A larval survey of dengue vectors was conducted from July to November 1966 and from May to November 1997 in Chiangmai Province, Thailand. Three villages in urban, transition, and rural areas were selected for the survey to clarify the spatial distribution of Ae. aegypti and Ae. albopictus along an urban-rural ecological gradient. The average number of Ae. aegypti larvae in larvitraps was higher in the urban area than in the rural area, as we expected, whereas the opposite was found for Ae. albopictus, rural area > urban area. A house survey of larvae-inhabiting containers showed significant differences in the number and composition of these containers among the study areas. Significant differences were also found in the average distance between houses, average tree height, and average percentage of vegetation cover for each house. The seasonal pattern of rainfall recorded in each study area did not show great differences among the study areas. The response of Ae. aegypti and Ae. albopictus to the urban-rural gradient is discussed in relation to the possibility of applying geographic information system techniques to plan the control strategy and surveillance of dengue vectors.

SummaryWe analysed the population genetic structure and differentiation regarding vector competence for a dengue virus of 15 Aedes aegypti samples collected from Chiang Mai in northern Thailand. Based on polymorphism of 10 isoenzyme loci, genetic differentiation was confirmed among samples collected in different subdistricts (high F ST values and P < 0.05). Based on infection rate for a dengue 2 virus, susceptibilities were similar in mosquitoes collected in San Nuea subdistrict and in Choeng Doi subdistrict, and were heterogeneous in populations sampled in other subdistricts. These findings are discussed and related to insecticide treatments.

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A field study was conducted to evaluate the effects of indoor residual house spraying of Bifenthrin and Deltamethrin on malaria vector population of Anopheles minimus s.l., from April 1999 to April 2001 at rural villages in Mae Hong Son province, northern Thailand. Nine villages in Mae Hong Son province were selected for the present study (three villages for control and three villages each for insecticide spray). The residual spray of Bifenthrin (25 mg/m ) showed greater adulticiding effects on An. minimus s.l. population than Deltamethrin (20 mg/m ). In Bifenthrin treated villages, a clear decrease in biting density of An. minimus s. l. was found in human bait collection as well as animal bait collection after the insecticide spray. In all of the three villages, the average density after insecticide spray was significantly lower than that observed before the spray. The effects of Deltamethrin on An. minimus s. l. density was found only in one village out of the three treated villages. A significant decrease in parous rate after insecticide spray was found in all the villages sprayed with Bifenthrin, whereas no significant changes were observed in control villages. The average parous rate in the villages treated with Deltamethrin became significantly higher after the insecticide spray. These results clearly suggested that the residual spray of Bifenthrin (25 mg/m ) was more effective than Deltamethrin (20 mg/m ).