An experimental hut trial of bednets was undertaken in The Gambia to compare six different permethrin treatments, including a wash-resistant formulation, and to test the effect of thorough hand washing of the nets. The wash-resistant formulation on nylon and cotton nets deterred Anopheles gambiae Giles (sensu lato) from house entry. All treated bednets killed a significantly higher number of A. gambiae (sensu lato) than untreated nets. The ability to kill mosquitoes was not diminished with the wash-resistant and Wellcome formulations after washing unlike the normal ICI formulation, this result being consistent with the permethrin residues found by gas liquid chromatographic (GLC) analyses to persist on the nets after washing. The mortality on treated cotton nets was significantly less than on treated nylon nets. Of the unwashed nets only the cotton net was significantly repellent. Both washed and unwashed treated nets significantly reduced the proportion of human bloodfed mosquitoes that survived and were potentially able to transmit malaria. Although the wash-resistant formulation might prove more efficient at killing mosquitoes with time, as compared to the other treatments, in The Gambia, where women wash their nets frequently, we suggest that the cheapest and most readily available formulation is used on nylon nets where possible.

Mosquitoes were caught in bedrooms in Tanzanian villages by human-biting catches and in light-traps set close to occupied untreated bed nets. Catches by each method were carried out on pairs of nights in the same week at different seasons and in different villages. The pairs of adjacent catches by the different methods showed a strong correlation. Analysis of the ratio between the catches by the two methods on pairs of nights in the same week indicated that on average three light-traps caught about the same number of mosquitoes as a team of two human catchers. The ratio did not differ significantly between Anopheles gambiae Giles (sensu lato), A. funestus Giles, and Culex quinquefasciatus Say, nor between the villages, or between times when mosquito populations were high or low. The distribution of numbers of ovarian dilatations differed significantly between catches in different villages and seasons but not between pairs of catches by the two methods. Similarly, the parity and sporozoite rates agreed between pairs of light-trap and house-resting catches, but differed markedly between villages and seasons. Thus it is concluded that light-traps used in conjunction with bed nets catch a representative sample of the vectors which would have bitten humans in bedrooms in this area.

A significant effect of moon phase on light-trap catches of Anopheles species was observed during a longitudinal study carried out in a forested belt of Madhya Pradesh, India, inhabited by tribal people. However, moon phase does not seem to have any effect on the proportion of adult Anopheles culicifacies Giles in the total catch, or the parity rate in this species, which is the vector responsible for perennial transmission of malaria in the area.

The two alpha-cyano pyrethroid insecticides lambda-cyhalothrin and alpha-cypermethrin were tested as bednet treatments at a target dose of 20 mg m-2. To establish their efficacy, female pyrethroid resistant and susceptible Anopheles stephensi Liston, and the F1-hybrids were allowed to fly freely in a room with a human subject under an impregnated net. Both treatments provided good personal protection by significantly reducing the number of blood fed mosquitoes compared to an untreated control net. Mortality after 24 h was significantly higher for the alpha-cypermethrin treated net when compared to lambda-cyhalothrin. For each insecticide there were no significant differences in the proportion of susceptible homozygotes and F1-hybrids found dead after a 24 h holding period, which suggests that there would be no selection for pyrethroid resistant heterozygotes by either of the insecticides.

The times for knockdown of resistant and susceptible strains of both Anopheles stephensi Liston (DUB234 and BEECH) and A. gambiae Giles (RSP and KWA), on exposure to a permethrin impregnated net were determined to evaluate the effect of age on susceptibility to permethrin. The mean knockdown times of pyrethroid resistant strains of A. stephensi and A. gambiae were significantly shorter if they were 10 days old (either fed or unfed), as compared with those of newly emerged mosquitoes. Based on KT50 values, the resistance ratios between resistant and susceptible A. stephensi were 3.5 and 2.2 for the newly emerged and 10-day-old mosquitoes, respectively. After 1 h exposure to paper impregnated with 0.25%permethrin, mortality of newly emerged individuals of the susceptible and resistant strains of A. stephensi was significantly less than when they were 10 days old. Irritability by permethrin was less with the resistant strain of A. stephensi. In the other strains irritability declined significantly with age. Pre-exposure of susceptible mosquitoes of BEECH and IRAQ strains of A. stephensi for a short time to permethrin did not increase tolerance when the mosquitoes were re-exposed 24 h later. Because of the age effect and the possibility of an effect of pre-exposure under some circumstances, confirmatory tests on newly emerged F1 individuals reared from wild collected mosquitoes are recommended in monitoring for resistance in wild populations.

