PLANT TOLERANCE OF GALL-INSECT ATTACK AND GALL-INSECT PERFORMANCE

We examined plant tolerance of gall-insect attack and gall-insect perf ormance in rosinweed (Silphium integrifolium, Asteraceae) and its apic al meristem galler Antistrophus silphii (Hymenoptera: Cynipidae). Gall densities were varied in field rosinweed populations, while gall dens ities, water, and nutrients were varied for rosinweed in an experiment al garden. Field plants grew under prevailing resource and competitive conditions, but garden plants grew free from competition, so gall-ins ect impacts, rosinweed regrowth, and gall-insect performance were obse rved under widely different growing conditions. Seasonal measures of r osinweed growth and leaf physiology, and end-of-season measures of bio mass, reproduction, gall-wasp emergence, growth, sex ratios, and paras itism were made for both experiments. Rosinweed poorly tolerated Antis trophus gall damage in the field. Galls reduced plant height, leaf are a, and inflorescence production. Rosinweed diverted biomass to stems, but produced no regrowth from axillary meristems. In the garden, rosin weed was much more tolerant of Antistrophus gall damage. Galls initial ly reduced plant height and leaf area, but axillary meristems grew pro fusely after gall formation, producing nearly all galled plant inflore scences and more than replacing leaf area initially lost to gall forma tion. Water- and nutrient-supplemented rosinweed were most tolerant of gall damage, experiencing little loss of total biomass or reproductiv e output. Field rosinweed failed to mount a tolerance-enhancing regrow th response because galls, resource availability, and competition comb ined to constrain axillary meristem growth. Gall-wasp performance was largely independent of rosinweed tolerance. Emergence, growth, sex rat ios, and parasitism were comparable in field and garden, and only slig htly affected by resource availability. Gall-insect performance may be buffered from environmental variation, disconnecting plant and herbiv ore population dynamics. Rosinweed's poor tolerance of gall damage may typify forb responses to herbivory in highly competitive grassland pl ant communities.