POPULATION-GROWTH OF THE PEA APHID, ACYRTHOSIPHON-PISUM (HARRIS) (HOMOPTERA, APHIDIDAE), AND PLANT-RESPONSE TO APHID NUMBERS IN COMMERCIALLY GROWN FIELD PEAS IN MANITOBA

Pea aphids, Acyrthosiphon pisum (Harris), were sampled weekly or biweekly over the summers of 1985 and 1986 in a total of 15 commercial plantings of Century, Trapper, or Triumph field peas across Manitoba. Pea aphid populations rose more quickly in fields of Trapper than in Century or Triumph fields. The owners of all Trapper fields sampled initiated aerial application of insecticide for pea aphid control; at the time of spraying in 1985, plants in Trapper fields had significantly greater numbers of aphids in sweep samples than such samples from Century or Triumph fields. In 1986, Triumph plants supported greater numbers of aphids than Century or Trapper plants. Triumph plants remained green longer than plants of the other cultivars; in 1985 pea aphid numbers on this cultivar were highest on the last date sampled. Yield components were measured in sprayed and unsprayed plots within the commercial fields. When data were averaged for each cultivar, there were no significant differences in yield per area or in 1000 seed weight between sprayed and unsprayed plots. Data indicated that existing economic thresholds are too low for Trapper peas. However, one of the fields of Trapper peas that we sampled had significantly reduced numbers of pods per plant, yield per square metre, and weight of 1000 seeds in the unsprayed plot; this field had the largest population of pea aphids of any field sampled, with aphid numbers peaking at 48.5 +/- 9.2 (SE) per plant tip during pod formation and filling. Significant yield losses also occurred in unsprayed plots of a Triumph field, which had a peak aphid population of 4.8 +/- 1.6 per plant stem at pod maturation.