting should start now,” Bailey said.

Recently, the largest captures of night-flying armyworm moths have been in the Bootheel counties and southwest Missouri. Moderate numbers were trapped in northeast Missouri at Novelty and Mexico.

Recent spring rains and winds from the southwest may have brought large numbers of armyworm moths into Missouri from as far south as the Gulf of Mexico.

The four-year cycle of armyworms is linked to the population cycles of beneficial insects that control the worms, Bailey said. When the army is on the march, the beneficial insects build up. As the armyworms die off, the beneficial populations decline.

“Last fall we saw very few beneficial insects,” he said.

For more information, see “Potential for True Armyworm Problems in Grass Crops” in the April 21, 2009, issue of Integrated Pest & Crop Management, a newsletter published by the MU Plant Protection Programs. The newsletter is available online at

TAGS: Management
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