The effectiveness of ultra-low-volume applications of malathion at a rate of 3 US fluid ounces per acre in controlling Aedes aegypti in Thailand*

Bull. Org. mond. Sante' 1970, 42, 27-35 Bull. Wld Hlth Org. The Effectiveness of Ultra-Low-Volume Applications of Malathion at a Rate of 3 US Fluid Ounces per Acre in Controlling Aedes aegypti in Thailand* CLIFFORD S. LOFGREN,' HUGH R. FORD,2 ROBERT J. TONN,3 Y. H. BANG4 & PHAICHITT SIRIBODHI5 Ultra-low-volume (ULV) aerial applications of malathion at a rate of 3 US fl oz per acre (219 ml/ha) for the control of Aedes aegypti and Culex pipiens fatigans were eva- luated in 2 areas near Bangkok, Thailand. A housing project of the Royal Thai Air Force received 2 applications a week apart while the village ofBang Khen received I application. Bioassays, oil-sensitive red dye cards, landing-rate counts, ovitraps, and light-traps were used to evaluate the effects of the treatment. A comparison of pre- and post-treatment indices indicated good control of A. aegypti following all treatments. The results of these tests when compared with those of an earlier study using 6 US fl oz/acre (438 ml/ha) showed that the higher rate of application rate was more effective. Only fair control of C. p. fatigans was obtained. A comparison of light-trap collections before and after treatment demonstrated fairly good over-all control of other species of mosquitos for 3-4 days following each of the 2 treatments on the Air Force housing area. Studies by Kilpatrick, Tonn & Jatanasen (1970) and Lofgren et al. (1970a), have shown that Aedes aegypti can be effectively controlled by means of ultra-low-volume aerial applications of malathion applied at a rate of 6 US fl oz/acre (438 ml/ha). In both reports, 2 applications spaced 4 days apart reduced the collecting rate of biting adults by over 90% and the number of positive ovitrap collections from about 500% to 0%. Kilpatrick, Tonn & Jatanasen (1970) were unable to obtain satisfactory control of A. aegypti with malathion applied at the rate of 3 US fl oz/acre * This study was supported jointly by US Public Health Service Research Grant No. CC 00174 from the National Communicable Disease Center, Atlanta, Ga., USA, and by the World Health Organization. 1 Entomologist, Entomology Research Division, US Department of Agriculture, Gainesville, Fla., USA. I Agricultural research technician, Entomology Research Division, US Department of Agriculture, Gainesville, Fla., USA. ' Project Leader, WHO Aedes Research Unit, Bangkok, Thailand. I Entomologist, WHO Aedes Research Unit, Bangkok, Thailand. 5 Deputy chief, Preventive Medicine Section Royal Thai Air Force Medical Corps. (219 ml/ha) by a Cessna-180 aircraft in test areas I square mile (2.6 kM2) or less in size. This is the dosage of malathion normally used for mosquito control in the USA and was the rate used to control Culex quinquefasciatus during the epidemic of St Louis encephalitis in Dallas, Texas, in 1966. The lower rate of application for the control of A. aegypti would result in considerable financial benefit and thus make it easier for developing coun- tries to employ this means of controlling disease vec- tors. As a result, it was decided that the WHO Aedes Research Unit would evaluate this dosage once again, using on this occasion a larger C-47 aircraft. One reason for the poor results obtained previously could have been the drifting of insecticide outside the relatively small test area. As a result, larger test areas were selected to permit the treatment of a buffer or border zone, 1/2 mile (0.8 km) or more wide, around the actual evaluation site. METHODS AND MATERIALS Two trial plots were selected; the first was an officers' housing