Western Tussock Moth Caterpillar

May 07, 2019

Written by Mary Louise Flint


 Two sizes of tussock moth caterpillars and leaf damage caused by their chewing.

May is a good time to observe western tussock moth caterpillars.You can find these colorful and hairy caterpillars chewing holes in leaves on oaks and many other trees on our nature reserve in spring. Their scientific name is Orgyia vetusta.

Tussock moth caterpillars spend the winter in egg masses on tree trunks and branches.During the previous summer, their mother will have laid 100 or more eggs in a large group and covered it with hairs and scales from her body, often placing it on or near an old cocoon case.When spring arrives and leaves start opening, the eggs hatch into small black caterpillars that spread out to feed.  Some will use silk strands and hairs to float in the wind to other trees.

 Newly hatched caterpillars on top of a hair-covered egg mass.

As caterpillars grow, they become hairier and more colorful. Mature caterpillars will have red spots on their sides, yellow markings with hairs, four prominent white tufts of hair (tussocks) on their backs, two large black tufts on the head and a single posterior tuft.Tussock moth caterpillar hairs can detach and irritate human skin---so donít pick them up.The oldest caterpillars can grow to almost 2 inches long.

 The oldest caterpillars are colorful with tufts of hair.

Caterpillars chew holes in leaves.Once caterpillars have finished feeding in late spring, they find a place to pupate on a branch, or twig, or even on a building or other object. They spin a loosely woven brownish or grayish cocoon that is covered with hairs.  

 Caterpillars pupate in woven, hairy cocoons such as these three on a tree trunk.

After a few weeks, the metamorphosis is complete and the adult moths emerge.  Female tussock moths are flightless with greatly reduced wings.Males are brown mottled moths with feathery antennae.Females release a pheromone to attract the night-flying males for mating, and the malesí large antennae allow them to easily sense it.

 The female tussock moth has vestigal wings and can't fly. 

 The male tussock moth has feathery antennae and flies at night. Photo by Joyce Gross. joycegross.com


After mating, the female lays masses of eggs on tree trunks and branches, covering them carefully with scales and hairs from her body.  There they stay until next spring when the cycle begins again.

Author:  Mary Louise Flint, Docent, Effie Yeaw and Extension Entomologist Emeritus, University of California, Davis

Photos:   All photos by Jack Kelly Clark, University of California Statewide IPM Program except the photo of the male moth by Joyce Gross.  Used with permission.