Most damage to alfalfa fields is done in late spring and summer by the larvae, which feed inside leaf buds at the tips of the stems and later attack the leaves. In a heavy infestation, the leaves are so badly shredded that the field takes on a frosted appearance. The adults spend the winter in debris in or near alfalfa fields. In the spring, these adults mate and lay their bright yellow eggs , in clusters of about 10, inside alfalfa stems. These eggs hatch, in 8 to 12 days, into pale yellow larvae with a black head.
Older larvae are bright green with a prominent white stripe along the back.
In 10 to 14 days the larvae are fully grown and are about 3 in. Mature larvae pupate in cocoons spun on plant foliage or on debris on the ground. Adults emerge in late June or early July, feed on second crop alfalfa, then in early fall, seek hibernating quarters. Following harvest, an insecticidal spray may also be necessary to protect new growth against larvae remaining in the stubble. Consult your provincial agricultural specialist for chemical control recommendations.