project of the Royal Thai Air Force located on the old airport highway directly 2452 -27- C. S. LOFGREN AND OTHERS across from the military section of the Don Muang Airport. Most of the housing in this area is in the form of 2-storey wooden constructions with 4 or more dwellings under the same roof. A 1-larva- per-container survey of this area made before the trial indicated that in 93 % of the premises A. aegypti were breeding, an average of 3.0 containers being positive for breeding out of an average of 8.9 con- tainers per premises. The second plot encompassed the Don Muang New Market at Bang Khen, which is located imme- diately south of the Royal Thai Air Force housing area. There were both old and newly constructed cement shophouses, and single- or 2-storey wooden house on the site. The C-47 aircraft used to apply the malathion was obtained on loan by the World Health Organi- zation from the Royal Thai Air Force. The aircraft was equipped for insecticide spraying with a system developed by personnel of the Entomology Research Division, Agricultural Research Service, US Depart- ment of Agriculture, that utilizes a short boom located about 30 inches (75 cm) under the fuselage and directly below the cargo doors. It has been described previously by Glancey, Ford & Lofgren (1969) and Lofgren et al. (1970b). The applications of malathion were made on 29 November 1968, and on 6 December 1968. The Don Muang and Bang Khen plots are adjacent to each other and were treated at the same time on 6 December but only Don Muang was treated on 29 November. Climatic conditions during both treatments appeared to be ideal. The winds at ground level during the first treatment were less than 2 miles/h (3.2 km/h); during the second application they were about 3-5 miles/h (4.8 km/h-8.0 km/h). In the first test the wind blew at right angles to the flight pattern and in the second test the wind blew at an angle of 600 to the flight pattern. It was obvious in both tests that the wind velocity at the spraying altitude (150 feet; 46 m) was considerably higher than at ground level. The aircraft was flown at a speed of 150 miles/h (241 km/h) and a swath of insecticide 500 feet (152 m) wide was laid down. The spray boom was fitted with 10 No. 8004 Tee-Jet flat-fan tips which gave an insecticide delivery rate of 31/2 US gallons per minute (13.2 1/min) under a pump pressure of 46 lbf/in2 (1.1 kgf/cm2). The evaluation methods for determining the effec- tiveness of the malathion treatments were the same as previously described by Lofgren et al. (1970a). Three evaluation sites were established in each area and all A. aegypti landing-rate counts were made at these sites from 0900 to 1230 hours. A mosquito scout was assigned to each site and collections were made for 20 minutes in each of 8 houses. Landing counts of Culex pipiens fatigans were made from only the 3 sites at the Royal Thai Air Force housing project; these counts consisted of 2-hour collections made from 1900 to 2100 hours from 1 house at each site. Both male and female mosquitos were collected and counted. Ovitraps (see Kilpatrick, Tonn & Jatanasen, 1970) were employed at all evaluation sites, 10 traps being us-d per site, and the paddles collected every 48 hours. Pretrial evalua- tions for all these methods were made prior to the aerial application of insecticide and a check area was used for comparisons during the evaluation. Bioassays of the treatment with caged A. aegypti adults and larvae were made. Droplet rate of deposi- tion was monitored with oil-sensitive red dye cards. In addition to these assay methods for A. aegypti and C. p. fatigans, the Preventive Medicine Branch of the Royal Thai Air Force set up 3 