Alfalfa webworm

Webworms are dark green caterpillars with two white stripes and black spots on their backs. They are often found within webbed leaves. They vary in size up to almost 1.5 inches in length.
Webworms overwinter as larvae in the ground adjacent to their fall food host. Moths emerge in early spring and lay eggs on leaves of host plants. Larvae will feed for 3 to 5 weeks.