Armyworms eat Missouri fescue pastures, hayfields; wheat fields next

Source: University of Missouri Extension

Armyworms, which can strip pastures and hayfields bare as they march across the landscape, are showing up in Missouri, warns a University of Missouri (MU) entomologist.

Most reports came from near Joplin and eastward, and in east central Missouri around Hermann and Montgomery City, says Wayne Bailey, MU entomologist. "I had about 50 calls on Thursday (May 23). Most reports were on fescue fields," he says.

Some people were concerned about sawfly larvae, but those don’t cause economic damage. The sawfly larvae have legs the entire length of the body. True armyworms have four pairs of legs at mid-body. Those have dark brown or black triangles at their ankles.

Early scouting to detect the young larvae makes control easier, Bailey says. The immature worms feed at night. Scouting at dusk and early morning will be more successful than at mid-day. "You must get down into the grass to find them," Bailey said. "They start feeding down low and work up. Just looking at the top of the grass, you will miss them."

Armyworms are easier to control when small than when they become large brown, white-striped worms feeding in a mass. The economic threshold for spraying is seven to 15 larvae per square yard. "With the value of the hay crop, it may pay to use the lower number this year," Bailey said. "As an alternative to spraying, cut the forage for hay."

Wheat, which is starting to set seed heads, will be vulnerable to the armyworm invasion. "Usually armyworms appear in fescue fields two weeks before being found in wheat fields. Scouting should start now," Bailey added.

Spraying armyworms in wheat fields is trickier because of pre-harvest intervals after spraying. Different sprays have different intervals, ranging from seven to 30 days.

There are no pre-harvest intervals for fescue. "Read the label for the interval for the pesticide to be used," Bailey says.

The location of the armyworm infestations depends on where the winds blow the adult moths flying in from the south, Bailey says.

Armyworms strip the leaves from the wheat. This stops the filling of seed heads. Sometimes the worms crawl up the seed stalk and clip the stem about 2" below the seed head.

Both types of damages reduce yields.

"Scouting the fields requires getting out of the pickup truck," Bailey warns. "If you see damage from the truck window, it's probably too late."

Parting the grass leaves and looking down into the canopy can reveal the immature worms early.

Young larvae are light green. Mature worms, about 1.5" long, are brown with long horizontal stripes.

For help in identifying pests and on controls, contact your local Extension specialist.