Abstract A community of insect herbivores has established on eucalyptus species in California following their introduction from Australia. A number of the species are under complete or partial biological control. A response to introduction of additional pest species into the complex has been the application of systemic insecticides to infested trees. Natural enemies that have been introduced to control the various pest species feed on the nectar of treated trees, and thus may be affected by these pesticides. In this study, Eucalyptus rudis trees were treated at label rates with the neonicotinoid systemic insecticide imidacloprid and nectar sampled at 5 months post-treatment during the spring bloom. The concentration of imidacloprid and its toxic metabolites in nectar was measured by ELISA at 660 ppb. Adults of the encyrtid egg parasitoid Avetianella longoi that were fed floral nectar collected from treated trees had significantly lower survival and reproductive fitness than adults fed nectar from untreated trees. In feeding bioassays, in which the adults were fed a range of concentrations of imidacloprid in sugar water, the LC50 for A. longoi was 212 ppb imidacloprid. Bioassays were also conducted with the braconid larval parasitoid, Syngaster lepidus. The LC50 for S. lepidus was 288 ppb imidacloprid. The insecticide appears to be concentrating in the nectar at higher levels than reported from other plant species and at concentrations exceeding the LC50 for two important parasitoids. If tree treatments become widespread as a result of continual introductions of new eucalypt herbivores, established biological control programs could be at significant risk.