Blood-feeding behaviour of the malarial mosquito Anopheles arabiensis: implications for vector control

Feeding behaviour of the malaria vector Anopheles arabiensis Patton (Diptera: Culicidae) was monitored for 12 months (March 2003-February 2004) in the Konso District of southern Ethiopia (5 degrees 15'N, 37 degrees 28'E). More than 45 000 An. arabiensis females were collected by host-baited sampling methods (light-traps, human landing catches, cattle-baited traps) and from resting sites (huts and pit shelters). In the village of Fuchucha, where the ratio of cattle : humans was 0.6 : 1, 51% of outdoor-resting mosquitoes and 66% of those collected indoors had fed on humans, human baits outdoors caught > 2.5 times more mosquitoes than those indoors and the mean catch of mosquitoes from pit shelters was about five times that from huts. Overall, the vast majority of feeding and resting occurred outdoors. In the cattle camps of Konso, where humans slept outdoors close to their cattle, approximately 46% of resting mosquitoes collected outdoors had fed on humans despite the high cattle : human ratio (17 : 1). In both places, relatively high proportions of bloodmeals were mixed cow + human: 22-25% at Fuchucha and 37% in the cattle camps. Anthropophily was also gauged experimentally by comparing the numbers of mosquitoes caught in odour-baited entry traps baited with either human or cattle odour. The human-baited trap caught about five times as many mosquitoes as the cattle-baited one. Notwithstanding the potential pitfalls of using standard sampling devices to analyse mosquito behaviour, the results suggest that the An. arabiensis population is inherently anthropophagic, but this is counterbalanced by exophagic and postprandial exophilic tendencies. Consequently, the population feeds sufficiently on humans to transmit malaria (sporozoite rates: 0.3% for Plasmodium falciparum and 0.5% for P. vivax, by detection of circumsporozoite antigen) but also takes a high proportion of meals from non-human hosts, with 59-91% of resting mosquitoes containing blood from cattle. Hence, classical zooprophylaxis is unlikely to have a significant impact on the malaria vectorial capacity of An. arabiensis in Konso, whereas treating cattle with insecticide might do.     

Preliminary lack of evidence for simian odour preferences of savanna populations of Anopheles gambiae and other malaria vectors

The behavioural response to several culicine and anopheline mosquitoes to the odour of alternative hosts (human vs monkey) arranged in a choice set-up using odour-baited entry traps (OBETs) was assessed in a field experiment in south-eastern Senegal. The experimental protocol followed procedures analogous to those adopted in olfactometer laboratory tests. Two adult Cercopithecus aethiops and a child of similar mass slept inside separate tents and their odours were drawn to each one of two paired OBETs so that approaching mosquitoes could experience both odour-laden streams before "choosing" to fly against one of the two air currents and into the trap. The traps were set up in a riverine forest clearing near the town of Kedougou, where primates (Papio papio, Cercopithecus aethiops, and Erythrocebus patas) are common. A total of 192 mosquitoes belonging to 4 genera was captured during 8 trap nights. All major human malaria vectors including Anopheles gambiae sensu lato, An. funestus, and An. nili, which constituted the bulk of the trap catch (N = 153), clearly expressed a preference for human odour, with > 90% of captured mosquitoes caught in the human-baited trap. A sub-sample of specimens belonging to the An. gambiae complex caught in both traps was identified by rDNA-PCR and RFLP as An. gambiae sensu stricto molecular form S (7/10), and An. arabiensis (3/10). The only species that did not show a preference for the alternative odour-laden air streams, among those caught in significant numbers, were mosquitoes of the genus Mansonia, with both Ma. uniformis and Ma. africana weakly preferring human odour, but not at a statistically significant level. These results are in accordance with the hypothesis that the strongly anthropophilic feeding preferences of An. gambiae did not evolve from an ancestral association with non-human primates.     

