The life cycle of some aphid species involves seasonal switches between unrelated summer (secondary) and winter (primary) host plants. Many of these “host-alternating” species, belonging to the sub-family Aphidinae, produce two return migrant forms on secondary host plants in autumn. Winged females (gynoparae) are produced first; these locate the primary host and deposit their sexual female offspring (oviparae). Later, males are produced on the secondary host and these locate the primary host independently before mating with the oviparae. The mechanisms of primary-host location by gynoparae and males are reviewed in this paper. Studies with several aphid species indicate that both forms are able to respond to volatile cues released by their specific primary host plant. Plant odours may also enhance or modify the responses of return migrants to the sex pheromone released by mature oviparae. Aphids are also able to sample non-volatile plant chemicals after landing, but there have been very few detailed investigations of the behaviour of return migrants at the primary-host-plant surface. Recent experiments with gynoparae of the black bean aphid, Aphis fabae Scopoli, show that these insects detect primary-host-specific cues during stylet penetration of peripheral plant tissues, and these stimuli promote settling and reproduction. Similar behavioural studies with males are required to shed light on the processes of speciation and reproductive isolation in host-alternating aphids.
