Young azalea caterpillars are small, green worms that grow into medium, purple worms and then into large, black and yellow-striped worms with red heads and prolegs. They are sometimes called Labor Day worms, because so many folks discover them around Labor Day. Azalea caterpillars are gregarious in the larval stage, which makes their control relatively easy. However, because the worms feed in groups, they often completely defoliate a portion of a plant before they are detected. There is only one generation per year. The adult moths emerge in early summer and deposit eggs in masses of 80 to 100 on a leaf. As the larvae mature, they consume more and more of the leaf. Most of the damage occurs in August and September. Azalea caterpillars can be shaken from the shrub and trampled underfoot. Sevin, Orthene, Marvrik, and pyrethrin-based insecticides should give adequate control as well if you don’t mind using insecticides.