Mosquito responses to carbon dioxide in B West African Sudan savanna village

Abstract. Mosquito responses to carbon dioxide were investigated in Noungou village, 30 km northeast of Ouagadougou in the Sudan savanna belt of Burkina Faso, West Africa. Species of primary interest were the main malaria vectors Anopheles gambiae S.S. and An.arabiensis, sibling species belonging to the An.gambiae complex. Data forAn.finestus, An.pharoensis, Culex quinquefasciatus and Mansonia uniformis were also analysed. Carbon dioxide was used at concentrations of 0.04-0.6% (cf. 0.03% ambient concentration) for attracting mosquitoes to odour-baited entry traps (OBETs). The ‘attractiveness’ of whole human odour was also compared with CO₂, emitted at a rate equivalent to that released by the human bait. In a direct choice test with two OBETs placed side-by-side, the number of An.gambiae s. I. entering the trap with human odour was double the number trapped with CO₂, alone (at the human equivalent rate), but there was no significant difference between OBETs for the other species of mosquitoes. When OBETs were positioned 20 m apart, again CO₂, alone attracted half as many An.gambiae s.l. and only 40% Anlfunestus, 65% Ma.uniformis but twice as many An.pharoensis compared to the number trapped with human odour. The dose-response for all mosquito species was essentially similar: a linear increase in catch with increasing dose on a log-log scale. The slopes of the dose-response curves were not significantly different between species, although there were significant differences in the relative numbers caught. If the dose-response data are considered in relation to a standard human bait collection (HBC), however, the behaviour of each species was quite different. At one extreme, even the highest dose of CO₂, did not catch more An.gambiae s.1. than one HBC. At the other extreme, the three highest doses of CO₂, caught significantly more Ma.unifonnis than did one HBC. An.pharoensis and Cx quinquefasciatus showed a threshold response to CO₂, responding only at doses above that normally released by one man. An.funestus did not respond to CO₂, alone at any dose in sufficient numbers to assess the dose response. Within the An.gambiae complex, An.arabiensis 'chose' the CO₂,-baited trap with a higher probability than An.gambiae S.S. Also An.arabiensis, the less anthropophilic of the two species, was more abundant in CO₂,-baited OBETs than in human bait collections.     

Zoophily of Anopheles arabiensis and An. gambiae in Madagascar demonstrated by odour-baited entry traps

In Madagascar we used odour-baited entry traps (OBETs) for host choice tests of wild female anopheline mosquitoes (Diptera: Culicidae) at representative localities on the East and West sides of the island (villages Fenoarivo and Tsararano, respectively) and at the southern margin of the central plateau (Zazafotsy village, 800 m altitude). No insecticide house-spraying operations have been undertaken at these villages. Odours from a man and a calf of similar mass, concealed in different tents, were drawn by fans into separate OBETs set side by side. Traps were alternated to compensate for position effects, and different pairs of individual baits were employed for successive replicates. Totals of 266 An. funestus Giles sensu stricto and 362 An. gambiae Giles sensu lato were collected in 48 trap nights during March-June 1999. For each mosquito species the 'index of anthropophily' was defined as the proportion of females caught in the human-baited trap. For An. funestus this index was found to be consistently greater than 0.5 (value for random choice between traps/hosts), indicating that this species 'preferred' human to calf odour (index=0.83). Conversely, the index of anthropophily for An. gambiae s.l. indicated they 'chose' calf in preference to human odour (index=0.26). No significant differences of relative preference for calf or man were detected between villages; geographical variance accounted for <8% of the total experimental variance. Molecular identifications of 181 specimens of the An. gambiae complex (approximately 50% of the samples) revealed only An. arabiensis Patton at Tsararano and Zazafotsy, but >97% An. gambiae Giles sensu stricto at Fenoarivo, in accordance with prior knowledge of the differential distributions of these sibling species on the island. Predominant zoophily (i.e. intrinsic 'preference' for cattle odours) by both An. arabiensis and An. gambiae s.s. in Madagascar contrasts with their greater anthropophily in continental Africa.     

