1. Plant secondary metabolites can govern prey–predator interactions by altering the diet breadth of predators and sometimes provide an ecological refuge to prey. Brassicaceae plants and their specialist pests can be used as a model system for understanding the role of chemically mediated effects restricting the diet breadth of natural enemies, and consequently the occurrence of enemy‐free space for the specialist pest.2. The objective of the present study was to test the performance of the generalist predator Episyrphus balteatus De Geer (Diptera: Syrphidae) fed on the specialist herbivore Brevicoryne brassicae L.(Homoptera: Aphididae), reared on two different brassica species: black mustard (Brassica nigra), a wild species with high levels of sinigrin; and canola (Brassica napus), a cultivated species without sinigrin.3. The preference and performance of the predator and the performance of the prey were measured. Sinigrin was quantified by high‐performance liquid chromatography in both leaf samples and aphids reared on the two host plants.4. The cabbage aphid performed better on canola than on black mustard. The performance of the predator on this aphid when reared on canola was clearly better than when reared on black mustard. Females had a higher overall preference for cabbage aphids reared on canola than on black mustard.5. The ability of aphids reared on plants with high glucosinolate content to reduce the performance of their generalist predators indicates that the presence of B. nigra may provide enemy‐free space for the cabbage aphid from its predator, a concept that has useful application in the context of biological control for agricultural systems.

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