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Two species of armyworms march on Central Texas

UVALDE - Hordes of fall armyworms and true armyworms have invaded several Central Texas counties, and the assault will likely continue, warned a Texas Cooperative Extension entomologist here.

Dr. Noel Troxclair, entomologist at the Texas A&M University System Agricultural Research Station and Extension Center, said the populations of these two armyworm species has increased significantly in recent weeks.

"This increase has caused problems for many area producers as there has been an associated increase in the destruction of green grass crops in the area," he said. "In areas of Atascosa, Frio, Medina and Zavala counties, pastures and hayfields have been completely stripped. And the affected area is probably more extensive and will continue to expand."

Extension agents in these and nearby counties have been asked to alert producers to be on the lookout for possible armyworm infestations.

"We're trying to get producers in the region to scout their pastures, hayfields and early-planted grains for armyworms because any green grass crop may be at risk," he said.

In spite of their names, fall armyworms and true armyworms are not worms but the "immatures" or caterpillars of moths.

Troxclair pointed out while fall armyworms feed around the clock, true armyworms feed primarily at night and will not be found farther up on plants during the day.

"Producers should look for armyworms down in the crowns of plants and under debris on the soil surface," he said.

The acceptable quantity "threshold" for fall armyworms is three larvae ? inch long or longer per square foot, he added. For true armyworms, the threshold is three larvae ? inch long or longer per foot of drill or four per square foot.

Troxclair said the easiest way to distinguish between the two species is that the fall armyworm has an inverted, cream-colored "Y" shape which contrasts with its dark brown head capsule.

"The true armyworm doesn't have this feature," he said, "and the head capsule is a lighter brown."

Troxclair added that it's more than likely a fall armyworm if feeding during the daytime. "There are different ways to manage these two species," he said. "If a producer has armyworms in a hayfield, it may be possible to mow and let it dry down. And if there's livestock, the producer can run those animals in the affected area to eat the grass before the armyworms get to it. But these methods require close monitoring to ensure any surviving larvae don't prevent grass regrowth." Troxclair said chemical control can be effective and recommended that producers use an insecticide licensed for use on true armyworms and fall armyworms.

"The products typically licensed for use in these two species have malathion, carbaryl or methomyl as their active ingredient," he said. "While some insect control can be achieved by non-chemical means, pesticides like these provide a more effective and broader control of these pests. However, it is a good idea for producers to determine the extent of infestation and compare that with the cost of treatment to help determine which way to go."

For more information on true and fall armyworms, contact Troxclair at (830) 278-9151.

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