The spotted tussock moth occurs in four geographic races. The western race occurs across a broad swath of the western United States and western Canada. The eastern race occurs throughout central and eastern Canada and the far northeastern U.S. and down the Appalachian mountain chain. Another race occurs along the northwest U.S. Pacific coast and the fourth along the California coast.
The “tussock” designation derives the description of clumps of grasses and the similarity in appearance of the tufts of hair-like growths on the body of the caterpillar and adult moth. The moth is in the tribe of tiger moths and is sometimes called the yellow-spotted tiger moth. Taxonomically a tribe is the classification above genus but below the family. Caterpillars in this group are also termed “woolly bears” because of the furry body coverings.
Adults are active from May until July and the caterpillars are seen in July through September, only going through one generation each year. The larvae grow through 5 stages, the final one being the caterpillar seen in these photos in which the front and rear ends have white plumes sprouting out of the bristly black bodies and the middle section is yellow to orange. The body coloration suggests a palindrome description, appearing identical at the head and rear ends. The only way to discern the head from the rear end is to see the individual moving. The video shows the caterpillar moving briskly along the trail.
Larvae feed on a variety of hardwood tree leaves, favoring willows and poplar, but will also dine on other hardwoods, such as oaks, birch, maples, and alder.
An article in the journal Allergy and Rhinology documented a case from Ohio of an allergic reaction in a young child to chemical toxins administered by contact with a Spotted tussock moth caterpillar. The anaphylactic reaction produced a rash over much of the child’s body which was treated and cured with no after effects. The toxins were apparently transmitted to the child through punctures from the hairy fur of the caterpillar as the boy handled it over the course of 20 minutes. This reaction has been described as a rare event and only seen in persons with allergy sensitivities. The hair-like fur contains these repellent chemicals and can impart a burning or stinging sensation when handled.
The distinctive black and orange coloration and white plumes, or lashes, of the caterpillar serve as a warning to bird predators of the non palatability of the caterpillar. The caterpillars are also able to produce a clicking sound which is thought to serve as a warning to potential bat predators that the caterpillar is distasteful, a kind of audible warning for a nocturnal predator in place of coloration which would be invisible at night.
The Butterflies and Moths of North America site reports that the Spotted tussock moth population is abundant and stable. The caterpillar photographed here was found in September 2021 on the Sandia Crest trail at about 10,000 feet elevation.