SEASON AND INTENSITY OF WATER-STRESS - HOST-PLANT EFFECTS OIL LARVAL SURVIVAL AND FECUNDITY OF NEODIPRION GILLETTEI (HYMENOPTERA, DIPRIONIDAE)

A 2 (season of stress) x 3 (stress level) x 6 (host genotype) factoria l design experiment was used to examine the influence of host plant st ress on larval feeding performance. Total instar survival and female c ocoon mass of a pine sawfly Neodiprion gillettei (Rohwer), were measur ed under greenhouse conditions in response to feeding on ponderosa pin e, Pinus ponderosa Douglas ex Law., seedlings subjected to water stres s treatments during either the early-season root growth or early-seaso n shoot growth period. Performance of N. gillettei depended on the gro wth period in which seedlings were stressed. Performance decreased whe n larvae fed on seedlings that were subjected to moderate and high str ess intensities during the shoot growth period compared with stress tr eatments during the root growth period. In addition, the potential fec undity of N. gillettei varied among open-pollinated families of ponder osa pine in response to feeding on seedlings that were either moderate ly or highly stressed during the shoot growth period. Potential fecund ity increased in response to larvae feeding on open-pollinated familie s having a low fine root:current-year foliage ratio compared with fami lies having a high fine root:current-year foliage ratio. This may indi cate that differential response for both herbivore and host to water s tress is caused in part by a genetic component in the host. Furthermor e, the interactions among timing of stress, level of stress and host g enotype, shown in this study to influence sawfly performance, demonstr ate how different combinations of these factors may result in opposing conclusions concerning the effects of host plant stress on herbivore performance.