Effectiveness of different insecticides and formulations against Aedes aegypti larvae in ant traps in Bangkok, Thailand.

320 NOTES Effectiveness of Different Insecticides and Formulations against Aedes aegypti Larvae in Ant Traps in Bangkok, Thailand * by Y. H. BANG, Entomologist, and ROBERT J. TONN,' Project Leader, WHO Aedes Research Unit, Bangkok, Thailand Ant traps are commonly used throughout the tropics to keep ants out of food cabinets. The traps are found in a variety of shapes and sizes; in Thai- land, earthenware traps are most popular, but plastic, glass and metal containers are also found. There is usually an ant trap for each leg of a cabinet. Ant traps are excellent larval habitats for Aedes aegypti and surveys in Bangkok-Thonburi have shown that there is an average of about 2 traps per house, from 35% to 70% of them being infested with larvae. These traps are good breeding sites since they are usually in shaded areas, are not likely to be disturbed, and frequently contain organic debris which may furnish a source of nutrients. This paper reports the findings of a study to evaluate 4 compounds used in ant traps to prevent A. aegypti infestation. The study was part of 4 projects carried out to provide background infor- mation for a pilot control scheme now being under- taken by the WHO Aedes Research Unit (ARU) in Bangkok. Materials and methods The study took place in the Bang Pho area of Bangkok which was one of the 14 study sites used by the ARU for surveys and baseline insecticide testing. The study area was divided into 5 sections. Each section was checked for ant traps and about 20 houses per section were selected for the experi- ment. From each group of 20 houses, 17 units of 4 ant traps were used. All ant traps were of the earthenware type which consist of an inner dry area in which the leg of the cabinet stands, a partition, and an outer area filled with water. Prior to the study, the traps were filled with water (approxi- mately 250 ml). Before treatment with insecticide, all the traps were examined for larvae 5 times during * This study was supported jointly by Public Health Service Research Grant No. CC 00174 from the National Communicable Disease Center, Atlanta, Ga., USA, and the World Health Organization. a Present address: Project Leader, WHO Japanese Ence- phalitis Vector Research Unit, Seoul, Korea. a period of 6 weeks (day 1 and weeks 1, 3, 5 and 6 before treatment). Insecticides tested were Abate, Dursban, and fenthion. Common salt (sodium chloride) was also tested. The formulations, con- centration and dosages are listed in Table 1. The granular materials were weighed in the laboratory prior to their application. The liquid formulations were dispensed by an ejection-type handsprayer calibrated to deliver 1.25 ml per jet. All treatments took place on the same day and each trap was examined weekly for larval infestation, being also refilled with water at that time. The number of weeks during which the insecticide furnished protection was determined by subtracting 1 week.from the time 1 or more traps per unit were positive for 2 consecutive weeks. Results The number and percentage of ant traps containing water and A. aegypti larvae in the 5 sections studied at Bang Pho are shown in Table 2. About 75% of the total number of traps contained water and about 64% had larvae. In this survey, about 95% of the 388 ant traps examined were made of earthenware (Table 3). The number of experimental ant traps infested with larvae at each inspection prior to treatment is given in Table 4. The index of infestation ranged from 55% to 90% with an average of 68% 1 day before treatment. An analysis of variance indicates that the positive ant traps on that day were evenly distributed among the blocks (sections) as were the treatments themselves. Table 5 shows the protection (in weeks) of ant traps containing A. aegypti larvae after insecticide treat- ment