Synonyms and other taxonomic changes
Halysidota (also spelled Halesidota, Halisidota) argentata
Explanation of Names
Latin for "silvered" from argentum - "silver", referring to the silver spots on the forewings.
Caterpillars to 37 mm. (1 1/2 inches)
Adult wingspan to about 5 cm. (2 inches)
British Columbia, western Washington and Oregon, mountains of northern California (plus San Bernardino County in southern California), mountains of Arizona, New Mexico, Colorado and Utah, spilling over a little into neighboring parts of Mexico, Texas, Nevada and Wyoming
Forests with stands of Douglas fir
Adults fly in July and August
Douglas Fir (Pseudotsuga menziesi), but not uncommon on other conifers. There are also a few reports of them eating a wide variety of other plants.
Adults lay eggs in July and August. Larvae feed from August until winter, at which time they spin webs for shelter and hibernate (they sometimes come out and feed on warm days, even as early as January). During the late spring they disperse throughout the tree for a short while, then in May and June find secluded places to pupate.
The larvae are gregarious feeders, staying together in a dense colony on a single branch until it's completely defoliated. This is unsightly and somewhat alarming, but unless there are lots of colonies or the tree is small, the damage to the tree as a whole is minimal.
Note of Caution: Like several kinds of wooly-bear-type caterpillars, these have venomous, stinging hairs, which can cause a burning sensation and/or a rash in sensitive people - look, but don't touch!