Ultra-low-volume ground aerosols of technical malathion for the control of Aedes aegypti L

Bull. Org. mond. Sante 11971, 45, 805-817 Bull. Wld Hlth Org. | Ultra-Low-Volume Ground Aerosols of Technical Malathion for the Control of Aedes aegypti L. C. P. PANT,1 G. A. MOUNT,2 SUJARTI JATANASEN 3 & H. L. MATHIS 4 The efficacy ofmalathion aerosols appliedfrom the ground was evaluatedfor the control of Ae. aegypti. Several techniques for applying the aerosols to an inhabited locality were investigated, and it was found that excellent control of adult mosquitos could be obtained with a dosage of 438 ml/ha. Two treatments carried out 3 days apart enabled the adult mosquito population to be reduced by 99o%, and it took about 2 weeks to regain its pre- treatment level. Schedules of treatment and strategies to be applied during an epidemic are discussed. In 1966, the World Health Organization estab- lished a research unit in Bangkok, Thailand, to study Aedes aegypti L.-the vector of dengue haemorrhagic fever-and to develop suitable methods for its control. Two current techniques are larvicid- ing with 1% Abate 5 sand granules 6 and ultra-low- volume (ULV) aerial spraying of technical malathion (Kilpatrick et al., 1970; Lofgren et al., 1970). Both these methods could be very useful in preventing or halting epidemics of haemorrhagic fever were it not for their limitations in some situations. Abate larviciding has the disadvantage of not killing in- fected adult female Ae. aegypti, so its use does not allow the transmission of infection to be interrupted immediately. Also, as has been demonstrated,7 it is too time-consuming for emergency use. ULV aerial spraying may be considered too costly, especially if 1 Project Leader, WHO Aedes Research Unit, Bangkok, Thailand. 2 WHO Consultant, Research Entomologist, US Depart- ment of Agriculture, Gainesville, Fla., USA. 3Director of Epidemiology, Communicable Diseases (Control) Division, Ministry of Public Health, Bangkok, Thailand. 4 Entomologist, WHO Aedes Research Unit, Bangkok, Thailand. 5 O,O,O',O'-tetramethyl O,O'-thiodi-p-phenylene phosphoro- thioate. 6 Bang, Y. H., Tonn, R. J. & Sujarti Jatanasen (1969) Unpublished document WHO/VBC/69.164; Bang, Y. H. & Pant, C. P. (1970) Unpublished document WHO/VBC/71. 253. 7 Bang, Y. H. & Pant, C. P. (1970) Unpublished document WHO/VBC/71.253. a more economical method of killing adult mosquitos is available. The scarcity of aircraft, pilots, and spray systems also limits the use of the technique. For this reason, WHO decided to investigate the ULV appli- cation of malathion aerosols from the ground as an alternative method of vector control. The objectives of the present study were to evaluate this method in the field conditions peculiar to South-East Asia, and determine the optimum dose of malathion for the satisfactory control of Ae. aegypti. MATERIAL AND METHODS AerosoI apparatus A Leco ULV cold aerosol generator (Mount et al., 1970) 8 was used to atomize technical malathion (960%). Mount et al. (1968) and Mount (1970) reported that ULV cold aerosols were more effective against adult mosquitos than high-volume thermal aerosols. Other advantages of ULV aerosols are that they do not require solvents, carriers, and additives, thus obviating the need for formulating facilities, and that they do not produce-as thermal aerosols do-dense fogs that constitute a traffic hazard. Lofgren (1970) has critically reviewed and summarized the literature on ULV application of insecticides. So far, the Leco ULV cold aerosol generator has 8 Manufactured by Lowndes Engineering Company, Valdosta, Ga., USA. 2772 - 805 806 C. P. PANT AND OTHERS been tested mostly in open fields. In order to develop techniques of application in an inhabited area and to investigate the efficacy of this technique against Ae. aegypti, which rests primarily indoors, it was considered desirable to conduct tests in progres- sively larger areas. Tests were first made in two small suburbs of Bangkok; then a large suburb- Huay Kwang-was treated several times; and finally the whole town of Sri Racha was treated three times. Test areas Before beginning the trials in Huay Kwang and Sri Racha, members of the Aedes Research Unit familiarized themselves with the spraying techniques, methods of calibrating vehicle speed, dosages of insecticide, discharge rates, etc., by trying them out in two small suburbs of Bangkok-Don Muang and Ban Gruoy-each with an area of about 2 ha and a well planned road system. Huay Kwang. This is a government housing area of 29 ha in the north-east sector of Bangkok, with abouLt 15 000 inhabitants living in some 1 700 houses. The houses usually have two storeys, are made of wood, and consist of 2-4 subunits. They have many windows and their main entrances face the road. Most of the roads, which have houses on both sides, run from east to west; two of them run from north to south and the area in the central section bordered by these roads is approachable by vehicles from