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1 Natural and recurring disturbances caused by fire, native forest insects and pathogens have interacted for millennia to create and maintain forests dominated by seral or pioneering species of conifers in the interior regions of the western United States and Canada. 2 Changes in fire suppression and other factors in the last century have altered the species composition and increased the density of trees in many western forests, leading to concomitant changes in how these three disturbance agents interact. 3 Two-and three-way interactions are reviewed that involve fire, insects and pathogens in these forests, including fire-induced pathogen infection and insect attack, the effects of tree mortality from insects and diseases on fuel accumulation, and efforts to model these interactions. 4 The emerging concern is highlighted regarding how the amount and distribution of bark beetle-caused tree mortality will be affected by large-scale restoration of these fire-adapted forest ecosystems via prescribed fire. 5 The effects of fire on soil insects and pathogens, and on biodiversity of grounddwelling arthropods, are examined. 6 The effects of fire suppression on forest susceptibility to insects and pathogens, are discussed, as is the use of prescribed fire to control forest pests.

The diameter of galls formed on shoots of arroyo willow, Salix lasiolepis, and initiated by the tenthredinid sawfly Euura lasiolepis strongly influenced the pattern of attack by the ectoparasitoid Pteromalus sp. Gall diameter is regulated by the plant; significant differences in mean gall diameter persisted between adjacent clones for 5 yr of study, and a genotypic influence is suggested because cuttings grown in a common environment maintained the relative gall size characters of the parental clones. Mean gall diameters per clone ranged from 3.5 to 7.5 mm. For a parasitoid with a mean ovipositor length of 1.89 mm (which must reach a host in the gall center from the gall exterior), many potential hosts in the larger galls could not be attacked. Attack by Pteromalus was concentrated in the first two galls on a shoot, and these were, on the average, the smallest galls on shoots. To avoid the effect of synchrony of attack phenology with production of small galls, the effect of gall diameter was examined only on galls in positions 1 and 2 on stems. Attack was still concentrated on the smaller galls, so we rejected the Random Attack Hypothesis and accepted the Gall Diameter Hypothesis. Winter precipitation affected gall diameter only after the driest winter in the winters 1978—1979 to 1982—1983, and a significant negative correlation between winter precipitation and percent interval parasitism was evident in only one of four clones tested for this effect. Gall diameter was strongly controlled by the ovipositing female, with minor, if any, influence by larval feeding. Therefore, Pteromalus attack could not have reduced gall diameter by more than 1.9% or 0.11 mm on average. Pteromalus attack on smaller galls was not the result of retarded gall growth subsequent to attack; the ectoparasitoid females were selecting smaller galls. The long—term relationship between mean gall diameter per clone and percent interval parasitism over 31 clone years from 1979 to 1983 was consistent with the Gall Diameter Hypothesis: a significant negative relationship was evident, accounting for 35% of the variance.

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