The dense stands that developed increased the abundance of insect and disease epidemics, and when combined with fire exclusion, significantly altered the composition and structure of these forests. Historically, western pine beetle (Dendroctonus brevicomis), pine engraver (Ips spp.), fir engraver (Scolytus ventralis), Douglas-fir tussock moth (Orgyia pseudotsugata) were insects associated with regularly burned areas. In most years bark beetles occurred at endemic levels in ponderosa pine, Douglas-fir, and grand fir killing large and weakened trees that were struck by lightning, infected by root disease (Armillaria spp.), or too old to resist attack. Pine engraver and fir engraver beetles attacked young, densely stocked ponderosa pine or removed trees scorched by low-intensity surface fires and/or trees severely infected with disease. Sometimes disease and insect infestations increased during droughts when trees were stressed.