both directions. However, the two peripheral sections to the east and west are unapproachable from their eastern and western boundaries, which made it necessary, for spraying purposes, to back the vehicle into the roads in those areas. Thus, with the excep- tion of the peripheral sections, Huay Kwang is an ideal area for treatment by motorized ground equipment. Makkasan, a similar railway housing locality approximately 5 km south of Huay Kwang, was used as a control (check) area. Sri Racha is a coastal town about 100 km south- east of Bangkok on the Gulf of Siam, and lhas some 1 700 houses and a total population of about 16 000. Spread over an area of 142 ha, it is a typical town in Thailand, with a heavily populated central section containing a market and shops and surrounded by low- and high-income housing units. The periphery is less dense in population and buildings. Roads run in all directions and the network, which is not as extensive and uniform as in Huay Kwang, is per- haps more representative of the road networks in this region, where epidemics of dengue haemorrhagic fever may occur. Most of the houses face roads that can be approached by motor vehicles. Approximately 1.5 km east of Sri Racha is a small suburb with about 200 houses. As this is typical of other sectors of Sri Racha, it was used as a control area. Both Sri Racha and this control area showed a high density of Ae. aegypti. Visual larval surveys were carried out in the treated area before and after the insecticide applications. Aerosol treatmenits The techniques of application were perfected during the earlier small-scale tests in Don Muang and Ban Gruoy. It was found that the best treatment period was the early hours of the morning and that, in order to deliver about 438 ml/ha in an inhabited locality, the vehicle should travel at about 5 km/hour and the flow-meter should be set at the maximum discharge rate. The effective use of this machine depends largely on the road network. Aerosols can move considerable distances down-wind, provided that the wind speed is low, so houses away from the main road can be treated by discharging aerosol towards them from a suitable point. In Huay Kwang, five treatments were carried out. During the first four, the nozzle of the aerosol generator was pointed to the rear of the vehicle; dur- ing the fifth treatment, it was pointed towards the left of the vehicle so that the aerosol could be directed towards the houses on the near side of the road. 1 Thus, two journeys were needed for the houses on both sides of the road to be treated. The aerosol generator was mounted on a wooden platform built to fit on to a Landrover truck. The flow-meter was set at the maximum discharge rate for all treatments except the first. The total amount of insecticide used was calculated by weighing the insecticide container before and after use. All the treatments were carried out early in the morning. Table 1 gives details of the five treatments applied in Huay Kwang. By the time the treatments in Sri Racha had been carried out, the field workers were already well versed in the techniques of application, and the nozzle was always directed from the left side of the vehicle. In order to treat some houses that were not im- mediately approachable by road, the vehicle was placed at a point of vantage according to the direc- tion of the wind, and the insecticide was discharged I In Thailand, traffic keeps to the left. MALATHION AEROSOLS TO CONTROL AEDES AEGYPTI 807 Table 1. Aerosol treatments applied in Huay Kwang Treatment No. and date of application Data 1 2 3 4 5 a 6/9/70 20/9/70 11/10/70 24/10/70 8/11/70 Discharge rate (ml/min) 91 136 149 130 105 Calculated dosage obtained (mI/ha) 154 468 512 344 475 Total insecticide used (kg) 5.7 { 16.4 13.5 9.0 6.1 a Evaluated in the central sector only. The peripheral sector also was treated, but the quantity of insecticide used there was not taken into account in calculating the total quantity used. Table 2. Aerosol treatments applied in Sri Racha Treatment No. and date of application Data 1 2 3 18/11/70 24/11/70 27/11/70 Discharge rate (ml/min) 115.7 125 130 Average dosage obtained (ml/ha) It 285 329 344 Total insecticide used (kg) 44.5 51.2 53.0 a Since the road network was denser in the heavily populated centre of the town, the dosage used there must have been close to 438 ml/ha. towards the houses from that point. Table 2 gives details of the three treatments carried out in Sri Racha. Evalutation Ae. aegypti landing rates. Fixed catching stations were located in the treated and control areas as follows: Huay Kwang: 56 stations except for the last treat- ment, when 40 stations were used. Control area for Huay Kwang (Makkasan): 18 stations. Sri Racha: 64 stations. Control area for Sri Racha: 24 stations. A team of 6-10 mosquito scouts collected all the mosquitos that landed on their legs between 08.00 hours anid 10.40 hours. Each scout spent 20 minutes per station and worked at 6-8 stations each