Interactions of a biological agent,Trichosirocalus horridus(Panzer), with a commonly used herbicide for thistle control, 2,4-dichlorophenoxyacetic acid (2,4-D), were examined using the thistle weevil's response to low-level 2,4-D sprays on two species of thistles at different growth stages, and thistle response to 2,4-D and weevil infestation. Two hundred sixty transplanted thistle rosettes of two thistle species at three developmental stages, naturally infested withT. horriduslarvae, were randomly selected in spring; half were sprayed with 15% of the recommended field dosage of 2,4-D. Plants caged 2 d later yielded a total of 2,246 weevils within 10 wk. Number of adults emerging from sprayed plants was less than half of that from unsprayed plants. Plant age or growth stage affected time and level of adult emergence, and thistle species affected level of emergence. Yield of adults from plants sprayed with 2,4-D, however, was not different from yield derived from healthy plants of the less-preferred thistle species or developmental stage. Combination ofT. horridusinfestation and the reduced dosage of 2,4-D caused 85% thistle mortality compared with 7% mortality in unsprayed weevil-infested thistles and 55% mortality in weevil-free rosettes sprayed with the reduced dosage of 2,4-D.