Studies on the dispersal of Aedes Aegypti at two localities in Delhi

WORLD HEALTH ORGANIZATION ORGANISATION MONDIALE DE LA SANTE WHOjVBC/72.388 ENGLISH ONLY STUDIES ON THE DISPERSAL OF AEDES AEGYPTI AT TWO LOCALITIES IN DELHI l R. Reuben, M. WHOjiCMR Research Unit 2 & 3 Ring Road, by 2 . 3 Yasuno and K. N. Pan1cker on the Genetic Control of Mosquitos Kilokri, New Delhi-14, India ABSTRACT The dispersal of a wild population of Aedes aegypti was compared with that of two laboratory strains, one heterozygous for the silver thorax marker and the other for a chromosomal translocation. Most wild mosquitos . . ~ ' were recovered within 50 m of the breeding site. However, females dis- persed significantly further than the males. The dispersal of males of the silver marker strain did not differ from that of wild males. Most recoveries of the silver marker and the translocation were from ovitraps within 50 m of the release point, although the maximum distance from which a recovery was made was over 200 m away. When mosquito control by genetic methods is considered, it is essential to know whether released insects will disperse satisfactorily into a natural population. As part of a programme investigating the feasibility of this approach, studies on the dispersal of Aedes aegypti from their breeding place were made at two sites in Delhi wi~h mosquitos captured in the field and two laboratory reared strains. At one site preliminary releases with males heterozygous for a chromosomal translocation, and at the other, males heterozygous for the marker silver thorax (Rai, 1971), were made, to compare the dispersal of these two strains to that of field mosquitos. Methods The study sites previously described by Reuben et al. (1972) were two tire dumps and adjacent areas at Model Basti and Shastri Nagar. Two methods of evaluation were employed; namely, the capture of marked adults within and at different distances away from the dump, and in the case of genetically marked males, the recovery of eggs from black jar ovitraps (Fay & Eliason, 1966) situated at various distances from the release point. Field adults were captured landing on human bait or resting indoors, marked with fluorescent dyes and metallic dusts as described by Reuben et al. (loc. cit.) and released in the'dump. Males heterozygous for the silver marker were also released at a single point in the Shastri Nagar dump. Recaptures of coloured and genetically marked mosquitos were made in the dump and in 1 Senior Scientist, WHO/ICMR. 2 WHO Ecologist. 3 Research Assistant, WHO/ICMR. The issue of this document does not constitute formal p:.~blication. It should not be reviewed, abstracted or quoted without the agreement of the World Health Organization. Authors alone are responsible for views expressed in signed articles. Ce document ne constitue pas une publication. II ne doit faire !'objet d'aucun compte rendu ou resume ni d'aucune citation sans l'autorisation de !'Organisation Mondiale de Ia Sante. Les opinions exprimees dans les articles signes n'engagent que leurs auteurs. wHojvBc/72.388 page 2 houses for several days and eggs were recovered from ovitraps in the weeks following the releases. From August to October paddles in the ovitraps were changed three times a week and at weekly intervals in November and December. Eggs from Model Basti, where the trans- location heterozygous males were released, were conditioned by being kept moist for 72 hours and then checked for hatchability. Since the average hatchability of normal eggs was 95%, paddles showing less than 80% egg hatch were considered to be the result of a cross between males carrying the translocation and normal females. Larvae hatching from eggs from Shastri Nagar were raised in the laboratory and the resulting adults