Daily News Update, Sept. 13, 2007 


Conditions ideal for devastating fall armyworm invasion

Dr. Chris Sansone, Extension entomologist at San Angelo, said fall rains can trigger armyworm onslaughts that can destroy pastures, small grain fields and lawns almost overnight.

"We're seeing all types of armyworms this year; the true, the yellow-striped and the beet armyworm, but the biggest number is the fall armyworm," said Sansone. "Conditions are ideal for a real invasion of these pests in coming weeks. This insect can build up large numbers seemingly overnight, causing crops and lawns to disappear before your eyes.

"The adults lay thousands of eggs, and by the time the larvae or 'caterpillars' are big enough to see, the damage is done."

Sansone said the tiny larvae chew the green layer from leaves and leave a clearing or "window pane" effect. He said the first three larval stages or "instars" do little damage and are easy to control. The final two stages are big and tougher to control. They can eat 85 percent of the total foliage consumed by the caterpillar.

"That's the reason it's so important to find the infestations before the caterpillars get too large," Sansone said. "Small larvae are easier to control and most of the damage can be prevented if the infestation is caught early. Individual fields and yards need to be scouted carefully, because the moths don't lay eggs consistently across an area. Often a pasture or small grain field will be infested while the field across the road is armyworm-free.

"San Angelo has had a number of lawns with armyworms in one yard and none in the neighbor's."

Sansone said the caterpillars won't kill an established lawn, but new lawns or those weakened by disease or other pests can be killed by the sheer volume of leaf blades the pests eat.

"Homeowners have an almost overwhelming number of control choices," Sansone said. "Products containing deltamethrin, cyfluthrin, cypermethrin and cyhalothrin are effective. These are sold under a wide variety of names brands. Carbaryl or 'Sevin' has been a standard for many years."

Sansone said homeowners should read the label and make sure the insecticide is labeled for lawns or turf.

Small grain producers should consider the stage of their crop when treating against armyworms. In the seedling stage, just three larvae per square foot can ruin a stand. Later in the tiller stage, it takes seven to 10 larvae to cause serious damage.

Treating pastures of native or improved grasses is often not cost-effective once all the economic factors are considered, Sansone said.

Fall armyworms are worst in the autumn when adult moths are carried into an area with cold fronts that trigger rain.

The adult fall armyworm moth is 1.5 inches across the wings. The hind wings are grayish-white and the front pair dark-gray, mottled with lighter and darker splotches and a whitish spot near the extreme tip.

Fall armyworms are a threat until the first hard freeze. The larvae range from 1.25 to 1.5 inches long and can have shades of brown, green or black. They can be distinguished from other armyworms by a prominent white, inverted Y-shaped line on the front of the head.

"Fall armyworms are easily controlled if homeowners and producers monitor their fields closely," Sansone said. "The key is proper identification and finding the infestation before the larvae become too large."