There is phenotypic variation among individual trees of interior Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) in their susceptibility to defoliation from 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 non-diapausing laboratory colony, and foliage from grafted ramets (6 to 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. Laboratory and field arenas were used to test adult oviposition behavior at various scales. When offered a choice among susceptible, resistant, and artificial twigs, female moths laid more egg masses (54%) on susceptible foliage. Conversely, no preference was detected when resistant versus susceptible Douglas-fir twigs were the only oviposition substrates available. Results from experiments using whole grafted trees suggested that female moths are reluctant to fly, and typically lay most of their eggs near the site where they emerge. Western spruce budworm feeding and oviposition behavior do not appear to be important factors associated with Douglas-fir resistance. However, weak female flight capabilities may indirectly affect larval distributions and should be investigated further.
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA