Early and Late-Season Insecticide Applications in Seed Alfalfa: Impact on Pest Species and Yield2

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Abstract
Alfalfa is grown for seed on approximately 9,500 ha in Alberta, Canada. Pest insects, such as Lygus spp. and the alfalfa plant bug, Adelphocoris lineolatus (Goeze), (Heteroptera: Miridae) can reduce seed yield if they are not controlled. However, little is known of the damage caused by these pest insects during late seed formation and development. Field studies were conducted during 1983 and 1986 to determine the effect of early- and late-season applications of the insecticides trichlorfon and phosmet alone and combined on Lygus spp., alfalfa plant bug and seed yield of alfalfa. Effects were dependent on the time of application, year, and insect species. Lygus spp. densities for late-season trichlorfon treatments were significantly (P < 0.05) lower than the control. Trichlorfon and phosmet had no effect on densities of alfalfa plant bugs in 1983, but some late-season trichlorfon treatments did reduce plant bug densities in 1986. Seed yields in plots receiving late-season trichlorfon were generally greater than in the control in both years.

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