Life Sciences: Division of Biology, Imperial College London
Doi
Abstract
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