BEHAVIORAL AND ELECTROPHYSIOLOGICAL RESPONSES OF THE FEMALE MALARIA MOSQUITO ANOPHELES-GAMBIAE (DIPTERA, CULICIDAE) TO LIMBURGER CHEESE VOLATILES

Limburger cheese, previously shown to attract female Anopheles gambiae Giles, was solvent extracted and chemically fractionated into acid an d non-acid fractions. The extracts and aliquots of headspace odour of the cheese were analysed by gas chromatography and electron impact mas s spectrometry. Nineteen saturated and unsaturated aliphatic fatty aci ds, ranging in carbon chain length from C-2 to C-18, were detected. Th e most abundant acids (>1 mg/g of cheese) identified in the acid extra ct were ethanoic, propanoic, butanoic, hexadecanoic and 9-octadecenoic acid. The same compounds were identified in analyses of headspace sam ples but only trace quantities of the less volatile acids (C-10 to C-1 6) were present, whilst C-18 acids were absent. Behavioural responses of female A. gambiae towards a range of dilutions of the acid extract (in diethyl ether) were recorded in a windtunnel bioassay. The undilut ed extract was found to be repellent, but became highly attractive (P much less than 0.001) at lower doses, and was still significantly attr active (P<0.001) when diluted 10(6) times. A synthetic mixture of 12 o f the more abundant aliphatic acids identified in the acid extract was found to be significantly attractive (P<0.001) when diluted 10(8) tim es. Electroantennographic (EAG) studies showed significant and reprodu cible responses to (saturated) Limburger cheese headspace. At doses hi gher than 0.1%, the synthetic mixture of 12 acids elicited significant ly higher EAG amplitudes than the solvent control (paraffin oil). EAG responses were recorded from mosquitoes stimulated with C-5 to C-8 aci ds, that were characterized by significant dose-dependencies. Weaker, though significant EAG responses were obtained with the less volatile acids (C-9 to C-14). Only hexadecanoic acid did not elicit a detectabl e response. The electrophysiological and behavioural responses obtaine d with fatty acids isolated from Limburger cheese suggests that togeth er they could act as a kairomone for female A. gambiae. The implicatio ns of this are discussed together with the occurrence and bacterial pr oduction of these compounds on human skin.