Alfalfa Damage from Acyrthosiphon kondoi and Economic Threshold Studies in Southern California1

Abstract

The so-called blue alfalfa aphid (BAA), Acyrthosiphon kondoi Shinji and Kondo, was first found in North America in 1974. It is far more damaging to alfalfa than the pea aphid, A. pisum (Harris) but is easily killed with chemicals used on alfalfa for aphid control. In southern California, a BAA population of 10–12/stem on new regrowth during the Jan., Feb. and Mar. cutting cycles will later reduce yields because natural enemies are not usually present to suppress populations. Damage appears to start at ca. 20 aphids/stem. When the BAA builds up after the crop is one-half or more grown (25–30 cm tall) the plant can withstand 40–50 or more BAA/stem with little or no loss in yield. Damage is most severe when the aphids attack the new regrowth following the winter cuttings. The BAA disappears in early spring when temperatures reach 27°C.