Oviposition preferences in pine sawflies: A trade-off between larval growth and defence against natural enemies

The oviposition preferences and performance of a monophagous pine sawfly (Neodiprion sertifer) were studied in the held on 61 naturally regenerated Scots pine trees. The preference of sawfly females, measured as the proportion of available shoots on a tree with egg batches, was found to be positively correlated with tree height and needle length in a multiple regression analysis. Females also seemed to prefer trees with high resin acid (diterpenoid) concentrations. Low variation among trees with respect to resin acid concentration, together with positive covariance between these concentrations and needle length, made it difficult to evaluate the role of resin acids. Even though pupae weighed less and larvae suffered higher mortality when protected from natural enemies on trees preferred by females, there were benefits associated with oviposition on such trees, i.e. uncaged larvae were less vulnerable to parasitoids on high resin acid trees. This indicates that females preferred trees on which their offspring gained enemy-free space over trees on which performance was high, which also was suggested by the weak interaction found between the effect of caging (enemy exposure) and oviposition preference on survival in early instars.