light-trap stations to make general mosquito collections. The species of the mosquitos collected was determined. A number of collections were made before the trials began and collecting was continued after treatments until the insect populations recovered. Unfortu- nately, no light-traps were used to make comparisons outside the treated area. RESULTS Data from bioassays and red dye cards Data from the bioassays and the rate of droplet deposition are recorded in Tables 1 and 2. A com- plete kill of adult A. aegypti mosquitos placed out- doors was obtained in the first test at Don Muang and in the test at Bang Khen. The kill in the second test at Don Muang varied from 86% at site 2 to 100% along the road. In only 1 instance was 100% mortality of adults placed indoors obtained (site 2, first test at Don Muang); the same site had the lowest kill in the second test (20%). The kill of adults placed indoors at Bang Khen was generally higher than that at Don Muang but this is to be expected since the houses in the Bang Khen area are more open and more typical of Thai houses in general than the military housing. Larval mor- talities did not exceed 19.2% in cups placed indoors and ranged from 25% to 100% in cups placed outside. The highest larval kill occurred in cups placed outdoors in the Bang Khen test area (91 %- 28 AEDES AEGYPTI CONTROL BY APPLICATION OF MALATHION AT 3 US FL OZ/ACRE 29 TABLE I MORTALITY OF CAGED ADULTS AND LARVAE OF AEDES AEGYPTI AND ADULTS OF CULEX PIPIENS FATIGANS *AND RATE OF DROPLET DEPOSITION ON RED DYE CARDS EXPOSED TO MALATHION APPLIED AT A RATE OF 3 US FL OZ/ACRE; a DON MUANG C. p. fatigans Rate of droplet Adults Adults deposition per jin b No. of Mortality No. of Mortality insects (%) insects ( Average Range First application (29 November 1968) Inside house Outside house Inside house Outside house Inside house Outside house Outside, along road Control area 175 200 100 150 125 125 225 125 13 43 0 58 0 70 92 0 88 70 85 100 116 96 72 100 95 100 109 99 85 90 111 96 125 100 77 100 100 0 104 104 105 186 125 100 28 100 100 11 Inside house Outside house Inside house Outside house Inside house Outside house Outside, along road Control area 125 125 125 125 100 105 250 125 125 Second a 0 62 0 77 0 25 98 0 pplication (6 December 1968) 81 100 100 100 65 100 200 100 a 219 ml/ha. b No. of droplets per cm' in parentheses. 73 98 20 86 91 99 100 20 2 3 4 0.5 (0.04) 26.4 (4.09) 2.1 (0.32) 27.8 (4.31) 1.0 (0.15) 46.8 (7.22) 28.2 (4.37) 0-1.3 (0-0.101) 15-40.5 (2.32-6.28) 0.5-3.5 (0.08-0.54) 16.8-42.8 (2.60-6.63) 0-4.5 (0-0.70) 30.8-67.8 (4.77-10.35) 16.8-38.5 (2.60-6.00) 2 0.8 (0.12) 12.6 (2.00) 0.1 (0.01) 15.7 (2.43) 1.4 (0.21) 11.0 (1.70) 25.1 (3.99) 0-1.8 (0-0.28) 1-25.3 (0.15-2.00) 0-0.5 (0-0.08) 9.3-22.5 (1.44-3.48 0.8-2.3 (0.12-0.35) 1.3-1 9.5 (0.20-3.02) 16-42.3 (2.47-6.62 3 I I C. S. LOFGREN AND OTHERS TABLE 2 MORTALITY OF CAGED ADULTS AND LARVAE OF AEDES AEGYPTI AND RATES OF DROPLET DEPOSITION ON RED DYE CARDS EXPOSED TO MALATHION APPLIED AT A RATE OF 3 US FL OZ/ACRE; a BANG KHEN Site | jLocatIon of Larvae | Adults | Rate of droplet depositionSite insects and ______________ __peInb No. red dye cards No. of Mortality No. of Mortality in sects (%) insects ( Average Range Inside house 125 2 100 93 7.6 0-16.8 (1.17) (0-2.60) 1 Outside house 125 91 100 100 33.6 28.8-36.8 (5.20) (4.46-5.55) 2 Inside house 125 19 100 98 2.1 0-7.5 (0.32) (0-1.16) Outside house 125 98 100 100 39.5 33.5-43.0 (6.12) (5.20-6.66) Inside house 125 14 100 97 2.3 0-7.8 (0.35) (0-1.20) 3 Outside house 125 100 100 100 34.7 25.3-51.0 (5.22) (3.90-7.90 Control area 125 0 100 20 a 219 ml/ha. b No. of droplets per cm2 in parentheses. 