More than 15 species of capitulum-feeding insects colonized a randomized garden plot of yellow starthistle, globe artichoke, cultivated safflower, andCirsium creticum(Lam.) D'Urv. in northern Greece in 1985. Among these insects were seven potential biocontrol agents of yellow starthistle: the cynipid wasp,Isocolussp.; the tephritid flies,Chaetorellia australisHering,Terellia uncinataWhite, andUrophora sirunaseva(Hering); and the curculionid beetles,Bangasternus orientalis(Capiomont),Eustenopus villosus(Boheman), andLarinus curtus(Hochhuth). These potential agents only attacked yellow starthistle plants, which were grown from seed collected in Greece and the United States. This host specificity data was used to justify the subsequent release ofC. australisandE. villosusin the United States. The relative intensity of interspecific interactions among capitulum-feeding insects of yellow starthistle is discussed, as well as the potential effectiveness ofE. villosusas a biocontrol agent. This study points to the usefulness of the open field test in host specificity determination of insects for biological control of weeds.