The young caterpillar is densely covered with yellow setae. The older caterpillar’s setae are either pale yellow or white. All instars have thin, black setae on the first and third abdominal segments. On the eighth abdominal segment, there is one tuft of black setae. The caterpillar will reach a length 50 mm (2.0 in). While there are numerous reports of the larval hairs of this species sometimes causing skin irritation in humans, there is no evidence that they possess any form of venom.
The American dagger moth can be seen from April to September throughout its range. Caterpillars can be seen from July to October.