100%). Mortalities of larvae in cups placed along the road in Don Muang was 92% in the first test and 100% in the second. The deposition of droplets of insecticide on the red dye cards was very uniform in each of the test areas. The best coverage occurred in the Bang Khen area with the average number of droplets on cards placed outdoors ranging from 33.6 to 39.5 per in2 (5.2-6.3 per cm2). Surprisingly, in the Don Muang area, which was treated at the same time, the average number of droplets ranged from 11 to 15.7 per in2 (1.7-2.4 per cm2). This difference is difficult to explain but it could be associated with the types of houses and the direction and velocity of the wind. It was noted that the number of droplets per card varied greatly from one card to another. The cards had been placed, alternately, on either side of the street between the rows of houses. With a fairly strong wind blowing, and considering that all the houses were 2 storeys high, the cards placed on one side of the street may have been sheltered by the houses. It is also possible that many fine droplets were not deposited in the area despite the ½/2-mile (0.8-km) border zone; if this happened, it would favour a higher rate of deposition in the Bang Khen area because the wind was blowing in that direction. The former explanation, however, is probably more logical since the number of droplets on cards placed along the road was 28.2/in2 (4.3/cm2) in the first test and 25.1/in2 (3.9/cm2) in the second; this road is located on the upwind side of the plot and closer to the border zone than the assay sites are. The rate of deposition of droplets on cards placed inside houses was low in all 3 tests, averaging from 0.1 to 7.6 per in2 (0.015-1.18 per cm2). A rather better penetration of the houses in the Bang Khen area was obtained, corresponding to the higher kill in the bioassays in this area. Droplets were also collected on glass slides treated with silicone; the slides were waved gently through the air after malathion had been sprayed directly overhead and for 2 swaths upwind by the aircraft. The droplets were measured under a microscope and the mass median diameter was calculated according 30 AEDES AEGYPTI CONTROL BY APPLICATION OF MALATHION AT 3 US FL OZ/ACRE TABLE 3 EFFECTIVENESS OF 2 TREATMENTS WITH MALATHION APPLIED 7 DAYS APART a AT A RATE OF 3 US FL OZ/ACRE b IN CONTROLLING AEDES AEGYPTI IN THE DON MUANG TEST AREA Pre- Percentage reduction on day indicated treatment (counted from first treatment) Site No. count(mosquitos/ man/hour) 1 3 4 5 8 10 12 114 17 19 1 5.4 91 100 91 91 100 100 100 91 100 91 2 5.8 83 100 83 74 100 100 40 100 91 66 3 5.1 90 100 80 90 100 90 100 51 80 71 Average 5.4 88 100 85 85 100 97 79 82 91 76 Control area 10.7 - 63 - - - 58 - - 48 - a Second application made on day 7. All collections were made between 0730 and 1200 hours. b 219 ml/ha. to the method of Yeomans (1949). The droplets ranged in size from 6,u to 114 ,u and the mass median diameter was 54 ,u. Studies of droplet size made in the USA both before (Glancey et al., unpublished data) and after these trials indicated that the mass median diameter of sprays produced by No. 8004 flat-fan tips operated at aircraft speeds of 150 miles/h (242 km/h) should range from 30 ,u to 40 ,u. It appears, therefore, that in the tests reported here, a completely representative sample of the spray droplets was not obtained. Mosquito evaluation The landing-rate-collection data for A. aegypti at the 2 test sites are given in Tables 3 and 4. Because of a change in staff, the landing counts made at the check area differ for the two trials. The collections reported in Table 3 were made by a single scout but those in Table 4 are averages of collections made by 2 scouts. The