Host Plant Quality and Deme Formation as Determinants of the Distribution and Abundance of a Gall-forming Herbivore

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2003-12

Authors

Egan, Scott P.

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Abstract

Here I link variation in host plant quality to herbivore fitness variation at the local scale (between adjacent plants) with the process of demic adaptation operating at the patch scale to provide a synthetic explanation to the patchy distribution of the specialist gall forming wasp Belonocnema treatae (Hymenoptera:Cynipidae) within populations of its host plant, Quercus fusiformis. In this gall former system gall densities vary by orders of magnitude between adjacent host trees; host trees supporting high gall densities are rare (<5%) and appear across the landscape as habitat islands within a seaofhost plants supporting only low herbivore densities. To understand the relationship between herbivore fitness variation and variation in the abundance and distribution of the herbivore at the local and patch scales a manipulative transplant experiment was performed. Mated sexual generation B. treatae females from each of five high gall density trees were placed on (a) their respective four nearest neighbor conspecifics that exhibited low gall density, (b) the four alternative high density trees, and (c) their natal trees (control). Each treatment (source population+ rearing site) was replicated three times. Linear mixed effects ANOV A was used with the host tree, on which development took place, set as a fixed factor and the herbivore source population set as a random factor to test for effects of local variation in plant quality on herbivore fitness and deme formation. For each replicate, I recotded: {a) gall initiation success(# of galls established/# oviposition scars), (b) total number of galls produced, ( c) leaf abscission rates, (d) gall developmental rate, (e) individual gall size, (fand g) percent of galls developing to established minimum emergent size thresholds for B. treatae in the absence and presence of natural enemies, and (h) overall emergence success(# of galls producing a B. treataeltotal # galls). Results from the test oflocal quality (high density vs. respective near-neighbor low density trees) demonstrate that leaf gall density per tree equates with host quality. Transplanted populations of gall formers exhibited greater gall initiation success (p < 0.05), total number of galls produced (p < 0.001), average gall size (p < 0.001), and percent reaching natural enemy critical size threshold (p = 0.05) on focal trees compared to low density neighbors. Within this mosaic oflocal quality variation, comparison of herbivore performance on natal and novel high density trees demonstrate that the herbivore has undergone adaptation at the level of the individual host plant (deme formation) as shown by lower leaf abscission rate (p < 0.001 ), and higher developmental rate (p < 0.001), average gall size (p < 0.001), percent reaching tree critical threshold (p < 0.001), and overall emergence success (p < 0.001). Together these results from these linked experiments demonstrate convincingly that (a) heavily galled focal trees are surrounded by host trees of relatively lower quality to the herbivore and (b) that deme formation has taken place. These results highlight the effects of host plant variation on individual life stage components of gall forrnet success in the absence of natural enemies within and among demes and will be used to illustrate how differences in host plant quality are further amplified in the presence of natural enemies.

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Keywords

Host plants, Phytophagous insects, Herbivores

Citation

Egan, S. P. (2003). <i>Host plant quality and deme formation as determinants of the distribution and abundance of a gall-forming herbivore</i> (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.

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