Mosquito repellence and mosquitocides of B. alata, B. pilosa and C. ambrosioides

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Author: T.C.Kazembe and S.Nkomo

Introduction

Mosquito repellency of plants were well known before the advent of synthetic chemicals (Curtis et al., 1989), but the most commonly used mosquito repellents are synthetic chemicals that mostly contain Deet in their formulations. However, Deet is associated with disadvantages that stem principally from its activity as a solvent of paints, vanishes, and some plastic and synthetic fabrics (Oshaghi, M. A., et al., 2003). Other disadvantages of Deet arise from concerns over the human toxicity reactions after its application and these vary from mild to severe (Tawatsin, A. et al., 2001). The most successfully used mosquitocide is DDT. However, some countries have discontinued its use as a result of environmental concerns. The interest in anti-mosquito products of plant origin is being revived because of the drawbacks associated with the continued application of synthetic compounds, some of which have led to widespread development of insecticide resistance (Choochote, W., et al., 2004). Extracts of several plants have been studied as possible mosquito repellents revealing the existence of natural repellents with good efficacy (Tawatsin, A. et al., 2001). Some people mainly in rural areas of many tropical countries including Zimbabwe burn plant materials using glowing charcoal to produce smoke which repels or kills mosquitoes and flies (Bell et al., 1996; Chadwick, 1975; Curtis, 1999; Moore, 2005; Palsson et al., 1999; Seyoum et al., 1999; Stephen et al., 1995; White, 1973; Ziba et al., 1994). In this study we report on some investigations which we have carried out on the potential of Blumea alata, Bidens pilosa, and Chenopodium ambrosioides as mosquito repellents and mosquitocides. These plants are widely used in Zimbabwe, particularly in the District of Hurungwe in Mashonaland West Province. Blumea alata Blumea alata belongs to the Aristeraceae family. Its leaves may be rubbed on to skin to repel mosquitoes and flies, or they may be dried at room temperature and ground to powder and placed in a loose porous cloth and hung in the house to repel mosquitoes and flies, or the leaf powder may be burnt on hot metal or glowing charcoal to repel or kill mosquitoes and flies. Its essential oils have been reported to exhibit antibacterial and antifungal activities (Kapswara and Gundidza, 1998), the main constituents being terpinen-4-ol (27.6 %), germacrene-D (15.4 %), sabinene (8.0 %) and á-terpine (5.5 %) (Mwangi et al., 1994). Bidens pilosa Bidens pilosa is an annual herb that belongs to the Asteraceae family. Its medicinal uses are quite extensive. Infusions have been used as a tonic and stimulant. Decoctions are drunk for coughs and juice of leaves is dropped into the eye for conjunctivitis. Young shoots are chewed for the relief of rheumatism and strong decoctions of leaves are drunk for relief of inflammation. The plant is used for rheumatism, heartburn, coughs, and fits in children. It is also used to repel pests in stored grain as well as aphids, ants, beetle, cabbage root fly, caterpillars, crickets, mites and termites. The plant is believed to contain relatively high omega-3-fatty acids including á-linotemic acid, eicosapentanoic aid, and docosahexaenoic acid. These are said to elevate high-density lipoprotein, which may be linked to coronary diseases and hypertension treatment (Mohan et al., 2000). Other active constituents include chalcones, aurone glycosides, volatile oils, acylated okanin glycoside, gallic and oxalic acids and phenolic astringents. Some of these, particularly the volatile oils, are considered responsible for the mosquito repellency. Chenopodium ambrosioides Chenopodium ambrosioides (Shona name: Mbanda) is a member of the chenopodiaceae family which originated from Central America. It is an annual herb that grows to a height of 40 cm with toothed, oval leaves that grow to a length of 4 cm and a width of 1 cm. The plant has a very strong odour and produces small green flowers and seeds which are very small, green when fresh, but black when dry. The plant is used to treat worm infections in humans and livestock. In Zimbabwe it is also used to treat convulsions. The constituents of its essential oils include ascaridole (68-80 %), isoascaridole, p-cymene, limonene, and x-terpinene (Beckstrom-Sternberg, 1994). Materials and methods Mosquito repellence a) Plant extracts Fresh leaves of each of B. alata, B. pilosa and C. ambrosioides were collected from Hurungwe District, Mashonaland West Province, Zimbabwe in January, 2007, and dried in the shade at room temperature for approximately 2 weeks, then ground to powder using a mill in the Department