Foliar nutrients and induced susceptibility: genetic mechanisms of Douglas-fir resistance to western spruce budworm defoliation

We conducted greenhouse defoliation experiments with clones of interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) over 3 years to assess the role of foliar nutrients as a resistance mechanism to western spruce budworm (Choristoneura occidentalis Freeman) defoliation. The grafted clones were derived from mature trees (i.e., ortets) that showed resistance or susceptibility to budworm defoliation in the forest. Current-year foliage was analyzed for concentrations of nitrogen (N), sugars (sucrose + fructose + glucose), phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), manganese (Mn), copper (Cu), iron (Fe), and zinc (Zn). We computed rank correlations between foliar nutrient levels in the ortets and their corresponding clones to test the null hypothesis that foliar chemistry does not have a genetic basis (H(0)1). Foliar concentrations of sugars and P were under genetic control to some degree, but concentrations of other nutrients were not. We used analysis of variance to test the null hypotheses that foliar chemistry does not change in response to budworm defoliation (H(0)2) and that it is not different between resistant and susceptible clones (H(0)3). We rejected H(0)2 for sugars, P, K, Mn, and Zn; defoliation by the budworm changed levels of these nutrients and had divergent effects on concentrations of P, K, and Zn in resistant clones. We concluded that induced susceptibility, whereby defoliation alters foliar nutrients to make trees more favorable for insect feeding, appears to be an important determinant of Douglas-fir resistance to the western spruce budworm. Failure to reject H(0)3 implies that previously reported differences between the foliar nutrient levels in resistant Douglas-firs and those in susceptible Douglas-firs in the forest are probably caused by induced susceptibility.