pretrial collections showed that the 3 sites at Don Muang had comparable A. aegypti popula- tions. However, the A. aegypti population after treatment fluctuated more at site 2 than at the other 2 sites. The average percentage reduction in landing rates showed a fairly good control of A. aegypti for about 20 days with the 2-treatment schedule (75 %- 100% reduction). More variation was observed among the 3 sites at Bang Khen. At site 2, the mos- quito population was initially higher than at the other 2 sites and the reinfestation was more rapid. Good control lasted for only 1-5 days at sites 1 TABLE 4 EFFECTIVENESS OF A TREATMENTWITH MALATHION APPLIED AT A RATE OF 3 US FL OZ/ACRE a IN CONTROLLING AEDES AEGYPTI IN THE BANG KHEN TEST AREA Pre- Percentage reduction on treatment day indicated (counted Site No. count from day of treatment)(m os quitos/manl hour) 1 3 5 7 10 12 1 6.5 100 100 85 54 15 0O 2 10.0 90 70 35 25 25 0b 3 5.1 92 100 100 92 80 80 Average 7.2 93 86 65 49 40 0 Control area 6.0 0 - 20 - 6 - a 219 ml/ha. b Collections on twelfth day exceeded pretreatment collec- tions. and 2; however, good control lasted throughout the study period at site 3. The average percentage reductions showed that a second application of insecticide was needed after 3-4 days to maintain satisfactory control. No good explanation can be given for the large reduction in the check area for the Don Muang plot. The ovitrap data are given in Tables 5 and 6. There is an excellent correlation between landing- 31 C. S. LOFGREN AND OTHERS TABLE 5 EFFECT OF 2 TREATMENTS WITH MALATHION APPLIED 7 DAYS APART AT A RATE OF 3 US FL OZ/ACRE a ON DEPOSITION OF EGGS BY AEDES AEGYPTI IN OVITRAPS IN THE DON MUANG TEST AREA Site No. 2 3 Average Control area Percentage of positive paddles before, and at indicated days after, first application of malathion b Pretreatment 37 33 73 48 42 2 4 10 30 20 20 50 0 40 0 13 6 8 10 10 20 40 o 0 10 17 - 25 10 12 0 10 0 3 0 0 0 0 14 0 10 0 3 25 16 18 0 30 40 23 10 20 40 23 25 a 219 ml/ha. b Paddles were changed every other day. collected. rate counts and the number of positive paddles at the 3 sites at Don Muang. Although site 2 did not have the highest percentage of positive ovitraps in the pretrial evaluation, some ovitraps were positive on each day except the first collection after the second spraying. The results in the Bang Khen plot showed complete cessation of oviposition at sites 1 and 3 up to the end of the sixth day and at site 2, 10% of the paddles were positive on the fourth and sixth days after treatment. However, oviposition returned to TABLE 6 EFFECT OF A SINGLE TREATMENT WITH MALATHION APPLIED AT A RATE OF 3 US FL OZ/ACRE a ON DEPOSITION OF EGGS BY AEDES AEGYPTI IN OVITRAPS IN THE BANG KHEN TEST AREA Percentage of positive paddles before, and at indicated days after, application of Site No. malathion treatment 2 4 6 8 10 12r 2 3 Average Control area 45 55 45 48 0 50 30 27 42 25 0 10 0 3 10 0 0 50 10 20 25 0 10 45 60 20 130 22 33 25 a 219 ml/ha. b Paddles were changed every other day. The day indicated is the day the paddles were collected. The day indicated is the day the paddles were the pretreatment levels on the eighth day. In general the ovitrap data indicated that oviposition by A. aegypti can be significantly reduced for about 1 week by 1 ultra-low-volume application of mala- thion but complete cessation of oviposition was not obtained in the present trial. Both the ovitrap data and the landing-rate counts indicated that the double treatment at Don Muang was more effective than the single application at Bang Khen. The results of the landing-rate counts on C. p. fatigans are presented in Table 7. Control was poor and the maximum