Electroantennogram and behavioural responses of the malaria vector Anopheles gambiae to human-specific sweat components

Afrotropical malaria vectors of the Anopheles gambiae complex (Diptera: Culicidae), particularly An. gambiae sensu stricto, are attracted mainly to human hosts. A major source of human volatile emissions is sweat, from which key human-specific components are the carboxylic acids (E)- and (Z)-3-methyl-2-hexenoic acid and 7-octenoic acid. Electrophysiological studies on the antennae of An. gambiae s.s. showed selective sensitivity to these compounds, with a threshold at 10(-6) g comparable to that of known olfactory stimulants 1-octen-3-ol, p-cresol, isovaleric acid, and lower than threshold sensitivity to L-lactic acid and the synthetic mosquito repellent N,N-diethyltoluamide (DEET). A combination of the acids released at concentrations > 10(-5) g in wind tunnel bioassays significantly reduced the response to CO2, the major attractant released by human hosts, for strains of An. gambiae s.s. originating from East and West Africa. Field trials with odour-baited entry traps (OBETs) in Burkina Faso showed that 7-octenoic acid significantly increased (by 1.7-fold) the catch of females of An. gambiae sensu lato (comprising two sibling species: An. arabiensis Patton and An. gambiae s.s.) in OBETs baited with CO2, whereas combinations of the acids significantly reduced the catch in CO2-baited traps (by 2.1-fold) and in whole human odour-baited traps (by 1.5-fold). The pure (E) and (Z) geometric isomers of 3-methyl-2-hexenoic acid gave comparable results to the (EIZ) isomer mixture. These results provide the first experimental evidence that human-specific compounds affect the behaviour of highly anthropophilic An. gambiae s.l. mosquitoes. The compounds appear to inhibit the upwind flight' response to known long-range attractants, and may serve either to mask' the attractants present or, more probably, to 'arrest' upwind flight when mosquitoes arrive at a host under natural conditions. In the final approach to hosts, vectors are known to reduce their flight speed and increase their turning rate, to avoid overshooting the source. In our experimental apparatus, these changes in flight behaviour would reduce the number of mosquitoes entering the ports of the collection devices.     

Differential responses of mosquito sibling species Anopheles arabiensis and An. quadriannulatusto carbon dioxide, a man or a calf

Field studies on responses of two mosquito sibling species, Anopheles arabiensis Patton and An. quadriannulatus Theobald, to a man, a calf and different release rates of carbon dioxide (man, calf and cow equivalents) were conducted in north-eastern South Africa. Various combinations of baits were compared in two-choice tests, using two mosquito nets, placed 2.5 m apart and 10 cm off the ground. Mosquitoes attracted to the baits were able to enter the nets from below and were collected by means of a suction tube. In a two-choice test between a man and CO₂ (human equivalent, 250 ml/min), 81% of the An. quadriannulatus were caught with CO₂. The reverse was seen for An. arabiensis , where only 20% of the total catch was caught with CO₂ compared to man. High release rates of CO₂ (cow equivalent, 800 ml/min) attracted significantly more An. quadriannulatus than the low release rate (250 ml/min), whereas no significant effect of the release rate of CO₂ on the total catch of An. arabiensis was seen. In the latter species, up to 33% of the attraction of human emanation is attributable to carbon dioxide. Anopheles quadriannulatus was equally attracted to a calf and CO₂ (calf equivalent, 180 ml/min). Catches of other mosquito species showed consistent differences between all treatments which appear to be associated with differences in host-preference, suggesting that the importance of CO₂ in host-seeking behaviour of mosquitoes increases with the degree of zoophily.     

Host selection by Anopheles arabiensis and An. quadriannulatus feeding on cattle in Zimbabwe

In the Zambezi valley, mosquito females of the Anopheles gambiae Giles complex (Diptera: Culicidae) were collected from a hut containing pairs of cattle distinguishable by known DNA markers. DNA was extracted from the blood-fed mosquito abdomens and primer sets for ungulate and mosquito DNA loci were used to identify the mosquito sibling species and individual host source(s) of their bloodmeals. The 67 mosquitoes comprised a mixture of An. arabiensis Patton (31%) and An. quadriannulatus Theobald (69%). DNA from one or both of the cattle present in the hut was detected in 91% of samples. When the hut contained an adult and a calf, the percentage of bloodmeals from the adult, the calf and adult + calf were 58%, 27% and 15%, respectively; the trend towards meals from the adult host was consistent but not always significant. When the pair of cattle comprised two adults of roughly equal size and age, then mosquitoes generally showed no significant bias towards feeding from one individual. There was no significant difference in the pattern of host selection made by An. arabiensis and An. quadriannulatus but the former had a significantly higher percentage (20%) of mixed meals than An. quadriannulatus (9%). These two members of the An. gambiae complex appear to be less selective in their choice of cattle hosts compared to day-active Diptera such as tsetse and Stomoxys, possibly because the hosts are generally asleep when Anopheles are active and there is therefore less selective pressure to adapt to host defensive behaviour. The slight bias of Anopheles towards older and/or larger cattle may be related to the host's larger surface area.     

