Though the climate of the coastal regions usually does not favor development of destructive populations, it may be a serious problem in the warm Central Valley. They are nearly spherical and pearly white. The body is marked with longitudinal white, reddish brown, or sometimes black stripes. The head is usually orange or brick red, as is the fourth body segment, which is distinctly humped and has two prominent black tubercles or spines. Each body segment also has less-distinctive black tubercles.
Caterpillars rest with the hind end elevated.
The forewings are reddish to grayish brown and are often darkest along the hind margin. The hindwings are off-white to light gray or brown. Even if completely defoliated, trees that are otherwise healthy can be expected to recover. When defoliation occurs on walnuts, the nuts are subject to sunburning. In autumn, caterpillars drop to the ground and spin silken cocoons.