Behavioural and electrophysiological responses of the female malaria mosquitoAnopheles gambiae(Diptera: Culicidae) to Limburger cheese volatiles

Abstract

Limburger cheese, previously shown to attract femaleAnopheles gambiaeGiles, was solvent extracted and chemically fractionated into acid and non-acid fractions. The extracts and aliquots of headspace odour of the cheese were analysed by gas chromatography and electron impact mass spectrometry. Nineteen saturated and unsaturated aliphatic fatty acids, ranging in carbon chain length from C2to C18, were detected. The most abundant acids (>1 mg/g of cheese) identified in the acid extract were ethanoic, propanoic, butanoic, hexadecanoic and 9-octadecenoic acid. The same compounds were identified in analyses of headspace samples but only trace quantities of the less volatile acids (C10to C16) were present, whilst C18acids were absent. Behavioural responses of femaleA. gambiaetowards a range of dilutions of the acid extract (in diethyl ether) were recorded in a windtunnel bioassay. The undiluted extract was found to be repellent, but became highly attractive (P«0.001) at lower doses, and was still significantly attractive (P<0.001) when diluted 106times. A synthetic mixture of 12 of the more abundant aliphatic acids identified in the acid extract was found to be significantly attractive (P<0.001) when diluted 108times. Electroantennographic (EAG) studies showed significant and reproducible responses to (saturated) Limburger cheese headspace. At doses higher than 0.1%, the synthetic mixture of 12 acids elicited significantly higher EAG amplitudes than the solvent control (paraffin oil). EAG responses were recorded from mosquitoes stimulated with C5to C8acids, that were characterized by significant dose-dependencies. Weaker, though significant EAG responses were obtained with the less volatile acids (C9to C14). Only hexadecanoic acid did not elicit a detectable response. The electrophysiological and behavioural responses obtained with fatty acids isolated from Limburger cheese suggests that together they could act as a kairomone for femaleA. gambiae. The implications of this are discussed together with the occurrence and bacterial production of these compounds on human skin.

Olfaction in vector-host interactions

“…It is also worth noting that compounds and blends of compounds that are attractive at a particular concentration, may be repellent or cause inhibition at higher concentrations (e.g. Knols et al 1997. Therefore, single compounds and blends should be tested across a range of concentrations in a methodical way, similar to range testing doses of drugs or toxins.…”

“…gambiae females to land at the odour source, whereas a mixture of 22 aliphatic carboxylic acids, which have been identified from human sweat, did not elicit a landing response (Healy and Copland 2000). However, a blend of 12 aliphatic carboxylic acids was found to be attractive when highly diluted (Knols et al 1997). A similar blend, although repellent or inhibitory on its own, was attractive when combined with ammonia and lactic acid .…”

“…gambiae contained at least one neuron that responded to ammonia and 1-butylamine. ORNs in GP sensilla were found responding to oxocarboxylic acids, short-chain carboxylic acids and lactic acid, compounds that are shown to attract mosquitoes or to enhance landing responses (Braks et al 2001, Healy and Copland 2000, Knols et al 1997.…”

Olfactory and behavioural responses of tsetse flies, Glossina spp., to rumen metabolites

“…However, the amounts of these C 1 -C 8 acids released in these experiments were 6-875 times higher than the amount of p-cresol released, and 1.4-250 times higher than the amounts of individual carboxylic acids released in our experiments. It is known that high concentrations of carboxylic acids can act as repellents for the malaria mosquito, Anopheles gambiae, where they can decrease the eYcacy of approaches to mixtures that are otherwise active (Knols et al 1997). Carboxylic acids and oxycarboxylic acids have already been reported as important chemostimuli in the sensory ecology of diVerent haematophagous arthropods such as mosquitoes (Cork and Park 1996;Knols et al 1997), sandXies (Dougherty et al 1999), ticks (Donzé et al 2004) and triatomine bugs (Guerenstein and Guerin 2001).…”

“…It is known that high concentrations of carboxylic acids can act as repellents for the malaria mosquito, Anopheles gambiae, where they can decrease the eYcacy of approaches to mixtures that are otherwise active (Knols et al 1997). Carboxylic acids and oxycarboxylic acids have already been reported as important chemostimuli in the sensory ecology of diVerent haematophagous arthropods such as mosquitoes (Cork and Park 1996;Knols et al 1997), sandXies (Dougherty et al 1999), ticks (Donzé et al 2004) and triatomine bugs (Guerenstein and Guerin 2001). It is quite possible that such acids may also inXuence tsetse Xy behaviour, but the pertinent mixture and doses remain to be elucidated.…”