alfalfa caterpillar, larva of the alfalfa butterfly, Colias eurytheme, a member of the family Pieridae. Found throughout most of Mexico, the United States, and S Canada, it is sometimes a serious pest of alfalfa, clover, and other legumes in the SE United States. It usually overwinters as a pupa, the orange adult emerging in early spring, when large numbers of these butterflies may be seen fluttering low over alfalfa fields. The female lays several hundred eggs on the undersides of leaves. The larvae are fully grown in 12 to 15 days; there are two generations each season in the northern part of the range and up to seven in the south. Low cutting of infested alfalfa, which exposes the larvae to sun and predators, aids control. The alfalfa caterpillar is classified in the phylum Arthropoda, class Insecta, order Lepidoptera, family Pieridae.