Smoke is the most widely used means of repelling mosquitoes utilised in the rural Tropics. Waste plant materials are frequently burned in Sri Lanka as a mosquito repellent, even though indoor residual spraying has been carried out by the government for many years (Silva, 1991). In rural Guinea-Bissau, 86% of residents used an unimpregnated bed net in conjunction with mosquito coils or plant-based smoke (Palsson and Jaenson, 1999a). In the Solomon Islands, a recent survey revealed that fire with coconut husks and papaya leaves was the most prevalent form of personal protection from mosquitoes, being used by 52% of residents (Dulhunty et al., 2000). Tests in Papua New Guinea found that the traditional practice of burning mango wood (Magifera spp.), coconut husks (Cocos nutifera), wild ginger leaves (Alpinia spp.), and betelnut leaves (Areca catechu) repelled mosquitoes (Vernede et al., 1994).
Ongore et al. (1989) surveyed a population in Kenya and discovered that 16% burned Lantana rhodesciense leaves and 16% burned various waste plant materials, including sisal leaves and rice husks. In Sierra Leone and Ghana people burn orange peels to drive out mosquitoes, while in Ghana and the Gambia the leaves of neem (Azadirachta indica) and the baobab tree (Adansonia digitata)
are burned (Aikins et al., 1994). Members of the Lamiaceae (Section 22.3.1) and Lippia spp. (Section 22.3.4) are frequently burned as protection from mosquitoes throughout Africa.
There have been few studies to measure whether the use of smoke actually prevents malaria. Snow et al. (1987) showed that churai (Daniellia oliveri) smoke used as a repellent in the Gambia did not significantly reduce the incidence of malaria in children (see Section 22.3.2). However, a study in Sri Lanka found that the use of traditional fumigants did protect against malaria (van der Hoek et al., 1998). The difference in the results of the two studies may be related to erratic use of churai in the Gambia. It is also very likely that the massive entomological inoculation rate (EIR) in the Gambia requires more effective means of personal protection from infected bites than offered by the churai. In Sri Lanka the lower EIR and less anthropophilic vector mosquito An. culicifaces may mean that burning local plant repellents is a viable means of malaria prevention. In Sri Lanka, Holy Basil (Ocimum sanctum), tulsi (Sanskrit name), is commonly burned, as are the seed husks from oil extraction from neem (A. indica) and the butternut tree (Madhuca longifolia) (Silva, 1991). These seeds contain insecticidal and repellent agents.
Burning wood and adding repellent plants to it probably works in several ways. First, the smoke may disguise human kairomones and disrupt convention currents essential in mosquito host location. Second, burning may release repellent or irritant molecules; the molecules released by the plants also may be insecticidal, e.g., pyrethrum found in Chrysanthemum spp. The dried flowers are commonly made into joss sticks for use against mosquitoes and are extremely effective, providing up to 86% reduction in biting when used over an 8-hour period (Charlwood and Jolley, 1984).
Plants from the Basil family are used commonly in East and West Africa as mosquito repellents (Dalziel, 1937; Kokwaro, 1976). Many species of the Lamiaceae are strongly aromatic and toxic to insects, such as Ocimum spp. and Hyptis spp. (cited in Palsson and Jaenson, 1999a).
The essential oils from the species of this genus contain linalool, linalol, linoleic acid, p-cymene, estragole, eucalyptol, eugenol, citral, thujone, ocimene, camphor, methyl chavicol, oleic acid, and many other terpenes, all of which are effective repellents. It grows rapidly under a range of climatic conditions, although it is best adapted to a drier climate. The essential oil of Ocimum basilicum (see Figure 22.3) is larvicidal, producing 100% mortality of Culex pipiens fatigans at 0.12% concentration (Chavan and Nikam, 1982).
In Tanzania, traditionally fresh Ocimum spp., kivumbasi (local name), are burned, and freshly cut twigs of O. suave and O. canum are placed in the corners of rooms to prevent mosquitoes from entering (Stephens et al., 1995; White, 1973). The latter method was field-tested in Guinea-Bissau, West Africa, by Palsson and Jaenson (1999b), who showed that fresh O. canum (also known as O. americanum) provided 63.6% protection from mosquito biting for 2 hours.
In Zimbabwe, Ocimum spp. leaves are rubbed on the skin as a method of repelling mosquitoes (Lukwa, 1994). When the juices from the leaves of O. suave and O. canum were spread on the legs of human volunteers, there was approximately 50% reduction in the proportion of female An. gambiae mosquitoes that were engorged with blood (White, 1973). A 250 mg/ml concentration of dried O. canum leaves in ethanol provided 70% repellency against Ae. aegypti for 2 hours (Lukwa et al., 1996). In Thailand, a 25% concentration of O. canum essential oil in ethanol was tested on three mosquito species. This formulation provided 3, 4, and 8 hours protection from the bites of Ae. aegypti, An. dirus, and Cx. quinquefasciatus, respectively (Tawatsin et al., 2001). Interestingly, when mixed with 5% vanillin, the protection times increased greatly for each mosquito species since it reduces the evaporation rates of repellents (Spencer, 1974).