Reduced susceptibility to DDT in field populations of Anopheles quadriannulatus and Anopheles arabiensis in Malawi: evidence for larval selection

Abstract.  Bioassays for insecticide resistance in adult mosquitoes were conducted on samples of Anopheles gambiae Giles s.l . (Diptera: Culicidae) species collected as larvae from breeding sites in the lower Shire Valley, Malawi. The results indicate full susceptibility to permethrin, deltamethrin and malathion, but reduced susceptibility to DDT in one sample from Thom (LT₅₀ of 8.39 min for females and 25.09 min for males). Polymerase chain reaction-based species identification of the mosquitoes assayed revealed a mixture of Anopheles arabiensis Patton and Anopheles quadriannulatus (Theobold). The LT₅₀ did not differ significantly between species. Genotyping of the L1014F and L1014S kdr alleles showed all mosquito specimens to be homozygous wild type; thus the reduced susceptibility detected is not attributable to target site insensitivity and instead is likely to be metabolic in nature. Anopheles quadriannulatus is characteristically zoophagic and exophilic. Indeed, of 82 Anopheles collected through knockdown collections within dwellings, only one was An. quadriannulatus and the rest were An. arabiensis . They are unlikely, therefore, to have been exposed to selection pressure arising from insecticide-treated net usage or to DDT indoor residual spraying. Therefore, it is suggested that this example of reduced susceptibility to DDT in An. quadriannulatus reflects selection in the larval stages.     

The effect of repellents Ocimum forskolei and deet on the response of Anopheles stephensi to host odours

The behavioural response of Anopheles stephensi Liston (Diptera: Culicidae) to incubated host odours (from human and goat) and to human odour in combination with a repellent plant, Ocimum forskolei (Labiatae), or deet (N, N, diethyl-toluamide) (20%) was tested in a dual-port olfactometer. An. stephensi was significantly attracted to both host odours compared with air alone, but showed no preference when given a choice between both host odours simultaneously. In choice tests, the addition of deet to human odour did not significantly divert mosquitoes to human odour alone, but did divert them to goat odour. O. forskolei combined with human odour diverted mosquitoes to goat or human odour alone. Combinations of human odour and O. forskolei, and human odour and deet were both as unattractive as air alone, and attracted mosquitoes equally when offered simultaneously. The results indicate that use of O. forskolei as a repellent would be beneficial in reducing vector biting if used in communities in areas with partially zoophilic mosquito species such as An. stephensi, and where animals are present.     

Attraction of mosquitoes to primate odours and implications for zoonotic Plasmodium transmission

Vector-borne diseases often originate from wildlife and can spill over into the human population. One of the most important determinants of vector-borne disease transmission is the host preference of mosquitoes. Mosquitoes with a specialised host preference are guided by body odours to find their hosts in addition to carbon dioxide. Little is known about the role of mosquito host preference in the spillover of pathogenic agents from humans towards animals and vice versa. In the Republic of Congo, the attraction of mosquitoes to primate host odours was determined, as well as their possible role as malaria vectors, using odour-baited traps mimicking the potential hosts of mosquitoes. Most of the mosquito species caught showed a generalistic host preference. Anopheles obscurus was the most abundant Anopheles mosquito, with a generalistic host preference observed from the olfactory response and the detection of various Plasmodium parasites. Interestingly, Culex decens showed a much higher attraction towards chimpanzee odours than to human or cow odours. Human Plasmodium parasites were observed in both human and chimpanzee blood, although not in the Anopheles mosquitoes that were collected. Understanding the role of mosquito host preference for cross-species parasite transmission provides information that will help to determine the risk of spillover of vector-borne diseases.     

