Cassida rubiginosa (Mueller) (Coleoptera: Chrysomelidae) adults laid eggs on Canada thistle where they were released, regardless of thistle density. Densities of C. rubiginosa and plant development varied within a field and between patches of Canada thistle in natural populations. Ramets infected with the rust disease, Puccinia punctiformis (Strauss) Roehl. (Uredinales: Pucciniaceae), were not more attractive than healthy ramets to C. rubiginosa. Ramets systematically infected with the rust disease did not bloom and suffered high mortality. Eucelatoria dimmocki (Aldrich) (Diptera: Tachinidae) and Itoplectis conquisitor (Say) (Hymenoptera: Ichneumonidae) parasitized C. rubiginosa larvae. Spilochalcis side (Walker) (Hymenoptera: Chalcididae) emerged from pupae. No parasitoids were found attacking oothecae.