The host specificity ofEustenopus villosus(Boheman) was studied in the laboratory using field-collected adults from Greece and overwintered progeny of these adults. No-choice and two-choice host specificity tests were carried out in the laboratory. Thirty-four plant species, in addition to yellow starthistle (Centaurea solstitialisL.) (as the control) from Greece and the United States, were tested. Under no-choice test conditions in the laboratory, adults showed to some degree a rather broad feeding spectrum but with heaviest feeding on yellow starthistle. Oviposition was restricted to some species in the genusCentaurea, with heaviest oviposition on yellow starthistle. Under two-choice conditions (test plant caged with yellow starthistle control), oviposition occurred only on yellow starthistle. The literature on field hosts and host specificity tests indicate a high level of larval host specificity to yellow starthistle. A laboratory study showed that the combined effects of adult and larval feeding on yellow starthistle can reduce seed production by 98.8% under laboratory conditions; this also indicates that the species has excellent potential as a biological control agent.