The aphids Aphis coreopidis (Thomas), Acyrthosiphon pisum (Harris), Acyrthosiphon soiani (Kaltenbach), and Myzus persicae (Sulzer) produce droplets at the tips of their cornicles when attacked by several species of insect predators. Alarm pheromones volatilizing from these cornicle droplets repel nearby aphids from their feeding sites. The release of cornicle droplets can also be elicited by various physical stimuli. Depending upon the type of stimulus and the aphid species involved, 50 to 90% of aphids produce droplets and 62 to 76% of droplets emit biologically active levels of pheromone. Aphids respond to the alarm pheromones at a distance of 1–3 cm from freshly secreted droplets and repellent activity is retained up to 60 minutes after secretion. Alarm pheromones are perceived by primary and secondary sensoria located on the aphids antennae.