Larvae are present during June-October. Head and body are yellowish green. There is a broad white line along each side, bordered above and below by thin red lines. Prolegs and thoracic legs are red. Raised tubercles bearing whorls of branched spines occur on each segment along the back of the body.
Spines are poisonous and cause severe nettling when brought in contact with skin.
The number of generations per year varies within the range. Two generations probably occur in Alabama. Importance The Io moth is scarce in Alabama. While trees are listed as hosts, the caterpillars are actually rarely found on foliage of trees.