High populations usually result from a flight of butterflies into the field when the alfalfa is less than 6 inches high. Factors contributing to economic populations are slow and uneven growth of the crop, lack of parasites, and hot, dry weather.
Infestations vary greatly from season to season and from one field to another.
Eggs hatch into green caterpillars in 3 to 10 days. Full grown caterpillars are up to 38mm long and are usually distinguished from other common caterpillars on alfalfa by their velvety green bodies and narrow, white stripe on each side of the body through which runs a fine, pale red stripe. Alfalfa Insect Forage Control .