A Douglas-fir tussock moth caterpillar has tasted the older needles, but trundles off in search of fresh, young needles. Photo by Jeff Mitton.
By Jeff Mitton
Within a few days, a blue spruce was transformed from a bright and handsome tree to a sad, brown structure, retaining only its oldest needles on its lower branches. A horde of Douglas-fir tussock moths, Orgyia pseudotsugata, were eating its needles, filling the air with fragrance of spruce needles.
It was an alarming and fascinating sight. Thousands of colorful caterpillars were throughout the tree, most of them chewing needles, others dangling from silk threads anchored to upper branches. I got closer to watch individual caterpillars, but after a few minutes I had blobs of green paste in my hair and on my shirt; after a little thought I realized these blobs were the defecations of masticated and digested spruce needles that were raining through the tree. I backed away, brushing myself off and trailing silk threads.
The larger of the caterpillars were colorful and attractive. Long hairs project in all directions, forming an outer defensive barrier. These hairs penetrate soft tissues and are anchored by their microscopic barbs, producing inflammation, swelling and itching. Two tufts, sometimes called pencils, of long black hairs with black tufts project forward and up from the head. Another pencil projects up and back from the rear, but the leading edge is composed of short, golden hairs while the trailing edge has long, black tufted hairs. Perhaps the most distinctive structures are four tussocks, dense bundles of white hairs tipped with buff. Black and white bristles project from brilliant red buttons on the caterpillar’s sides and back.
The Douglas-fir tussock moth is a native to western North America, and its geographic range is coincident with the range of Douglas-fir; both species are found in British Columbia, all of the western states and the mountains of Northern Mexico. Locally, the preferred host seems to be Douglas-fir, for about a decade ago an infestation killed many Douglas-firs on the north-facing slopes of the canyons west of Boulder. But the moths can utilize all of the true firs and the spruces, and in California a recent epidemic took many white firs in Yosemite National Park.
Caterpillars use silk to help them get around. They can attach a strand to a branch and lower themselves to another branch or the ground. Or, if they let out a long strand that is caught by the air, they can be lofted downwind a considerable distance. Perhaps they learned this by watching spiders.
Some of the largest caterpillars were spinning cocoons as a protective casing in which they would pupate. Cocoons were attached to branches and were made primarily of silk, but the caterpillar's long hairs were woven in with the silk.
Pupae are in cocoons for 10 to 18 days, and then the adults emerge. Males have tan and brow mottled wings with wingspans of 1 to 1.7 inches. Females have only rudimentary wings, are flightless, and will almost certainly spend the rest of their lives in the same tree. As they emerge from the cocoon, females release an attraction pheromone and mate on or near the cocoon. Their eggs are laid on the cocoon and will hatch in the following spring.
Tussock moth populations are usually held at low levels by predators and disease. Mountain chickadees and nuthatches eat egg masses during the winter. Parasitic flies lay eggs on the caterpillars and after the eggs hatch the fly larvae penetrate and consume the caterpillar. When moth populations escape these predators and develop an epidemic, a naturally occurring nuclear polyhedrosis virus spreads, killing caterpillars and pupae, causing epidemics to crash.
Although the blue spruce looks very sad, I don't think it will die. Most Douglas-firs must be defoliated several consecutive years before their energy reserves are exhausted and unable to produce new needles in the spring.
Epidemics of Douglas-fir tussock moths have impacts on human health. Breezes loft fragments of caterpillar hairs from the forest and bring them to town, and if they are inhaled or come into contact with the eyes, they trigger a set of allergy symptoms called tussockosis. As caterpillars west of town defoliate trees, they also induce itchy skin, rash, watery eyes and sneezing in unsuspecting, allergic people in town.
Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.
July 3, 2014