Differential responses of the yellow fever mosquito, Aedes aegypti, and the Scottish biting midge, Culicoides impunctatus to human host odours
Behavioural studies using a Y-tube olfactometer confirmed the differential attractiveness of human volunteers to the yellow fever mosquito, Aedes aegypti. Volatile chemicals were collected from all volunteers by air entrainment. The extracts were analysed behaviourally with Ae. aegypti and the Scottish biting midge, Culicoides impunctatus in a Y-tube olfactometer. The behavioural responses were similar to those previously observed to the hands of the volunteers. Electrophysiological responses of Ae. aegypti and C. impunctatus, to volatiles from the air entrainment extracts, were recorded using coupled gas chromatography-electroantennography (GC-EAG). EAG-active compounds were tentatively identified using GC-mass spectrometry (GC-MS) and were confirmed by peak enhancement. A quantitative and qualitative analysis of compounds within the extracts revealed significant differences in chemical profiles. The mean absolute amounts of benzaldehyde, 6-methyl-5-hepten-2-one, octanal, nonanal, naphthalene, decanal and geranylacetone were significantly greater in the unattractive group than the attractive group. Similarly, the mean relative amounts of 6-methyl-5-hepten-2-one, octanal and decanal in the unattractive group were significantly greater than the attractive group. Five compounds caused significant reductions in behavioural responses of Ae. aegypti when presented alongside a human hand in the Y-tube olfactometer, thus accounting for the lack of attraction towards the volunteers’ hands in the unattractive group. The identification of such behaviourally active compounds could lead to improved control technologies, whereby such compounds could potentially be incorporated into new, safe and natural repellents against biting insects.