Susceptibility of Culex pipiens fatigans to fenthion in Rangoon, Burma, 1963-69.

MALATHION-RESISTANT ADULT ANOPHELES ALBIMANUS IN EL SALVADOR 631 Georghiou (1969) discussed the resistance poten- tial of anophelines and indicated that, to his know- ledge, no consistent and direct treatment by choline- sterase inhibitors (organophosphorus compounds or carbamates) had been applied against field populations of anophelines, except on a very limited basis. He further pointed out, however, that indirect selection has been taking place through the agri- cultural use of large quantities of organophosphorus compounds and carbamates within the proximity of Anopheles breeding areas. He recognized incipient tolerance to malathion in An. albimanus in Central America, particularly in Guatemala and Nicaragua, where intense parathion selection pressure is applied for six months each year in the cotton-growing areas. Anopheles mosquitos inhabiting areas in close proximity to cotton acreage in El SalVador have likewise been subjected to such organophosphorus pressure and the data presented here show a correla- tion between the cotton-growing areas and resistant populations of An. albimanus. REFERENCES Georghiou, G. P. (1969) Wld Rev. Pest Control, 8, 86-94 Mobley, L. A. (1955) Cotton in Central America, National Cotton Council of America, Memphis, Tenn. Smith, R. F. & Reynolds, H. T. (1968) Effects ofmanipula- tion of cotton agro-ecosystems on insect pest popula- tions. In: Conference on the Ecological Aspects of International Development, Warrenton, Virginia, 9-11 December 1968 Susceptibility of Culex pipiens fatigans to Fenthion in Rangoon, Burma, 1963-69 * by L. S. SELF1 & MAUNG MAUNG TUN2 The only locality in Rangoon with a history of organophosphorus spraying is Kemmendine. Lar- viciding with fenthion (OMS-2) emulsifiable concen- trate began in March 1966. The susceptibility of Culex pipiens fatigans larvae has been tested in Kemmendine and 9 other locali- ties of Rangoon during the past 6 years. The data were analysed, in 1969, for the entire period, using the computer at WHO headquarters, Geneva, Switzerland, and reference can be made to the WHO Information Circular on Insecticide Resistance 3 for the complete results. The 95% fiducial limits for the LC,0 and LC95 values, and the slope of the probit regression line are given for each larval test. * From the WHO Filariasis Research Unit, Rangoon, Burma. 1Entomologist. Present address : WHO Japanese Encephalitis Vector Research Unit, Seoul, Korea. ' Entomologist, seconded from Directorate of Health, Burma. ' Information circular on insecticide resistance, insect behaviour and vector genetics, an unpublished WHO docu- ment, VBC/IRG/70.9. A limited number of copies of this document is available to persons officially or professionally interested on request to Distribution and Sales, World Health Organization, 1211 Geneva, Switzerland. In this report, the computed LC50 and LC,5 values on the computer print-outs are summarized according to locality and year. A conclusion will be given on whether any appreciable change has occurred in the susceptibility of C. p. fatigans during 31/2 years of larval control. Methods Larval tests. The WHO procedure (WHO Expert Committee on Insecticides, 1963) was used through- out, and fenthion solutions were routinely despatched to Rangoon. The experience gained from many tests showed that 10 concentrations ranging from 0.001 ppm to 0.01 ppm normally provided 3-5 mortality counts between 10% and 90%. Disposable plastic cups holding 250 ml of demineralized water were preferred to glass beakers which required repeated cleaning. After control in Kemmendine, it was never possible to obtain 3 or more replicates for any test. When larvae became extremely scarce due to progressive reductions in adult density, suitable re- gression lines were often obtained using concen- trations of0.002,0.0035, 0.004,0.0045 and 0.006 ppm. 2586J NVJ 1iE Sometimes only a few larvae were found and were exposed only to 0.02 ppm. Only field larvae were exposed after 1966: pre- viously, larvae were sometimes derived from wild gravid females. After control in Kemmendine larvae were found during standardized larval surveys, and clean dippers and other equipment were set aside for the collections. Adult tests. A few tests were carried out in 1964 and 1969 in Kemmendine and two other localities. Adult females were reared from field-collected larvae, offered glucose, and exposed for 1 h when 1-2 days old. Mortality counts were made after 24 h. The 6 concentrations