checked for the presence of the silver marker. Results 1. Dispersal of field mosquitos At Shastri Nagar the distribution of field adults was determined by comparing the average number of individuals collected per dwelling at various distances from the dump. All mosquitos were presumed to have originated in the dump since little breeding other than at this site was observed. Between 22 and 29 June collectors remained in each house until they could find no more mosquitos, whereas from 20 June to 12 July they collected for 20 minutes per house. In both cases the number of mosquitos collected per structure fell rapidly as the distance from the dump increased, and between 85-98% of the mosquitos collected were within 30m of the, dump (Table 1). Females flew further from the dump than did males, as indicated by the rapid decline in the recapture of males as the distance from the dump increased. The recapture rates for males and females within the first 20m from the dump were significantly different (X2 = 6.05, P<0.05, d.f = 2) for data collected between 29 June and 12 July. Additional evidence that female mosquitos flew further than the males is provided by a study of the sex ratio in captures of landing and resting mosquitos in and around the dump. The sex ratio of adults raised from eggs from Shastri Nagar was 1:1. However, in collections from within the dump the ratio of males to females was 1:1.2 and in collections from houses outside the dump the ratio was 1:4.2, indicating that more females than males had left the dump and entered houses. Between 29 June and 4 July, 4101 field mosquitos (1157 males + 2044 females) marked with fluorescent and metallic dusts were released at various sites within the Shastri Nagar dump. There was a gradual movement of the mosquitos from the dump as the study progressed. One day after release, 79% of the recaptures were made within the confines of the dump, on the second day 26% of them were in the dump, 66% within 10 m of it and 7% beyond 10 m, on the third day 16% of recaptures were within the dump, 72% within 10m and 12% beyond this distance and by the fourth day only 3% were captured in the dump, 73% within 10 m of it and 24% beyond 10m (Table 2). 2. Dispersal of genetic marker strains On the afternoons of 2 and 3 August, 2072 silver males were released at a single point within the Shastri Nagar dump. Recaptures were made within the dump and in the houses for three days following releases. Most of the recaptures were within 30m of the dump (Table 3) and mainly in buildings to the north and the south where the houses shared a common wall with the dump. The dispersal of the silver males was very similar to that of the dispersal of field males described above. Between 7 and 27 August a total of 49 134 silver males was released again in the Shastri Nagar dump. Adults from eggs from 50 ovitraps were checked weekly after 7 August for the presence of the marker (Table 4). In the first week the marker was recovered within and close to the dump. In the second week it was recovered from an ovitrap 100m away. Most of the recoveries were however, from within 50 m of the dump, but recoveries from bvitraps inside the dump were relatively few. Fig. 1 shows the locations of ovitraps which were positive for the marker. WHOjVBC/72.388 page 3 At Model Basti 46 215 translocation heterozygous males were released in the dump between 8 and 30 August. Eggs were collected from 72 ovitraps and the distance from the release site of those traps which showed reduced hatchability in the weeks after the initiation of releases is shown in Table 5. The results obtained were similar to those obtained at Shastri Nagar. Reduced hatchability was first noticed in paddles from traps in and close to the dump and later from traps