Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores

[ Although the effects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra-and interspecific), most studies have analyzed these effects separately. We compared the effects of genotypic diversity of big-leaf mahogany Swietenia macrophylla with that of tree species diversity on two specialist caterpillars (Hypsipyla grandella stem borers and Phyllocnistis meliacella leaf miners) and three generalist leafhoppers (Cicadellidae) feeding on mahogany in a large-scale (7.2 ha) forest diversity experiment in southern Mexico. The experiment consisted of fifty-nine 21 x 21-m plots, with 64 tree saplings each (3-m spacing between plants). Plots were either mahogany monocultures or species polycultures of four species (including mahogany) and - within each of these two plot types - mahogany was represented by either one or four genotypes. Throughout a five-month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no effect of mahogany genotypic diversity on either specialist caterpillars or generalist leafhoppers, and this result was consistent across levels of tree species diversity. In contrast, species diversity had significant effects on both specialists but neither of the generalist herbivores. Specifically, species diversity lowered H. grandella attack at the middle of the sampling season, but increased attack at the end of the season, whereas P. meliacella abundance was consistently reduced. Such effects were not mediated by effects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not influence spider abundance. This study is one of few to directly compare sources of plant diversity, and uniquely compares such effects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity effects outweigh those of genotypes, and our results suggest that such effects are stronger on specialist than generalist herbivores.