Feeding and non-feeding alates and variously aged feeding apterae of Myzus persicae were tested for their sensitivity to the alarm pheromone, (E)-β-farnesene. The dosages at which 50% of test aphids responded to pheromone were; 0.03 ng for non-feeding alates, 0.06 ng for feeding alates, O.lng for older nymphs and young adult apterae, 3 ng for 1st and 2nd instars, and 200 ng for old apterous adults. Aphids in groups are more sensitive to pheromone than single, isolated aphids. Aphids which do not disperse when exposed to pheromone are more sensitive to tactile stimuli and are less apt to be captured by a predator than nonexposed aphids.