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.

alfalfa or lucern, perennial leguminous plant (Medicago sativa) of the family Leguminosae (pulse family), the most important pasture and hay plant in North America, also grown extensively in Argentina, S Europe, and Asia. Probably native to Persia, it was introduced to the United States by Spanish colonists. Of high yield, high protein content, and such prolific growth that it acts as an effective weed control, alfalfa is also valued in crop rotation and for soil improvement because of the nitrogen-fixing bacteria in its nodules. The several varieties of the species grow well in most temperate regions except those with acid soil or poor drainage. The alfalfa belt of the United States centers chiefly in the northern and western parts of the country. Young alfalfa shoots have been used as food for humans and have antiscorbutic properties. Carotene and chlorophyll for commercial use are extracted from the leaves. Alfalfa is also called medic, the name for any plant of the genus Medicago—Old World herbs with blue or yellow flowers similar to those of the related clovers. Black medic (M. lupulina) and the bur clovers (M. arabica and M. hispida) are among the annual species naturalized as weeds in North America and sometimes also grown for hay and pasture. Alfalfa is classified in the division Magnoliophyta, class Magnoliopsida, order Rosales, family Leguminosae.

Perennial, clover-like legume (Medicago sativa). It is widely grown primarily for hay, pasturage, and silage. It is known for its tolerance of drought, heat, and cold, and for its improvement of soil by nitrification (see nitrifying bacteria) due to bacteria associated with its roots. The plant, which grows 1–3 ft (30–90 cm) tall, develops numerous stems that arise from a much-branched crown at soil level, each bearing many three-leaved leaflets. Its long primary root—as long as 50 ft (15 m) in some plants—accounts for its unusual ability to tolerate drought. Its remarkable capacity for regeneration of dense growths of new stems and leaves following cutting makes possible as many as 13 crops of hay in one growing season. Alfalfa hay is very nutritious and palatable, high in protein, minerals, and vitamins.

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