TRADE-OFFS IN RESPONSES TO HOST PLANTS WITHIN A POPULATION OF A GENERALIST HERBIVORE, CHORISTONEURA-ROSACEANA

Evolutionary constraints on the ability of herbivores to efficiently u se a set of phytochemically similar hosts, while maintaining a high pe rformance on phytochemically different hosts, are central in explainin g the predominance of host specialization in phytophagous insects. Suc h feeding trade-offs could be manifested within insect populations as negative genetic correlations in fitness on different host species. We tested the hypothesis that feeding trade-offs were present within a p opulation of the obliquebanded leafroller, Choristoneura rosaceana (Ha rris). Components of fitness were measured in families originating fro m an apple orchard that were fed on four host-plant species in the lab oratory. Under the conditions of this experiment, all across-host gene tic correlations were strongly positive, suggesting that this populati on comprised true generalists. With the exception of diapausing propen sity, the heritability of the fitness components tended to be lower in caterpillars fed on apple leaves than in insects fed other hosts. Thi s suggests a constraint on the selective response of the fitness compo nents in the orchard environment.