Short-term effects of defoliation by sawflies (Hymenoptera: Diprionidae) on above- and below-ground growth of three ponderosa pine genotypes

Diprionid sawflies (Hymenoptera: Diprionidae) feeding on ponderosa pine, Pinus ponderosa Dougl. ex Lawson, divide food resources by feeding at different times during the season and on different aged trees. Whereas some species prefer adult trees, others have specialized on pole-size trees, and yet others on seedlings. Some attempts have been made to determine whether conifer growth compensates in response to herbivore damage as do certain nonwoody plants; however, the impact of defoliation in conifers may depend on the growth stage of trees. The presence or absence of compensatory growth in seedlings is critical because for most pine species, early growth allocation can impact growth in the long term. We induced attacks by Neodiprion gillettei Rohwer on 1-yr-old ponderosa pine seedlings to determine the effect of defoliation on above- and below-ground growth. Three pine half-sib families whose parent trees were either susceptible or resistant to pine sawflies were used in the experiment. Growth was evaluated through destructive sampling. We found no evidence of seedling compensatory growth in response to herbivores. Defoliation reduced both above- and below-ground biomass. Stable root:shoot ratios indicated that foliage removal resulted in an approximately equivalent reduction in below-ground biomass. Differences in growth patterns among genotypes were found as well.