morning. The mosquitos were brought for identification and dissection to the laboratories, where their species, sex, and abdominal condition were determined. The females were dissected in order to find out what pro- portion of them were parous, using the techniques developed by Detinova (1962). The density of the mosquito population was observed several days before the treatment and at intervals of 1-5 days afterwards until it was well on the way to recovering its pretreatment level. Ovitraps. CDC-type I ovitraps, consisting of 1/2-litre jars painted black and containing hardboard paddles for oviposition, were kept, one inside and one outside each station, for 48 hours. Then the paddles were removed and brought back to the laboratory. Ovitraps were not placed in the Sri Racha control area as high mosquito densities pre- vailed there throughout the trial period. Bioassays. Bioassays were carried out during some treatments by placing 15 laboratory-fed Culex fati- gans adults, 3-4 days old, in wire-frame cages (10 x 6 x 6 cm) covered with mosquito netting. Before the treatments were begun these cages were placed in 15-25 houses in different parts of the trial area. The cages were kept near the walls but not oppo- site the main entrances of the houses, and were removed 1 hour after the treatment. The mosquitos were then transfered to paper cups for 24 hours, to recover. The mortality, immediate and after 24 hours, was recorded. Six similar cages kept in the untreated area served as a control. Droplet densities. Droplet densities were deter- mined by placing 2 Kromekote cards 2 (10.5 x 14 cm) vertically and horizontally inside rooms in the Huay 1 Center for Disease Control, US Department of Health, Education, and Welfare. ' Manufactured by Champion Paper and Fiber Co., Hamilton, Ohio, USA. 808 C. P. PANT AND OTHERS Kwang area. In Sri Racha, 2 cards per room were placed, horizontally. In Huay Kwang, 2 cards were placed in each of 9 upstairs and 9 downstairs rooms on both the north and south sides of the roads (total: 72 cards). The same procedure was adopted for Sri Racha, except that 6 upstairs and 6 downstairs rooms north, south, east, and west of the main roads were used (96 cards in all). The cards were kept on or near the side walls of the rooms-that is, not facing the main entrances-so as to avoid direct exposure to the incoming insecticide aerosol. A dye with Rhodamine B base1 was added to the technical malathion at the rate of 1 g/litre. The cards were brought back to the laboratory and two one-inch square (6.45 cm2) samples were selected at random from each card and examined under a binocular stereoscope. All the droplets impinged on these samples were counted. Following objections raised by some inhabitants of the treated houses, dye was not used during the last two treatments in Sri Racha. Droplet size. About 650 droplets selected at random on diagonal lines drawn on the cards were measured with a calibrated eye-piece micrometer. The measurements of the spots were corrected by applying a spread factor 2 and the resulting data were used to prepare frequency distribution tables with 5-,um intervals; median and mass median diame- ters were then calculated. Estimates of droplet size were also made by impinging the aerosol droplets on silicone-treated 3 glass microscope slides. Samples of over 200 droplets collected outside and inside treated houses were measured with an ocular micrometer (magnifica- tion: 280 x). The diameters of the droplets were estimated by correcting for a spread factor of 0.4. The mass median diameters were computed accord- ing to the methods of Yeomans (1949) for estimating the size of impinged droplets. Fly counts. The total number of flies alighting on a grid placed in the market area of Sri Racha were counted during 15-minute periods before and after treatment, to study the effect of the treatment on flies. I Tetraethyl rhodamine produced by Fluka AG, Buchs SG, Switzerland. ' Cynamid International (1967) Malathion LV concentrate ground application with knapsack mistblowers (unpublished Technical Information document). 3 Dri-Film SC-87, produced by General Electric Co., 49-10 Northern Boulevard, Long Island City, New York 11101, USA. RESULTS Effect on mosquito densities of ULV malathion aerosols appliedfrom the ground On the basis of the Ae. aegypti landing rates in the areas studied, Table 3 shows the percentage reduc- tions in mosquito densities observed on the 14 days after the treatments. In Huay Kwang, where the treatments were carried out at intervals ranging from 13 to 21 days, the mosquito densities were allowed to return almost to their pretreatment levels, the object being to develop optimum dosages and tech- niques that could be applied during the large-scale experiment in Sri Racha. It should be recalled here that the first four treatments at Huay Kwang were carried out with the nozzle of the aerosol generator pointing towards the rear of the vehicle. Thus the aerosol was produced