Sampling Adults by Animal Bait Catches and by Animal-Baited Traps

Chapter 5 Sampling Adults by Animal Bait Catches and by Animal-Baited Traps The most fundamental method for catching female mosquitoes is to use a suit able bait to attract hungry host-seeking individuals, and human bait catches, sometimes euphemistically called landing counts, have been used for many years to collect anthropophagic species. Variations on the simple direct bait catch have included enclosing human or bait animals in nets, cages or traps which, in theory at least, permit the entrance of mosquitoes but prevent their escape. Other attractants, the most widely used of which are light and carbon dioxide, have also been developed for catching mosquitoes. In some areas, especially in North America, light-traps, with or without carbon dioxide as a supplement, have more or less replaced human and animal baits as a routine sampling method for several species (Chapter 6). However, despite intensive studies on host-seeking behaviour no really effective attractant has been found to replace a natural host, and consequently human bait catches remain the most useful single method of collecting anthropophagic mosquitoes. Moreover, although bait catches are not completely free from sampling bias they are usually more so than most other collecting methods that employ an attractant. They are also easily performed and require no complicated or expensive equipment. HUMAN BAIT CATCHES Attraction to hosts Compounds used by mosquitoes to locate their hosts are known as kairomones, that is substances from the emitters (hosts) are favourable to the receiver (mosquitoes) but not to themselves. Emanations from hosts include heat, water vapour, carbon dioxide and various host odours. Wright (1975) considered warmth and humidity were the main attractants of mosquitoes to humans, and doubted whether there was any skin odour involved in host attraction, but Khan (1977) believed that in addition to skin temperature and skin colour, body odour and other factors were involved. Price et al. (1979) concluded that female Anopheles quadrimaculatus were mainly attracted to humans by chemicals eman ating from the skin, while studies by Schreck et al. (1981, 1990) showed that there were unidentified attractants to mosquitoes in the sweat from human 349 M. W. Service, Mosquito Ecology © Springer Science+Business Media Dordrecht 1993 350 MOSQUITO ECOLOGY: FIELD SAMPLING METHODS subjects. There are two types of sweat, eccrine sweat which comes from most body surfaces but especially from the palms of the hands and soles of the feet, and apocrine sweat from the axillary, perigenital and perianal regions. [S]-Lactic acid (formerly called L-lactic acid) is in fact produced by glycolysis in the eccrine sweat glands, and excess remains in the final secretion-sweat. Most mammals have apocrine-type sweat glands; birds lack sweat glands. Schreck et al. (1990) found that sweat from the face and hands generally elicited the great est response from mosquitoes, and there were significant differences between the attractiveness of sweat from the hands of different people. However, of the 12 mosquito species tested four showed no response, while another four species were only weakly attracted to human sweat: the species most attracted was Aedes aegypti, followed by Aedes albopictus and Anopheles albimanus. It was pointed out by Schreck et al. (1990) that attractant substances from the skin (e.g. sweat) might contaminate equipment used in mosquito behaviour studies, and that traps frequently handled might catch a disproportionate share of mosquitoes. Bar-Zeev et al. (1977) summarised the available information on the factors that appeared to attract Aedes aegypti to humans. They also carried out labora tory studies on responses to carbon dioxide, relative humidity, temperature and emanations from a human forearm, and confirmed the attractancy of [S]-lactic acid (Acree et al., 1968; Smith et al., 1970). More recently Kusakabe & Ikeshoji (1990) found that lactic acid, heat, black colour, movement and sound were all to some degree attractive to both sexes of Aedes aegypti but carbon dioxide was not attractive. In an interesting paper Gillett (1979) discussed possible mechan isms by which mosquitoes orientate upwind to hosts in the absence of visual cues; he also presents some pertinent physical characteristics of wind speed near the ground. Takken & Kline (1989), Takken (1991) and Lehane (1991) have briefly summarised what is known about substances that attract mosquitoes to baits and odour-baited traps, while the role of carbon dioxide in host attraction has been reviewed by Gillies (1980). McIver (1982) listed five types of stimuli that have been shown to elicit host responses in mosquitoes, namely vision, heat, water vapour, carbon dioxide and