There is phenotypic variation among individual trees of interior Douglas-fir (Pseudotsuga menziesii variety glauca [Beissn.] Franco) in their resistance to defoliation by the western spruce budworm (Choristoneura occidentalis Freeman). We are evaluating the factors associated with this resistance using grafted clones derived from parent trees that are resistant versus susceptible to budworm defoliation in the field. We tested the potential role of feeding and oviposition behavior in determining the resistance using C. occidentalis larvae and moths from our nondiapausing laboratory colony, and foliage from grafted ramets (6–8-yr-old potted trees) of resistant and susceptible Douglas-firs. In the larval feeding bioassay, fifth instars were offered a choice between branches from susceptible and resistant ramets. There was no difference in the proportion of current-year buds and shoots consumed, indicating that larvae do not discriminate between foliage from susceptible and resistant trees. When offered a choice in cage arenas made of metal screen, female moths laid significantly more of their egg masses on susceptible foliage (54%), compared with resistant foliage (34%), artificial foliage (6%), or nonfoliage material (6%). Conversely, no oviposition preference was detected when resistant versus susceptible Douglas-fir twigs were the only substrates available in paper bag arenas. Western spruce budworm feeding and oviposition behavior do not appear to be important factors associated with Douglas-fir resistance.
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