From Auburn University/ACES:
AUBURN, Ala.—Fall armyworms have been a problem for most Alabama farmers in pastures and hayfields during the summer months. Alabama Extension entomologists have spotted fall armyworms in soybean fields in north Alabama.
Dr. Ron Smith said armyworm numbers statewide are higher than they have been in a very long time.
“The biggest issue for farmers is detecting the armyworms in time to treat pastures and hayfields, especially in central Alabama,” Smith said. “In north Alabama, the armyworms are in soybean fields. While some of the worms may have moved from a pasture to the soybeans, there are armyworm eggs being laid in soybean fields.”
Armyworm Movement and Scouting
Smith said earlier in the season, the armyworms move from a ravaged pasture to a field of tender soybeans. But now, worms are going directly to the soybean plants to deposit their eggs. There are two strains of armyworms, one prefers grasses and soybeans the other prefers cotton. In this case, the armyworms in north Alabama are the grass-eating strain.
Reed said about three weeks ago fall armyworms were the primary caterpillar species leading farmers to spray in some north Alabama soybean fields.
Now in the Tennessee Valley and other parts of the state, the fall armyworm is a member of a complex of foliage feeding caterpillars infesting soybeans that include green cloverworms, soybean loopers, pod worms (cotton bollworm), a few yellow striped armyworms and velvetbean caterpillars. Pod worms are generally feared for their bloom and pod feeding, but early in August they were observed feeding on foliage as well as blooms in double-cropped soybeans.
Alabama Cooperative Extension System conservation crop specialist Dr. Dennis Delaney said soybeans would offer tender vegetation, which is what armyworms feed on in pastures.
“Just like in pastures, armyworms can eat most of the soybean plant,” Delaney said. “The armyworms tend to eat the leaves and leave the tougher part of the plant. They also often move from grass weeds in soybean fields to soybean plants when the grass is eaten up, or killed by herbicides.”
Delaney said their movement from plant to plant is quick, and an infested field can be seriously damaged in a short amount of time. After bloom, the threshold for lower leaf loss is 20 percent or less.
Dr. Kathy Flanders, an Alabama Extension entomologist, said she recommends treatments in pastures when there are more than two caterpillars per square foot. One way to determine the number of caterpillars in a field is to physically look for them, but sometimes finding them is difficult.
In the field, Smith said the soybean threshold is normally six to eight caterpillars per row foot. The sweep net threshold for fall armyworms is 1.5 medium to large size caterpillars per sweep, and a foliage loss potential of more than 20 percent in soybeans in the reproductive stage.
Armyworm Life Cycle and Treatment
It takes about 30 days for a female armyworm to develop into a mature, egg-laying adult moth. The length of this cycle coincides with reports of armyworms in the state. Some of the first armyworm reports were in late June. Producers are now seeing a recurrence of armyworm issues they thought were taken care of earlier in the summer.
Armyworm control is best done before the final molting stage.
“Fall armyworms in soybeans can be most economically controlled with a pyrethroid insecticide at a mid-label rate,” Smith said. “Armyworms are a pest that can thankfully be controlled at a rate of two to three dollars per acre.”
For more information, visit www.aces.edu. There are many available fall armyworm resources for both the livestock producer and the row crop producer. There is a map of Alabama counties with confirmed armyworm outbreaks here. The Alabama Extension Forage IPM Guide and Soybean IPM Guide both contain updated lists of registered insecticides for armyworm control. Other information can be found on the Alabama Forages page.