Given particular site and stand conditions, then outbreaks are often triggered by disturbances that lower the resistance of previously healthy trees.

EXAMPLES

Fir engraver beetle outbreaks are often associated with stands that have been weakened by root disease, storm damage or drought (slide).

Southern pine beetle "spots" often start around a previously healthy tree that has been weakened by lightning (slide).

Mountain pine beetle outbreaks often start in areas where trees had been growing vigorously (= thick phloem) but which were recently weakened by a disturbance like drought, flooding, insect defoliation, etc. (= lowered resistance) (slide).

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