Preference-performance linkage in a herbivorous lady beetle: consequences of variability of natural enemies

We investigated the relationship between oviposition preference and offspri ng performance in a herbivorous lady beetle Epilachna pustulosa on two cooc curring plant species, thistle Cirsium kamtschaticum and blue cohosh Caulop hyllum robustum, in 1994 and 1995. The relative importance of bottom-up eff ects by host plants and top-down effects by natural enemies on offspring pe rformance were determined using field and laboratory experiments. In both y ears, egg density on blue cohosh was significantly higher than on thistle. A laboratory experiment demonstrated that larval survival from hatching to adult emergence was significantly higher, and developmental period shorter when larvae were reared on blue cohosh compared to thistle. The positive pr eference-performance linkage varied between years in the field. Top-down ef fects had a different impact on larval survival on the two host plant speci es. Arthropod predators, a lady beetle Harmonia axyridis and an earwig Forf icula mikado, considerably depressed immature survival on thistle, while th ey were negligible on blue cohosh. Although the lack of effective predation increased larval survival on blue cohosh, it led to defoliation due to inc reased larval feeding late in the season. Because of severe intraspecific c ompetition, old larvae had significantly lower survival on blue cohosh than on thistle. In 1994, as larval survival decreased due to defoliation on bl ue cohosh, the overall survival rate was significantly higher on thistle th an on blue cohosh. This survival pattern was opposite to that found in the laboratory experiment. In contrast, in 1995, the increase in predatory lady beetles on thistle caused greater larval mortality. Thus, the overall surv ival was significantly lower on thistle than on blue cohosh, although sever e intraspecific competition occurred on blue cohosh as it had in 1994. Cons equently, the offspring performance on the two host plants is largely deter mined by the relative importance of arthropod predation determining larval survival on thistle and host plant defoliation reducing late larval surviva l on blue cohosh. These results indicate the important role of spatial and temporal variability of natural enemies on the preference-performance linka ge of herbivorous insects.