Effect of trichlorfon sprays and alfalfa ( Medicago sativa L.) cultivars on pea aphid, Acyrthosiphon pisum (Harris)

Abstract

Trichlorfon has been cited as being a selective insecticide that has only minimal detrimental impact on natural enemies. However, when applied to alfalfa in Minnesota, trichlorfon has been found to enhance pea aphid populations to an even greater extent than does carbaryl, an insecticide notorious for inducing aphid outbreaks. We found a direct relationship between the number of trichlorfon applications that plots received during a regrowth period and the consequent pea aphid numbers that these plots supported. Pea aphid showed appreciable tolerance to both trichlorfon and carbaryl in laboratory dip tests. In small plots, effects of trichlorfon sprays on beneficials are transitory because of the short residual life of the insecticide and the relative mobility of many natural enemies. This gives trichlorfon the appearance of selectivity. Interactions between aphid parasitism, cultivar resistance, and insecticide were observed. Cv. Gladiator had the most resistance to pea aphid, with cv. Kanza a close second, and cv. Weevlchek the most susceptible. Pea aphids from trichlorfon-treated alfalfa plots had higher levels of parasitism, primarily Aphidius ervi Hal., than did pea aphids from untreated plots, suggesting a density-dependent response by the parasites to the insecticide-enhanced pea aphid numbers. Cultivar differences in pea aphid numbers were not sufficient to show a consistent effect on percentage parasitism.