. Insect and disease conditions in the United States, 1979-83 : what else is growing in our forests?. Forest insects United States; Trees Diseases and pests United States; Plants Disease and pest resistance. ^ Douglas-fir Tussock Moth On the Downswing Written by Leon F. Pettinger Its populations can explode. Within 1 year, a minuscule population of Douglas-fir tussock moth can grow to outbreak numbers that strip the foliage from the treetops. But when populations are low, the insect's feeding goes unnoticed, and its life stages are difficult to find. The caterpillars, the larval stage of the t
. Insect and disease conditions in the United States, 1979-83 : what else is growing in our forests?. Forest insects United States; Trees Diseases and pests United States; Plants Disease and pest resistance. ^ Douglas-fir Tussock Moth On the Downswing Written by Leon F. Pettinger Its populations can explode. Within 1 year, a minuscule population of Douglas-fir tussock moth can grow to outbreak numbers that strip the foliage from the treetops. But when populations are low, the insect's feeding goes unnoticed, and its life stages are difficult to find. The caterpillars, the larval stage of the tussock moth, prefer to feed on Douglas-fir, grand fir, and white fir. When grown as ornamentals, spruces, especially blue spruce, are common hosts. The tussock moth's preferred hosts vary according to location. In the Pacific Northwest and the northern Rocky Mountains, the larvae feed on Douglas-fir and grand fir. In the Great Basin, the Southwest, and California, the lar- vae prefer white fir. In the central Rocky Mountains, they most often damage ornamental trees, such as spruce (fig. 1). When populations build up enough to completely defoliate their preferred hosts, the larvae will move to other trees, such as pine, larch, and spruce. The Douglas-fir tussock moth completes its life cycle within 1 year and passes through four stages: egg, larva, pupa, and adult. The larvae go through five or six stages, known as instars, growing larger with each instar. Full-grown larvae are striking caterpillars (fig. 2). In late July and August, the mature larvae spin a thin cocoon, or pupal chamber, of silken webbing mixed with body hair. About 10 to 18 days after the larvae pupate, the adult moths emerge. The wingless females cannot fly. Attached to their cocoons, they emit a chemical messenger, called a pheromone, to attract the males (fig. 3). After they mate, the females lay eggs, and the pattern repeats itself (fig. 4). From 1979 to 1983, outbreaks were relatively small. In effect, th
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