We examined plant tolerance of gall-insect attack and gall-insect performance in rosinweed (Silphium integrifolium, Asteraceae) and its apical meristem galler Antistrophus silphii (Hymenoptera: Cynipidae). Gall densities were varied in field rosinweed populations, while gall densities, water, and nutrients were varied for rosinweed in an experimental garden. Field plants grew under prevailing resource and competitive conditions, but garden plants grew free from competition, so gall-insect impacts, rosinweed regrowth, and gall-insect performance were observed under widely different growing conditions. Seasonal measures of rosinweed growth and leaf physiology, and end-of-season measures of biomass, reproduction, gall-wasp emergence, growth, sex ratios, and parasitism were made for both experiments. Rosinweed poorly tolerated Antistrophus gall damage in the field. Galls reduced plant height, leaf area, and inflorescence production. Rosinweed diverted biomass to stems, but produced no regrowth from axillary meristems. In the garden, rosinweed was much more tolerant of Antistrophus gall damage. Galls initially reduced plant height and leaf area, but axillary meristems grew profusely after gall formation, producing nearly all galled plant inflorescences and more than replacing leaf area initially lost to gall formation. Water- and nutrient-supplemented rosinweed were most tolerant of gall damage, experiencing little loss of total biomass or reproductive output. Field rosinweed failed to mount a tolerance-enhancing regrowth response because galls, resource availability, and competition combined to constrain axillary meristem growth. Gall-wasp performance was largely independent of rosinweed tolerance. Emergence, growth, sex ratios, and parasitism were comparable in field and garden, and only slightly affected by resource availability. Gall-insect performance may be buffered from environmental variation, disconnecting plant and herbivore population dynamics. Rosinweed's poor tolerance of gall damage may typify forb responses to herbivory in highly competitive grassland plant communities.
