Genetic variation in resistance mechanisms of Douglas-fir to western spruce budworm herbivory

I investigated genetic variation in resistance mechanisms of Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) to western spruce budworm (Choristoneura occidentalis Freeman) herbivory. Five independent, but complimentary studies are included in this dissertation. The first study compared genetic structure and composition of allozymes between resistant and susceptible trees by using starch gel electrophoresis on megagametophytes. I found that resistant trees had a higher frequency of the most common alleles, a larger proportion of homozygous genotypes, and lower allelic heterozygosity compared to susceptible trees. Tree heterozygosity was not related to tree radial growth rate. The second study compared budburst phenology, photosynthetic compensation, and growth rate among clones from resistant and susceptible trees. I found that clones of resistant trees had later budburst phenology, and higher growth rates than clones of susceptible trees, whereas photosynthetic compensation overall did not differ. Differences in budburst phenology and growth rate were consistent across mature trees, their clones, and their half-sib seedling progenies indicating strong genetic control of these traits. The third study focused on effects of tree genotype on budworm performance. I found that tree genotype influenced budworm performance, and this influence was mediated by the degree of synchrony between budworm feeding and budburst phenology. The fourth study focused on the role of monoterpenes in Douglas-fir resistance to budworm defoliation. I found that foliage total monoterpene concentration varied widely over years, but the fractional composition of monoterpene components was very consistent. Higher total monoterpene concentration and monoterpene/N ratio negatively influenced budworm fitness. The effectiveness of foliage monoterpenes as a resistance mechanism of Douglas-fir to budworm defoliation may be mediated by the degree of synchrony between budworm feeding and budburst phenology and foliage N concentration. The fifth study compared the effect of artificial and budworm defoliation on the growth and biomass allocation of half-sib seedling progenies of resistant and susceptible trees. I found that artificial defoliation could effectively simulate the effects of budworm defoliation on seedling total biomass but not biomass allocation. Seedlings from resistant trees grew faster than seedlings from susceptible trees, suggesting a role of inherently high growth rate as a resistance mechanism.