on the road, afterwards mov- ing towards the houses with the wind. The wind varied in velocity and direction from one treatment morning to the next, although generally the wind was westerly and of low speed. Treatments 1 and 2 both produced a reduction of about 50% in landing rates, although the latter was carried out with a much higher dosage than the former. This was perhaps due to a road accident, which delayed treatment 2 until an upward and erratic movement of the wind, peculiar to that region towards noon, had started. This may have caused some of the aerosol to drift upwards and to dissipate too fast. Treatments 3 and 4, performed at the target dosage rate of 438 ml/ha, reduced Ae. aegypti landing rates by 69.2% and 75.8%, respectively. The calculated dosages were 512 and 344 ml/ha, respectively. For treatment 5 in Huay Kwang, the technique was modified, the aerosol generator being rotated 900 to the right so that its nozzle pointed to the left, and the insecticide was discharged towards the houses on the near side of the road. The target dosage for this treatment also was 438 ml/ha. As can be seen from Table 3, the modification of the technique resulted in a spectacular improvement, a 94.5% reduction in the landing rate being obtained 1 day after treatment. Based on the experience obtained in Huay Kwang, the whole town of Sri Racha was treated on 18, 24, and 27 November 1970 at the target dosage rate of 438 ml/ha. Owing to differences in the density of the road network between the small, heavily populated town centre and the larger, less populated peripheral area, the calculated average dosage rate achieved was lower than 438 ml/ha. However, the actual dosage MALATHION AEROSOLS TO CONTROL AEDES AEGYPTI Table 3. Reduction in landings of adult female Ae. aegypti on human bait a Locality TreatmentNo. 1 R b 51.6 2 R 50.5 Huay Kwang 3 R 69.2 4 R - 5Lc 94.5 Sri Racha 1 L 2 L 3 L 82.2 92.9 98.8 % reduction 1-14 days after treatment 5 6 7 8 9 10 11 a Percentage of average of 2 landing catches before the first treatment. b R = Nozzle directed towards the rear of the vehicle. c L = Nozzle directed towards the left of the vehicle. Table 4. Results of dissecting female Aedes aegypti before and after treatments at Huay Kwang and Sri Racha Findings Gut condition (%) u nfed fed semigravid gravid Ovarian stage b (%) 2 3 4 5 Proportion of parous females (%) No. of mosquitoes dissected H Control > Before contr treat- ment 1 41.0 24.2 33.3 14.9 18.6 15.2 21.5 30.1 33.3 22.6 27.0 18.2 _ ) ',A 19.4 21.5 13.1 9.4 37.3 244 c 19.3 30.1 15.0 12.0 41.6 120 d 9.1 33.3 15.2 3.0 25.0 33 luay Kwang No. of days after treatment 2 3 4 33.3 21.2 21.2 24.2 48.5 6.1 21.2 12.1 12.1 31.8 33 46.4 9.1 27.3 18.2 36.4 18.2 27.3 0 18.2 44.4 22 42.9 14.3 21.4 21.4 57.1 0 21.4 7.1 14.3 25.0 14 Control area 5 2/5 ' 0 3/5 0 2/5 0 3/5 0 0 2/5 5 33.4 25.0 26.0 15.6 29.9 28.5 26.0 8.9 6.7 52.0 I191 e Sri Racha Before No. of days aftertreatment ment 1 2 3 22.5 13.5 36.0 27.9 13.5 22.5 36.0 17.1 10.8 57.6 111 d 53.3 20.0 16.7 10.0 50.0 23.3 16.7 3.3 6.7 17.9 30 75.0 1/2 ' 8.3 1/2 8.3 0 8.3 0 75.0 2/2 8.3 0 8.3 0 0 0 8.3 0 18.2 0 12 2 a Percentages were not calculated where fewer than 10 mosquitos were dissected. b After Christophers' classification, modified by Puri (1 957). c Average of 5 catches. df Average of 2 catches. e Average of 4 catches. 809 1 C. P. PANT AND OTHERS achieved in the town centre must have been close to that level. Table 3 shows the results of the three treatments at Sri Racha, which resulted in reductions in the landing rates of female Ae. aegypti of 82.2 %, 92.9%, and 98.8 %, respectively. By the time treatment 2 was carried out-6 days after treatment 1-the landing rates had recovered by 86%. The interval between treatments 2 and 3 was only 3 days and, judging by their combined effects, it was these two treatments that produced the best results of the whole series. One day after the third treatment at Sri Racha, the landing rate of female mosquitos was only 0.1 per man-hour, and after 3 days it was still less than 1. Five days after treatment, the rate was 3.7 per man-hour, and thereafter there was a pro- gressive increase until, 14 days after treatment, it had returned almost to the pretreatment level. Throughout this observation period, the landing rates in both control areas showed no drastic reduc- tion. Thus the reduction in the density of Ae. aegypti in the treated area can only be ascribed to the malathion aerosol. Effect of aerosol treatment on the proportions of mosquitos found to be unfed, in ovarian stage 1, or parous Table 4 shows the effects of treatment on the proportions of mosquitos found to be unfed, in the ovarian stage 1, or parous. As can be expected with an insecticide treatment producing a high mortality in adult mosquitos, the parous rate dropped markedly in Huay Kwang except after the third treatment. Particularly impressive are the figures for Sri Racha, where this rate was reduced by 70-100% compared with the