Importance of inter- vs. intra-specific variation in host quality on an herbivore: Testing non-wheat host grass species with wheat stem sawfly, Cephus cinctus

Oligophagous and polyphagous herbivores can experience variation in performance (e.g., size and fecundity) across plant host species. This is especially the case for herbivores with endophagous larvae that remain in a single host plant, where a mother’s oviposition choice influences host plant quality experienced by her offspring. One such generalist herbivore is the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae). This internal stem-mining sawfly can have larvae survive and complete their life cycle in several cultivated and uncultivated native and exotic grasses. We tested the influence of interspecific host quality variation on C. cinctus larval post-diapause performance by measuring adult body size and female fecundity (egg number set at adult emergence) of emerged adults from exotic cheatgrass/downy bromegrass (Bromus tectorum L.) and smooth bromegrass (B. inermis Leyss), and native slender wheatgrass (Elymus trachycaulus (Link) Gould ex Shinners (= Agropyron trachypaulum), and intraspecific variation by comparing performance from three B. inermis populations. Surprisingly, variation in C. cinctus male and female body sizes, female egg number, and female egg number per mg body mass, across the three B. inermis populations was as large (or even slightly larger) than across the three host grass species tested. This experiment was not conducted under common garden conditions. Thus, variation in C. cinctus performance collected across three B. inermis populations might be due in part, or fully, to differences in abiotic conditions experienced across the three sites rather than differences in host quality. Larvae were collected from B. tectorum, and E. trachycaulus, and one B. inermis population, were all collected from the same SW Montana location. Future experiments to examine intra- vs. inter-specific host grass variation on C. cinctus performance could compare C. cinctus performance from a population of each test grass from each test location, or infest these all under under common garden conditions. Host plant quality is important to C. cinctus, since female fecundity influences female fitness.