THE EFFECT OF HOST VARIABILITY ON GROWTH AND PERFORMANCE OF THE INTRODUCED PINE SAWFLY, DIPRION-SIMILIS

Diprion similis (Htg.) (Hymenoptera: Diprionidae) was reared in the laboratory on Pinus banksiana Lamb. and Pinus strobus L. from the second stadium through adult emergence. Groups of larvae were fed current-year or previous years' foliage from specific trees. Host species had a significant, but limited, effect on D. similis growth and performance. Foliar age had a stronger and more consistent effect on development. In contrast with reports for other diprionid species, previous years' needles had a consistently greater adverse effect on D. similis performance than did new growth. This suggests that there is a conflict between the avoidance of host tissues with high allelochemical concentrations and the avoidance of those with reduced nutrient content. Larval survival did not vary between foliar treatments, which suggests that the detrimental effects of host diet are chronic rather than acute. Female fecundity was strongly associated with cocoon weight, but the relationship varied with host diet and diapause incidence, as did the relative reproductive potential. Substantial between-tree variability in insect performance indicates a potential for resistance breeding programs. Tree rankings for each performance variable were highly consistent, which would permit the development of an expeditious screening procedure.