Implications of the Within-Canopy Oviposition Preference, Abundance, and Larval Performance Patterns of a Host Specific Cynipid gallformer

Abstract

Gallformers oviposit at the site of larval development and thus promote selection for a linkage between the oviposition behavior of fertile females and larval performance. The host vigor hypothesis (HVH) predicts this linkage will occur on host tissues with relatively high growth rates that can be identified by large tissue size. However, both predictions may be subject to system-specific developmental and/or intra-specific competition constraints that may occur at different scales within host plants. We examined patterns and relationships of oviposition preference, gall abundance, and larval performance of the leaf-galling generation of Belonocnema treatae Mayr (Cynipidae: Hymenoptera) on its host plant, Quercus fusiformis Small at both within-canopy and within-leaf levels. Over two consecutive gallformer generations, leaves were collected from 16 trees according to previously documented within canopy patterns of carbon uptake (upper > mid > lower and SE > SW = NE > NW) which, along with abiotic correlates, are known to influence phytophage habitat suitability. At each of the 12 strata x direction sites/tree, ovipuncture density, initiated gall density, gall size, and gall-former performance(# mature galls/ovipuncture) were estimated from an arbitrary sample of 100 leaves (total 1200 leaves per tree) to document within-canopy herbivory patterns. Threeway ANOV A was used to test for main effects and interactions of direction, strata, and host plant on ovipuncture density, initiated gall density, and performance followed by post-hoc REFGWQ tests to detect consistent herbivory patterns among trees. To examine within-leaf herbivory patterns and test the HVH, leaf area was estimated for all second year leaves attacked by B. treatae and correlated with gallformer performance and abundance. At the within-canopy level, gall and ovipuncture densities were highest in the low stratum, whereas performance was highest in the upper stratum. Leaf-level performance and abundance were not consistently correlated with each other or leaf size. These results suggest that larval performance is enhanced on more physiologically active regions within host canopies, but preferred oviposition sites are not linked to larval performance and neither oviposition preference nor larval performance is influenced by tissue size.