SEASONAL OCCURRENCE AND NUMBER OF GENERATIONS OF LYGUS-LINEOLARIS AND LYGUS-BOREALIS (HETEROPTERA, MIRIDAE) IN SOUTHERN MANITOBA

The seasonal occurrence and number of generations of Lygus lineolaris, (Palisot de Beauvois) were studied on alfalfa (Medicago sativa L.), everbearing strawberries (Fragaria X ananassa Duchesne), oilseed rape (Brassica napus L.), and stinkweed (Thlaspi arvense L.) in field plots at Winnipeg, Manitoba, and in commercial alfalfa and oilseed rape fields throughout the agricultural areas of Manitoba in 1990-1993. Lygus borealis (Kelton) was studied only during 1993. Lygus lineolaris had two non-overlapping generations in southern Manitoba. Overwintered adults migrated onto host plants during the first 3 weeks of May, and females containing chorionated eggs were found in relatively small numbers for 5-6 weeks until the last 3 weeks of June. First-generation nymphs were present from the last half of May until the last half of July. During the last 2 weeks of June, the overwintered adults disappeared suddenly and were replaced completely by first-generation adults. First-generation adults eclosed during the last 2 weeks of June and in July, and first-generation females containing chorionated eggs were present for about 6-7 weeks until mid-August. Second-generation nymphs first appeared in late July or early August and most completed development by the end of September. Second-generation adults were found from the first half of August until they hibernated in October and November. Most second-generation females contained previtellogenic eggs and, therefore, were in reproductive diapause. In 3 of the 4 years, the overwintered adults appeared to include all second-generation adults plus 0-20% of the first-generation adults. The data from commercial alfalfa and oilseed rape fields indicated that the seasonal occurrence oft. lineolaris was relatively uniform in most or all agricultural areas of Manitoba. Lygus lineolaris has one generation (the second generation) on oilseed rape, and the eclosion and dispersal of reproductively active first-generation adults were well synchronized with the bud and flower stages of this host plant in southern Manitoba. In 1993, L. borealis had two generations and its seasonal occurrence was similar to that of L. lineolaris.