The co-evolution of herbivore impact: field estimates of impact in older and newer gallmaker–Solidago interactions

Background: Novel associations of specialist insect herbivores with host plants are frequently assembled. If herbivore and host co-evolve following their first interactions, we would expect increases over evolutionary time in plant tolerance to attack, and usually decreases over time in insect virulence (because most insect herbivores depend for resources on continued survival of their hosts). The result should be a pattern of decreased herbivore impact through time. This is the ‘evolution-of-impact hypothesis’. Organism: Two specialist gallmakers of the goldenrods Solidago altissima L. and S. gigantea Ait.: Rhopalomyia solidaginis/capitata (Diptera: Cecidomyiidae), and Eurosta solidaginis (Diptera: Tephritidae). Site of experiments: Tallgrass prairie at two sites in Iowa, USA. Predictions: For both gallmakers, S. gigantea is the novel host and S. altissima the older one. We therefore predict a higher impact of both gallmakers on S. gigantea than S. altissima. However, we predict a similar impact on each species of mechanical damage, with which both hosts have long evolutionary experience. Methods: We measured the aboveground biomass of galled and ungalled ramets from the field to assess herbivore impact by each gallmaker on each species. We used stem width–biomass relationships for ungalled ramets, applied to stem widths of galled ramets, to estimate the potential biomass of galled ramets in the absence of attack. We calculated impact as potential minus actual biomass. We also re-analysed similar data for a third gallmaker of the same hosts, Gnorimoschema gallaesolidaginis (Lepidoptera: Gelechiidae) from Heard and Kitts (2012). We assessed impacts of mechanical damage with field and greenhouse experiments of clipping and defoliation damage imposed on ramets of each species. Results: All three gallmakers had a higher impact on the novel host (S. gigantea) and, combined across all datasets, the pattern of higher impact on the novel host was highly significant. Impacts of mechanical damage did not differ between host species. Conclusions: Patterns in impacts of goldenrod gallmakers on their hosts are consistent with the evolution-of-impact hypothesis.