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 performance of pine sawfly larvae (Neodiprion sertifer) and on the oviposition behaviour and fecundity of adult females were studied in a subarctic area. Two anatomical traits and resin acid concentrations of pine needles from experimental trees were measured and bioassays undertaken to determine their effect on herbivores. Twice as many females oviposited on trees receiving only ambient rain (dry control) as on irrigated trees. Larval growth performance was also slightly better on dry control trees. However, larval survival, relative larval growth rate, female fecundity and oviposition preference did not differ between acid-treated (pH 3; both H2SO4 and HNO3) and irrigated control (pH 6) trees. So, the oviposition preference of European pine sawfly is modified more by the amount of rainfall than by its pH. The concentrations of measured 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 results suggest that the oviposition preference expressed by this eruptive species is a more sensitive indicator of host plant quality than larval performance. Secondly, for N. sertifer, needle resin acid content and related structural characteristics modify larval performance and female oviposition behaviour more than needle surface thickness.