OVIPOSITION AND LARVAL PERFORMANCE OF EUROPEAN PINE SAWFLY IN RELATION TO IRRIGATION, SIMULATED ACID-RAIN AND RESIN ACID CONCENTRATION IN SCOTS PINE

The effects of irrigation and simulated acid rain on the growth perfor mance of pine sawfly larvae (Neodiprion sertifer) and on the ovipositi on behaviour and fecundity of adult females were studied in a subarcti c area. Two anatomical traits and resin acid concentrations of pine ne edles from experimental trees were measured and bioassays undertaken t o determine their effect on herbivores. Twice as many females oviposit ed on trees receiving only ambient rain (dry control) as on irrigated trees. Larval growth performance was also slightly better on dry contr ol trees. However, larval survival, relative larval growth rate, femal e fecundity and oviposition preference did not differ between acid-tre ated (pH 3; both H2SO4 and HNO3) and irrigated control (pH 6) trees. S o, the oviposition preference of European pine sawfly is modified more by the amount of rainfall than by its pH. The concentrations of measu red total and some individual resin acids showed significant negative correlations with larval survival and relative growth rate. There was a weak correlation between resin duct area, expressed as a proportion of needle cross-sectional area and female oviposition choice. Our resu lts suggest that the oviposition preference expressed by this eruptive species is a more sensitive indicator of host plant quality than larv al performance. Secondly, for N. sertifer, needle resin acid content a nd related structural characteristics modify larval performance and fe male oviposition behaviour more than needle surface thickness.