of Chemistry, University of Zimbabwe. The powder of each was placed in a 500 cm3 distillation flask and the distillation apparatus set up. The distillation flask was heated to about 120oC over a Bunsen burner collecting the distillates into 50 cm3 conical flasks. Oils were separated from the aqueous phases using separation funnels yielding B. alata (16.0 cm3), B. pilosa (19.0 cm3), and C. ambrosioides (17.5 cm3), which were kept at 40C until shortly before bioassaying when 15 cm3 of each dissolved in ethanol and the volume made up to 50 cm3 in volumetric flasks. b) Repellent bioassay Three different treatment methods (determination of landing time, dose finding experiments and determination of protection time) were used to study the repellent activity of the dry distilled extracts of the three test plants against laboratory-reared Aedes aegypti female mosquitoes after they were applied to the human skin. A. aegypti mosquitoes bite during the day, hence the experiments were carried out between 08.00 h and 16.00 h. c) Preparation of extract mixtures The extract solutions in ethanol were mixed in equal proportions by volume to give composite mixtures which were used to investigate the possibility of synergism in their repellence activities. Thus 7 cm3 solutions of each of B. alata, B. pilosa, and C. ambrosioides were mixed to give 14 cm3 composite mixtures of each of B. alata/B. pilosa (Ba/Bp), B. alata/C. ambrosioides (Ba/Ca), and B. pilosa/C. ambrosioides (Bp/Ca). d) Preparation of mosquitoes Female laboratory-reared Aedes aegypti mosquitoes (40 in number), free from pathogens were placed in a 5-litre capacity cage with a mosquito netting on top and a sleeve on the side that was used to introduce the mosquitoes into the cage. The mosquitoes were fed on 10 % sucrose solution but starved for 24 hours prior to the experiment. e) Repellent test procedure Laboratory repellent tests were conducted using the human-bait technique of the WHO (1996) standard method with modifications by the Blair Research Institute, Ministry of Health and Child Welfare (Lukwa , N., 1999). Three human volunteers were used in the experiments. An area of 5 cm x 5 cm (25 cm2) was marked and cut open on each glove which was used for the experiments. The edges of the cut area were lined with masking tape and 40 randomly selected mosquitoes were placed in the 5-litre cage via the sleeve. f) Determination of landing time Landing time is the average time required for the first mosquito to land on the exposed area and attempt to take a blood meal. An untreated hand was exposed to the mosquitoes to determine the landing time which was then recorded. The procedure was repeated 10 times and the average landing time calculated. Average landing time = (30+25+34+29+33+40+37+26+36+30)1/10 = 32 seconds. g) Exposure time The repellence was monitored at 30-minute intervals, each volunteer putting the test hand in the cage for the first 3 minutes of every half hour (Tawatsin et al., 2001). h) Repellence test procedure The repellence of the volatile oils was evaluated using the human-bait technique (WHO, 1996), the testing period lasting for 6 hours. Aedes aegypti being a day biter, the tests were carried out between 08.00h and 16.00 h, at 25-300C and relative humidity of 60-80 %. Repellence was calculated according to the number of mosquitoes that were prevented from landing compared to the number of mosquitoes that landed on the control (Oshaghi et al., 2003): % Repellence = [(C-T)/C] x 100 Where C is the number of mosquitoes that landed on the control and T is the number of mosquitoes that landed on the treated areas of the subjects. i ). Dose finding experiments A series of repellence experiments was run starting with the application of a 0.5 cm3 dose of the plant extract solution in ethanol (Section a). The 0.5 cm3 dose was applied on to the open area of the hand and the % repellence during the 3-minute exposure time was determined. The dose was increased by 0.5 cm3 for each succeeding experiment, calculating the % repellence after each dose, until a dose that gave 100 % repellence during the 3-minute exposure time was achieved (Figure 1). This dose was the minimum amount of extract that gave complete protection from mosquito landing and was used in all experiments. The results indicated that the minimum dose for B. alata and C. ambrosioides was 2 cm3 and that for B. pilosa was 2.5 cm3. Hence in all experiments involving B. alata and C. ambrosioides, the dose used was 2 cm3 and that for B. pilosa was 2.5 cm3. The procedure was repeated for repellent mixtures, giving 1.5 as the dose required for B. alata/C. ambrosioides, and 2 cm3 for B. alata/B. pilosa and B. pilosa/C. ambrsioides mixtures (Figure 2), and these were used in all experiments involving the respective mixtures.

