August 6, 2010

Soybean growers in many areas of the state are reporting feeding damage from caterpillars (Figure 1). Correctly identifying the caterpillar causing the damage can be helpful in planning the most effective treatment. Following is a quick guide to the three most common species in Nebraska.

Four other species also have been reported, but are less common. They include the yellow-striped armyworm, painted lady butterfly, garden webworm, and forage looper.

For information on treatment thresholds and determining when treatment is necessary, see Decision-Making Guide for Defoliating Insects in Soybean.

Green Cloverworm

Larvae are pale green with one or two yellow/white stripes extending down each side of the body. They have three pairs of prolegs in the middle of the body. Small caterpillars can hang from a silk and appear to be suspended in mid-air. Larvae wiggle violently when touched. Caterpillars (Figure 2) chew soybean leaves primarily between the leaf veins. Adults are dark brown or black moths with spotted wings. Their wingspan is approximately 1 inch. Adult moths at rest are dark brown and have a triangular appearance.

Biocontrol. Green cloverworm caterpillars are attacked by several types of beneficial insects, including spined soldier bugs and several species of wasps.Cotesia wasps attack very young green cloverworms. The wasp larva completes its feeding within the green cloverworm and then emerges to pupate. Pupae of this wasp are noted as small yellow wooly cocoons on soybean leaves (Figure 3).

Rogas nolophanae wasps also have been reported. These wasps attack older green cloverworms rather than the younger cloverworms attacked by the Cotesia wasps. Activity is indicated when you see the parasitized bodies of green cloverworm caterpillars that appear as greyish and brown hardened bodies on top of soybean leaves. Holes in these “hardened bodies” indicate that the wasp has completed its life cycle within the caterpillar’s body and exited the mummy.

Yellow Woolly Bear Caterpillar

Yellow woolly bear caterpillars (Figure 4) are not uncommon, but are not uniformly distributed, in part due to egg-laying. The adult moth lays a large number of eggs on a single leaf. Upon hatching the caterpillars skelotonize that particular leaf, and then disperse out from that point.  Woolly bear caterpillars tend to eat more of the leaf, including leaf veins and leaf edges, than do green cloverworms.

Alfalfa Caterpillar

The alfalfa caterpillar (Figure 5) has a velvety look with a white stripe and a faint yellow stripe along its side. These caterpillars feed on legumes, such as alfalfa, clovers, and soybeans, and can eat a great deal of leaf mass. High populations of alfalfa caterpillars are more frequently found in alfalfa than in soybeans. We’ve received many questions about the large numbers of yellow butterflies near small pools of roadway waters. These are orange-sulphur butterflies, the adult stage of the alfalfa caterpillar.

Michael Rethwisch
Butler County