In the Brazilian Amazon Hyptis sp., locally called Hortela-do-campo, is traditionally burnt and leaves are rubbed on the skin in order to keep mosquitoes away (Sears, 1996). Here, its repellent activity is associated with its strong smell. In West Africa the fresh plant is sometimes used or the aerial parts of the H. suveolens are placed on charcoal and the resulting smoke repels the mosquitoes (Palsson and Jaenson, 1999a), although thermal expulsion of the plant volatiles actually attracted mosquitoes (Seyoum et al., 2002b). In Tanzania, freshly picked and bruised sprigs of H. suaveolens, hangazimu, are hung in the house to try to prevent mosquitoes from entering (Curtis et al., 1991). However, fresh H. suaveolens did not cause reduction in biting when hung in an experimental hut (Curtis and Lines, 1986, unpublished). In comparison, when tested in Guinea-Bissau, the fresh plant was able to provide approximately 70% protection from biting for 2 hours (Palsson and Jaenson, 1999b). The smoldering plant provides the most effective protection. Nicholson and Lines (1987, unpublished) showed that there was a 10-fold reduction in biting in the presence of hangazimu smoke. Similarly, Palsson and Jaenson (1999b) showed that smoldering H. suaveolens provided approximately 84% protection for 2 hours against An. gambiae, whereas Seyoum et al. (2002b) found only a 20.8% reduction in biting.
There are few published accounts of these species of plants being used traditionally as personal protection against mosquitoes. In the Brazilian Amazon, the leaves are either rubbed on the skin or burnt to produce smoke (Sears, 1996). Barnard (1999) tested several concentrations of the Mentha piperita essential oil against Ae. aegypti and determined that with 100% concentration the protection time was 45 minutes; this was reduced to 30 minutes when the concentration was 25%. Field tests of M. piperata in India against An. culicifaces, Anopheles annularis, and Anopheles subpictus provided 85% protection over 11 hours (Ansari et al., 2000). However, these results are inflated as insect collectors not wearing repellent collected the mosquitoes from baits wearing repellent. It therefore appears that this is not a very effective repellent. Evaporating the essential oil of M. piperita at room temperature caused knockdown of several mosquito species (ICMR, 2000). A related plant, Mentha arvensis (Japanese mint), has extremely high vapour toxicity to insects (see Figure 22.4). It is widely grown throughout the Tropics for its essential oil. The leaves yield up to 80% menthol, which is the insecticidal ingredient contained in this species. Mosquito repellent chemicals are also contained in the plant, including menthone, limonene, beta pinene, alpha pinene, and linalool (Lee et al., 2001). Fumigants evaporated using heat or mosquito coils containing the plant may repel and kill mosquitoes. Evaporating the pure essential oil with a kerosine lamp caused a 51.94% reduction in mosquito biting in field trials (Moore, in preparation). The essential oil also has larvicidal effects on mosquitoes, its LD50 being 83.8 ppm against An. stephensi (Kumar and Dutta, 1987). The plant is able to reproduce asexually via a rhizome, making propagation easy and rapid.
Thyme oil at 100% is repellent against Aedes quadrimaculatus, An. albimanus, and Ae. aegypti for at least 30 minutes, when applied to cloth (USDA, 1947-1964). Most recently, varying concentrations of the essential oil of red thyme were tested in the laboratory against Ae. aegypti and An.
albimanus (Barnard, 1999). At 100% concentration it provides 135 and 105 minutes protection against Ae. aegypti and An. albimanus, respectively, and at 25% concentration, the protection time was 45 minutes for both species of mosquitoes. It was also determined that mixtures of essential oils were, in fact, no more effective than the essential oils alone. In the former USSR, a local method against biting insects was tying thyme stick, Thymus serpyllum, with thick cotton, drying these, and then burning them. Rubtzov tested this method and reported 85 to 90% protection for 60 to 90 minutes in the open air (Rubtzov, 1946, cited in Curtis et al., 1991).
22.3.2 Daniellia oliveri (Fabaceae)
The local names churai, santango, and santao refer to resins and wood commonly burnt indoors in western Africa to prevent mosquitoes from entering at night (Palsson and Jaenson, 1999a; Lindsay and Janneh, 1989; Bockarie et al., 1994). In several field trials, it was determined to be an effective, accepted, and cheap form of personal protection. In Guinea-Bissau, smoke from the burning bark of D. oliveri reduced biting from mosquitoes by 74.7 and 77.9% in comparison to the control in two separate field experiments (Palsson and Jaenson, 1999b). In Banjul, Gambia, santango reduced biting on human subjects by 77%, which was more than a permethrin mosquito coil, but less than deet soap (Lindsay and Janneh, 1989).
It is essential, however, to determine whether the product can reduce malaria as well as mosquito biting. Bockarie et al. (1994) performed entomological studies comparing households where wood had been burnt and where wood was not burnt. There was a significantly higher number of female An. gambiae in exit traps of houses where wood had been burnt. This showed that the environment created indoors while burning wood was not favorable for resting mosquitoes. However, the proportion of blood-fed mosquitoes was not greatly different between the two types of households, which indicated that they were still successful in blood feeding. Laboratory experiments by Curtis and Hill (1986, unpublished) showed that burning churai does not inhibit feeding success of mosquitoes. Churai is therefore reducing the apparent nuisance of mosquitoes, but not necessarily reducing the biting index sufficiently. This finding is substantiated by evidence from the Gambia, where it was determined that the burning of D. oliveri did not have any impact on the incidence of malaria in children (Snow et al., 1987).
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