of impregnated papers in the WHO kit did not always provide a suitable range of mortalities because some populations had no survivors at 0.2% and 0.4 %. The tests, however, were satisfactory for detecting possible resistance and for obtaining comparative data. In 1969, some females were directly exposed after collecting them off human bait and resting inside houses. The collections in Kemmendine posed a considerable challenge because 1 man working for 1 hour could be expected to capture only about 1 adult mosquito. Results Larval tests. The results are shown in Table 1. In unsprayed localities, the mean LC50 values fell within the narrow range of 0.0025 ppm-p.0041 ppm. Larvae from 5 widely separated localities had LC80s ranging from 0.0031 ppm to 0.0034 ppm. Moreover, the variations in the LC,5 values were not great (0.0055 ppm-0.0093 ppm). It is quite evident that larval populations in Rangoon and out- lying suburbs (north and south Okkalapa) did not differ markedly in their susceptibility to fenthion. In the 3 successive years of the control operation at Kemmendine the larval LC50 remained at a level which was not significantly greater than the highest pre-spray LC50 values obtained. The LC95s were variable but also showed that susceptibility had not decreased. The values still remain similar to data in unsprayed localities. The few Kemmendine larvae exposed only to 0.02 ppm died. The highest LC50 and LC,,5 values recorded in Kemmendine after control are 0.0067 and 0.012 ppm, respectively. Corresponding values for unsprayed localities were 0.0084 (south Okkalapa) and 0.034 ppm (Myoma), respectively. The range of susceptibility values for all larval tests (159) is shown in Table 2. The results indi- cate that the regression lines for the Kemmendine population have not been displaced. Many of the LC96 values in Kemmendine after control fell between 0.004 and 0.005 ppm. A colony established in January 1966 from field material in Kemmendine was routinely tested throughout August 1969. There was little variation in its response to fenthion, and the mean LC50 and LC.5 values were 0.0028 and 0.0051 ppm, respectively. Adult tests. The susceptibility of all populations reared from larvae was similar, and the results are shown in Table 3. The wild adults in Kemmendine appear to have been less susceptible than those in Ahlone: the mortalities, at 0.2% and 0.4 %, were much lower for the former. No wild adult in Kemmendine survived the highest concentration (3.2%Y.) to which they were exposed. Discussion The slight decrease in larval LC95 values after control should be noted. By 1969, all the populations tested were derived from Pegu jars and water drums except one that was obtained from a soakage pit (LC,5 of 0.0096). The slightly steeper regression lines for these container populations may be asso- ciated with their reduced genetic range and also with the smaller number of replicates that was used for those tests. Oviposition by two females could produce enough offspring for a larval test, whereas the larger populations in the unsprayed polluted sites must be derived from many more parents. Rosen's (1967) observation that larvae show a high susceptibility during the peak monsoon rains is interesting because it coincides with the period of lowest adult density, except in those localities (south and north Okkalapa) with many pit latrines. The residual field dosage (1.0 ppm) in Kemmendine is about 100 times the laboratory LC.5 value. There has been no field or laboratory evidence that this dosage has failed to kill all larvae in drains and other polluted sites, at least those larvae present at the time of spraying. An explanation for the differences in susceptibility of larval-reared adults as opposed to wild adults in Kemmendine cannot be given without additional studies. The wild adult population in Kemmendine after 1966 had the distinctive feature of being older than populations in unsprayed localities, and also of emerging from breeding sites with much lower larval densities (WHO Filariasis Research Unit, unpublished data). It is also worth noting that SUSCEPTIBILITY OF CULEX P. FATIGANS TO FENTHION IN RANGOON, 1963-69 q c 0 c a Co 0 c o~~~ 01% . o co co o o 0 o 0400~~~~~~~~~-30 *e S S o Co go 660 iC o o o co 0 C 00 0q ~~~~~~qc*qC; 5 CD_to c oco o o o o o o o~~~~~2 o o C_D 0 C4~~~~~0 04~ c6 Cccc -0 0 c 0 004 0oBo o o cc cc c cX o. o c*o q.oocc cc c c ~ ~ ( _ 0 -- 0 In ~ ~ ~ 0c,xnu o~~~ co.5 o. . o . o . .o o ~ ~ oq c c 0 Cs04 _04_0 0 h04 _ N ot S q 0o 0 a5 =_ UH 0 F- corCD o. 0 S 0 -i IIS -J F1-goc ci aa0 q o 0C o ~ ~ ~ ~ ~~~~~~~o o 04 ~ ~ ~ c~~~~~~c 0 C to o c o o o 0 C go N- c~~~~~~~~~~~C D_ qS LO qO~~~~~~~~i LO w Lor- go go~ ~~ o S 1t q R q F--~~0. . 