Season and Intensity of Water Stress: Host-Plant Effects on Larval Survival and Fecundity of Neodiprion gillettei (Hymenoptera: Diprionidae)

Abstract

A 2 (season of stress) x 3 (stress level) x 6 (host genotype) factorial design experiment was used to examine the influence of host plant stress on larval feeding performance. Total instar survival and female cocoon mass of a pine sawfly, Neodiprion gillettei (Rohwer), were measured under greenhouse conditions in response to feeding on ponderosa pine, Pinus ponderosa Douglas ex Law., seedlings subjected to water stress treatments during either the early-season root growth or early-season shoot growth period. Performance of N. gillettei depended on the growth period in which seedlings were stressed. Performance decreased when larvae fed on seedlings that were subjected to moderate and high stress intensities during the shoot growth period compared with stress treatments during the root growth period. In addition, the potential fecundity of N. gillettei varied among open-pollinated families of ponderosa pine in response to feeding on seedlings that were either moderately or highly stressed during the shoot growth period. Potential fecundity increased in response to larvae feeding on open-pollinated families having a low fine root :current-year foliage ratio compared with families having a high fine root :current-year foliage ratio. This may indicate that differential response for both herbivore and host to water stress is caused in part by a genetic component in the host. Furthermore, the interactions among timing of stress, level of stress and host genotype, shown in this study to influence sawflv performance, demonstrate how different combinations of these factors may result in opposing conclusions concerning the effects of host plant stress on herbivore performance.