SEASONAL OCCURRENCE OF LEPIDOPTEROUS PESTS OF CRUCIFEROUS CROPS IN SOUTHWESTERN QUEBEC IN RELATION TO DEGREE-DAY ACCUMULATIONS

Eggs, larvae, and adults of diamondback moth, Plutella xylostella (L.) , and imported cabbageworm, Artogeia rapae (L.), were monitored on cab bage, broccoli, and brussels sprouts [Brassica oleracea (L.) var, capi tata, 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 sampl ing permitted detection of three to four generations on each cultivar type. However, adult counts in pheromone traps were not correlated wit h the number of eggs and larvae on plants. The date at which each gene ration appeared and their duration varied little between the three Bra ssica cultivars. An average of 352.7 degree-days (DD) above 7.3 degree s C were required to complete one generation, but the considerable ove rlap 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 genera tions of this species on each cultivar type. The first A. rapae eggs w ere found during the last week of May, at least 2 weeks before adults were observed. However, butterfly counts were generally correlated wit h the number of larvae on plants. The dates at which each generation o f A. rapae started were similar between cultivar types, and an average of 319.7 DD above 10.0 degrees C was required to complete one generat ion.