rate observed before treatment. As expected, there was a noticeable increase, imme- diately after treatment, in the proportion ofmosquitos with an empty gut or ovaries at stage l. Ov'itraps Table 5 shows the data on ovitraps found to con- tain eggs in the treated and control areas. It will be seen that, in the Makkasan control area, the per- centage of such traps and the mosquito densities stayed at a high level throughout the period of observation. Mosquito densities in the Sri Racha control area remained high throughout the period of the trial. Therefore, ovitraps were not set up there. However, it is reasonable to assume that the same high rates for traps with eggs would have been maintained here also. All the treatments reduced the percentage of such traps, and here again treatment 5 in Huay Kwang and all the treatments at Sri Racha gave spectacular results. On the third day after treatment 5, in Huay Kwang, none of the out- door ovitraps contained eggs and only 2.5% of the indoor traps did. In the control areas, nearly 50% of both outdoor and indoor ovitraps contained eggs. At Sri Racha, no ovitraps with eggs were recorded until 5 days after treatment 3. Even 12 days after treatment, the percentage of such traps was extremely low, again confirming the high mortality that the last two treatments produced in mosquitos. Bioassays Table 6 shows the results of bioassays carried out in the two areas. High mortalities (74.3-100%) were consistently obtained in Huay Kwang, where the road network was extensive and uniform. In Sri Racha, the bioassay mortalities (48.9-98.9o%) were lower, perhaps because the more widely spaced and irregular road network made it difficult to disperse the aerosol as thoroughly. It will be seen that the average dosage achieved in Sri Racha was lower than that obtained in Huay Kwang. In Sri Racha the control mortalities after a 24-hour holding period were 27.8 % and 22.7 %, as the mosquitos had to be carried long distances. However, the immediate mortalities were nil, so both observations have been reported. Bioassay mortalities in upstairs and downstairs rooms were not significantly different; apparently the aerosol is able to work its way throughout a house. Droplet densities Table 7 shows the wide range of droplet densities measured on Kromekote cards for the five treatments in Huay Kwang and the first treatment in Sri Racha. For the same treatment, the density was very high in some houses and very low or zero in others, although the bioassay mortalities were high even in the latter. In Huay Kwang, droplet densities on the cards placed horizontally were much higher than on those placed vertically, and generally cards placed down- stairs received more droplets than those upstairs. Since the droplets ultimately would move down- wards, this finding is not surprising. However, it is apparent that the aerosol moved into the houses and penetrated all the rooms, although the distribution may not have been completely uniform throughout the area. 810 811MALATHION AEROSOLS TO CONTROL AEDES AEGYPTI Table 5. Percentage of ovitraps found to contain eggs, in Huay Kwang, Sri Racha, and Makkasan Huay Kwang Date No. of days Indoors Outdoors ,after treatment ndoors OutdoorsI 6.8 20.8 6.9 48.0 51.0 44.0 45.0 (Treatment No. 1 ) 7.9 1 15.0 26.0 14.9 8 4.4 4.3 20.9 (Treatment No. 2) 21.9 1 2.1 0.0 28.9 8 8.5 4.2 1 1.10 (Treatment No. 3) 12.10 1 9.5 2.3 16.10 5 26.7 17.8 19.10 8 36.4 36.4 24.10 (Treatment No. 4) 26.10 2 20.9 14.6 28.10 4 2.5 8.1 30.10 6 18.6 27.3 2.11 9 26.2 34.1 8.11 (Treatment No. 5) 9.11 11.11 10.5 2.6 1 3 2.6 0.0 Makkasan (control area) Date Indoors 21.8 32.0 8.9 47.0 19.9 58.0 29.9 22.0 13.10 25.0 27.10 58.8 5.11 I 47.1 10.11 47.1 Sri Racha 23.9 1 8.7 84.2 17.11 (Treatment No. 1) 18.11 1 9.7 6.7 24.11 (Treatment No. 2) 25.11 1 0.0 3.1 27.11 (Treatment No. 3) 28.11 1 0.0 3.7 30.11 3 0.0 0.0 2.12 5 0.0 0.0 9.12 12 6.9 13.8 Note: As very high mosquito den- sities prevailed throughout the study period in the control area of Sri Racha, ovitraps were not kept there. Outdoors 58.0 39.0 33.0 17.0 11.8 50.0 44.4 22.2 812 C. P. PANT AND OTHERS Table 6. Bioassay mortalities (%) in Huay Kwang and Sri Racha Huay Kwanga Treatment North SouthN.. ControlO. Upstairs | Downstairs Upstairs Downstairs 2 80.0 (105) b 79.0 (105) 81.9 (105) 74.3 (105) 0.0 (90) 3 100.0 (210) 100.0 (210) 100.0 (210) 88.1 (210) 0.0 (90) 5 90.0 (180) 78.8 (165) 79.4 (165) 93.9 (165) 0.0 (90) Sri Racha North South East West Treatment Mrai otoNo. Mortality Down _ Dows Down- Down- Con r l Upstairs stairs Upstairs stairs Upstairs stairs Upstairs stairs Immediate 95.6 87.8 84.0 98.9 85.6 68.9 87.8 96.7 0.0 After 24 hours 98.9 97.8 96.0 98.9 96.7 94.4 93.3 100.0 27.8(90go) (75) (90) (90) (90) (90) (90) (90) Immediate 53.3 48.9 82.2 65.6 92.2 85.6 84.4 82.2 0.0 2 After 24 hours 77.2 76.7 i 85.6 87.7 98.9 97.8 100.0 94.4 22.7(90) (90) (90) (90) (90) (90) (90) (90) (75) a Only mortalities after 24 hours are shown for Huay Kwang. b The figures in