A measure of the specific impact for herbivore–plant interactions was used to test whether 7 insect species differ in the severity of their effects on young cruciferous plants. Specific impact was estimated as a biomass conversion ratio (reduction in plant biomass per unit gain in insect biomass) based on dry weights. The armyworm Mamestra configurata (Walker) had a specific impact of 4–5 on both canola, Brassica napus L., and mustard, Sinapis alba L., in the laboratory, and a similar specific impact of 3.4 was estimated for a natural field population. Two herbivore densities showed little if any difference in the specific impact for M. configurata on canola, nor for the flea beetle Phyllotreta striolata (F.), which had impacts of 81 and 96 at the 2 densities tested. The flea beetle Phyllotreta cruciferae (Goeze) had a specific impact of 148 on canola, not significantly different from that of its relative, P. striolata, which feeds in a similar way. The specific impact of the beetle Entomoscelis americana Brown was 3.5, similar to that of M. configurata, and both species had lower specific impacts than another folivore, Plutella xylostella (L.), with an impact of 12. The specific impacts of 2 aphids, Lipaphis erysimi (Kaltenbach) and Myzus persicae (Sulzer), were similar with values of 12 and 16, respectively. Although different herbivores showed substantial differences in their impacts on canola, no consistent pattern of difference was found between herbivores belonging to the leaf-chewing and sap-feeding guilds, nor were there consistent differences between generalist and specialist herbivores.