The effect of continuous, intensive grazing by cattle on aphid populations was examined in the first growth cycle of ‘Alfagraze’ and ‘Apollo’ alfalfa which are tolerant and not tolerant to grazing, respectively. Populations were almost entirely pea aphid, Acyrthosiphon pisum (Harris). The effect of grazing on aphid populations was examined in small plot exclusion studies in 1991 and 1992, and the effects of grazing and use of an early insecticide application with ≤7 day grazing restriction were examined in large plot exclusion trials in 1993 and 1994. Grazing reduced aphid populations by 66% to 90% when numbers exceeded ~1 aphid per stem. Populations were not significantly reduced by grazing when numbers did not exceed 1 per stem. Permethrin reduced aphid numbers for up to 7 wks and was more effective than carbofuran. Effects of grazing were similar regardless of insecticide usage. Coccinellid beetle populations in 1993 and 1994 were composed of Coccinella septempunctata L., Harmonia axyridis (Pallas), and Coleomegilla maculata DeGeer. Coccinellid numbers were much lower in grazed than ungrazed plots when aphid numbers were reduced by grazing in 1993, but were not greatly affected by grazing in 1994 when aphid numbers were low. Coccinellid numbers paralleled trends in aphid numbers. Continuous, intensive grazing or the early application of an insecticide at a low rate followed by grazing were effective approaches for suppressing pea aphid in alfalfa.
2 Department of Crop and Soil Sciences, University of Georgia, Athens, GA 30602.