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