Matching plant defence syndromes with performance and preference of a specialist herbivore

Abstract

Summary 1. Plants use various strategies to cope with opponents. According to the plant defence theory these traits are predicted to covary across taxa and were shown to be grouped into several syndromes for Apocynaceae. Specialist herbivores tolerate or detoxify components of the plants' chemical weapons. Their development might mirror the putative defence syndromes of their hosts. This hypothesis was tested by measuring nutritive values and potential defence properties of seven species of Brassicaceae, considering leaf age. Effects of these traits were assessed on various lifehistory traits of the oligophagous sawfly Athalia rosae. 2. Positive correlations were found between particular plant traits. A hierarchical cluster analysis assembled plants in three distinct groups with either low nutritional quality or higher nutritional quality together with either only chemical or with chemical and mechanical defences. Although young and old leaves of each species grouped within the same clusters, age was a significant source of variation, demonstrating that ontogenetic changes in plant quality influence associations. 3. The correlations of several life-history parameters of A. rosae with each other and with plant traits were investigated. Mortality rates, developmental times and adult mass were correlated and important for insect fitness. Preference of adult females was largely influenced by larval performance. Three distinct clusters were determined, with fitness of this specialist highly depending on host plant quality. 4. Multivariate studies of the long-term performance and preference of the specialist in relation to the plant defence syndromes revealed general implications for plant - insect interactions, namely that insect traits mirror the defence syndromes of their hosts. Larval performance and adult preference were more influenced by general mechanical and chemical than by chemical plant defence traits, to which this specialist is adapted. 5. The 'plant defence syndromes hypothesis' is of general importance, however, as defence strategies and nutritional value change drastically throughout ontogeny, tissue age should be considered, and modifications on single trait associations are needed. Clusters forming plant syndromes mostly matched with clusters determined from the parameters of a specialist herbivore. Further research is needed on generalist performance, which might be differently influenced by the plants' defence strategies.