PARENT SESSION
Oral Session # 89: Insect Ecology II.
Presiding: R Sears
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 101.
Host plant quality and demic adaptation as determinants of the distribution of a gall-forming herbivore.
Egan, Scott *,1, Cryer, Greg1, Ott, James1, 1 Southwest Texas State University, San Marcos, Texas, 78666
ABSTRACT- Hypotheses correlating variation in host plant quality and herbivore fitness, when linked with the hypothesis of local (demic) adaptation, provide a synthetic explanation for the patchy distribution of insect herbivores within host plant populations. We tested both the among-plant fitness variation and demic adaptation hypotheses for the host-specific leaf-galling insect, Belonocnema treatae (Hymenoptera:Cynipidae), on its host, plateau live oak, Quercus fusiformis. A manipulative transplant experiment was performed where mated females from populations of each of six high gall density trees (focal trees) were bagged onto branches of (a) their respective four nearest-neighbor hosts (1 per cardinal direction), (b) the five alternative focal trees, and (c) their natal trees. Each treatment -- nearest-neighbor, alternative focal, and natal -- was replicated three times at the level of the individual tree. For each replicate, we recorded: (a) oviposition intensity (# ovipositor insertion scars/leaf), (b) oviposition success (# galls established/# oviposition scars), (c) individual gall size (diameter), and (d) emergence success (# of gall producing a B. treatae/total # of galls). Herein we report on gall size, an index of host plant quality. Mixed model ANOVA showed gall size to be greater (p < 0.001) on natal trees (&xover; = 5.10mm ± 0.28; N = 2109 galls) than on neighboring trees (&xover; = 3.16mm ± 0.28; N = 2734 galls). Repeated measures ANOVA was used to test for demic adaptation. Gall size was significantly greater (p < 0.05) on natal trees (&xover; = 4.53mm ± 0.38; N = 2116 galls) than alternative focal trees (&xover; = 3.24mm ± 0.19; N = 7337). These results suggest that (a) heavily galled focal trees are surrounded by host trees of lower quality to the herbivore and (b) the herbivore has undergone local adaptation at the level of the individual host tree (i.e. deme formation).
Key words: host plant quality, gall former, deme formation, cynipid-oak system