When white spruce is infested with spruce beetle broods, Dendroctonus rufipennis (Kirby), more beetles are produced than when Lutz and Sitka spruce are infested. In spite of host suitability differences, outbreaks of the spruce beetle have been more frequent and severe in stands of Lutz spruce than in white or Sitka spruce. Host suitability may be as important as host susceptibility and weather conditions in the development of spruce beetle outbreaks in south central Alaska. Cool summer temperatures and high precipitation limit the rate of development and growth of beetles in maritime Sitka spruce stands of southeast Alaska, but in white spruce stands of interior Alaska, cold winter temperatures usually help to maintain endemic levels. When these environmental factors are ameliorated, however, spruce beetle populations increase rapidly to epidemic levels.