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<br />tree vigor. Severe weevil outbreaks have been recorded during <br />years of low spring soil moisture (Stevens [12]). Increasing <br />the amount of spring precipitation or the duration of snow <br />cover in plantations would be expected to improve tree vigor <br />and decrease the incidence of tree killing by this pest. <br /> <br />T:ree phenol.ogy. - Onset of shoot elongation ("bud-burst") and radial <br />growth ("cambial initiation") is species-specific for conifers grow- <br />ing in the study area. However, growth initiation varies annually <br />in response to spring weather (Fowells [13]). Insect development <br />is generally synchronized with host phenology su~h that feeding <br />life-stages occur simultaneously with new growth. For example, <br />egg hatch of the most serious defoliator of new needles of white <br />fir (Douglas-fir tussock moth) is synchronized with bud-burst. <br />Wickman [14] has found that both processes respond to accumulated <br />day-degrees within the American River Basin. Thus, prolongation of <br />snowpack from 5 to 30 days would not induce asynchrony and starve <br />the insects, but merely retard egg hatching and flushing of new <br />growth. Such a delay would have little effect on populations of <br />this and perhaps other tree defoliators. <br /> <br />Diseases that infect new needles and shoots--western gall <br /> <br /> <br />rust, many needle rusts and needle casts--are most damaging when <br /> <br /> <br />young shoots and primary inoculum are produced simultaneously and <br /> <br />1-11 <br />