further away from the dump. The maximum distance reached was more than 200 m in four weeks. Relatively few paddles from the traps in the dump and at its periphery showed reduced hatchability. Discussion Although in desert conditions Aedes aegypti has been recaptured as far as 2.5 km from the point of release (Wolfinsohn & Galun, 1953), most studies indicate the average distance flown by this species to be less. Sheppard et al. (1969) found that they averaged 37 m in 24 hours. In the present studies it appeared that dispersal was limited to less than 50 m from the dump. However, the number of houses sampled or ovitraps distributed per unit area decreased with distance from the point of release. Thus there may be more emigration from the dump than appears from the results obtained and further studies are required to derive a quantitative expression for outward movement. Recent studies on dispersal, using ovitraps as well as adult traps to detect movement, have shown that genetically marked A. aegypti also have a relatively short flight range (Fay & Craig, 1969, Bond et al., 1970 and Hausermann et al., 1971). Males of the silver marker strain moved 38 m downwind in one day (Fay & Craig, 1969) and females of the same strain were captured about 200m and 300m from the release site a few hours after release. About 70% of all eggs laid by silver marker females were within 500 m of the release point and 99% within 600 m. (Hausermann et al., 1971). Possibly because most ovitraps were situated close to the release point (less than 200m) dispersal in the Delhi studies was considerably less. Sheppard et al. (1969) found no difference in the distances flown by males and females. However, at Shastri Nagar female mosquitos dispersed further, probably because of the lack of blood sources at the dump. The male has no need for a blood meal and would not disperse as readily from the breeding site. At both sites breeding was in used tires. Since tires are more attractive to ovipositing females of A. aegypti than ovi trap.s; (Bond & Fay, 1969) those traps adJacent to tires would have reduced efficiency resulting in a relative infrequency of recoveries. This was evident in our studies as few recoveries of the marker were made from ovitraps within and close to the dump. Hausermann et al. (1971) demonstrated differences in dispersal of various genetic strains of A. aegypti. However, at Shastri Nagar dispersal of males heterozygous for the silver thorax marker did not differ from that of field males and only a slight difference between the dispersal of the adult mosquitos and the recoveries of the silver marker.from ovitraps was observed. Acknowledgement We acknowledge gratefully Dr K. S. Rai, Notre Dame University; this study was performed with mosquitos released for his genetic studies in the field. We are also indebted to Drs S. G. Suguna and K. K. Grover who identified markers. Our thanks are also due to Dr G. C. LaBrecque, ProJect Leader, for his constant help and to Dr W. W. Macdonald for a very valuable discussion. Finally, we are grateful to Mr Darsham Singh, Social Scientist, and Miss R. Jolly, Health Educator, for their help in the field and to the Biological Aides of the unit for their good work. WHO/VBC/72. 