Caterpillar 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.
Consequently, during the critical summer period, fields must be checked at least once a week to detect developing populations. The larger larvae are most destructive. In contrast to armyworms, alfalfa caterpillars do not skeletonize leaves and will consume the midrib. This wasp stings very small alfalfa caterpillars and lays an egg inside. The egg hatches and the wasp larva consumes the body contents of the caterpillar.