Experimental huts in Côte d’Ivoire were used to evaluate the pyrethroid alpha-cypermethrin, the non-ester pyrethroid etofenprox, the organophosphate pirimiphos-methyl and the carbamate carbosulfan on bednets against pyrethroid-resistant Anopheles gambiae Giles. To test for selection for the resistance gene by the treated nets, A. gambiae collected live or dead from the huts were kept and analysed for the presence of the kdr gene using a new polymerase chain reaction followed by sequence-specific oligonucleotide probing (PCR–SSOP) for kdr-genotyping. Deliberately holed bednets freshly treated with pirimiphos-methyl or carbosulfan caused over 90% kill of A. gambiae s.s. and Culex spp. However, the mortality with alpha-cypermethrin or etofenprox treated nets was similar to that with untreated nets. Bloodfeeding of A. gambiae s.s. on the sleepers under the nets was only significantly reduced by alpha-cypermethrin and carbosulfan. Tests of the residual activity of the bednets after seven months showed that pirimiphos-methyl had lost its efficacy while carbosulfan still performed well. Once again the pyrethroid treated nets gave similar results to the untreated nets. Selection for the kdr-allele by alpha-cypermethrin and etofenprox, but not by carbosulfan, was indicated by PCR–SSOP genotyping of mosquitoes. Thus carbamates such as carbosulfan, or organophosphates of longer persistence than pirimiphos-methyl and of low mammalian toxicity, would seem to be a promising alternative to be used on bednets, particularly in areas of pyrethroid resistance.

A dual port olfactometer was used to study the response of Anopheles gambiaeGiles sensu stricto to odours of human and animal origin. Human odour consisted of human skin emanations collected on a nylon stocking, which was worn for 24 h. This was tested alone or together with 4.5% carbon dioxide, the concentration in human and cattle breath. Cattle odours consisted of cow skin emanations and/or carbon dioxide. Cow skin emanations were collected by tying a nylon stocking (‘cow sock’) around the hind leg of a cow for 12 h. Anopheles gambiae s.s. was consistently highly attracted by human odour, which is consistent with the high degree of anthropophily in this mosquito. Anopheles gambiae s.s. was not attracted by human or cattle equivalent volumes of carbon dioxide and this gas did not enhance the effect of human skin residues. Furthermore, A. gambiae s.s. showed a high degree of aversion to cow odour. When human odour and cow odour were tested together in the same port, mosquitoes were still highly attracted, indicating that whilst cattle odour may deter A. gambiae s.s., these mosquitoes can detect human odour in the presence of cattle odour. It was concluded that carbon dioxide plays a minor role in the host seeking behaviour of A. gambiae s.s., whilst host specific cues such as human skin residues play a major role and very effectively demonstrated anthropophilic behaviour in the laboratory.

Flight activity in strain 16cSS of Anopheles gambiae (sensu stricto) has been investigated using an acoustic actograph. The peak activity was concentrated within the first hour after light off. After recording their activity, individual mosquitoes were examined for their chromosome inversion karyotype. No significant relationship was found of any of the karyotypes to total activity over the 2 hr after light off or the time at which 50% of their activity had been completed.

The results are discussed in relation to the possible widespread introduction of mosquito bednets and the possible selection of mosquitoes biting abnormally early before people go to bed.