reduction (82 %) occurred TABLE 7 EFFECTIVENESS OF 2 TREATMENTS WITH MALATHION APPLIED 7 DAYS APART AT A RATE OF 3 US FL OZ/ACRE a IN CONTROLLING CULEX PIPIENS FATIGANS IN THE DON MUANG TEST AREA Site No. 2 3 Average Control area Pre- Percentage reduction on treatment day indicated (counted from count first treatment)(mosquito/ man/hour) 0.5 3.5 5.5 7.5 10.5 12.5 36.5 45 76 44 81 86 59 16.3 39 26 26 57 54 0 28.8 36 72 51 97 69 0 27.2 42.1 41 29 65 43 82 13 74 10 11 14 a 219 ml/ha. 32 AEDES AEGYPTI CONTROL BY APPLICATION OF MALA1'HION AT 3 US FL OZ/ACRE 33 the night following the second application of mala- treatment, the reduction was again 91 %; it remained thion. It was obvious that higher dosage rates or at 88 %-90 7% of the pretreatment level for 3 days more frequent applications are needed to control but the mosquito population then increased rapidly. this species. Table 8 contains the records of light-trap collec- DISCUSSION AND CONCLUSIONS tions for the 17 species of mosquitos found in the test area. Only Anopheles nigerrimus, A. annularis, ULV aerial spray trials with malathion were con- Culex gelidus, C. p. fatigans, C. tritaeniorhynchus ducted by the WHO Aedes Research Unit in Thailand summerosus and Mansonia uniformis were collected in during 1968. The first set of trials using a single- sufficient numbers to show the effects of the spraying. engined aircraft demonstrated that 6 US fi oz/acre All these species showed a decided drop in number (438 ml/ha) were necessary to achieve adequate immediately after the first treatment. The 4 pre- control of A. aegypti populations (Kilpatrick, Tonn treatment collections were averaged and the percent- & Jatanasen, 1970). However, they were of the age reductions were calculated from this value. There impression that 3 US fi oz/acre (219 ml/ha) might be was a definite reduction in the population after the sufficient if a larger aircraft were used. In the current first treatment (91 %); 5 days later, the collections were series of trials using a C-47 aircraft both application 61 % of the pretreatment average. After the second rates were tested again. Two applications of 6 US fi TABLE 8 LIGHT-TRAP COLLECTIONS OF MOSQUITOS BEFORE AND AFTER EACH OF 2 TREATMENTSWITH MALATHION APPLIED AT A RATE OF 3 US FL OZ/ACRE a IN THE DON MUANG TEST AREA rotal number of male and female mosquitos collected on day indicated Species of mosquito Pretreatment 14 13 2_ Anopheles nlgerrimus A. peditaeniatus A. sinensis A. aconitus A. annularis A. vagus Culex fuscocephalus C. gelidus C. p. fatigans C. tritaeniorhynchus summerosus C. rubithoracis C. brevipalpis Ficalbia minima F. chamberlaini Mansonia annulifera M. uniformis Uranotaenla sp. 61 0 0 0 27 0 0 60 10 18 3 0 2 5 0 17 10 14 0 0 0 12 0 0 6 0 0 0 0 11 0 528 0 13 0 100 4 0 61 9 16 4 0 0 0 37 0 Post-treatment 1 10 1I112 13 14 5 11 111 13 114 115 240 0 0 0 51 3 0 6 4 5 0 0 0 23 0 0 0 0 0 24 0 0 0 20 0 0 4 0 42 0 0 0 4 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 16 7 0 0 0 55 0 3 9 4 0 0 0 0 5 0 54 0 0 0 7 0 0 3 0 3 0 0 0 0 0 4 0 113 0 0 0 5 0 0 36 0 0 0 12 0 0 0 2 2 0 0 0 1 0 0 0 0 0 0 14 5 0 0 20 0 0 0 0 0 0 5 0 0o 0 0 0 0 0 6 0 25 0 0 0 6 0 0 2 0 0 0 0 0 7 0 30 0 5 0 0 0 0 0 0 0 0 23 0 0 2 3 0 0 0 0 2 0 0 0 0 0 5 0 123 265 214 1 194 0 0 12 0 2 4 0 0 0 0 0 7 0 0 0 0 12 2 0 5 0 17 0 0 0 3 3 26 0 0 0 0 16 0 0 16 6 18 0 0 0 0 0 30 0 0 0 0 10 0 0 4 5 6 0 0 0 5 0 10 0 199 0 0 0 9 0 0 4 6 10 0 0 0 8 0 19 0 a 219 ml/ha. Totals 213 45 773 351 31 i67 54 80 71 135!57 31 41 143 35 151 333 300 234 255 ~~1 _I C. S. LOFGREN AND OTHERS oz/acre were used in the first trial at Nakhon Sawan and the results of this test are presented in another paper (Lofgren et al., 1970a). In the