relative to the untreated traps found positive. The proportion of positive untreated traps ranged from 50% to 100% during the 23 weeks of the study. Every treatment except the 0.02% salt caused 100% mortality of the larvae when first administered. Along with the 0.02% salt, the 2 for- mulations of fenthion were the first to show failure. Of these, the fenthion granules at 5 ppm had the 2371G 321EFFECTIVENESS OF INSECTICIDES ON AEDES AEGYPTI LARVAE IN THAILAND TABLE 1 INSECTICIDES, FORMULATIONS AND DOSAGES TESTED IN ANT TRAPS IN BANGKOK Treat- ple Concen- Amount ment Insecticide Formulation Concen- dosage tration applied No. tration(%W (p) applied pratta p ant trap I Abate Sand granules 1 5 1 0.25 g 2 Abate Sand granules 1 10 1 0.250 g 3 Abate Clay granules 5 5 5 0.025 g 4 Abate CEli granules 5 10 5 0.050 g 6 Abate Emulsifiable concen- 50 5 0.1 1.250 ml trate 6 Abate Emulsifiable concen- 50 10 0.2 1.250 mlI trate 7 Dursban Granules 10 5 10 0.0125 g 8 Dursban Granules 10 10 10 0.0250 g 9 Dursban Emulsifiable concen- 40.8 5 0.1 1.25 ml trate 10 Dursban Emulsifiable concen- 40.8 10 0.2 1.25 ml trate 11 Fenthion Granules 2.5 5 2.5 0.05 g 12 Fenthion Granules 2.5 10 2.5 0.10 g 13 Fenthion Emulsifiable concen- 50 5 0.1 1.25 ml trate 14 Fenthion Emulsifiable concen- 50 10 0.2 1.25 ml trate 15 Salt Granules - 0.02% - 5 g 16 Salt Granules - 0.04% - 10 g 17 Control 0 ] 0 0 0 TABLE 2 NUMBER AND PERCENTAGE OF ANT TRAPS CONTAINING WATER AND AEDES AEGYPTI LARVAE IN BANG PHO AREA OF BANGKOK PRIOR TO INSECTICIDE TREATMENT Number of Section ant traps examined With water With Aedes larvae Percentage No. % No. of those examined A 80 61 76.3 48 60.0 B 78 59 75.6 56 71.8 C 70 53 75.7 45 64.3 D 73 54 74.0 48 65.8 E 80 58 72.5 45 56.3 Average 76.2 57.0 74.8 48.4 63.8 ,% 0TABLE 3 PREVALENCE SURVEY OF ANT TRAPS IN BANG PHO AREA OF BANGKOK No. No. No. of ant traps a Section of houses of houses Total examined with traps 1 2 A 20 17 62 10 72 B 20 18 72 0 72 C 20 18 76 0 76 D 20 20 84 4 88 E 20 18 76 4 80 Total 100 91 370 18 388 a Type 1, earthenware with a separate central area; Type 2, all others. f NOTES TABLE 4 PERCENTAGE OF ANT TRAPS INFESTED WITH AEDES AEGYPTI LARVAE BEFORE TREATMENT a IN BANG PHO AREA OF BANGKOK Insecticide and Dosage formulation to proposed be used b (ppm) Abate 1% G Abate 1% G Abate 5% G Abate 5% G Abate EC Abate EC Dursban 10% G Dursban 10% G Dursban EC Dursban EC Fenthion 2.5% G Fenthion 2.5% G Fenthion EC Fenthion EC Salt Salt Control 5 10 5 10 5 10 5 10 5 10 5 10 5 10 0.02% 0.04% Average per treatment Week before treatment lAverage 6 100 60 85 60 80 70 70 65 60 30 95 65 50 80 75 20 55 65.9 5 3 95 80 75 85 55 80 75 65 37 20 75 65 80 100 80 25 50 67.2 95 80 75 95 45 85 44 70 63 50 100 70 70 90 65 60 65 71.9 I I IC 70 65 56 90 65 56 60 85 51 40 56 75 45 75 60 70 75 64.4 60 55 90 80 70 70 60 80 55 60 55 85 55 80 60 70 75 68.2 84.0 68.0 76.2 82.0 63.0 72.2 61.8 73.0 53.2 40.0 76.2 72.0 60.0 85.0 68.0 49.0 64.0 67.5 a Average of 5 sections using 20 ant traps per treatment. b G = granules; EC = emulsifiable concentrate. c One day before treatment; percentage of positive ant traps was not significantly different either between treatments or between sections. earliest failure. Traps containing Abate sand granules and Dursban 10% granular formulation at 5 ppm never reached the level of infestation of the controls, even after 23 weeks of study. The number of weeks of protection against re- infestation ranged from 4 to 17.2 weeks (Table 6). Analysis of variance indicates that the protection in weeks of the larvicidal treatments was highly signi- ficant. Dursban granular formulation had an average protection period of about 17 weeks. Abate granules at an applied dosage of 10 ppm were next with a period of 14 weeks. The next most effective treat- ment was 0.04% salt which lasted for more than 11 weeks. In general, granular formulations were better than emulsifiable concentrates (EC) at