AbstractYellow starthistle (Centaurea solstitialisLinnaeus) (Compositae), native to Eurasia and the Mediterranean Basin, is a spiny and poisonous, naturalized, invasive weed of grasslands and other environments in the western USA.Chaetorellia australisHering is a trivoltine natural enemy of yellow starthistle whose larvae feed on developing seeds within capitula. Following host-specificity studies in Greece and the USA,C. australiswas first introduced from Greece into the USA in 1988 for the biological control of yellow starthistle. Field samples of yellow starthistle capitula revealed that 10.3% (four years after initial release) and 18.0% (three years after initial release) of capitula at Colfax, Washington, and Merlin, Oregon, respectively, were infested by ≥ 1C. australis(mature larvae or empty puparia). Both sites also contained naturalized, weedy populations of,Centaurea cyanus, a second host-plant ofChaetorellia australis. The fly was also recovered fromCentaurea cyanuscapitula two years after its release at Kendrick, Idaho. The potential importance of the presence ofC. cyanusforChaetorellia australisand biological control of yellow starthistle is discussed. At the Colfax site, all infested yellow starthistle capitula contained 1C. australis, while at the Merlin site 28.8% of infested capitula contained > 1C. australis, with a mean of 1.36C. australisper infested capitulum. A preliminary seed destruction impact study at the Merlin site showed that there was a 83.4% reduction in seeds per capitulum infested by ≥ 1C. australis, and the presence of > 1C. australisresulted in a 95.4% reduction in seeds per infested capitulum.
