46Ravindran and WilliamsMATERIALS AND METHODStest of independence was per<strong>for</strong>med to establishChennai (longitude: 80° 15'E, latitude: 13° 05'N) isthepreferences towards breeding at various well depths.capital city of the state of Tamil Nadu, India, and is Habitat specific preferences were studied in fewsituated in the northeastern part of the state. Minimum selected habitats namely open wells, bed pools,mean and maximum mean temperatures recorded puddles and hoof-prints present in outskirts of periduring1990–2000 were 19.8° C and 38.5° C, urban localities with or without ongoing agriculturalrespectively. Maximum rainfall is during Northeast activity. Any water stagnation with a surface area ofmonsoon (October–December). <strong>The</strong> study was2less than 1 m was classified as a puddle and those withundertaken in Chennai metropolitan area in western2 2more than 1 m but less than 50 m as bed pool.outskirts of the city during the period 1992–1999. Random surveys were undertaken and immaturesAgricultural activities have been totally withdrawn in were collected using well nets (wells), ladles (bedareas bordering the city due to ongoing urbanization pools and puddles) and spoons (hoof-prints).and partially withdrawn in areas away from the Interspecific association and index of association inoutskirts of the city. <strong>The</strong> area represents an ecological relation to breeding habitats of various mosquitotransition zone.vector species were calculated using the method ofCole (1949) and Whittaker and Fairbanks (1958).Studies on mosquito breeding in rice fields wereDuring sampling, immatures collected were countedcarried out in agricultural fields located atinstar-wise. Fourth instar larvae and pupae werePoonamallee and Thirukandalam areas, which are 10brought to laboratory and reared until emergence <strong>for</strong>km and 20 km away from the outskirts of Chennai.identification.Observations were carried out from September 1992to September 1993, covering one long-duration and RESULTS AND DISCUSSIONtwo short-duration crops. Five fields were selectedand regularly observed <strong>for</strong> mosquito breeding and A total of 11 Anopheles, 10 Culex and four Aedespopulation structure on a weekly basis during each species were collected from various breeding habitatscrop. A quadrat with a dimension of 33 x 33 x 20 cm in the study site (Table I). Among important disease2enclosing an area of 0.1m was used as a sampler vectors, An. stephensi and An. culicifacies, the urban(Chandrahas, 1990). Ten samples were taken/plot. and rural malaria vectors, respectively, Cx.Sampling was confined to previously earmarked tritaeniorhynchus, Cx. vishnui and Cx. pseudovishnui,sampling sites, which included four corner, four the potential vectors of JE in peninsular <strong>Indian</strong> regioncentral and two random sites within 0.5 m from the and Ae. aegypti, vector of dengue were present. In ricebund. During each sampling occasion, height of rice fields, no malaria or filaria vectors were observedplant, depth of standing water and peak noon water breeding. Similar results were obtained at Maduraitemperature were recorded with other in<strong>for</strong>mation (John Victor and Reuben, 1999) and Pondicherrysuch as fertilizer used.(Chandrahas and Rajagopalan, 1979). Among vectorsof JE, Cx. tritaeniorhynchus was predominant beingIn Irrigation wells, study period covered a pre- encountered during all the 25 weeks of observationmonsoon (August–October) and post-monsoon carried out over a period of three rice growing seasons,(November–February) period during 1998–1999. whereas Cx. vishnui and Cx. pseudovishnui wereObservations were carried out in irrigation wells in observed breeding during seven and one occasionsriceland areas in Vanagaram area near Poonamallee, only. Breeding pattern of Cx. tritaeniorhynchuswhere irrigation wells suitable <strong>for</strong> the study were differed from observations of Reuben (1971) andfound. <strong>The</strong> conditions <strong>for</strong> selection of a well were Rajendran and Reuben (1991) who reported intenseabsence of fishes, no floating vegetation and breeding at the latter stages of plant growth. In Tamilaccessibility <strong>for</strong> sampling from all sides. Weekly Nadu, JE virus isolations have also been reported inobservations were carried out in ten irrigation wells. A An. subpictus, Cx. infula, Cx. whitmorei and Cx.well net with a 20 cm diameter was used as a sampler. fuscocephala, all of which breed in ricefields (PhilipImmature samples were collected by pulling well nets Samuel et al., 2000). Except Cx. infula and Cx.through a distance of 1 m along the edges of the wall. whitmorei, all other species were found breeding inRepeated sampling in the same area was avoided. rice fields.Depth of water table and peak noon water temperaturewas recorded during each visit. Chi square analysis <strong>for</strong> <strong>The</strong> density of immatures during the three cultivation
Mosquito breeding in riceland agro-ecosystem near Chennai47seasons is given in Fig. 1 and 2. Anopheles and Culexdensity was high during first week aftertransplantation. <strong>The</strong>reafter, intensity in breedingdecreased. In Culex species, another peak in thedensity of immatures was observed during fourth tosixth week after transplantation. Standing watermaintained <strong>for</strong> a period of two weeks aftertransplantation provided opportunity <strong>for</strong> intenseexploitation of the habitat <strong>for</strong> breeding. Reduction inthe density of immatures after this period, is due toalterations in daily irrigation practices that arecontrolled by various factors like soil texture, acreageand the number of crops raised in a year, erratic powersupply and rainfall. Fields are usually irrigatedimmediately after cessation of standing water tomanage adequate water supply. Such en<strong>for</strong>cedintermittent irrigation due to lack of sufficientirrigation water has also been reported by Rajagopalanet al. (1990) and Russell et al. (1942) in areas in theadjacent erstwhile South Arcot district. In addition,rice cultivation practices such as deweeding,application of fertilizer and pesticides also reduce thedensity of immatures. In the present study, nopesticides were applied though application of urea as afertilizer was undertaken be<strong>for</strong>e and three weeks aftertransplantation.50004500400035003000Density/m 2 250020001500100050000 I II III IV V VI VII VIII IX XWeeksTransplantationAug/Sep Jan/Feb Jun/JulFig. 1. Anopheles immature density in different rice growing seasons.50004500400035003000Density/m250020001500100050000 I II III IV V VI VII VIII IX X2Aug/Sep Jan/Feb Jun/JulTransplantationWeeksFig. 2. Culex immature density in different rice growing seasons.
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