parenthesis represent the number of mosquitoes tested. Droplet size Fig. 1 shows the frequency distribution of droplet sizes, arranged in 5 ,tm intervals. The median value was 21 ,um and the mass median diameter was 26 ,.m, calculated according to the method of Rathburn (1970). The droplets collected on glass slides ranged in size from 1 to 50 ,tm, a distribution similar to that ob- tained with Kromekote cards. Mass median dia- meters measured 18 and 21 ,tm for droplets collected on indoor and outdoor slides, respectively. This small difference confirmed good penetration of the aerosol into the houses. The estimates of mass median diameter were lower with the glass slides than with the Kromekote cards-probably because a larger proportion of small droplets was sampled with the impinged droplet method. Visual larval survey The results of visual larval surveys carried out in both areas before and after the treatments are given 40 ae 30 C6 a0 CL 20 CL 10 L 0 0 5 10 15 20 25 30 35 40 45 Droplet size in pLm (median: 21p.m) WHO 10900 Fig. 1. Frequency distribution of droplets according to diameter in micrometres. _rn I1..... . MALATHION AEROSOLS TO CONTROL AEDES AEGYPTI Table 7. Average number and range of malathion droplets per square inch (6 45 cm2) inside houses in Huay Kwang and Sri Racha Huay Kwang Treatment North J South No. Upstairs Downstairs Upstairs Downstairs 1 H a 19.6 41.2 39.1 44.3 (0.0-63.0) b (0.0-191.0) (0.5-157.5) (0.0-177.0) H 38.1 46.4 189.0 182.4 (1.5-185.5) (0.0-245.0) (0.0-474.5) (0.0-651.5) 2 V c - 7.8 50.0 7.0 (0.0-30.5) (0.0-112.0) (0.0-76.0) H 183.2 136.9 72.9 118.8 (2.0-644.0) (0.0-630.0) (0.0-289.0) (0.0-535.0) V 1.0 26.3 2.2 4.9(0.0-2.5) (0.0-103.0) (0.0-5.5) (0.0-23.5) H 70.8 75.7 28.0 25.5 (0.0-364.0) (1.0-383.5) (0.0-178.5) (0.0-168.5) 4 V - 11.7 - 7.9 (0.0-55.5) (0.0-23.5) H 40.1 25.7 188.7 291.4(0.5-151.0) (0.0-121.0) (0.0-755.0) (37.0-830.5) V 1.6 1.6 13.3 35.2(0.0-4.5) (0.0-10.0) (0.0-63.0) (0.0-183.0) Sri Racha d Upstairs Downstairs North 114.4 480.2 (0.0-538.0) (0.0-1056.0) South 2.8 70.1 (0.0-10.5) (0.0-246.5) East 146.0 81.2 (1.0-648.0) (3.5-232.5) West 160.2 733.6 (3.0-422.5) (227.5-1845.5) a H = horizontal cards. b The range is shown in parentheses. c Vertical cards. d Horizontal cards only. 813 C. P. PANT AND OTHERS Table 8. Results of visual larval surveys before and after treatment at Huay Kwang and Sri Racha Huay Kwang Sri Racha Findings Before After Before Aftertreatment treatment treatment treatment 18/8/70 3/12/70 a 21/9/70 16/12/70 No. of houses examined 199 150 76 97 No. of houses with larvae 189 142 73 78 Aedes index 95.0 94.7 96.1 80.4 No. of containers examined 1 713 1 261 887 957 No. of containers with larvae 665 522 424 353 Receptacle index 38.8 41.4 47.8 36.9 Breteau index 334.2 348.0 557.9 363.9 Total outdoor containers 585 367 615 554 Outdoor containers with larvae (%) 43.9 45.2 45.5 43.5 Total indoor containers 1 128 894 272 403 Indoor containers with larvae (%) 36.2 39.8 52.9 27.8 a 25 days after fifth treatment. in Table 8. At Huay Kwang, no significant dif- ferences were observed before and 25 days after the last treatment. At Sri Racha, statistically significant differences in the Aedes Index (p <0.01), receptacle index (p <0.01), Breteau index (p <0.05), and the percentage of indoor receptables containing larvae (p <0.01) before and 19 days after the last treatment were observed; however, no such significant dif- ferences were found in the proportions of outdoor containers with larvae. It is possible that some larvicidal effect may have been produced indoors, since the droplets have more time to settle there than outdoors, where it is windy. The visual larval survey in Huay Kwang was made some time after treatment, so this difference was not noticed there. Fly counts Table 9 shows the results of fly counts made before and after treatment in the market area of Sri Racha. Reductions of 64-91 % were observed in counts made outside the market. Few flies were counted inside the market itself, yet reductions of 47-100% were obtained 1 day after treatment. Table 9. Fly counts at Sri Racha before and after the 3 treatments a No. of flies counted Location 1 week before 1 day after 6 days after 1 day after 1 day after 1st treatment 1 st treatment 1 st treatment 2nd treatment 3rd treatment Outside market 212 28 82 19 26 (87.1) b (64.2) (91.0) (87.7) Inside market 21 7 13 0 11 (66.6) (38.0) (100) (47.6) a Basedlon observation, for 15 minutes, of a wooden grid 1 m x I m. The treatments were performed on 17, 24, and 27 November 1970. b1The figures in parenthesis show the percentage reduction in the count. 