Behavioral response of the malaria mosquito, Anopheles gambiae, to human sweat inoculated with axilla bacteria and to volatiles composing human axillary odor.

The responses of Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to odors from male and female axillary sweat incubated with human axilla bacteria were recorded in a dual-choice olfactometer. Staphylococcus epidermidis was selected for its low odor-producing pattern, Corynebacterium jeikeium for its strong Nα-acylglutamine aminoacylase activity liberating carboxylic acids including (R)/(S)-3-hydroxy-3-methylhexanoic acid (HMHA) and Staphylococcus haemolyticus for its capacity to liberate sulfur-containing compounds including (R/S)-3-methyl-3-sulfanylhexan-1-ol (MSH). Anopheles gambiae behavioral responses were evaluated under (i) its responsiveness to take off and undertake sustained upwind flight and (ii) its discriminating capacity between the two olfactometer arms bearing a test odor in either one or both arms. Experiments were conducted in the presence of carbon dioxide pulses as a behavioral sensitizer. Anopheles gambiae clearly discriminated for the olfactometer arm conveying odor generated by incubating any of the three bacteria species with either male or female sweat. Whereas An. gambiae did not discriminate between male and female sterile sweat samples in the olfactometer, the mosquito consistently showed a preference for male sweat over female sweat incubated with the same bacterium, independent of the species used as inoculum. Sweat incubated with C. jeikeium rendered mosquitoes particularly responsive and this substrate elicited the strongest preference for male over female sweat. Tested on their own, neither HMHA nor MSH elicited a clear discriminating response but did affect mosquito responsiveness. These findings serve as a basis for further research on the odor-mediated anthropophilic host-seeking behavior of An. gambiae.

An in vitro assay for testing mosquito repellents employing a warm body and carbon dioxide as a behavioral activator.

We describe here an in vitro behavioral assay for testing mosquito repellents applied in a dose-based manner to a warm body (34 degrees C) in test cages. The system was used to assess the sensitivity of 4-6-day-old Anopheles gambiae to the insect repellent diethyl methyl benzamide (deet). These tests were made in the absence and presence of additional carbon dioxide (CO2) applied as a pulse to activate mosquitoes in the cages. In the absence of the CO2 pulse the mosquitoes hardly responded to the warm body. Increasing the CO2 level in the cage by 1,000 parts per million caused a 25-fold increase in the number of landings by mosquitoes on the warm body in 2-min tests. This mosquito activation allowed the measurement of a significant reduction in the number of landings to bite on the warm body with increasing doses of deet (0.4 to 3.8 microg/cm2). An asymptotic nonlinear model fitted to the repellency data in the presence of CO2 allowed estimation of the effective dose of deet that reduced landings to bite by 50% (ED50) at 0.95 microg/cm2 (5 nmol/cm2) and the corresponding ED95 at 4.12 microg/cm2 (21.5 nmol/cm2). This in vitro bioassay has the advantage of permitting a fast throughput of test products under standardized conditions and is suitable for screenings designed for the purpose of discovering lead products with as yet unknown human toxicological and dermatological profiles.