Effects of experimentally altered plant moisture stress on the performance of Neodiprion sawflies

For five years we studied the effects of experimentally altered levels of plant moisture stress on Pinus ponderosa (Dougl. ex Laws.) on the performance of Neodiprion autumnalis and N. fulviceps. We could not consistently support the hypothesis that high plant moisture stress leads to improved herbivore performance. An alternative hypothesis that sawflies performed better on non-stressed trees was supported in some years. Sixty trees were divided into three treatments: trenching; watering, and control. Plant moisture stress differed significantly among treatments, being highest on trenched trees and lowest on watered trees. Mean N. autumnalis survival rates from first larval instar to cocoon spining differed significantly among treatments over a four-year period, with mean survival being lowest on experimentally trenched trees. Neopridion fulviceps survival from first larval instar to cocoon spinning did not differ among treatments. No correlation existed among survival rates on trees between years, indicating that trees did not tend to be consistently susceptible or resistant. Inclusion of the inital levels of total terpenes, D_bh and age for each tree as covariates did not improve the explanatory power of the analysis of variance on treatment effects.