Host-mediated volatile polymorphism in a parasitic plant influences its attractiveness to pollinators.

Host-plants can mediate the interactions between herbivores and their mutualists and also between parasitic plants and their mutualists. The present study reveals how a hemiparasitic plant parasitizing three host species gives rise to three distinct hemiparasite-host neighborhoods which differ in terms of volatile composition and pollinator attractiveness. The study was performed in a population of the mistletoe Tristerix verticillatus infecting three different species of hosts occurring in sympatry within a small area, thus exposing all individuals studied to similar abiotic conditions and pollinator diversity; we assessed the effect of hosts on the hemiparasites' visual and olfactory cues for pollinator attraction. During the study period, the hemiparasite individuals were flowering but the hosts were past their flowering stage. We collected volatile organic compounds from the hemiparasite and its hosts, measured floral display characteristics and monitored bird and insect visitors to inflorescences of T. verticillatus. We showed that: (1) floral patches did not differ in terms of floral display potentially involved in the attraction of pollinators, (2) hosts and hemiparasites on each host were discriminated as distinct chemical populations in terms of their volatile chemical profiles, (3) insect visitation rates differed between hemiparasites parasitizing different hosts, and (4) volatile compounds from the host and the hemiparasite influenced the visitation of hemiparasite flowers by insects. The study showed that a species regarded as "ornithophilic" by its floral morphology was actually mostly visited by insects that interacted with its sexual organs during their visits and carried its pollen, and that host-specific plant-volatile profiles within the T. verticillatus population were associated with differential attractiveness to pollinating insects.

Selection of Nothofagus host trees by the aphids Neuquenaphis staryi and Neuquenaphis edwardsi.

Leaf volatiles were collected from three Nothofagus species growing in close proximity in Los Ruiles National Reserve, Chile. The volatile preparation from leaves of No. alessandrii were attractive to the specialist aphid, Neuquenaphis staryi, but not to the generalist aphid, Ne. edwardsi, while the volatile preparations of No. dombeyi and No. glauca were attractive to Ne. edwardsi, but not to Ne. staryi. This reflects the pattern of aphid/host-plant associations. Alpha-Agarofuran was found to occur in all leaf volatile preparations and was shown by electroantennography and olfactometry to be attractive for both Neuquenaphis spp., suggesting it may be the Nothofagus host-recognition factor for Neuquenaphis. The factor(s) mediating Ne. stayi's specialization on No. alessandrii remain to be identified.