Host choice and host leaving in Rhopalosiphum padi (Hemiptera: aphididae) emigrants and repellency of aphid colonies on the winter host.

Host choice and winter-host leaving in emigrants of bird cherry-oat aphid, Rhopalosiphum padi (Linnaeus), were investigated in the laboratory. In settling choice tests, emigrants collected from the winter host, Prunus padus, preferred this plant over a summer host, oats. Emigrants which had left P. padus for up to 24 h did not express a preference as a group, and those which had left for 24-48 h preferred oats. Eighty seven percent of emigrants caged as fourth-instar nymphs on P. padus leaves abandoned the host by the second day of adult life, and apparently did not subsequently return to the leaf. In an olfactometer, P. padus leaves which had supported spring generations of R. padi were repellent to emigrants. Volatiles were entrained from uninfested and R. padi-infested P. padus using cut twigs in the laboratory as well as intact twigs on a tree in the field. Entrainment extracts from uninfested P. padus had no effect on emigrants in the olfactometer, whereas those from twigs infested with nymphal emigrants were repellent. The study indicates that in R. padi, host-alternation is driven by behavioural changes which occur in individuals as well as between morphs.