DIFFERENTIAL HERBIVORY IN AN OAK POPULATION - THE ROLE OF PLANT PHENOLOGY AND INSECT PERFORMANCE

Individual trees in a North Florida population of Quercus geminata, (s and live oak) vary widely in rates of herbivory by a leaf-mining moth (Stilbosis quadricustatella). Some oaks are always heavily infested wh ile neighboring trees remain lightly attacked. We conducted field expe riments in 1991 and 1992 to determine if leafminer performance on heav ily and lightly attacked trees contributed to this pattern. We also co mpared the leaf production phenology, leaf area, and stem length of he avily and lightly attacked trees to determine if tree phenotypic trait s influenced herbivory. Our results indicate that: (I) Lightly attacke d trees produced leaves earlier in the spring than heavily attacked tr ees, and the difference among trees in leaf production phenology expla ined 61% of the variation in leafminer densities (P = 0.008). (2) Ligh tly attacked trees produced significantly smaller leaves than heavily attacked trees, but leaf size explained only 13% of the variation in h erbivory (P = 0.048). (3) There were no differences in larval performa nce on heavily and lightly attacked trees in either 1991 or 1992, with the exception of egg mortality, which was significantly higher on lig htly attacked trees in 1991 (P = 0.04). (4) Host plants caused greater leafminer mortality than natural enemies, but neither differed signif icantly between heavily and lightly attacked trees. Our transfer exper iments and observations of wild leafminers revealed little difference in leafminer performance on heavily and lightly attacked Quercus gemin ata trees. However, tree phenotypic traits differed, especially the ti ming of spring foliation, which occurred significantly earlier among t rees with low rates of herbivory. These results suggest that leafminer ovipositional preference, rather than performance, could be an import ant determinant of differential herbivory. There may be an unknown sel ective advantage to preferential oviposition on trees with delayed spr ing leaf production.