Effect of Swathing and Clear-Cutting Alfalfa on Insect Populations in Southern Alberta

Abstract

Swathing alfalfa ( Medicago sativa L.) for hay greatly reduced the populations of several pest insects and their insect predators for 2 and often for 4 wk after swathing in southern Alberta. Clear-cutting alfalfa reduced pest and predator insects usually for 3-4 wk, but the predator populations recovered rapidly in some fields. The pest insects were Hypera postica (Gyllenhal), Acyrthosiphon pisum (Harris), Lygus spp., Adelphocoris lineolatus (Goeze), and Sitona scissifrons Say. Predators were Aeolothrips fasciatus (L.), Nabls alternatus Parshley, Orius tristicolor White, and Coccinellidae. There was a difference in insect populations among fields because of management of the alfalfa during the growing season. These results have led to the recommendation for southern Alberta that, if pea aphid populations are high and the alfalfa is near the 10% bloom stage, the crop should be cut rather than treated with an insecticide.