Soroka, J. J. and L. F. Grenkow. 2012. When is fall feeding by flea beetles (Phyllotretaspp., Coleoptera: Chrysomelidae) on canola (Brassica napusL.) a problem? Can. J. Plant Sci. 92: 97-107. Two cultivars of Brassica napus canola were seeded in mid-May and early June in three field experiments in each of 3 yr near Saskatoon, Saskatchewan, to determine the effects of late-season flea beetle feeding on seed yields. In the first experiment, canola was sprayed with insecticide late in the summer to eliminate naturally-infesting flea beetles. In the second, 1×1×1.5 m screen cages were placed over early- and late-seeded canola at flowering and infested with flea beetles as canola matured. In the third investigation, sleeve cages were placed over individual plants and infested with 100 flea beetles. Flea beetles had no detrimental effects on early-seeded canola in any experiment, but did affect seed yields of late-seeded plots in some trials. Over two cultivars in 1 year, late-seeded plants in cube cages infested with about 350 flea beetles per plant when lower pods were turning from translucent to green in colour reduced yield by 241 kg ha^-1 over control yields. Seed weights in these late-seeded plots were decreased from 2.68 g per 1000 seeds in uninfested cages to 2.44 g per 1000 seeds in infested cages. Populations of 100 flea beetles per plant in sleeve cages had no effect on harvest parameters in any seeding date or year.