Figure 1

Figure 2

Mosquitocides

The powdered leaves (Section a) were used in the evaluation of the potential of the three plants as mosquitocides, using 50 individual 5-8 day old laboratory-reared female A. aegypti mosquitoes for each replicate test. Ten were placed in each of the 3 standard plastic cups. Ten of the remaining 20 were used for the control and the other ten for the standard. The plastic cups were covered with mosquito netting on top. The control was treated similarly to the test experiment except that there was no addition of plant powder to the glowing charcoal. (i) Mosquitocides test procedure The inhabitants of Hurungwe District, Mashonaland West Province, Zimbabwe, burn the plant materials on glowing charcoal or on hot metal sheets to produce smoke that will act as mosquitocide. In this project the leaf powders were burnt on glowing charcoal to release the mosquitocides. Three replicate cups were placed around glowing charcoal making sure that the direct heat from the charcoal did not affect them. The leaf powder was placed on the glowing charcoal and the set up was quickly covered using a 1m3 cardboard box for 3 minutes, and recorded the number of knocked down mosquitoes, maintaining the recorded knocked down mosquitoes at room temperature. Cotton wool previously soaked in 10% sucrose was placed on top of the cups which were then put in a box covered with a cloth and left for 24 hours. The number of mosquitoes which died was recorded and the percentage mortality was calculated in both the exposure and control cups. The experimental procedure was repeated with the other replicate experiments changing the position of the cups. (ii) The percentage mortality The percentage mortality was calculated using the expression: % Mortality = [(Number of dead mosquitoes) / (Total number of mosquitoes)] x 100. The percentage number of dead mosquitoes in the control experiment was represented by C and the percentage number of dead mosquitoes in the exposure experiment was represented by E. If the value of C that was calculated was between or equal to 5% and 20% then a corrected value for exposure mortality was calculated using Abbott's formula (Abbott, 1925): Corrected exposure mortality = [(E-C) / (100-C)] x 100. Mortality rates less than 80% after 24hours post-exposure time indicate insect resistance. Rates between 80-90% suggest the possibility of resistance that needs to be clarified and above 98% indicate insect susceptibility (Kamau and Vulule, 2006). (iii) The percentage knockdown The percentage knockdown was calculated using the expression: % Knockdown = [(C-T) / (10-T)] x 100 where C is the number of mosquitoes knocked down in the exposure experiment, T is the total number of mosquitoes knocked down in the control experiment. The percentage knock down was calculated for all the replicates so that the mean % knock down could be determined. Commercial mosquito coils which contain transfluthrin as active ingredient was used as the standard, and the results compared using the t-test. (iv) Dose-finding experiments for mosquitocides A series of knockdown experiments was run starting with the application of a 0.2 g dose of the plant leaf powder onto glowing charcoal (Section i) and exposing 10 mosquitoes in a cup for an exposure time of 3 minutes, the experiment being done in replicates, and the number of knocked down mosquitoes recorded. In the next set of replicate experiments the dose was increased by 0.2 g to 0.4 g and the procedure repeated, recording the number of knocked down mosquitoes. The procedure was repeated, increasing the dose by 0.2 to 0.6 g, then to 0.8 g, and finally to 1,0 g. calculating the % knock down after each dose, until a dose that gave 100 % knockdown after the 3-minute exposure time was achieved (Figure 3). This dose was the minimum amount of powder that gave complete knockdown of mosquitoes and was used in all experiments. The results indicated that the minimum dose for B. alata and C. ambrosioides was 0.8 g and that for B. pilosa was 1.0 g. Hence in all experiments involving B. alata and C. ambrosioides , the dose used was 0.8 g and that for B. pilosa was 1.0 g of powder.

