The efficacy ofBangasternus orientalisCapiomont against its hostCentaurea solstitialisL. was determined at two different sites (low and high elevation) in California over a 2-yr period. Host plant and weevil phenology were delayed at the high elevation site. Consequently, the initiation of secondary flower buds (high elevation site) was extended well into November, long after the ovipositing adult weevil population had expired. More than 50% of the egg-bearing test heads did not become infested with mature pupae because of larval mortality. After seed reduction by the weevil, 70% of the seeds remained in the test heads. Our data suggest thatB. orientalisappears to be ineffective by itself as a biological control agent of yellow starthistle in California.