Goldenrod stem galler preference and performance: effects of multiple herbivores and plant genotypes

Few studies have examined how the preference-performance relationship of an herbivore for different genotypes of its host plant is affected by the presence and/or feeding activity of other members of the herbivore assemblage. In an outdoor garden, we manipulated the abundance of three common herbivores, the meadow spittlebug, a leaf beetle, and an aphid, on replicate 1-m² plots of 16 different genotypes of tall goldenrod, Solidago altissima. Adults of the goldenrod stem galler, Eurosta solidaginis, were subsequently released into the garden to oviposit among the host plants. Oviposition preference was strongly influenced by plant genotype and the presence of two of the herbivores, spittlebugs and leaf beetles. The effects of the herbivores were additive: the presence of leaf beetles reduced preference by 6%, spittlebugs by 18%, and both herbivores combined by 25%. Plant genotype-herbivore species interaction effects on stem-galler preference, which would indicate the presence of genetic variation among goldenrod genotypes in their norms of reaction for their acceptability as a host to the stem galler, were absent in this study. The performance of the stem galler was also significantly affected by goldenrod genotype, but in general was not affected by the presence of herbivores early in the season (the exception was a positive correlation between the proportion of ramets infested by all herbivores and gall size). Overall, we could find no correlation between preference and performance. This is in accord with results from previous studies on this system that were performed in the absence of herbivores, suggesting that the presence of herbivores in this study did not qualitatively alter the preference-performance relationship. We suggest that the lack of a positive correlation between host-plant preference and larval performance may reflect a constraint on the discriminatory ability of female stem gallers preventing them from selecting the best hosts among plants that differ in genotype and level of environmental stress (e.g., presence of interspecific herbivores).