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Armyworm Arsenal | How to Control Armyworms in Soybeans


Justin Turner doesn’t plan on a fight with fall armyworms in soybeans every year. But he knows he’d better be ready to rumble if they invade.

Turner farms in northeast Louisiana at MerRouge. His soybean and corn rotation can face excessive insect infestations if control measures aren’t taken in the warm, humid and wet climate that often sees heavy fall rains. And if fall armyworms (FAW) strike, they’d better be controlled before they can chomp away at leaves and eventually defoliate a field.

FAW, as well as common armyworms and the southern armyworm variety, have sporadic cycles on the region’s crops. FAW can be absent for two or three years, then sneak up on growers in a flash.

“They’re pretty easy to control,” says Turner, who’s a regional crop consultant when he’s not farming. “But if it’s too wet to spray, they can get away from you.”

FAW were actually bad in 2010 due to dry weather. They prefer foliage, but with the dry summer, they “moved from grassy road ditches and thickets out into lush irrigated soybean and rice fields,” says Turner.

FAW look similar to corn earworms and are the larvae of small, night-flying moths, university entomologists point out that. The larva has a prominent white inverted Y-shape on its head. They generally eat foliage and are easily seen. They are ferocious feeders and can strip off leaves and tender stems very rapidly, moving into pods when not controlled.

“The mistake is for producers to wait too long before they spray,” says Monti Vandiver, a Texas AgriLife Extension integrated pest management (IPM) specialist in Muleshoe. “It’s one of those pests that are easily remembered by people who have seen what they can do. They can be devastating, but can be managed.

“Typically, they can be controlled most effectively when they are a half-inch long or smaller. But if they get too large, they are difficult to kill. Sometimes you see mixed populations with small and large worms. If you can kill 90% of the small ones, you still might not get adequate control of the entire population.”           

The trouble with foliage feeders

Turner says the trouble with foliage feeders is the bigger they get, the more they eat. “They aren’t as bad as loopers, but they’re still pretty bad,” he says.

Turner’s FAW insecticide spray program involves a 1.6-2.6-oz. application of Baythroid and ½ lb. of Orthene. “It usually involves the pyrethroid plus the Orthene acephate,” he says. “If I have stink bugs with them, I’ll bump up the acephate to ¾ lb.”

Vandiver says he would also choose a pyrethroid if infestations were light. “If it’s heavy, I would lean toward Intrepid,” he says.

Other insecticides recommended for FAW control include: Lorsban 4EC, 1-1.5 pt./acre; Steward 1.25 SC, 9.2 oz./acre; Prolex, 3.2-3.8 oz./acre; Karate Z 2.08CS, 1.5-1.8 oz./acre; Lannate 2.4LV, 1.5 pt./acre; Lannate 90SP, 0.6 lb./acre; Intrepid 2F, 4-6 oz./acre; Tracer 4SC, 1.5-2 oz./acre; Larvin 3.2F, 1 pt./acre; and Mustang Max 0.8EC, 3.2-4 oz./acre. Vandiver says a new insecticide, Belt from Bayer, is approved for FAW control on corn and cotton and could become labeled for soybeans.

Ric Bessin, University of Kentucky entomologist, saysFAW are not as difficult to control in soybeans as in corn. “The problem with FAW in corn is you can’t get insecticide to it since the worms are in the whorl,” he says.

“FAW can be one of the more difficult insect pests to control in corn. Late-planted fields and later-maturing hybrids are more likely to become infested. FAW causes serious leaf feeding damage as well as direct injury to the ear and ear shank. While FAW can damage corn plants in nearly all stages of development, it will concentrate on later plantings that have not yet silked,” says Bessin.

Vandiver says FAW will feed on ears in a way similar to corn earworm but with a larger appetite. “They will also feed on much harder kernels,” he says. “Yield losses from kernel feeding can be secondary to shank feeding which can cause whole ears to drop to the ground prior to harvest.”

He adds that Yield Guard Bt corn does not prevent FAW infestations but may slow them down a slightly. “Herculex Bt corn appears to be more effective in suppressing FAW but should not be depended on for complete control,” says Vandiver, adding that there is no established economic threshold for FAW. “Conventional and Bt corn should be closely monitored for this pest. Late-planted full-season corn is most susceptible to damage.”

Bessin says adult moths move northward in progressive stages from overwintering sites along the Gulf Coast region and begin to appear in the Midsouth area in late June or early July. “The spherical gray eggs are laid in clusters of 50-150, usually on the leaves,” he says. “Egg masses are covered with a coating of moth scales or fine bristles. Larvae hatch in 3-5 days and move to the whorl.”

Unlike common armyworm, FAW feeds during the day and night, but are usually most active in corn in the morning or late afternoon, he says.

Very early symptoms of FAW resemble European corn borer infestation, southwestern corn borer and corn earworm. Small holes and ‘window-pane’ feeding in the leaves emerging from the corn whorl are common. Although initial symptoms of damage are similar, thresholds and control measures differ. Therefore it is important to find the live larvae and determine which insect is causing the damage.

Bessin adds that, like European corn borer, FAW can only be effectively controlled while the larvae are small. “Early detection and proper timing of an insecticide application are critical,” he says.

Identification of worms in a soybeans or corn can be done by looking at their legs, says Bessin. Most caterpillars have four pairs. Green cloverworms have only three pairs and loopers have two pairs.

Bessin notes that beet armyworms can also be an occasional problem for soybeans. “You usually have to turn to non-pyrethroid options,” he says. “Steward, Larvin or Tracer do a pretty good job.”

Other insecticides recommended for beet armyworms include both Lannate types and Intrepid.

For more on FAW and other armyworm control, consult your regional Extension entomologist or your crop consultant.

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