Volatiles functioning as host cues in a blend become nonhost cues when presented alone to the black bean aphid

Herbivorous insects recognize and locate their hosts by detecting characteristic blends of volatile compounds these plants emit. The possibility that insects may use the same compounds in a different context as nonhost cues has received relatively little attention. Volatiles normally emitted by the host but encountered without other host volatiles could theoretically function as nonhost cues. We hypothesized that insects might show a positive response to host volatile compounds when encountered together in a blend but avoid the same volatiles when encountered individually. To test this we examined the behavioural responses of the black bean aphid, Aphis fabae, to physiologically relevant doses of volatile compounds emitted by its host, Vicia faba, which had been previously implicated in host recognition. Of 15 volatiles tested for behavioural activity, 10 caused aphids to respond negatively, suggesting they were repellent. We then made a blend comprising each of these compounds at the concentration at which they elicited the most negative behavioural response. The resultant blend elicited a positive response, suggesting it was attractive/arrestant. This demonstrated that the same volatile compounds can function as both host and nonhost cues, depending upon the context in which they are perceived. Thus, background odour context needs to be considered for successful use of behaviourally active volatile compounds in integrated pest management strategies. Furthermore, the finding that odorants are perceived differently when combined suggests that there is an emergent property of odour perception whereby discrimination of odour quality can occur according to blend properties.