Spray Timing and Economic Threshold for the Pea Aphid, Acyrthosiphon pisum (Homoptera: Aphididae), on Field Peas in Manitoba

Six insecticide regimes were tested between 1981 and 1983 in plots of field peas to determine the optimal time to control the pea aphid, Acyrthosiphon pisum (Harris), and to vary the density of the pest so that a relationship between yield and aphid density could be determined. Aphid populations rebounded rapidly after an insecticide application so that one early application did not protect the crop. Two or more sprays were uneconomical because they did not enhance yield more than a single spray applied as soon as 50% of the crop had young pods. Because the weight of 1,000 seeds provided a more precise measure of aphid damage than weight of seeds per unit area, this measurement was used to relate yield to pest density. The economic threshold was estimated to be 9–12 aphids per sweep when the crop began to flower. This threshold varied from season to season due, in part, to variation in potential seed weight caused by variation in precipitation and heat stress. A relationship between plant samples and sweep samples is presented that showsthe threshold based on sweep samples is equivalent to 2–3 aphids per tip of stem.