In the last couple of weeks, we have received numerous reports of small green caterpillars in soybean and sunflower. The caterpillars have been described as producing webbing and moving extremely quickly when disturbed. They have ranged in color from green to brown, but all had a white stripe present in the middle of their backs. All these characteristics led to identifying the caterpillars as alfalfa webworms. Alfalfa webworms are not commonly a pest of crops. The preferred host for these caterpillars is alfalfa, on which they will occasionally cause severe defoliation. They are also known to periodically feed on corn, soybean and sunflower, but rarely cause enough defoliation to warrant insecticide management.
Caterpillar Identification
Early life stages of the caterpillars are pale yellow to green in color. As the caterpillars grow, they become dark brown to green. The older caterpillars have a clearly visible white stripe that runs down the middle of their backs. The caterpillars are often observed resting within a loose silken shelter. There are at least two generations of the alfalfa webworms in South Dakota.
Thresholds
In most cases, the alfalfa webworm caterpillars will not cause serious defoliation. However, for soybean, if defoliation is at 30% prior to flowering or 20% during flowering, insecticide management is justified to prevent yield losses. For sunflower, it is unlikely that this insect will cause any yield loss.
