THE EFFECTS OF ALFALFA PLANT BUG (HEMIPTERA, MIRIDAE) FEEDING LATE INTHE SEASON ON ALFALFA SEED YIELD IN NORTHERN SASKATCHEWAN

A 4-year field cage study was conducted to determine what effects feed ing of alfalfa plant bug, Adelphocoris lineolatus (Goeze), late in the season had on alfalfa seed yields. When zero, one, two, or four alfal fa plant bugs were placed in sleeve cages containing stems of alfalfa at the green pod stage of growth, there was a significant decrease in the number of pods per cage and per raceme, the number of healthy seed s, and the weight of seeds with two or four bugs per cage. Increasing the duration of infestation resulted in significant increases in the n umber of damaged seeds per cage. An insecticide field trial was conduc ted in north central Saskatchewan in 1988 to determine what effects th e occurrence of plant bugs had on seed yields of alfalfa. Alfalfa plan t bug, lygus bug (Lygus spp.), pea aphid [Acyrthosiphon pisum (Harris) ], and beneficial arthropod populations were monitored from 23 June to 1 September 1988. Seed yields were significantly greater in plots tha t were treated with trichlorfon in June and in June and August than in plots that were treated in August alone. The implications of a large alfalfa plant bug population late in the season to alfalfa seed produc tion in the area are discussed.