LEPIDOPTEROUS PESTS OF BRASSICA CROPS AND THEIR PARASITOIDS IN SOUTHWESTERN QUEBEC

To evaluate the potential of using parasitoids against lepidopterous p ests of Brassica oleracea (L.) in southwestern Quebec, we studied the relative importance of parasitoids and diseases of lepidopterous pests occurring on Brussels sprouts, and on early and late plantings of bro ccoli and cabbage. During the 2-yr study (1993-1994), Plutella xyloste lla (L.) and Artogeia rapae (L.) were the most important lepidopterous species on early and late plantings, respectively. Late broccoli plan tings were particularly infested by A. rapae. In total, 14 parasitoid and 3 hyperparasitoid species were recovered (15 Hymenoptera, 2 Dipter a). Plutella xylostella and A. rapae had the highest parasitism levels , with 5 and 6 parasitoid species, respectively. Overall, parasitoids accounted for 65 and 43%, and diseases accounted for 22 and 43% of the mortality of P. xylostella and A. rapae, respectively. Arrangement of the data into a parasitoid web showed that the most common parasitoid species was the newly established Cotesia rubecula (Marshall). It was the most important larval parasitoid of A. rapae and had the highest hyperparasitism level (11%). Most parasitoid species were restricted t o 1 host species, but all hyperparasitoid species were polyphagous and parasitized only the most common parasitoid species. Natural control by parasitoids and diseases was important only late in the season, and restricted to maturing larvae and to pupae. Control would be improved by focusing efforts earlier in the season, and on eggs and young larv ae of the pests.