814 MALATHION AEROSOLS TO CONTROL AEDES AEGYPTI 815 DISCUSSION To contain an epidemic of dengue haemorrhagic fever, the vector control method of choice is one that can eliminate the infected and infective adult mos- quitos rapidly and can be repeated to maintain con- trol. Furthermore, it should be applicable at short notice and should be economical. It has been shown that aerosols of technical malathion dispensed in ULV quantity from specially equipped aircraft are effective and capable of maintaining low densities of Ae. aegypti (Lofgren et al., 1970). This method is ideal for combating epidemics in large cities, pro- vided that aircraft, pilots, operating funds, etc. are readily available. However, in many countries of South-East Asia such ideal situations do not exist. Thus, in Thailand, epidemics have been reported in scattered townships and villages, necessitating the treatment of large and widely separated areas. Moreover, the means for undertaking such opera- tions are scarce in Thailand. Under such circum- stances, alternative methods must be envisaged. One that has proved to be practical, rapid, inexpensive, and applicable even in remote places is the dispersal of technical malathion by a Leco aerosol generator mounted on a motor vehicle. It has been shown that the density of Ae. aegypti can be drastically reduced by this method, using the optimum dosage of 438 ml/ha. Application was most effective when the nozzle of the apparatus was pointed towards houses. Evidently, owing to dif- ferences in the road network in different parts of a town, and because the outskirts occupy a larger area than the town centre, the actual average dose delivered may be lower. The aerosol penetrates inside the houses so that, despite the differences in droplet densities shown on special cards, high bioassay mortalities of caged mosquitos were consistently produced in the present study. The average droplet densities were higher than those reported by Lofgren et al. (1970) for aerial applications. Dyed malathion droplets on Krome- kote cards are more easily seen and counted than undyed droplets on the oil-sensitive papers used by the above-mentioned authors. The technique of collecting droplets on cards kept indoors has certain limitations: cards kept in a vertical position show very few or no droplets, whereas those kept horizon- tally collect falling droplets. The behaviour of such small particles and the way in which they are de- posited are difficult to study; hence, sampling devices have to be precise. The exact interval between the treatment of a house and the removal of the cards will naturally influence the numbers of droplets col- lected and, since this interval is not easy to determine in the field, quantitative comparisons in those cir- cumstances may not be reliable. However, the pro- cedure does show the general pattern of distribution of the insecticide. The mass median diameters obtained in this study (21 and 26 jum) are somewhat higher than the opti- mum range of 5-10 tum suggested by Mount (1970). It is possible that the efficiency of this ULV applica- tion method could be further increased by better atomization. The cold aerosol generator used in this study was operated at maximum air pressure (0.2 kgf/cm2), so increased atomization was not pos- sible. However, the heavy duty model of the Leco ULV aerosol generator is capable of air pressures of at least 0.4 kgf/cm2 and would thus be suitable for future studies, in South-East Asia, of insecticidal efficiency in relation to droplet size. Areas of a town that are completely cut off from road communications are difficult to treat. How- ever, the trial in Sri Racha, which is a typical coastal town of Thailand, showed that most houses are approachable by road. A vehicle such as that used in this study, or a 4-wheel-drive Jeep seems to be the most suitable for this purpose, particularly during the rainy season. Field workers with some experience soon learn how to move within a locality in order to treat most of the houses. Some houses can be treated by keeping the vehicle at a suitable distance and letting the aerosol move into them with the air. If there are still some houses or areas that cannot be treated, use could be made of smaller, portable machines such as Mity Moe Sr.' which also can deliver a ULV quantity of malathion and effectively reduce Aedes densities, as demonstrated by the Aedes Research Unit during trials carried out in 1970. For such town sectors 2-4%. malathion thermal fogs can also be applied, using a Swing- fog.2 Thus although it might be thought that an excellent road network is essential for such an opera- tion, most towns in Thailand have a reasonable road system allowing the use of such machines without too much difficulty. One Leco machine was able to cover the medium-sized town of Sri Racha in half a day. 1 Manufactured by the Buffalo Turbine Agricultural Equipment Co., Inc., 70 Industrial Street, Gowanda, New York 14070, USA. 2 Manufactured by Motan GmbH, D-7972 Isny (Allgau), Federal Republic of Germany. 