second trial 3 US fl oz/acre were applied to 2 plots near Bangkok. The excellent results obtained in the Nakhon Sawan trial showed that the higher rate of application was superior, regardless of the type of aircraft used. Observations on the effect of malathion at the rate of 3 US fl oz/acre (219 ml/ha) on other species of mosquitos showed that most of them were con- trolled to some degree but for only brief periods. The differences in reinfestation rates of the mosquito populations is related to breeding potential and migratory habits. Most of the recovery of A. aegypti in each treatment area would be directly connected to breeding potential rather than to migration since this species does not disperse over long distances. In these tests, large areas were treated and the test sites were surrounded by habitats not favourable to the breeding of A. aegypti. For the most effective control of mosquitos in epidemic situations it would be desirable to use a treatment that killed both larvae and adults. In Thailand, however, most of the breeding sites of A. aegypti are either indoors or covered or protected in such a way that insecticide sprays cannot readily reach them. Thus no insecticide applied as an aerial spray can do more than reduce the adult population and the number of treatments needed to keep A. aegypti suppressed will be determined primarily by the breeding pressure. Therefore, a thorough know- ledge of the mosquito species to be controlled is essential. Information on biting patterns, resting sites, flight range, population density, reproductive capacities and blood hosts should be known so that effective control procedures can be used. Much of this information has already been obtained from research carried out by the WHO Aedes Research Unit. On the basis of results presented in this paper and in Lofgren et al. (1970a), it can be concluded that good control of adult A. aegypti can be attained with ULV applications of malathion at the rate of 6 US fl oz/acre (438 ml/ha). This implies that the method can be used for rapid control of A. aegypti when epidemics of dengue haemorrhagic fever occur. However, further research on the ULV application method in conjunction with epidemiological studies is needed to determine definitely whether or not disease transmission can be stopped. Additional studies on dosage rates and the spacing of treat- ments would also be useful in establishing more cer- tainly the most effective and economical treatment procedures. It is possible that a dosage rate between 3 and 6 US fl oz/acre or an initial application of 6 US fl oz/acre followed by a lower rate of applica- tion would provide adequate control. In the mean- time, however, it is apparent from our tests that the higher rate gives excellent control of A. aegypti in these circumstances. ACKNOWLEDGEMENTS We wish to thank the Royal Thai Air Force and the Thailand Ministry of Public Health for their generous co-operation and assistance throughout the study. We are particularly grateful to Captain Ampol Javusombat and members of his crew for making the aerial applica- tions and to Major Somsawart Sudsatya for his help in evaluating the results. We are grateful also to Colonel Vere Thaikla for his administrative assistance. Finally, we wish to express our appreciation to the mosquito scouts of the WHO Aedes Research Unit, the Ministry of Public Health and the Department of Preventive Me- dicine of the Royal Thai Air Force for their help. RESUME EFFICACITE DES APPLICATIONS DE MALATHION SOUS ULTRA-FAIBLE VOLUME A LA DOSE DE 219 ml/ha DANS LA LUTTE CONTRE AEDES AEGYPTI EN THAILANDE On a procedd a des essais d'evaluation du malathion applique sous ultra-faible volume dans