We investigated the role of an entomogenic gut bacterium Pseudomonas fluorescens on larval development and survival of Culex vishnui Theobald, the mosquito vector of Japanese encephalitis, which breeds exclusively in rice field water in tropical regions. The bacterium could be isolated from the midguts of third and fourth instars of the mosquito. Pseudomonas fluorescens strain 1 (Ps1) was identified as the one vital to C. vishnui larval survival. The phenotypic characteristics of this strain are described. As part of the study, antibiotics were used to cure C. vishnui larvae of Ps1; thus cured larvae did not survive when subsequently reared in sterile or antibiotic-treated rice field water or tap water. In contrast, 93.3 -100% of cured larvae developed into normal adults when subsequently reared either in unsterile or in sterile rice field water or in tap water containing Ps1 bacteria (5 £ 10 3 ml). Thus the elimination of Ps1 from breeding water could be used as a strategy to suppress the mosquito growth, as part of an innovative approach to control Japanese encephalitis.

The mosquito Culex tritaeniorhychus is an established vector of Japanese encephalitis in India and many Asian countries. Entomological monitoring enables estimation of the abundance of the vector mosquitoes and therefore aids in vector management and disease control. In compliance with this proposition, an assessment of the pupal productivity of the mosquito C. tritaeniorhynchus was made from selected rice fields of West Bengal, India. The results are expected to provide the present status of mosquitoes in an endemic region of Japanese encephalitis. In course of sampling of the rice fields, the mosquito C. tritaeniorhyhchus was found in abundance with other mosquitoes like Culex bitaeniorhynchus, Anopheles subpictus, Anopheles vagus, Anopheles barbirostris. Application of multivariate analysis indicated that the plant height, water depth and temperature could explain the variations in the pupal productivity of C. tritaeniorhynchus in rice fields. The correspondence of the immature abundance of C. tritaeniorhynchus with plant height suggests persistence of the population throughout the paddy rice cultivation period. Using the plant height as an indicator, the abundance of the mosquito C. tritarniorhynchus can be predicted enabling application of appropriate strategies for population intervention and thus the possibility of the disease.

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The distribution of the mosquito larvae in the breeding habitats varies at the spatial scale depending on the availability of the resources and the predators. This proposition was assessed through the observation of the spatial distribution of Culex larvae (Culex tritaeniorhynchus) in artificially constructed rice field habitats. Using a binomial generalized linear model with logit link, the disparity in the abundance of the larvae was evaluated to justify the effects of light (light vs shade), vertical (surface vs bottom), and horizontal (wall vs center) distribution as explanatory variables. Under light availability, the spatial occupancy of the mosquito larvae was higher in the center than in the walls of the mesocosms. However, the larval orientation was higher on the surface than at the bottom of the mesocosms in all instances. In comparison to open spaces, the larval aggregation was higher in the presence of the floating vegetations like Azolla and Lemna, indicating that the habitat heterogeneity of the mesocosms influenced the distribution of the mosquito larvae in the available spaces. A reduction in the larval aggregation pattern in the spaces was observed in the presence of the predator (Anisops sp.) reflecting the possible evasion tactics of the mosquito larvae. The observations suggest that the mosquito larvae may utilize the vegetation in the rice field habitats quite effectively and occupy empty spaces of predators. The results may be considered as a prototype of the prospective localization of the mosquito larvae in the rice fields and help to frame the strategies of spraying the biopesticides to achieve optimal efficacy in mosquito regulation.

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