Preference-performance in a specialist sawfly on congeneric host plants

In herbivorous insects, the preference-performance hypothesis predicts that females will choose to oviposit on plants that maximize offspring development. Although there is evidence to support this hypothesis, several studies have shown that it does not always apply. Among the factors that may modulate the preference-performance relationship, diet breadth is a significant one, although no correlation has been found for monophagous insects. More case studies are needed, as more restricted diets are expected to require better discrimination ability. In this study, we explored the preference-performance linkage in a monophagous sawfly, Tequus schrottkyi (Konow) (Hymenoptera: Pergidae), which feeds as larva on the wild potato Solanum commersonii Dunal (Solanaceae). We used the cultivated potato, Solanum tuberosum L., as an alternative host plant for our experiments. Performance was evaluated from newly emerged larvae until adult emergence, raising individual larvae on potted plants or excised leaves. Female oviposition preference was studied in two-choice bioassays, in which individual females were offered whole plants of S.commersonii and S.tuberosum. The nutritional quality of the plants was assessed by measuring protein, non-structural carbohydrate, and water contents. The offspring performed better on their host plant, S.commersonii, resulting in more larvae completing their development to the adult stage. Moreover, the larvae developed faster and their cocoon weighed more when they were raised on their host plant. The females oviposited almost exclusively on S.commersonii, regardless of their previous food plant. The protein and non-structural carbohydrate contents did not differ between the plants, whereas water content was higher in the non-preferred S.tuberosum. The differences in water content and in secondary metabolites, which have been analyzed in a previous study, may play a role in female preference and offspring performance. The results are discussed within the framework of the preference-performance hypothesis and the chemical ecology of insect-plant interactions.