0. OlO _ d 6d J6 o ~ o o go o _ e r- c o qo q o 0 1 .04 18 o o o IoI0 ___ _ _ _ NI, c_ _ _ _ _ _ Ic Igc,, 0°o o t {o |N cc c c CO oe 0~- 0 =a- -H U) 633 m I- II- I~ C I o oE~ 1-C C 0 0 Cs 0 LUI -J I-- z 0 0 UlE z v- z I U. LU -J cn z cn Un IL (D 0 tn S co 0 'D1- 0-0 r c CMc c O 0 0-J- o o 0 0 s 3C ,CC -0Z0 0-.-- .0 * 0 ZCCO 10 TABLE 2 THE RANGE OF SUSCEPTIBILITY VALUES FOR 159 LARVAL POPULATIONS OF C. P. FATIGANS IN RANGOON No. of tests with LC*os within No. of tests with LC,ss within Concen- concentrations shown concentrations shown trations of fenthion Kemmendine Kemmendine Kemmendine Kemmendine(ppm) before after Other before after Other control a control b localities c control a control b localities c <0.001 1 I d I d >0.001 4 10 13 >0.002 4 15 27 4 6 >0.003 4 17 22 2 9 3 >0.004 1 6 17 2 15 18 >0.005 5 5 1 7 20 >0.006 3 2 5 11 >0.007 1 1 6 11 >0.008 1 4 4 >0.009 2 2 1 >0.01 6 4 9 >0.02 2 >0.03 3 a From July 1963 to December 1985. b From October 1966 to September 1969. c From July 1963 to September 1969. d The lowest LCse and LC.s values recorded were 0.00059 and 0.00070 ppm, respectively, In south Okkalapa in 1963. TABLE 3 SUMMARY OF ADULT SUSCEPTIBILITY TESTS WITH FENTHION AGAINST C. P. FATIGANS IN RANGOON Total no. Mortality (%) at concentrations shown LCsI LC,sYear Source of adults of females 5 ; exposed control 0.1% 0.2% 0.4% 0.8% 1.6% 3.2% ° Kemmendine 1964 Reared from larvae 1 500 0.0 17.7 58.4 94.4 100 0.18 0.41 1969 Reared from larvae 1 074 0.0 8.7 52.6 84.2 100 0.19 0.50 1969 Captured In field 304 0.0 0.0 10.9 66.6 94.8 97.4 100 0.33 0.82 Ahlone 1964 Reared from larvae 300 0.0 1.7 46.7 90.0 100 0.21 0.45 1969 Reared from larvae 355 3.0 18.5 56.0 93.8 100 0.18 0.44 1969 Captured in field 700 0.0 15.0 44.0 91.5 100 0.22 0.45 Kyaukmyaung 1964 Reared from larvae 500 0.0 2.0 45.0 95.2 100 0.21 0.40 1969 Reared from larvae 375 0.0 6.8 58.0 97.2 100 0.18 0.36 634 NOTES SUSCEPTIBILITY OF CULEX P. FATIGANS TO FENTHION IN RANGOON, 1963-69 635 the wild adults tested in Kemmendine had to be collected from many localities within that area. In conclusion it can be said that the results of this study have indicated that no appreciable change has occurred in the larval susceptibility of C. p. fatigans to fenthion despite 31/2 years of larval control. There has also been no evidence that adults in this area have become resistant to fenthion although a 2-fold increase in adult susceptibility values may have occurred. REFERENCES Rosen, P. (1967) Bull. Wld Hlth Org., 37, 301 WHO Expert Committee on Insecticides (1963) Wld Hlth Org. techn. Rep. Ser., No. 265, pp. 51-56 A Battery-Operated Light-Trap for Sampling Mosquito Populations * by M. W. SERvicE 1 Light-traps have been extensively used in North America for sampling mosquito populations. More recently they have been used to sample Anopheles (Sun, 1965; Chamberlain et al., 1964), including malaria vectors (Odetoyinbo, 1969). Some groups and species of biting flies are caught mainly in traps using lights emitting large amounts of ultraviolet radiation as well as visible light (Barr et al., 1963; Breev, 1963; Davies & Williams, 1962; Williams & Davies, 1957), while others are caught in greater numbers in traps having black lights which emit only, or predominantly, ultraviolet radiation (Belton & Pucat, 1967; Breev, 1958; Rowley & Jorgensen, 1967). Incandescent lights have also proved useful (Downey, 1962; Odetoyinbo, 1969; Service, 1969a, 1969b; Sudia & Chamberlain, 1962). Recently Mangum & Callahan (1968) showed that near- infrared lamps attracted Aedes aegypti (L.), a species not attracted to white light. Because of the recent interest in the possibility of sampling malaria vectors with light-traps, a small, portable, battery-operated light-trap was designed that could use different light sources. Description of trap Basically the trap (Fig. 1) is similar to the Penn- sylvania light-trap (Frost, 1957), except that, as in the CDC trap (Sudia & Chamberlain, 1962), a small fan is used to draw the catch down into a collecting bag. * This work received financial support from the World Health Organization. I The Nature Conservancy, Monks Wood Experimental Station, Huntingdon, England. FIG. 1. A MONKS WOOD LIGHT-TRAPa ........... ........... -04 a The slit In the collecting bag Is shown open. The light- sensitive photoelectric cell (arrowed) Is mounted on top of the ballast box In front of the switches. 2586K