The efficacy of lemon-scented tea tree oil (LSO), Leptospermum petersonii (FM. Bailey), was evaluated against the diamondback moth, Plutella xylostella (L.) under laboratory conditions. Feeding activity and development of larval stages were significantly reduced on broccoli leaves that had been dipped in LSO. Oviposition deterrence was also found when an adult stage was exposed to treated leaves. Fecundity dropped by >50% at concentrations >0.5%. The LC50 value for third instar larvae was estimated to be 2.93% 7 d after treatment. Experiments were also conducted under greenhouse conditions to assess the efficacy of LSO against the diamond-back moth. Our results suggest that LSO has modest potential for development as a botanical insecticide. The oil was also tested at concentrations from 0.5 to 6% for oviposition deterrence of an egg parasitoid of the diamondback moth, Trichogramma pretiosum (Riley). LSO deterred parasitization in choice tests but not in no-choice tests. LSO did not cause mortality of T. pretiosum during 24 h in a contact toxicity test. We conclude that LSO had no significant effects on the parasitoid, and therefore LSO is compatible with this biocontrol agent for integrated management of the diamondback moth.