Oviposition byTrichosirocalus horridus(Panzer) onCarduusthistles treated with low dosages of a commonly used herbicide for thistle control, 2,4-dichlorophenoxyacetic acid (2,4-D), was examined in greenhouse and small-plot field studies. Ovipositing weevils placed in cages in the greenhouse with rosettes treated with 5% of the recommended field dosage (1.68 kg/ha) responded to deteriorating leaf condition as a result of 2,4-D treatment rather than to the presence of the herbicide. Fewer eggs were laid in drying or dead leaves than in healthy leaves. Egg viability and larval mortality were not affected by the herbicide. In field studies, oviposition was not different between thistles sprayed with 15% of the recommended field dosage of 2,4-D and control; musk thistle was preferred over plumeless thistle for oviposition.