Laboratory colonization of Anopheles quadriannulatus from sympatry with other sibling species of the Anopheles gambiae complex in Zimbabwe

Abstract. A laboratory colony of the mosquito Anopheles quadriannulatus was established from a wild population occurring sympatrically with An.arabiensis in Zimbabwe. These sibling species are members of the An.gambiae Giles complex and were distinguished primarily by means of their specific polytene chromosome banding patterns. By using an ox-baited trap, we sampled selectively for the more zoophilic An.quadriannulatus. It was confirmed that An.quadriannulatus has the diagnostic slow allozyme of aspartate aminotransferase (AAT^95/95). In a mixed population under laboratory conditions, An.arabiensis displaced An.quadriannulatus within eight generations, without introgression. Colonization of An.quadriannulatus was facilitated by pooling the progeny from wild-caught mothers of confirmed identity and by using a specially adapted cage to promote mating.     

Laboratory studies on the olfactory behaviour of Anopheles quadriannulatus

The host preference of Anopheles quadriannulatus Theobald (Diptera: Culicidae), the zoophilic member of the malaria mosquito complex Anopheles gambiae Giles, was investigated in a dual‐choice olfactometer. Naïve female mosquitoes were exposed to CO₂, acetone, 1‐octen‐3‐ol, and skin emanations from cows and humans in various combinations. Their behavioural responses were recorded when they had entered one of either upwind traps from where the odours were being released. The mosquitoes did not respond to CO₂ when released at human or cattle equivalent concentrations. Too few mosquitoes responded to acetone to allow for a statistical analysis. The combination of CO₂+ 1‐octen‐3‐ol was repellent. Cow odour alone was slightly attractive, but there was a synergistic attractive effect of cow odour + CO₂. Surprisingly, the mosquitoes were attracted to human odour, and in a choice situation human odour was selected above cow odour + CO₂. Anthropophilic An. gambiae Giles s.s. was repelled by cow odour + CO₂ in contrast to An. quadriannulatus. In a choice situation, both mosquito species selected human odour above cow odour + CO₂. The implications of these results are discussed in the light of recent behavioural data from the field.     

Human skin microbiota and their volatiles as odour baits for the malaria mosquito Anopheles gambiae s.s

Host seeking by the malaria mosquito Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) is mainly guided by volatile chemicals present in human odours. The skin microbiota plays an important role in the production of these volatiles, and skin bacteria grown on agar plates attract An. gambiae s.s. in the laboratory. In this study, the attractiveness of volatiles produced by human skin bacteria to An. gambiae s.s. was tested in laboratory, semi‐field, and field experiments to assess these effects in increasing environmental complexity. A synthetic blend of 10 compounds identified in the headspace of skin bacteria was also tested for its attractiveness. Carbon dioxide significantly increased mosquito catches of traps baited with microbial volatiles in the semi‐field experiments and was therefore added to the field traps. Traps baited with skin bacteria caught significantly more An. gambiae s.s. than control traps, both in the laboratory and semi‐field experiments. Traps baited with the synthetic blend caught more mosquitoes than control traps in the laboratory experiments, but not in the semi‐field experiments. Although bacterial volatiles increased mosquito catches in the field study, trapping several mosquito vector species, these effects were not significant for An. gambiae s.l. It is concluded that volatiles from skin bacteria affect mosquito behaviour under laboratory and semi‐field conditions and, after fine tuning, have the potential to be developed as odour baits for mosquitoes.     

A comparative study on the efficacy of artesunate plus sulphadoxine/pyrimethamine versus artemether-lumefantrine in eastern Sudan

Background

The most important factor for effective zooprophylaxis in reducing malaria transmission is a predominant population of a strongly zoophilic mosquito, Anopheles arabiensis. The feeding preference behaviour of Anopheline mosquitoes was evaluated in odour-baited entry trap (OBET).

Methods

Mosquitoes were captured daily using odour-baited entry traps, light traps and hand catch both indoor and in pit traps. Experimental huts were used for release and recapture experiment. The mosquitoes collected were compared in species abundances.

