Eggs, larvae, and adults of diamondback moth, Plutella xylostella (L.), and imported cabbageworm, Artogeia rapae (L.), were monitored on cabbage, broccoli, and brussels sprouts [Brassica oleracea (L.) var. capitata, italica, and gemmifera] for 2 years in southwestern Quebec. The first eggs and adults of P. xylostella were observed during the first week of June, and the use of pheromone traps combined with plant sampling permitted detection of three to four generations on each cultivar type. However, adult counts in pheromone traps were not correlated with the number of eggs and larvae on plants. The date at which each generation appeared and their duration varied little between the three Brassica cultivars. An average of 352.7 degree-days (DD) above 7.3 °C were required to complete one generation, but the considerable overlap between generations reduced the usefulness of DD accumulations to predict P. xylostella occurrence. Visual counts of adult A. rapae and plant sampling of eggs and larvae permitted detection of three generations of this species on each cultivar type. The first A. rapae eggs were found during the last week of May, at least 2 weeks before adults were observed. However, butterfly counts were generally correlated with the number of larvae on plants. The dates at which each generation of A. rapae started were similar between cultivar types, and an average of 319.7 DD above 10.0 °C was required to complete one generation.