the same concentration. D-tests for the comparison of the mean number of weeks of protection with the insecticide were made. Of the 119 values, only 6 exceeded the confidence intervals of the D value (Q sx= 10.67) at the 5% level. These were all for the 2 Dursban granular treatments at 5 ppm and 10 ppm which were significantly different from fenthion EC (5 ppm and 10 ppm) and fenthion granules (5 ppm). Discussion Dursban granules and EC (5 ppm and 10 ppm) were the most effective residual larvicides of the 16 treatments tested in earthenware ant traps. The length of protection against natural infestation at 5 ppm and 10 ppm was 16 and 17 weeks, respectively. Treat- ment No. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 I- 322 EFFECTIVENESS OF INSECTICIDES ON AEDES AEG YPTI LARVAE IN THAILAND TABLE 5 PROTECTION OF ANT TRAPS CONTAINING AEDES AEGYPTI LARVAE AFTER INSECTICIDE TREATMENT a RELATIVE TO UNTREATED TRAPS FOUND POSITIVE IN BANG PHO AREA OF BANGKOK Treatment Insecticide and Dosage Week c No. formulation b (ppm) I Abate 1% G 5 >23 2 Abate 1% G 10 >23 3 Abate 5% G 5 13 4 Abate 5% G 10 >23 5 Abate EC 5 18 6 Abate EC 10 18 7 Dursban 10% G 5 >23 8 Dursban 10% G 10 >23 9 Dursban EC 5 16 10 Dursban EC 10 22 11 Fenthion 2.5% G 5 9 12 Fenthion 2.5% G 10 11 13 Fenthion EC 5 5 14 Fenthion EC 10 11 15 Salt 0.02% 10 16 Salt 0.04% 22 17 J Control 0 a Average of 20 ant traps, 5 ant traps per unit. b G = granules; EC = emulsifiable concentrate. CWeek in which larval reinfestation in treated ant traps reached that of controls. Both 1 % Abate sand granules and EC were better than either fenthion granules or EC. In the evalua- tion of the effectiveness of the larvicides, there appeared to be a close relationship between evalua- tion by determining the relative percentage of reinfestation and the number of weeks' protection per ant-trap unit. Abate sand granules (1 %) in ant traps were effective for about 3 weeks more than 5% Abate clay granules at the same dosage. In addition, the granular formulations, except 5% Abate clay gran- ules, were better than the EC formulations. There was also about 5 weeks difference in effectiveness between Dursban granular and EC formulations at the 2 concentrations. The period of effectiveness for salt (0.04%) ranged from 8 to 17 weeks with an average of 11.5 weeks. The protection of 1% Abate sand granules for ant traps was in the same range as for earthenware jars treated at an applied dosage of 1 ppm (Bang & Tonn, unpublished data). The effectiveness of the 3 formulations of Abate was prolonged only 3-4 weeks by increasing the dosage from 5 ppm to 10 ppm (Table 6). Furthermore, when the relative protection of ant traps treated with either Abate EC or 1% sand granules was compared with untreated ant traps, the residual life was not significantly improved by increasing the dosage (Table 5). The present data do not explain these observations. However, most of the ant traps contained various amounts of organic debris. This debris might reduce the residual life of Abate. Another possibility is the rapid evaporation of the water, leading to frequent water exchange which can also reduce the residual life. The protection, in number of weeks, was not TABLE 6 WEEKS OF PROTECTION OF ANT TRAPS BY THE VARIOUS TREATMENTS AND FORMULATION OF INSECTICIDES FROM NATURAL REINFESTATION OF AEDES AEGYPTI LARVAE IN BANG PHO AREA OF BANGKOK Treatment Insecticide and 1 Dosage Average No. formulation a (ppm) persection I Abate 1% G 5 11.4 2 Abate 1% G 10 14.2 3 Abate 5% G 5 7.8 4 Abate 5% G 10 11.8 5 Abate EC 5 9.0 6 Abate EC 10 13.8 7 Dursban 10% G 5 16.4 8 Dursban 10% G 10 17.2 9 Dursban EC 5 11.8 10 Dursban EC 10 12.0 11 Fenthion 2.5% G 5 4.4 12 Fenthion 2.5% G 10 7.4 13 Fenthion EC 5 4.0 14 Fenthion EC 10 4.6 15 Salt 0.02% 6.4 16 Salt 0.04% 11.5 17 Control 0 0.0 a G = granules; EC = emulsifiable concentrate. 