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The terpenoid and phenyl propanoid content of xylem resin as well as phloem nitrogen and carbohydrate levels of lodgepole pine trees infected with Armillaria root disease, Comandra blister rust, and dwarf mistletoe and check (asymptomatic) trees were determined. Comparisons were made to determine if differences existed that might influence their susceptibility to bark beetle attack. These variables were also contrasted with respect to aspect (north and south). Five volatiles (tricyclene, α-pinene, camphene, γ-terpinene, and bornyl acetate) were significantly higher in trees with one or more diseases than in check trees. Four volatiles (myrcene, camphor, 4-allylanisole, and γ-terpineol) were significantly lower in diseased trees. Camphene was the only resin constituent found to differ with respect to aspect, with a higher concentration on the north aspect of check trees. There were no significant differences in carbohydrate or nitrogen content with respect to aspect. The check trees were found to have significantly higher starch, total nitrogen, and free amino-N contents than diseased trees. Trees infected with Comandra blister rust were found to contain lower levels of reducing and nonreducing sugars than the other diseased trees and the check trees. Changes in terpenoids and phenyl propanoids in trees susceptible to mountain pine beetle attack suggest a biochemical basis for host selection. Key words: mountain pine beetle, lodgepole pine, Comandra blister rust, dwarf mistletoe, Armillaria root disease, resin chemistry, susceptibility.

Abstract. The effect of ponderosa pine (Pinus ponderosa Doug]. ex Laws) water stress, measured by xylem water potential, on oviposition preference and egg development of Neodiprion fulviceps was determined by two laboratory experiments in 1987 and 1988. In the first experiment, adult females were allowed to choose between foliage from water‐stressed, watered, and untreated control trees. Significant differences were found between treatments in xylem water potential in 1987 but not in 1988. Females chose water‐stressed trees and a positive correlation was found between female preference and xylem water potential in 1987 but not in 1988. In the second experiment, females were placed in globe cages with a single branch from each treatment and allowed to oviposit. Significant differences were found among treatments in egg survival in 1987 but not in 1988. These results indicate that sawflies can distinguish between stressed and non‐stressed trees and that this behaviour is adaptive in that egg survival was greatest on the preferred treatment.

Total flow of lodgepole pine oleoresin was investigated in relation to size of wound, diameter class, disease status, and aspect (north and south). The two wound sizes were 2.54 and 5.08 cm in diameter. The diameter classes were 15.24, 20.32, and 25.40 cm. The disease classes were Armillaria root rot (Armillaria spp.), lodgepole pine dwarf mistletoe (Arceuthobium americanum Nutt. ex. Engelm.), and Comandra blister rust (Cronartium comandrae Pk.). It was found that the 5.08-cm wound resulted in almost twice as much resin flow as the 2.54-cm wound after 24 h. No differences in flow were found between size classes with the 5.08-cm wound. However, there was a general trend for flow to increase with an increase in diameter class. The 15.24-cm diameter class had significantly less flow than the other two diameter classes with the 2.54-cm wound. There was also a general trend for greater total flow on the north aspect. There was a general trend for greater resin flow in the diseased trees. It was also found that disease status had a significant effect on tree growth. The comandra blister rust trees put on significantly less growth than trees in the other two disease classes. Radial growth was also found to be greater on the north aspect of all trees. Interestingly, the mountain pine beetle (Dendroctonus ponderosae Hopkins) usually attacks the north aspect first then spreads or maintains a strip attack on that aspect. The role of the constitutive (primary) defensive system in conifers needs further study to understand the interactions taking place, especially from the host selection point of view during the endemic periods. During epizootic periods resources are utilized as a result of availability. But during endemic periods, to maintain the insect populations, host selection would seem critical for survival. Key words: resin flow, Pinus conforta, Armillaria, Cronartium, Arceuthobium, mountain pine beetle.

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