Host-plant acceptance by aphids: cues initiating reproduction

The host-plant acceptance behaviour of aphids was examined, in two aphid species, the bird cherry-oat aphid, Rhopalosiphum padi and the pea aphid, Acyrthosiphum pisum, to elucidate the recognition factors used to assess plant suitability for parthenogenetic reproduction. In addition, host-plant chemistry was investigated to study possible cues that initiated reproduction. The probing and parturition behaviour of R. padi on barley, Hordeum vulgare, or bird cherry leaves, Prunus padus, were monitored by electrical penetration graph (EPG) coupled with a simultaneous video recording. The autumn winged gynoparae and the summer winged virginoparae initiated reproduction on their host plants, P. padus and H. vulgare, respectively, before phloem contact occurred and these findings suggest that phloem contact is not necessary for host-acceptance decisions. Host acceptance behaviour of A. pisum, on susceptible and phloem-based resistant line of Medicago truncatula was monitored and found to be similar on both plant lines. The phloem-based resistance mechanism did not affect parturition behaviour indicating that reproduction is initiated prior to phloem contact. The results also imply that sign chemicals used as host recognition cues located in a peripheral tissue rather than in the phloem of host plants. Bioassays were employed to characterise the sign chemicals within host-plants of R. padi. It was found that virginoparae reproduced similarly on a neutral medium such as water or holidic diet as well as host-plant aqueous extract, which suggests that particular stimulants may be not involved in host-plant recognition by this generalist morph. On the other hand, aqueous extracts of bird cherry leaves specifically stimulated parturition in autumn gynoparae. Bioassay-guided chemical fractionation showed that at least five secondary metabolites appear to be involved in host recognition by gynoparae