Alfalfa Caterpillar Colias eurytheme Photo Gallery Description and Biology: Larva / caterpillar: This larva is a grass-green color with a fine white stripe on each side of the body. The white stripe has a very fine red line running through it. The larvae have a velvety-appearance from dense short hairs on the body. When full grown, the larvae is 1½ inches long. Larvae feed for 2+ weeks. Alfalfa caterpillar feeding in soybean. Adult / butterfly: The adults of the alfalfa caterpillar are the familiar yellow butterflies that can be seen flying over alfalfa, soybean, and other flowering fields, particularly in August. The undersides of the wings are are solid yellow; the topside of the wings are bordered in black. Life cycle: The butterflies lay eggs singly on the undersides of leaves. A female can lay 200 to 500 eggs. The eggs hatch from 3 to 5 days. The alfalfa caterpillar butterfly is The larvae are brown at first, changing to the familiar green color common, flying over alfalfa fields in late July and August in ND. shortly afterward. the larvae grow rapidly. Full grown larvae attach to a stalk to form their chrysalis. The butterfly emerges 5 to 7 days later. There are at least two generations in North Dakota. Host(s): The caterpillars feed primarily on alfalfa, but can be found in other legumes, including soybean and clover. Damage/Symptoms: The caterpillar consumes foliage. They will consume a part of or the entire leaf. Newly planted fields are at greater risk as they are more vulnerable to defoliation. Comments: In alfalfa, the caterpillars are often controlled adequately through timely cutting, naturally occurring diseases, and several parasitic wasps; thresholds for treatment are relatively high at 10 larvae per sweep. Another method for assessing the need to treat is counting larvae per plant. Using this approach, the treatment threshold is one caterpillar per two plants. If many diseased or parasitized caterpillars are observed, relative to healthy caterpillars, control may not be warranted. In soybeans, this caterpillar should be considered along with others when assessing whether treatment thresholds of 20% defoliation during flowering or pod set might be reached. This defoliation level could occur when 4 to 8 larvae per row foot is found. .