not in Young The mortality effects of crofton weed (Ageratina adenophora, (Asteraceae)) acetone extract and biogas fermentation residue on the cabbage aphid Brevicoryne brassicae, and the risk posed to its parasitoid Diaeretiella rapae, were studied. Laboratory trials demonstrated the insecticidal activity of both substances, and this was confirmed by field experiments in which reductions of 73 and 71% aphid populations were recorded 3 days after treatment for the acetone extract and the biogas fermentation residue, respectively. The results of a laboratory contact toxicity test on inert substrates for parasitoid adults, and calculation of the Hazard Quotient (HQ) ratio, indicate a low risk for D. rapae, with HQ being consistently below the trigger value of 2. Brevicoryne brassicae (Aphidoidea): Host; Diaeretiella rapae (Aphidiidae): Parasite. English none