323 324 NOTES significantly extended by doubling the dosage of Dursban in both formulations or fenthion EC (Table 6) in treated ant traps. In many cases, when the outer portion of the ant trap was being filled with water, some was spilt into the inner ring. A. aegypti larvae were frequently found there, even though effective control occurred in the outer ring. This indicated that the insecticide was not able to penetrate the earthenware wall about 5 mm thick between the two areas. An Example of Filariasis Control from West Malaysia by T. WILSON, Professor of Tropical Hygiene, Liverpool School of Tropical Medicine, Liverpool, England In West Malaysia, the predominant filarial parasite is Brugia malayi, two forms of which are recognized in man. Wuchereria bancrofti occurs in restricted foci, mainly in isolated rural areas and transmitted by anophelines. The microfilariae of the periodic form of B. malayi have a markedly nocturnal periodicity in the peri- pheral blood; the vectors are anophelines or the Mansonia of open swamps; and infections in animals are rare. Microfilariae of the subperiodic form tend to be present in the peripheral blood throughout the 24 hours with a minor night-time peak; the vectors are the swamp-forest Mansonia; and infections in animals are common. Both forms of B. malayi cause similar clinical manifestations of filariasis-recurrent attacks of fever, lymphadenitis, retrograde lymphangitis and eventually elephantiasis, almost always of the legs below the knee. Treatment with diethylcarbamazine rapidly removes microfilariae of B. malayi from the blood, but this removal is accompanied by a sharp febrile reaction lasting for several days. In spite of this drawback, preliminary trials in Malaysia showed that mass treatment with diethylcarbamazine could be used as a method of filariasis control, and control teams were organized in the various States of West Malaysia where endemic filariasis was known to exist. The writer was given the opportunity of examining the work of these control teams between May and August 1968 as a short-term consultant on filariasis for the World Health Organization. Procedure Each 4-man control team is headed by a hospital assistant or health inspector. After a preliminary house-to-house census of each settlement or village, a night-blood survey is carried out; 20 mm3 of blood are collected from each individual by finger-prick, spread as a thick smear, dried overnight and stained with dilute Giemsa. The whole area of the film is examined, and the number of microfilariae and their species are recorded. At varying intervals after the blood survey, the team returns to administer treat- ment. Each person is weighed, and is given a dose of 5 mg of diethylcarbamazine citrate per kg of body-weight; the dose is seen to be swallowed. The team either stays in the settlement or revisits it daily for the next few days to give symptomatic treatment for the febrile reactions which develop in micro- filaria carriers within the first 24 hours. Similar supervised doses are given at weekly intervals to a total of 6 doses. This may involve 8-10 visits by the team in order to ensure that a high proportion of the persons in each settlement com- plete the treatment course with a total dose of approximately 30 mg/kg: the proportion who do so is reported to be about 80% in many areas. Follow- up blood surveys are made at varying intervals after the initial mass treatment, in many areas within about 2 years, and further treatment is limited to those found infected at these resurveys. Results The results from some areas where regular follow-up surveys have been made are summarized in Tables 1-4. The parasite in Kedah is periodic B. malayi, which probably has no animal reservoir of infection. The results indicate that the great reduction in micro- filaria rate obtained by the initial mass treatment can be maintained for years by periodic resurveys and the treatment of those found infected (Table 1). 2371H