Olfactory basis of host-recognition in the black bean aphid, Aphis fabae

Abstract

Behavioural and electrophysiological responses of winged virginoparous Aphis fabae to volatile compounds of faba bean, Vicia faba, were studied and semiochemicals used in host location identified. In olfactometer bioassays, aphids responded positively to V. faba volatiles from an intact plant. This response also occurred when volatiles from an air entrainment sample of a V. faba plant were tested. Coupled gas chromatography-electroantennography revealed the presence of 16 electrophysiologically active volatile compounds in the air entrainment sample and 15 of these were identified as (E)-2-hexenal, (Z)-3-hexen-1-ol, 1-hexanol, benzaldehyde, 6-methyl-5-hepten-2-one, octanal, (Z)-3-hexen-1-yl acetate, (R)-linalool, methyl salicylate, decanal, undecanal, (E)-caryophyllene, (E)-β-farnesene, (S)-germacrene D, and (E,E,)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. A synthetic blend consisting of all identified compounds in the same concentration and ratio as in the air entrainment sample elicited a similar behavioural response from the aphids as the air entrainment sample. Each compound was tested for behavioural activity individually at the same concentration as in the air entrainment sample and subsequently over a range of different doses. It was found that the response to the complete blend was not due to a response to a single compound. Dose response experiments also revealed ten of the compounds elicited negative behavioural responses from aphids. Further behavioural experiments revealed that these responses were context-specific and behavioural activity of individual compounds was different when they were presented alongside other compounds in the blend. It was hypothesised that a blend of host volatiles in a species-specific ratio may be used by A. fabae to recognise its host. To determine whether or not ratios of volatiles could provide a reliable cue to host seeking aphids, intra-specific and diurnal variation of ratios of volatiles emitted were investigated. Although considerable variation in ratios was observed the quantities of some pairs of compounds were positively correlated, indicating a degree of consistency in the ratios.