Figure 3

Results and Discussion

Table 1 Percentage mean repellency of plant extracts against Aedes aegypti mosquitoes Time in Hours 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 C. ambrosiodes 100 100 98 100 96 89 88 72 66 58 35 32 18 B. pilosa (Bp) 100 100 94 96 79 62 60 51 39 22 24 18 15 B. alata (Ba) 100 100 100 98 100 92 86 75 69 60 42 28 22 Ca/Ba 100 100 98 100 100 98 94 85 78 72 58 42 34 Ca/Bp 100 100 100 96 100 92 92 78 62 56 44 35 30 Ba/Bp 100 100 100 100 98 94 84 78 69 54 36 30 26 DEET 100 100 100 100 100 100 100 100 100 100 100 100 100 Ethanol 0 0 0 0 0 0 0 0 0 0 0 0 0

The results for the mean protection times in Table 1 indicate that Chenopodium ambrosioides would give 100% protection for 1.5 h, Bidens pilosa for 1.0 h, whilst Blumea alata gives 2.0 h. Whilst 100% protection would be the desirable, a product that gives 70% protection would still be considered useful. In that scenario, C. ambrosioides would be considered an efficient repellent for 3.5 h, B. pilosa for 2.0 h, and B. alata for 3.5 h. The mixture of B. alata- C. ambrosioides gives 100 % protection for 2.0 h, B. alata-B. pilosa for 1.5 h and B. pilosa - C. ambrosioides for 2.0 h. On the basis of a 70% cut-off, B. alata - C. ambrosioides would be an efficient repellent for 4.5 h. Thus B. alata and C. ambrosioides have reinforced one another's efficiency from the 3.5 h for each of the two to the 4.5 for the mixture. There might be a case for the two acting in synergism, not withstanding the unpronounced effect of either extract on the question of the 100 % efficiency where the 100 % efficiency of B. alata is 1.5 h and that for C. ambrosioides is 2.0 h but the mixture maintains the 100 % efficiency at 2.0 h. Thus, the efficiency of the mixture is clearly superior.

In the case of B. alata - B. pilosa mixture, the 100 % efficiency for the mixture is up to 1.5 h whereas that for B. alata is up to 2.0 h and that for B. pilosa is at 0.5 h and the effect of the 70 % efficiency for the individual extracts on that of the mixture do not appear particularly attractive. However, the mixture gives better results than either of the two up to 4.5 h. The two do not appear to favourably reinforce one another. The same argument would appear valid for the B. pilosa - C. ambrosioides mixture. Thus, a clear case for synergism has been established for the B. alata - C. ambrosioides mixture only. In mixtures containing B. pilosa, B. pilosa appears to lower the efficacy of the other extract. This observation would tend to mitigate against the tendency of some traditional healers to mix different herbs in an attempt to foster synergism between the different constituents. The table indicates that C. ambrosioides gave complete repellence up to 1.5 hours, B. alata for 2.0 hours and B. pilosa for 0.5 hour. The mean protection dropped to 88 % for C. ambrosioides, 86 % for B. alata and 51 % for B. pilosa 3.0 hours after application. The differences between C. ambrosioides and B. alata protection times were not significant after 3.5 hours, but the differences between these two and B. pilosa which had dropped to 51 % after 3.5 hours were significant. The mixtures B. alata-C. ambrosioides (Ba/Ca), B. alata-B. pilosa (Ba/Bp), and B. alata-C. ambrosioides (Ba/Ca) provided 95 % protection for the first 1.5 hours, and dropped to below 70 % after 4.5 hours for the Ba/Ca mixture, 3.5 hours for the Bp/Ca and Ba/Bp mixtures. The differences in protection time were not significant within the first 1.5 hours but became significant between 1.5 and 5.0 hours post application, the Ba/Ca mixture providing greater protection than the other two mixtures over the entire period. B. alata, C. ambrosioides, and B. alata / C. ambrosioides mixture provided similar protection up to 1.5 hours post application. The repellence of the mixture was still similar to that of B. alata up to 2.0 hours, but after 2.5 hours the mixture provided significantly higher repellence than the other two repellents. It provided complete protection for 2.5 hours, thus giving higher mean protection time than either of the single repellents: 2.0 hours for B. alata and1.5 hours for C. ambrosioides. There were statistical differences between the protection time of B. alata and that of B. alata / C. ambrosioides mixture at 95 % level (t = 5.9, df = 12) and also with C. ambrosioides (t = 6.85, df = 12). Thus the relationship between the two extracts is probably synergistic. The difference in repellence between the mixture and the individual plant extracts was statistically significant (F test, p = smaller than 0.05). The mean repellence for the mixture fell to below 70 % after 4.5 hours post application, where as B. alata provided 75 % protection for 3.5 hours and C. ambrosoides provided 72 % protection for 3.5 hours. There were no significant differences between the repellencies of B. pilosa, C. ambrosioides and B. pilosa / C. ambrosioides mixture during the first 1.5 hours post application. The repellence of B. pilosa dropped to less than 70 % after 2.0 hours when C. ambrosioides was still giving 96 % protection and the B. pilosa / C. ambrosioides mixture was giving 100 % protection. The repellence of C. ambrosioides only dropped to less than 72 % after 3.5 hours and the B. pilosa / C. ambrosioides mixture was still providing a mean protection of 78 %, though it then fell to below 70 %. The differencies between the two individual repellents and the mixture were significant at 95 % (t = 4.44, df = 12), indicating possible synergism between B. pilosa and C. ambrosioides extracts, though to a lesser extent than that for the B. alata and C. ambrosioides extracts. The differences between B. alata, B. pilosa and B. alata / B. pilosa mixture repellencies was not significant during the first 1.5 hours post application, with the mixture and B. alata giving 100 % repellence each, and B. pilosa giving 96 %. The repellence of B. pilosa dropped to below 70 % after 2.0 hours post application when B. alata was giving 100 % protection and the mixture was giving 98 % protection. The mixture gave mean repellence of 78 % up to 3.5 hours when B. alata gave 75 % up to 3.5 hours. There after, they both fell to below 70 %. The extracts of B. alata and B. pilosa gave different repellence behaviour although the two plants are members of the same Asteraceae family. Plants of the same family may have different repellent properties and variations in plant constituent composition may occur due to geographical locations (Choi et al., 2002 or Tawatsin et al., 2001). Percentage knockdown Table 2 Percentage mean knockdown against time Sample/time in minutes 0 10 20 30 40 50 C ambrosioides 0 20 95 100 100 100 B pilosa 0 25 87.5 100 100 100 B alata 0 32.5 92.5 100 100 100 Control 0 0 0 0 0 0