host odours. A major activator in host location is the concentration of carbon dioxide emitted by hosts, which mosquitoes detect by capitate pegs on their palps. An increase of only 0·01% in carbon dioxide concentration may be detectable, and the response is almost logarithmic to a saturation level of 0·05-0·5%. As the biological range of carbon dioxide concentration emanating from animals is be tween 3-5%, it is not surprising that artificial concentrations as great as 10% from dry ice or gas cylinders elicit little additional response. Carbon dioxide from animals is therefore 100 or more times greater than the background con centration of 0·02-0·04%, but yet much less than the concentration of about 100% at the release point emitted from gas cylinders or dry ice. Although dis charge rates can be altered, the mixing of the gas in the air at various distances from the trap-that is its concentration-will depend on local environmental conditions, which can be very variable in both time and space, and usually remain largely unknown in trapping experiments. ANIMAL BAIT CATCHES AND ANIMAL-BAITED TRAPS 351 In addition to carbon dioxide expired breath contains several organic com pounds (Teranishi et aI., 1972) including acetone (Crofford, 1976), some of which may be attractive to mosquitoes. Sutcliffe (1986) gives a good review of how blackflies locate their hosts, and much of this will be of interest to those concerned with host orientation by mosquitoes. Takken & Kline (1989) reported for the first time from field experiments that octenol had potential as a mosquito attractant. Later Kline et al. (1990) conducted field trials in Florida with unlit CDC-type light-traps baited with various combinations of attractants including: (1) CO2; (2) octenol; (3) octenol + CO2; (4) octenol + butanone + CO2; (5) lactic acid + CO2; (6) lactic acid + octenol + CO2; (7) honey; (8) phenols; and (9) phenols + octenol. Not surprisingly different mosquito species sometimes responded differ ently to these chemicals. Basically very few species are attracted in any numbers to octenol alone, but when octenol and carbon dioxide were used together there appears to have been a synergistic effect and a twofold or greater catch was obtained with most species of Aedes, Psorophora, Anopheles, Coquillettidia and Mansonia encountered in the area. With Culex species, however, there was little attraction to either chemical alone or in combination. But in contrast to these generalisations Aedes taeniorhynchus and Coquillettidia perturbans seemed to re spond to octenol alone. Honey (500 ml diluted with 300 ml 29% sodium chloride, then extracted overnight with 250 ml hexane in a liquidlliquid extractor, followed by concentration over a steam bath) was very attractive to Aedes taeniorhynchus (not Coquillettidia per turbans as stated in the paper's abstract). The presence of butanone seemed to decrease collections of all species. Laboratory experiments with Aedes aegypti and other species have suggested that mosquitoes might selectively feed on hosts having a rise in temperature due to viral or other parasitic infections (Gillett & Connor, 1976; Mahon & Gibbs, 1982; Turell et al., 1984). In laboratory experiments Day & Edman (1983) re ported that mice were more susceptible to feeding mosquitoes when they were infected with malaria, but in later experiments hypothermia had no significant impact on numbers of mosquitoes feeding on mammals (Day & Edman, 1984a). If infected hosts are more susceptible to biting mosquitoes, and this applies to human malaria, then there could be epidemiological consequences. This topic and other aspects of blood-feeding and host location are reviewed by Edman & Spielman (1988), while Bowen (1991) provides a good review of host finding cues. Arrival at bait Species which normally feed at twilight or during the night will often bite during the day if a suitable host is present. In England species which were essentially crepuscular and nocturnal were caught in large numbers during the day when ever bait catches were performed in sheltered sites, where unfed females were resting among the vegetation (Service, 1969a, 197Ib). In contrast few or no mosquitoes were caught during day-time catches in exposed areas such as in fields or on pathways. At night, however, baits in both sheltered and exposed areas were bitten (Service, 197Ib). It was concluded that although during the day mosquitoes resting among vegetation were not actively orientated to host 352 MOSQUITO ECOLOGY: FIELD SAMPLING METHODS feeding, they would nevertheless readily feed if a host was in the immediate area. At night under the influence of an endogenous biting rhythm adults actively flew in search of blood-meals and were consequently encountered in both sheltered and

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