Evidence for host‐associated races in a gall‐forming midge: trade‐offs in potential fecundity

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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, andI. imbricata.2. In the field,Borrichiasuffers the highest rate of galling andI. imbricatathe lowest. Common garden experiments also revealed highly significant differences in the attack rates of the three host species byAsphondylia. However, these differences depended on the source of the attacking midge population.3. Midges collected fromBorrichiaandI. frutescensattacked the host in which they developed almost exclusively, whereas those fromI. imbricataattacked bothIvaspp.4. EachAsphondylialarva develops in its own chamber within a gall.Borrichiagalls were 2.5–3.5‐fold less crowded than those fromI. imbricataandI. frutescens, respectively. Consequently, midges that developed inBorrichiawere significantly larger and eclosed with 30–40% more eggs than those that developed inI. imbricataandI. frutescens, respectively.5. Although performance ofAsphondylialarvae was lowest on the twoIvaspp., especiallyI. 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 ofAsphondylia, thereby favouring the formation of races at the level of plant genus.