816 C. P. PANT AND OTHERS It would probably take the same time to treat several smaller villages; for larger areas, several machines would be required if the treatment needed to be made rapidly. It has been suggested 1 on the basis of the feeding habits of the vector and the incubation period of the virus in mosquitos that, if the treatment has to be repeated during an epidemic, this may not be neces- sary until 10 days after the initial application, even if newly emerged mosquitos are found in the area. In 1 Pant, C. P. & Yasuno, M. (1970) Unpublished document WHO/VBC/70.242. the present investigation, mosquito densities had recovered to their original levels in about 14 days, but it is doubtful whether the transmission of infec- tion would have been resumed within 10 days. It has been shown that two treatments, carried out with a 3-day interval, were more effective than one. Therefore, in an epidemic-time and funds permitting-two initial treatments carried out with a 3-day interval, followed by another treatment after 10 days, should be sufficient. However, this can be confirmed only after a trial has been conducted in an actual epidemic situation. ACKNOWLEDGEMENTS Grateful thanks are expressed to Dr Prakorb Tuchinda, Director-General, Department of Medical Sciences, Yod- sae, Bangkok, who provided the staff and laboratory faci- lities to carry out this investigation; to Dr Boonluan Punthumachinda, Miss Prapaipuk Chattraphuti, and Miss Pimpa Paounpongkul, who assisted in the laboratory and field evaluation; to Mr Anek Ruangsri, who did the strenuous driving during the application of the aerosol; to Mr Wirat Samutrapong and Mr Ongart Chareonsook for their assistance and supervision of field staff; and to all other members of the Aedes Research Unit who gave their help. The co-operation of the US Department of Agriculture in providing a consultant is also gratefully acknowledged. RESUMI APPLICATIONS AU SOL D'AEROSOLS DE MALATHION TECHNIQUE SOUS ULTRA-FAIBLE VOLUME POUR LA LUTTE CONTRE AEDES AEGYPTI L. L'unite OMS de recherche sur Aedes, a Bangkok (Thailande), a procede en 1970 a plusieurs essais au sol d'aerosols de malathion technique afin de mettre au point une methode adequate et economique de lutte contre A. aegypti, vecteur de la dengue et de la fievre hemorragique en Asie du Sud-Est. Les aerosols sous ultra-faible volume (produits par un generateur Leco monte sur camion) ont l'avantage sur les aerosols thermiques de ne pas necessiter l'emploi de solvants, de vehicules inertes ou d'additifs. L'appli- cation au sol est parfois plus indiquee que l'application par voie aerienne: c'est le cas quand il faut combattre des epidemies atteignant de tres vastes territoires et lorsque les moyens materiels (avions) et humains (pilotes entraines et experimentes) ne peuvent etre reunis rapidement. Cette situation est frequente dans beaucoup de pays du sud-est asiatique. Quant aux traitements larvicides par l'Abate, ils sont incapables d'agir sur une epidemie en cours, n'ayant aucun effet sur Aedes adulte. Les essais ont et faits & Huay Kwang, faubourg de Bangkok, d'une superficie de 29 hectares, ou vivent environ 15 000 personnes, et a Sri Racha, ville cotiere couvrant 142 hectares et abritant 16 000 habitants. On a trouve que, dans les lieux habites, il est possible d'obtenir un taux d'insecticide de 438 ml/ha en dirigeant le jet d'aerosols lateralement vers les habitations, le generateur fonctionnant a pleine puissance et le vehicule transporteur circulant a la vitesse de 5 km/h. Grace aux cartes colorees, on s'est assur6 que les gouttelettes de malathion pen6- traient a l'interieur des maisons et que leur diametre moyen (21 ,um) etait proche de la valeur optimale. La m6thode a donne d'excellents resultats contre les mous- tiques adultes: deux traitements a 3 jours d'intervalle ont abaisse leur densit6 de 99 %. Les populations d'Aedes ont recouvre leur importance primitive 2 semaines environ apres le traitement. I1 semble cependant qu'en cas d'epidemie les applications ne doivent pas etre rep6tees avant un intervalle de 10 jours, en raison des pref6rences trophiques du vecteur et de la duree de la periode d'incubation de l'infection virale chez le moustique. Selon les auteurs, les applications au sol d'aerosols repr6sentent un excellent moyen de lutte contre A. aegypti adulte et devraient etre mises a l'epreuve lors d'une epidemie. MALATHION AEROSOLS TO CONTROL AEDES AEGYPTI 817 REFERENCES Detinova, T. S. (1962) Age-grouping methods in Diptera of medical importance, Geneva (World Health Organiza- tion: Monograph Series, No. 47) Kilpatrick, J. W., Tonn, R. J. & Jatanasen, S. (1970) Bull. Wld Hlth Org., 42, 1-14 Lofgren, C. S. (1970) Ann. Rev. Ent., 1970 Lofgren, C. S., Ford, H. R., Tonn, R. J. & Jatanasen, S. (1970) Bull. Wld Hlth Org., 42, 15-25 Mount, G. A. (1970) Mosquito News, 30, 70-75 Mount, G. A., Lofgren, C. S., Pierce, N. W. & Husmen, C. N. (1968) Mosquito News, 28, 99-103 Mount, G. A., Pierce, N. W., Lofgren, C. S. & Gahan, J. B. (1970) Mosquito News, 30, 56-59 Puri, I. M. (1957) A practical entomological course for students of malariology, Delhi (Government of India Health Bulletin No. 18). Rathburn, C. B. (1970) Mosquito News, 30, 501 Yeomans, A. H. (1949) Bull. US Dept. Agric., Butr. Ent. Plant Quarant., No. ET267