Geographic and Host-Associated Size Variation in the Parasitoid Wasp Torymus umbilicatus (Hymenoptera: Torymidae) in Florida: Implications for Host Survival and Community Structure

Abstract

Acquisition of enemy-free-space has been suggested to reduce selective pressure against host range expansion in phytophagous insects. The gall midge, Asphondylia borrichiae Rossi and Strong (Diptera: Cecidomyiidae), which attacks the stem tips of its 3 host plants produces a spherical tumor-like growth (= gall). Juvenile stages (larvae and pupae) of A. borrichiae develop inside the gall; the midge spends approximately 95–98% of its life cycle embedded within the gall. During these juvenile stages, A. borrichiae are parasitized by 4 species of hymenopterans. Previous studies have found that one of the most common and the largest parasitoid, Torymus umbilicatus (Gahan), tends to dominate large galls owing to its significantly longer ovipositor, which enables it to penetrate the biggest galls and reach larvae and pupae that become unavailable to the other 3 parasitoids, which have much shorter ovipositors. Moreover, previous studies suggest that the gall midge is diverging both morphologically and genetically in sympatry. The current study is the first to provide morphological evidence that T. umbilicatus, which is a dominant member of the parasitoid guild that attacks A. borrichiae, may also be diverging in sympatry along with its host. Female T. umbilicatus from sea oxeye daisy (Borrichia frutescens [L.] DC) were significantly larger than those from alternative host plants of the gall midge, dune elder (Iva imbricata Walter) and marsh elder (I. frutescens L). Additionally, size of female T. umbilicatus collected from 2 geographically distant sites were significantly different and these differences were consistent with a latitudinal gradient in size between plant species. Although T. umbilicatus were larger from galls collected from B. frutescens compared to I. frutescens at both sites, gall diameter demonstrated a significant decline along a south-north latitudinal gradient. However, a significant interaction between plant species and site suggests that differences in T. umbilicatus size (and most likely their gall midge host) is caused either by phenotypic plasticity of the species at the 2 sites, or these insects (T umbilicatus and gall midges) tend to be smaller with increasing latitude. Moreover, galls on I, frutescens, owing to their smaller size and increased crowding, decline in size at a greater rate than those from B. frutescens which produces significantly larger and less crowded galls.

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