Results

Anopheles arabiensis was found to account for over 99% of Anopheles species collected in the study area in Lower Moshi, Northern Tanzania. In experimental release/capture trials conducted at the Mabogini verandah huts, An. arabiensis was found to have higher exophilic tendency (80.7%) compared to Anopheles gambiae (59.7%) and Culex spp. (60.8%). OBET experiments conducted at Mabogini collected a total of 506 An. arabiensis in four different trials involving human, cattle, sheep, goat and pig. Odours from the cattle attracted 90.3% (243) compared to odours from human, which attracted 9.7% (26) with a significant difference at P = 0.005. Odours from sheep, goat and pig attracted 9.7%, 7.2% and 7.3%, respectively. Estimation of HBI in An. arabiensis collected from houses in three lower Moshi villages indicated lower ratios for mosquitoes collected from houses with cattle compared to those without cattles. HBI was also lower in mosquitoes collected outdoors (0.1–0.3) compared to indoor (0.4–0.9).

Conclusion

In discussing the results, reference has been made to observation of exophilic, zoophilic and feeding tendencies of An. arabiensis, which are conducive for zooprophylaxis. It is recommended that in areas with a predominant An. arabiensis population, cattle should be placed close to dwelling houses in order to maximize the effects of zooprophylaxis. Protective effects of human from malaria can further be enhanced by keeping cattle in surroundings of residences.     

Differential behaviour of Anopheles gambiae sensu stricto (Diptera: Culicidae) to human and cow odours in the laboratory

A dual port olfactometer was used to study the response of Anopheles gambiae Giles 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.     

Cannibalism and predation among larvae of the Anopheles gambiae complex

Among the aquatic developmental stages of the Anopheles gambiae complex (Diptera: Culicidae), both inter- and intra-specific interactions influence the resulting densities of adult mosquito populations. For three members of the complex, An. arabiensis Patton, An. quadriannulatus (Theobald) and An. gambiae Giles sensu stricto, we investigated some aspects of this competition under laboratory conditions. First-instar larvae were consumed by fourth-instar larvae of the same species (cannibalism) and by fourth-instar larvae of other sibling species (predation). Even when larvae were not consumed, the presence of one fourth-instar larva caused a significant reduction in development rate of first-instar larvae. Possible implications of these effects for population dynamics of these malaria vector mosquitoes are discussed.     

Identification and expression of odorant-binding proteins of the malaria-carrying mosquitoes Anopheles gambiae and Anopheles arabiensis

Host preference and blood feeding are restricted to female mosquitoes. Olfaction plays a major role in host-seeking behaviour, which is likely to be associated with a subset of mosquito olfactory genes. Proteins involved in olfaction include the odorant receptors (ORs) and the odorant-binding proteins (OBPs). OBPs are thought to function as a carrier within insect antennae for transporting odours to the olfactory receptors. Here we report the annotation of 32 genes encoding putative OBPs in the malaria mosquito Anopheles gambiae and their tissue-specific expression in two mosquito species of the Anopheles complex; a highly anthropophilic species An. gambiae sensu stricto and an opportunistic, but more zoophilic species, An. arabiensis. RT-PCR shows that some of the genes are expressed mainly in head tissue and a subset of these show highest expression in female heads. One of the genes (agCP1588) which has not been identified as an OBP, has a high similarity (40%) to the Drosophila pheromone-binding protein 4 (PBPRP4) and is only expressed in heads of both An. gambiae and An. arabiensis, and at higher levels in female heads. Two genes (agCP3071 and agCP15554) are expressed only in female heads and agC15554 also shows higher expression levels in An. gambiae. The expression profiles of the genes in the two members of the Anopheles complex provides the first step towards further molecular analysis of the mosquito olfactory apparatus.     

Development of an exposure-free bednet trap for sampling Afrotropical malaria vectors

