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.”
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.
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.
Life Cycle and treatment
It takes about 30 days for a female armyworm to develop into a mature, egg-laying worm. 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.
“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.”
Alabama Cooperative Extension System entomologists have been watching soybean looper numbers rise for several weeks. The major outbreak entomologists warned against earlier in the summer is happening in soybean fields in south and central Alabama.
Soybean loopers are also increasing in number in many north Alabama soybean fields. These are the key species triggering treatments during the third week of August in some fields in the Tennessee Valley for the complex of foliage feeding and pod feeding worms infesting double-cropped soybeans. The most recent year in which the soybean looper was a serious pest of north Alabama soybeans was 2012 when treatments were initiated the first week of September.
Extension researchers have pest traps scattered at locations throughout the state. High soybean looper trap counts in July gave entomologists reason to believe the looper numbers would be higher than-average in early August.
Reed and Smith are encouraging farmers to diligently scout soybean fields.
“The occurrence of soybean loopers is widespread in soybean fields in south and central Alabama at this time,” Smith said. “With widespread pest pressure, if a grower does not have a commercial scout, it is important to actively look for soybean loopers themselves.”
Adding insecticides to fungicide
Farmers in the Gulf Coast area of Alabama began adding insecticides to their fungicide sprays for some fields during the first half of August to slow the increase in soybean looper numbers.
Soybean looper larvae feed primarily on foliage. They will normally start feeding in the lower half of the plant canopy and move upward over time. Reed said scouts should try get their sweep nets into the lower half of the canopy if possible to get a more accurate estimate of soybean looper numbers. Soybean loopers are more difficult to dislodge from soybeans than other foliage feeding caterpillars.
“Early on, farmers will see leaves with a window pane effect,” Reed said. “The very small soybean looper larvae eat only the green portion of the leaf leaving behind the transparent cuticle layer. As larvae mature, they become more aggressive feeders and once soybean loopers begin feeding on the upper canopy, they can soon consume more than 50 percent of the foliage when numbers are high.”
Smith said soybean loopers are the most expensive pest to control. While there is more than one option, there are a limited number of control options available for soybean loopers—ranging from less than $10 per acre to nearly $20 per acre. Intrepid, Intrepid Edge, Belt, Besiege, Prevathon and Steward are the only four products available to farmers for soybean looper control.