Description of the Pest
Western tussock moth overwinters in the egg stage, and larvae begin emerging in March and April. Immature larvae are hairy and black. Mature western tussock moth larvae are large (almost 2 inches in length) caterpillars with numerous red and yellow spots and long tufts of hair. There are four white tufts of hair on the top of the first four abdominal segments, two black tufts on the head, and many grayish tufts over the body. Adult females are large, wingless, and predominantly gray. The males are winged moths. There is only one generation per year.
As the tree begins to leaf out, larvae feed on the foliage. Localized high numbers of larvae can completely defoliate trees. Orchard workers that touch larvae may get rashes caused by the urticating setae, or hollow hairs, connected to poison sacs under the larval skin.
Visually search trees for black caterpillars feeding on terminal growth and look for cocoons on major scaffolds. Levels of western tussock moth warranting treatment are very rare. If a treatment is needed, insecticides that are effective against navel orangeworm or obliquebanded leafroller are also effective against western tussock moth.
Organically Acceptable Methods
Sprays of Bacillus thuringiensis or the organic formulation of spinosad (Entrust) are acceptable for use in an organically managed orchard.