Most of the economically important genera of North American Scolytidae commonly infest trees weakened by a variety of agents including drought, natural disasters, diseases, or old age, and are therefore considered secondary insects (Rudinsky 1962). Outbreaks of defoliating insects have occasionally weakened trees to the point they have become susceptible to secondary bark beetle invasion. For example, secondary bark beetles killed 16.7% of the ponderosa pine (Pinus ponderosaLaws.) defoliated by the pine butterfly,Neophasia menapiaFelder&Felder, in central Idaho (Evenden 1940). Similarly, defoliation of white fir,Abies concolor(Lindl.), by the Douglas-fir tussock moth,Orgyia pseudotsugataMcDunnough, in California was followed by infestations of the fir engraver,Scolytus ventralisLeConte, and a roundheaded borer,Tetropium abietisFall (Wickman 1963). Unfortunately, the intensity of defoliation associated with such bark beetle outbreaks has seldom been documented and as a result we lack information on the degree of defoliation likely to render trees susceptible to bark beetle attack.