388 page 4 TABLE l. DISTRIBUTION OF MOSQUITOS IN BUILDINGS IN THE VICINITY OF SHASTRI NAGAR TIRE DUMP June 22-29 June 29 - Jul_y_ 12 Distance from No. of bldgs. Av.no./bldg. No. of bldgs. Av. no ./bld__g_. dump (m) searched Males Females searched . Males Females 0 1 24.0 38.0 1 22.5 39.1 1-10 16 7.9 26.3 16 6.1 29.1 11-20 11 0.4 1.6 8 1.6 12.1 21-30 10 o.o 1.7 ·13 0.9 4.6 31-40 3 o.o 0.3 6 0.6 3.0 41-50 9 o.o o.o 9 0.3 1.0 so + 46 0.1 0.1 15 0.1 0.2 TABLE 2. DISPERSAL OF MARKED MOSQUITOS RELEASED WITHIN THE TIRE DUMP AT SHASTRI NAGAR !Days after Sex.!:: release 0 I I l M 49 F 125 2~ M 3 F 19 3 M 0 F 7 4 M 0 F 1 5 M 0 F 2 6 M 0 F 0 7 M 0 F 1 8 M 0 F 0 ~ lM - 31-40 m; IF - 51-60 m. b - M = Male; F = Female. Distance from dum_E_ 1-10 11-20 9 3 31 1 4 0 49 2 2 0 29 2 1 0 21 5 0 0 11 1 0 0 6 1 0 0 7 1 0 0 1 0 (meters) 21-30 1 3 1 1 0 3 0 2 0 1 0 1 0 0 0 0 TABLE 3. DISPERSAL OF SILVER MALES FROM A RELEASE POINT IN THE SHASTRI NAGAR TIRE DUMP. 2072 MALES RELEASED ON THE AFTERNOONS OF 2 AND 3 AUGUST, AND RECAPTURED IN THE MORNINGS FROM 3-5 AUGUST !Distance from No. of No. of males captured WHOjVBC/72.388 page 5 dump collections Total per collection 0 12 27 2.2 1-10 44 65 1.3 11-20 6 4 o. 7 21-30 12 5 0.4 31-40 5 0 o.o 41-50 12 3 0.3 51 + 23 0 o.o . WHO/VBC/72.388 page 6 Date Aug. 13 20 27 Sept. 3 10 17 24 Oct. 1 8 15 22 29 Nov. 5 12 19 26 Dec. 3 10 17 Total TABLE 4. RECOVERY OF THE SILVER THORAX MARKER FROM OVITRAPS FOLLOWING THE RELEASE OF MALES HETEROZYGOUS FOR THE MARKER AT SHASTRI NAGAR FROM 7-27 AUGUST. TOTAL NUMBER OF OVITRAPS USED IS INDICATED IN PARENTHESIS AND FIGURES IN THE BODY OF TABLE INDICATE NUMBER OF PADDLES POSITIVE FOR SILVER THORAX MARKER Distance of ovitraps from_l)_er~her_y_ of dum.J2. Dump 0-25 m. 25-50 m. 50-75 m. 75-100 m. (4) (16) (8) (3) (3) 1 1 0 0 0 1 2 1 0 1 0 0 0 0 0 1 3 0 0 0 1 2 1 0 0 0 0 1 0 0 0 3 2 0 0 1 3 1 0 0 0 2 1 0 0 1 0 2 0 0 0 3 2 0 0 0 1 0 0 0 0 4 1 0 0 0 2 0 0 0 0 0 0 0 0 0 2 0 0 0 0 1 0 0 1 0 0 0 0 0 1 0 0 0 0 7 29 12 0 2 100-200 m. (16) 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 2 Date Aug. 9 16 23 30 Sept. 6 13 20 27 Oct. 4 11 18 25 Total TABLE 5. RECOVERY OF STERILE EGGS FROM OVITRAPS FOLLOWING THE RELEASE OF SEMISTERILE MALES AT MODEL BASTI FROM 8-30 AUGUST. TOTAL NUMBER OF OVITRAPS USED IS INDICATED IN PARENTHESIS AND FIGURES IN THE BODY OF TABLE INDICATE NUMBER OF PADDLES WITH EGGS SHOWING LESS THAN 80% HATCHABILITY wHojvBc/72.388 page 7 Distance of ovitraps from periphery of dump Dump Periphery 0-25 m. 25-50 m. 50-75 m. 75-100 m. 200 + m. (10) (10) (15) (12) (13) (5) (7) 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 1 1 0 0 0 0 0 1 2 0 0 0 0 0 1 0 0 0 1 0 2 5 1 0 0 1 1 1 4 2 1 0 0 0 2 3 0 0 0 0 1 2 2 0 0 0 0 1 2 3 0 3 0 0 0 0 0 0 0 0 0 '() 0 0 0 1 0 0 I 3 10 20 i 7 5 I Q 2 1 ' I i WHojvnc/72. 388 page 8 0:: C( (!) C( z 0:: ..... U) <l X U) (/) UJ a: 1- UJ :~:g 0 ¢ il ~ 0 N Q FIG. 1. MAP OF SHASTRI NAGAR SHOWING LOCATIONS OF OVITRAPS FROM WHICH THE SILVER MARKER WAS RECOVERED z .. 0 C( _. ct z a:: a:: U..IUJ :w:::w:: a: a: <let ::!2 a: a:: UJUJ >> _._. ii5ii5 a: a: oo IL.IL. U..II&J >> + I 11.11. ct<t a: a: 1-1- >> oo eo REFERENCES Bond, H.A., Craig, G.B. and Fay, R.W. (1970) Field mating and movement of Aedes aegypti. Mosq. News 30 : 394-402. Bond, H.A. and Fay, R.W. (1969) Factors influencing Aedes aegypti occurrence in containers. Mosg. News 29 113-116. Fay, R.W. and Craig, G.B. (1969) Genetically marked Aedes aegypti in studies of field populations. Mosq. News 29 121-127. Fay, R.W. and Eliason, D.A. (1966) A preferred oviposition site as a surveillance method for Aedes aegypti. Mosq. News 26 531-535. Hausermann, W., Fay, R.W. and Hacker, c.s. (1971) Dispersal of Genetically marked female Aedes aegypti in Mississippi. Mosq. News 31 : 37-51. Rai, K.s. (1971) Personal communication. Reuben, R., Yasuno, M., Panicker, K.N. and LaBrecque, G.C. (1972) The estimation of adult populations of Aedes aegypti at two localities in Delhi. In preparation. Sheppard, P.M., Mac~onald, w.w., Tonn, R.J. and Grab, B. (1969) The dynamics of an adult population of Aedes aegypti in relation to dengue haemorrhagic fever in Bangkok. J. Anim. Ecol. 38 661-702. Wolfinsohn, M. and Galun, R. (1953) A method for determining the flight range of Aedes aegypti (L). Bull. Res. Council of Israel 2 433-436. WHOjVBC/72.388 page 9