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Contrary to its name, the spruce budworm is not a worm, but the caterpillar of a moth. There are, in fact, a number of different species of spruce budworm, all belonging to the genus Choristoneura and all broadly similar in appearance, but the two most destructive insects are the Eastern Spruce Budworm, C. fumiferana and the Western Spruce Budworm, C. occidentalis. The caterpillars feed on various coniferous trees, such as spruces, firs and pines. The moths are found in many parts of the United States, including Alaska, and Canada; some species are found in Europe and Asia. It is the caterpillar that does the damage; the adult moth does not feed on plants.
The moths, which vary from grayish to brownish in color, lay their eggs late in the summer on the undersides of spruce needles. The larvae, after they hatch, do not begin feeding, but instead build structures out of silk among bark and small twigs, in which they hibernate through the winter. They emerge from hibernation the following spring, just before new growth begins on the host tree.
The caterpillars, which are brown with whitish spots — resembling small spruce twigs, initially feed upon old needles, but move on to budding new needles, which they seem to prefer, as soon as they are available. It is these attacks on the growing centers that cause the most damage. The larvae feed until they have matured, usually by late June. At this point, they build silk cocoons close to the feeding areas and pupate there. The moths appear about ten days later.
Obvious signs of attack by spruce budworm are brown-colored dead needles and stunted or deformed growth near the tips of branches. Closer inspection will reveal silk webbing spun between needles and twigs, and crumbly excreted material known as frass. Damage may be restricted to the growing tips, but in heavy infestations, other parts may be affected and severe defoliation may result. Trees that are stressed — for example, as a result of drought — are more vulnerable to attack by this insect and are more badly damaged. Healthy trees will normally recover, but repeated attacks on trees weakened by stress can eventually be fatal.
The spruce budworm has a number of natural predators, including birds, spiders and parasitic wasps. These usually limit budworm numbers, but from time to time, a combination of factors can produce a population explosion, resulting in extensive damage. Often, the best control policy consists of keeping the trees healthy and better able to resist or recover from attack by minimizing stress factors. In the event of a severe infestation, a number of common insecticides can be effective. The bacterium Bacillus thuringensis, which produces an insecticidal compound, has proved to be a successful method of biological control.