Parasitiods Utilizing the Same Host: Adaptive Nature of Differences in Size and Form

Abstract

Eleven hymenopterous parasitoid species commonly exploit one sawfly host, Neodiprion swainei. A comparison of the area of forewing, structure of ovary, and length of ovipositor of some of these species is revealing. The three guilds of parasitoids that attck the larval, eonymphal, and cocoon stages of the host, respectively, face different problems in host exploitation. Females of the larval parasitoid guild have large egg production and large wings. They are adapted to exploit a relatively abundant, easily discovered stage of the host, but their progeny are subject to high mortality while in the host. Females of the cocoon parasitoid guild have small wings and low egg production. They seek a scarcer, well—concealed stage of the host, and their progeny sufer less mortality. Members of the eonymphal parasitoid guild show intermediate characteristics. Among the cocoon parasitoids, the species have different ovipositor lengths, presumably allowing them to utilize segments of the host population, either different size classes or hosts that are buried at different depths in the forest litter. Comparison of ovipositor lengths indicates how similar the species can be in size, and therefore larval food exploitation, before competitive displacement prevents coexistence.