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Beetle populations often attain outbreak levels when host pines are moisture stressed during drought or following forest fire. Between 1987 and 1993, western pine beetle killed over 165,000 trees in south central Idaho.
The beetle devastated stands of large, over-mature, decadent pines in the first half of the twentieth century. Western pine beetles will still infest these stands where they occur. Single, large, old, slow growing or diseased pines are very susceptible to attack. Large old ponderosa pines surrounded by second growth mixed conifer stands are often at risk, a
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result of fires suppression and excessive competition from younger trees.
Over the last couple of decades the western pine beetle has become especially aggressive in second growth stands, killing trees six inches or larger in diameter, including apparently healthy trees.
Trees are characteristically killed in groups, primarily in dense, over-stocked stands of pure, even-aged pines, but mortality also occurs among dense clumps of ponderosa pines in stagnating mixed-conifer stands.
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