DOUGLAS-FIR NUTRIENTS AND TERPENES - POTENTIAL RESISTANCE FACTORS TO WESTERN SPRUCE BUDWORM DEFOLIATION

Differences in the nutritional quality of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) foliage may explain why some individual trees are more resistant or susceptible than others to damage from the western spruce budworm, Choristoneura occidentalis Freeman. We compared levels of foliar nutrients (nitrogen, sugars, phosphorus, potassium, calcium, magnesium, copper, manganese, zinc, iron, sodium, and sulfur) and allelochemicals (mono- and sesquiterpenes) between ''resistant'' (light defoliation history) and ''susceptible'' (history of heavy defoliation) Douglas-fir trees in 1989 and 1990. Phenology of bud burst and the radial growth rate of the trees were also examined. Susceptible trees had lower levels of N and sugars than resistant trees. These results agreed with published predictions from artificial diet bioassays. Moreover, susceptible trees had mineral/N ratios (for P, K, Ca, Mg, Cu, and Zn) which were closer to the optimum levels previously established in diet bioassays. There were no detectable differences in foliar concentrations of terpenes. However, susceptible trees had a greater proportion of total terpenes that were monoterpenes, whereas resistant trees had a greater percentage of oxygenated monoterpenes. Resistant trees also had delayed bud burst and shoot expansion compared to susceptible trees, and they had accumulated more radial growth over the past 25 yr, implying they were more vigorous than susceptible trees.