Variable plant quality and performance of the willow-feeding leaf beetle Galerucella lineola

Plants vary in their suitability to herbivorous insects owing to differences in chemical, morphological and other traits. In this thesis the focus is on how one optimal host (Salix viminalis) and one suboptimal host (S. dasyclados) influence performance of the leaf beetle Galerucella lineola. Four main questions were addressed: i) Is there a relationship between leaf phenolic composition and larval performance? ii) How does plant hybridization affect phenolic content and larval performance? iii) Why do G. lineola larvae prefer to feed in protected microsites? iv) Can leaf quality indirectly affect the risk of predation? Beetle larvae feeding on S. dasyclados showed lower survival , took longer to develop and were smaller than those feeding on S. viminalis. Larval performance was not correlated with the total concentration of phenolics. Nor did any individual phenolic compounds appear to influence the insect. However, in a multivariate analysis, larval performance was correlated with phenolic composition. Thus performance seems to be affected by interactions between several compounds rather than by single compounds. Hybridization affected insect performance. S. viminalis x dascylados hybrids were of intermediate suitability as food for G. lineola larvae compared with the parental plants. Furthermore, concentrations of most phenolic compounds were intermediate in the hybirds. However, strong heterosis was displayed for three compounds, the concentrations of which were significantly higher than the parental means. Larvae of G. lineola feeding in protected microsites were protected against desiccation but not against predators. Thus, the preference for protected feeding sites may be the result of selection pressure exerted by weather. G. lineola larvae feeding on the suboptimal willow suffered more from predation than those feeding on the optimal host. This was probably due to the slow-growing larvae being exposed to predators for a longer period. In addition, the defensive capabilities of the larvae may have been reduced. Thus, variation in larval performance related to host plant quality can be amplified by the action of predators.