Pea aphid, Acyrthosiphon pisum (Harris), populations on cultivars of field peas in Manitoba and their effects on pea yield
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The research for this project was undertaken from 1984 to 1987. Natural infestations of pea aphids, Acyrthosiphon pisum (Harris), were measured throughout the summer on five (1984) or six (1985, 1986) cultivars of field peas in small field plots near Glenlea, Manitoba. sarting from equal numbers of pea aphids per cage, in 1986 pea aphid populations were also monitored in 1 m3 field cages which contained plants of one of these six field pea cultivars. Throughout the summers of 1985 and 1986 pea aphids were also sampled in a total of nine Century, four Trapper and two Triumph commercial pea fields in several regions across the province. In all of these tests, pea aphid population growth patterns were similar among cultivars. Aphid numbers rose from low levels during the vegetative to blooming stages of plant growth in mid-July, peaked in late July or early August as pods developed and matured, and declined rapidly by mid- to late August as pea plants senesced. However, numbers of pea aphids at the time of population peak differed consistently and significantly with the cultivar upon which they grew. Peak numbers of pea aphids were larger on Triumph or Trapper plants than they were on Century or Tipu plants. In commercial fields, populations of pea aphids rose more rapidly on Trapper than they did on Century or Triumph plants. In field plots seed weight was the yield component most sensitive to aphid feeding. Triumph had significantly lower seed yields in infested than in control subplots in two out of three years. Seed weight was significantly reduced in infested subplots of Tara peas in one year. Because aphid numbers were low and generally occurred later than at flowering or pod initiation in Century peas, no yield losses due to pea aphids occurred in this cultivar in any of the tests. However, linear regression of seed weight over aphid density indicated that, of the cultivars tested, Century is most susceptible to increasing aphid numbers. Trapper seed weight was least related to aphid density despite the relatively high numbers of pea aphids occurring on this cultivar. In laboratory studies pea aphids had the greatest intrinsic rates of natural increase rm, on the cultivar Trapper and the smallest on the cultivars Tipu and Century. Ten days after infestation, the most antixenosis resistance was expressed by the cultivar Tipu, and the least by Triumph. After 20 days, Triumph still was most preferred by the aphid. Trapper appeared somewhat tolerant of the effects of pea aphid feeding.