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Armyworm outbreaks threaten southwest Missouri crops

Farmers in southwestern Missouri should scout for armyworm larvae, a pest that attacks fields en masse, causing extensive damage if not treated. Wheat, fescue and seedling corn fields are most at risk, said Wayne Bailey, University of Missouri Extension entomologist.

“They can sweep across a field like an army and completely defoliate a field,” Bailey said. “They go for green vegetation first, so right now it’s prime time for wheat.”

Newly hatched larvae are pale green and move like inchworms; full-grown larvae are dull brown and can be 1.5 inches long. Look for alternating light and dark stripes running the length of their bodies.

Adult moths migrate to Missouri in spring from southern states, targeting grass and small grains first. Due to migration paths this year, moths arrived later than usual and have bypassed MU Extension moth traps.

“We haven’t picked many up in traps, which is what’s so strange,” Bailey said. “Usually they come in large-enough numbers that we pick them up in the whole western part of the state.”

Outbreaks may occur after a cool, wet spring because of reduced natural pathogens, which can keep armyworm populations in check. So far, the area most at risk is between Lamar and Nevada, Mo. “The area may get larger if there are other moth flights,” Bailey said.

In wheat and fescue, armyworms feed on plant leaves, but they can also cut seed heads.

“They might cut off 2 to 3 percent the first day, 50 percent the next day, and finish it off the third day,” he said. “But if numbers are really high, they can take all the foliage off the wheat.”

Because damage can be rapid once cutting starts, the threshold for treatment is 2 percent to 3 percent of wheat heads being cut, Bailey said. “Once larvae start cutting heads, we suggest people spray more than if they’re just foliage-feeding.”

Make sure the cutting is caused by true armyworms and not mice or voles, he said. “The armyworm cutline is straight across. Rabbits, voles or mice cut at an angle. These animals tend to stack seed heads on the ground in piles, whereas armyworm cutting is scattered. They don’t feed on the heads. They just cut and crawl off.”

If larvae are not cutting, the threshold for wheat is an average of four or more half-grown or larger worms per square foot.

In seedling corn, larvae can cause total defoliation, which happens overnight, Bailey said. “They can take every leaf off, so you just have what little stem there was left next morning.”

Late planting this year has added to the risk, he said. While armyworm larvae can attack corn up to the full-tassel stage, plants now are small and especially vulnerable to worm feeding.

“Seedling corn is what we worry about most. You can always handle them on larger corn,” Bailey said. “Infestation usually occurs first on grasses like fescue and wheat. But because we’re so late with corn planting, they could easily go for seedling corn first.”

To prevent loss of corn, treat fields when 25 percent or more of seedling corn plants are damaged, he said. Several insecticides are labeled for true armyworm.

For all crops, frequent scouting is essential to avoid major economic loss. When scouting, bear in mind that larvae feed from the bottom of the canopy up and may be difficult to see with a quick glance, Bailey said.

“They’re night feeders when they’re small and you might not notice them unless you’re out there looking for them,” he said. “You really have to scout in early morning, late afternoon or at night.”

However, when armyworm populations are heavy or as worms get larger, they may also feed during the day, Bailey said. “They will lose their fear of light that keeps them night feeders.”

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