Host Selection and Larval Performance of Two Willow‐Feeding Sawflies

Abstract

Host selection by ovipositing females and larval performance of two willowfeeding sawflies, Nematus salicis and N. pavidus, were investigated in laboratory choice and feeding experiments. In the field N. salicis is more specialized in its host utilization than N. pavidus, which is commonly found on a great number of willow species. In the Multiple Choice Oviposition Experiment females of both sawfly species exhibited a strong preference for one of six willow species offered. In No Choice Oviposition Experiments N. salicis retained its high selectivity while N. pavidus laid eggs equally on several willows. Host utilization spectra of larvae, as measured by larval growth and survival, were noticeably wider than those of ovipositing females. In N. salicis larval performance was optimal on willows that were also preferred by females, but in polyphagous N. pavidus conformity between host selection by ovipositing females and the larval performance was negligible. Host plant spectra of these sawflies are largely determined by behavior of ovipositing females and not by food utilization efficiency or survival of larvae. We suggest that host specialization of the sawflies studied has developed from behavioral processes of adults and is guided by specific plant characteristics such as phenolic glycosides in willow leaves. The physiological specialization of larvae may proceed more slowly and be secondary. In general we emphasize the role of behavioral constraints and adaptations in the ecology and evolution of host utilization by herbivorous insects.