Does enemy-free space for eggs contribute to a leaf beetle's oviposition preference for a nutritionally inferior host plant?

1. Natural enemies are likely to influence the interactions between herbivo rous insects and their host plants. In particular, selection exerted by nat ural enemies could favour host-plant switches and cause, or maintain, ovipo sition preference for a host species that is nutritionally inferior to anot her acceptable host. 2. In a previous study, it was shown that larvae of the leaf beetle Oreina elongata perform better on Adenostyles alliariae (Asteraceae) than on Cirsi um spinosissimum (Asteraceae). Moreover, A. alliariae provides larval and a dult beetles with sequestrable chemical defences. However, in the field, eg g densities are much higher on C. spinosissimum than on adjacent A. alliari ae. 3. In this study, it was investigated whether this oviposition pattern coul d be maintained by C. spinosissimum, providing the eggs of O. elongata with better protection from natural enemies. In a field experiment, the surviva l of eggs was quantified on plants of each of the two species, with and wit hout enemy exclusion. 4. Egg survival was equal for both host species when enemies were excluded from the plants, but it was higher on C. spinosissimum than on A. alliariae when enemies were allowed to the plants. It was also experimentally tested whether the higher egg densities observed in the field on C. spinosissimum are actually due to oviposition preference by the beetle. In a no-choice t est, females laid more eggs on C. spinosissimum than on A. alliariae. 5. It can thus be confirmed that C. spinosissimum is really preferred for o viposition and it is concluded that this preference is likely to be maintai ned, at least partly, by a higher egg survival on C. spinosissimum due to e nemy-free space provided by this host plant.