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Abstract. 1. The gall midge, Asphondylia borrichiae, attacks the terminals of three plants in the aster family: Borrichia frutescens, Iva frutescens, and I. imbricata. 2. In the field, Borrichia suffers the highest rate of galling and I. imbricata the lowest. Common garden experiments also revealed highly significant differences in the attack rates of the three host species by Asphondylia. However, these differences depended on the source of the attacking midge population. 3. Midges collected from Borrichia and I. frutescens attacked the host in which they developed almost exclusively, whereas those from I. imbricata attacked both Iva spp. 4. Each Asphondylia larva develops in its own chamber within a gall. Borrichia galls were 2.5–3.5-fold less crowded than those from I. imbricata and I. frutescens, respectively. Consequently, midges that developed in Borrichia were significantly larger and eclosed with 30–40% more eggs than those that developed in I. imbricata and I. frutescens, respectively. 5. Although performance of Asphondylia larvae was lowest on the two Iva spp., especially I. frutescens, these hosts may provide an escape from natural enemies as a trade-off for reduced offspring performance. 6. Differences in host-plant species phenology may reduce gene flow among host-associated populations of Asphondylia, thereby favouring the formation of races at the level of plant genus. [ABSTRACT FROM AUTHOR]