Variation in Budburst Phenology of Douglas-fir Related to Western Spruce Budworm (Lepidoptera: Tortricidae) Fitness

Abstract

Variation in budburst phenology among individual trees of interiorDouglas-fir (Pseudotsuga menziesii var. glauca {[}Beissn.] Franco) mayinfluence their susceptibility to western spruce budworm (Choristoneuraoccidentalis Freeman) defoliation. We tested the hypothesis thatphenological asynchrony between Douglas-fir and the western sprucebudworm is a mechanism of resistance using clones derived from parenttrees that showed resistance versus susceptibility to C. occidentalisdefoliation in the field. Susceptible clones had earlier budburstphenology compared with resistant clones when they were grown in acommon greenhouse environment, demonstrating a genetic basis forparallel phenological differences exhibited by the parent trees. Wetested the importance of phenological asynchrony as a factor influencingfitness of C. occidentalis using two different greenhouse bioassayexperiments. One experiment compared western spruce budworm performanceon equivalent phenological stages of susceptible and resistant clones bymatching larval feeding to the columnar (fourth) bud development stageof each clone. Larvae reared on resistant clones had greater realizedfitness (i.e., number of F, offspring produced) than those reared onsusceptible clones when the influence of variation in budburst phenologywas minimized. In the other experiment, western spruce budworm larvaewere placed on all trees on the same date when approximate to50% of allterminal buds in the population were in the yellow (second) budburststage. Larvae reared on susceptible clones had greater realized fitnessthan those reared on resistant clones when the influence of phenologicalasynchrony was expressed. Our results suggest that resistant phenotypesof Douglas-fir have negative effects on survival and reproduction of C.occidentalis under the natural conditions that insects and treesexperience in the field. Genetic variation among trees in budburstphenology has an important influence on interactions between the westernspruce budworm and Douglas-fir.