Emission of alarm pheromone by non-preyed aphid colonies

The sesquiterpene (E)-beta-farnesene (E beta f) is the primary component of the alarm pheromone of most aphid species. It is released in response to physical stress including attack by natural enemies and causes aphids to cease feeding and disperse. E beta f also acts as a kairomonal cue for aphid natural enemies. In this study, we collected the headspace volatiles released by aphid colonies of different sizes. Gas chromatography-mass spectrometry analysis demonstrated the presence of E beta f in the absence of predator attack. A quadratic relationship was found between the released (E)-beta-farnesene amounts and aphid colony size. Behavioural impact of aphid alarm pheromone towards Episyrphus balteatus female oviposition behaviour was also demonstrated in this work. These results highlight the primary role of the small but continuous release of aphid alarm pheromone in mechanisms of decision-making by aphid predators during their foraging and egg-laying behaviour.