deux secteurs des environs de Bangkok (Thallande): un ensemble d'habita- tions militaires et le village de Bang Khen. Les applica- tions ont 6te effectu6es par epandage aerien 'a raison de 219 ml/ha. Les batiments militaires ont ete traites a deux reprises a sept jours d'intervalle; le village n'a fait l'objet que d'un seul traitement. Les effets des traitements sur les populations d'Aedes aegypti ont ete apprecies par la mesure des taux de cap- ture par homme et par heure et par la mesure des taux de ponte dans des pieges a oviposition. Quant a la reparti- tion et it la dispersion de l'insecticide, a l'interieur et 'a l'exterieur des habitations, elles ont dte estimees d'apres la mortalite d'A. aegypti adultes et de larves exposes dans les secteurs traites pendant les applications ainsi qu'a 34 AEDES AEG YPTI CONTROL BY APPLICATION OF MALATHION AT 3 US FL OZ/ACRE 35 I'aide de cartes colorees. On a en outre determine les taux de capture de Culex pipiens fatigans et denombre les moustiques de toutes especes captures au moyen de pieges a lumiere. Les resultats ont demontre l'efficacit6 des applications de malathion sous ultra-faible volume. Dans les bati- ments militaires, la reduction du taux de capture d'A. aegypti a et de l'ordre de 75-100% pendant une dur6e de 20 jours. On a aussi enregistre une forte reduc- tion des taux de ponte apres la ire application et une interruption quasi complete des ovipositions pendant une semaine apres le 2e traitement. Dans le secteur de Bang Khen, une seule application d'insecticide a amene une rdduction satisfaisante des populations d'A. aegypti adultes pendant 3-4 jours cependant que la ponte 6tait r6duite de plus de 90% pendant une semaine. Les essais biologiques ont permis d'enregistrer, apres chaque application, un taux de mortalite de 86 a 100% d'A. aegypti adulte a l'exterieur des habitations. Pour les moustiques exposes a l'interieur des maisons, le taux de mortalite a varie de 20 a 100%; il a ete generalement plus eleve (93 a 98 %) dans le village de Bang Khen en raison, probablement, d'une meilleure penetration de l'insecti- cide dans les maisons villageoises. A en juger par le d6p6t des gouttelettes de malathion sur les cartes colorees, la dispersion de l'insecticide dans les secteurs traites a Wt6 tres uniforme, en depit de variations lors des applications successives. En ce qui concerne C. p. fatigans, les resultats ont ete peu favorables et la reduction maximale du taux moyen de capture a atteint 82 %. Dans le secteur militaire, les captures 'a l'aide de pieges a lumiere ont diminue dans la proportion de 91 % imme- diatement apres chaque traitement. Les principales especes capturees ont ete: Anopheles nigerrimus, A. annu- laris, Culex gelidus, C. p. fatigans, C. tritaeniorhynchus summerosus et Mansonia uniformis. Bien que les applications de malathion sous ultra- faible volume 'a la dose de 219 ml/ha permettent de lutter efficacement contre A. aegypti adulte, on ne peut affirmer que ce dosage assure une destruction du moustique suffisante pour enrayer une epidemie de dengue a forme hemorragique. D'apres d'autres recherches menees par les auteurs, il semble que l'adoption d'une dose d'appli- cation de 438 ml/ha permettrait d'obtenir l'elimination rapide du vecteur. REFERENCES Glancey, B. M., Ford, H. R. & Lofgren, C. S. (1969) Mosquito News (in press) Kilpatrick, J. W., Tonn, R. J. & Jatanasen, S. (1970) Bull. Wld Hlth Org., 42, 1-14 Lofgren, C. S., Ford, H. R., Tonn, R. J. & Jatanasen, S. (1970a) Bull. Wld Hlth Org., 42, 15-25 Lofgren, C. S., Ford, H. R., Tonn, R. J. & Jatanasen, S. (1970b) Bull. Wld Hlth Org., 42, 157-163 Yeomans, A. H. (1949), Bull. US Dept. Agric., Bur. Ent. Plant Quarant., No. ET267