An exposure-free bednet trap (the 'Mbita trap') for sampling of Afrotropical malaria vectors was developed during preliminary studies of mosquito behaviour around human-occupied bednets. Its mosquito sampling efficiency was compared to the CDC miniature light-trap and human landing catches under semi-field conditions in a screen-walled greenhouse using laboratory-reared Anopheles gambiae Giles sensu stricto (Diptera: Culicidae). When compared in a competitive manner (side by side), the Mbita trap caught 4.1+/-0.5 times as many mosquitoes as the CDC light-trap, hung beside an occupied bednet (P < 0.000 1) and 43.2+/-10% the number caught by human landing catches (P < 0.0001). The ratio of Mbita trap catches to those of the CDC light trap increased with decreasing mosquito density. Mosquito density did not affect the ratio of Mbita trap to human-landing catches. In a non-competitive comparison (each method independent of the other), the Mbita trap caught 89.7+/-10% the number of mosquitoes caught by human landing catches (P < 0.0001) and 1.2+/-0.1 times more mosquitoes than the CDC light trap (P = 0.0008). Differences in Mbita trap performance relative to the human landing catch under noncompetitive vs. competitive conditions were explained by the rate at which each method captured mosquitoes. Such bednet traps do not expose people to potentially infectious mosquito bites and operate passively all night without the need for skilled personnel. This trap is specifically designed to catch host-seeking mosquitoes only and may be an effective, sensitive, user-friendly and economic alternative to existing methods for mosquito surveillance in Africa.     

Unexpected anthropophagic behaviour in Anopheles quadriannulatus

The strongly anthropophilic behaviour of Anopheles gambiae Giles sensu stricto (Diptera: Culicidae), the most important malaria vector in Africa, has been demonstrated by field and laboratory studies. Other members of the An. gambiae complex express varied degrees of anthropophily. Anopheles quadriannulatus (Theobald) species A and B are more zoophilic members of the complex and hence are considered to be of no medical importance. Olfactometer experiments with An. quadriannulatus species A have demonstrated attraction to both human and cow odour. To extend these olfactometer observations a choice experiment was conducted in an outdoor cage with a human and a calf as baits, using laboratory-reared mosquitoes. Anopheles gambiae s.s. (from Liberia) and two strains of An. quadriannulatus species A (SKUQUA from South Africa, SANGQUA from Zimbabwe), marked with different coloured fluorescent powders for identification purposes, were released simultaneously and given an equal opportunity to feed on either host. The experiment was repeated six times. Bloodmeals were identified using the precipitin technique. Anopheles gambiae s.s. showed highly anthropophagic behaviour, taking 88% of bloodmeals from the human host. In contrast, both strains of An. quadriannulatus fed with equal frequency on the human or the calf; the response to either host was not significantly different. These results confirm the olfactometer findings and demonstrate anthropophagic behaviour not previously recorded in this species. This finding has implications for prospective manipulation of host preference for genetic control purposes.     

Prospects for the Development of Odour Baits to Control the Tsetse Flies Glossina tachinoides and G. palpalis s.l.

Field studies were done of the responses of Glossina palpalis palpalis in Côte d''Ivoire, and G. p. gambiensis and G. tachinoides in Burkina Faso, to odours from humans, cattle and pigs. Responses were measured either by baiting (1.) biconical traps or (2.) electrocuting black targets with natural host odours. The catch of G. tachinoides from traps was significantly enhanced (∼5×) by odour from cattle but not humans. In contrast, catches from electric targets showed inconsistent results. For G. p. gambiensis both human and cattle odour increased (>2×) the trap catch significantly but not the catch from electric targets. For G. p. palpalis, odours from pigs and humans increased (∼5×) the numbers of tsetse attracted to the vicinity of the odour source but had little effect on landing or trap-entry. For G. tachinoides a blend of POCA (P = 3-n-propylphenol; O = 1-octen-3-ol; C = 4-methylphenol; A = acetone) alone or synthetic cattle odour (acetone, 1-octen-3-ol, 4-methylphenol and 3-n-propylphenol with carbon dioxide) consistently caught more tsetse than natural cattle odour. For G. p. gambiensis, POCA consistently increased catches from both traps and targets. For G. p. palpalis, doses of carbon dioxide similar to those produced by a host resulted in similar increases in attraction. Baiting traps with super-normal (∼500 mg/h) doses of acetone also consistently produced significant but slight (∼1.6×) increases in catches of male flies. The results suggest that odour-baited traps and insecticide-treated targets could assist the AU-Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC) in its current efforts to monitor and control Palpalis group tsetse in West Africa. For all three species, only ∼50% of the flies attracted to the vicinity of the trap were actually caught by it, suggesting that better traps might be developed by an analysis of the visual responses and identification of any semiochemicals involved in short-range interaction.