Table 2 reveals that B. alata, B. pilosa and C. ambrosioides are effective mosquitocides. They all exhibit 100 % knockdown of the test mosquitoes within 30 minutes of application, with mean mortality: B. alata (90 %), B. pilosa (90 %) and C. ambrosioides (92.5 %).

Conclusion Thus the results indicate that plant based mosquito repellents, generally giving an average of 2.5 hours repellence time, are effective for shorter periods compared to synthetic repellents which are known to give up to 8 hours protection. However, the study indicates that the plant based products may be enhanced by using them in the form of mixtures of different plant extracts. Thus, the mixture of B. alata and C. ambrosioides provided 72 % protection for 4.5 hours, compared to the individual plant extracts which gave protection above 70 % for a maximum of 3.5 hours and the differences between mixtures and individual plant extracts were generally significant. The three plants also showed mosquitocidal properties. Their knockdown as well as the mortality rates suggest that they can effectively be used as mosquitocides. The effectiveness of the plant mixtures as mosquitocides will in future be included in the test protocols so as to establish if there is the possibility of synergism or potentiation in their activities. Acknowledgements The authors are grateful to Dr Mutambu, the National Institute of Health Research for permission to use their laboratories during the course of the project and to messers Murahwa, Makuwaza, Lukwa, Chiwade (Technicians), Muchenje for their valuable technical support.

