Alfalfa caterpillars are larvae of the sulfur butterfly. They are green worms with a white, longitudinal stripe along each side of their bodies. They attain a length of 1/4 to 1/2 inches, and on close inspection appear to have a velvety skin. They are usually held in check by naturally occurring parasites and diseases. Infestations seldom reach economic levels before late summer. Damaging populations are most likely encountered in southwest Kansas fields that stand five or more weeks between cuttings. Control measures are justified when there are 10 worms per sweep of the net, and cutting will not take place during the next few days.