References Abbott, W. S., A method of computing the effectiveness of an insecticide. J. Econ. Entmol. 1925, 18: 265-266. Berkstrom-Sterberg, Stephen, M., Duke, A. J., Wain, K. K., Ethnobotany Database. http://probe.nalusda.gov:8300/cgi-bin/browse/ethnobotdb (ACEDB version 4.3- data version July 2004). Bell, D., Cameron, A.,Fernando, M., Pholsyna, K., Foley, D.,Bakote'e, B., and Bryan, J.H., Malaria in Honiara, Solomon Islands: vector studies, Southeast Asian J. Trop. Med. Public Health, 1996, 27, 372. Chadwick, P. R., The activity of some pyrethroids, DDT and Lindane in smock from coils for biting inhibition, knockdown and kill of mosquitoes (Diptera, Culicidae), Bull Entomol Res, 65. Choochote, W., Tuetun, B., Kanjanapothi, D., Rattanachanpichai, E., Chaithong, U., Chaiwong, P., Jitpakdi, A., Tippawangkosol, P., Riyong, D., and Pitasawat, B., Potential of crude seed extract of Celery, Apium graveolens L., against the mosquito Aedes aegypti (L.) (Diptera: Culicidae), Journal of Vector Ecology 2004, 29 (2): 340-346. Choi, W. S., and Park., B. S., Ku, S. K.,Lee, S. E., Repellent activities of essential oils and monoterpenes against Culex pipiens pallens.J. Am Mosq Control Association, 2002, 18 (4), 348-351. Curtis, C. F., Lines, J. D., Ijumba, J., Callaghan, A., Hill, N., and Karimazad, M. A., The relative efficacy of repellents against mosquito vectors of disease, Med Vet Entomol, 1987, 1, 109. Curtis, C. F., 1996, Introduction 1: An overview of mosquito biology, behaviour and importance in olfaction in mosquito-host interactions. Wiley, Chichester. P54-70. Jacoud, R. J. S., Contribuicao para oestudo farmacognostico do Argeratum conyzoides L., Rev. Bras. Farm., 1961, 42, (11/12): 177-197. Kamau, L. and Vulule, J. M., State of insecticide susceptibility in Anopheles arabiensis from Mwea rise irrigation scheme, Central Kenya, Malaria Journal, BioMed Centre. 2006, 5: 46. Kapswara, M. and Gundidza, M., Extraction, chemical characterization and biological activity of essential oils from Iboza riparia, Becium obavatum and Blumea alata. School of Medicine. Honours Publications. Department of Pharmacy, University of Zimbabwe, 1998. Lukwa, N., Nyazema, N. Z., Curtis, C. F., Mwaiko, G. L. and Chandiwana, S. K., People's perspectives about malaria transmission and control using mosquito repellent plants in a locality in Zimbabwe, Cent Afr J Med, 1999, 45, 64. Menut, C., Sharma, S., and Luthra, C., 1993, Aromatic plants of Tropical Central Africa, Part X-Chemical composition of essential oils of Ageratum houstonianum Mill. And Ageratum conyzoides L. PhD Thesis, UNESP, Jaotcabal. Mohan, L. K., Escott-Stump, S., Krause's Food, Nutrition & Diet Therapy, 10th Edition, W. B. Saunders, 2000. Moore, S. J., A methodology for developing plant-based products for use against Anopheles mosquitoes, London School of Hygiene and Tropical Medicine, University of London, London 2005. Mwangi, J. W., Achola, K. J., Lwande, W., Hassanali, A., Laurent, R., Constituents of the essential oil of Blumea brevipes (oliv. & hiern) willd. Department of Pharmacy, University of Nairobi, Kenya, 1994 Oshagi, M. A., Ghalandari, R., Vatandoost, H., Shayeghi, M., Kamali-nejad, M., Tourabi-Khaledi, H., Abolhassani, M. and Hashemsadeh, M., Repellent effect of extracts and essential oils of Citrus limon (Rutaceae) and Melissaofficinalis(Labatiae) against the main malria vector, Anopheles stephensi (Diptera: Culicidae), Iranian journal of Public Health,2003, 32, 4, Palsson, K. and Jaenson, T. G., Comparison of plant products used as mosquito repellents in Guinea Bissau, West Africa, Acta Trop, 1999, 72, 39. Seyoum, A., Palsson, K., Kung'a, S., Kabiru, E. W., Lwande, W., Killeen, G. F., Hassali, A. and Knols, B. G., Traditional use of mosquito-repellent plants in Western Kenya and their evaluation in semi-field experimental huts against Anopheles gambiae: ethnobotanical studies and application by thermal expulsion and direct burning, Trans R Soc Trop Med Hyg, 2002, 96, 225. Snow, R. W., Craig, M., Deichmann U. and Marsh K., Estimating mortality, morbidity and disability due to malaria among Africa's non-pregnant population. Buletin of the World Health Organization, 1999, 77: 624-640. Stephens, C., Masamu, E. T., Kiama, A. J., Kinenekejo, M., Ichimori, K., and Lines, J., Knowledge of mosquitoes in relation to the public and domestic control activities in the cities of Dar es Salaam and Tanga, Bull WHO, 1, 73, 97. Tawatsin, A., Wratten, S. D., Scott, R., Thavara, U. and Techadamarongsin, Y., Repellency of volatile oils from plants against three mosquito vectors, Journal of Vector Ecology, Vol 26, No 1: 76-82, 2001. White, G. B., The insect repellent value of Ocimum spp. (Labiatae): traditional anti- mosquito plants, East African Med J, 1973, 50, 248. Ziba, C., Slutsker, L., Chitsulo, L. and Steketee, R. W., Use of malaria prevention measures in Malawian households, Trop Med Parasitol, 1994, 45, 70.