Influence of Rice-farming Herbicide (2, 4-Dichlorophenoxyl Acetic Acid) on the Development of Culex pipiens pipiens (Diptera: Culicidae), a Major Swamp-breeding Mosquito Vector of Filariasis

Wetland rice-fields have been adjudged one of the most productive mosquito larval habitats, partly due to the influence of rice-farming chemical inputs on rates of development and survival of immature stages of mosquitoes. This study was, therefore, carried out to situate, appropriately, potential contributions of application of 2,4-Dichlorophenoxyl Acetic Acid (2,4-DAA) herbicide to mosquito production in rice-farming. The bio-assay involved rearing of approximately one-day old first instar larvae of Culex pipiens pipiens mosquitoes in a series of increasing concentration of 2,4-DAA, ranging from 0.18 – 0.72g/litre. The larvae were monitored daily for survival and duration of development of immature life stages. The results showed that Total Larval Duration (TLD) was significantly (P < 0.05) elongated from 8.39±0.79 days in the Control mosquitoes (i.e., larvae not exposed to herbicide) to about 15 days in those raised in the highest concentration (i.e., 0.72g/l) of herbicide tested. Though, duration of development was dependent on herbicide concentration, the pattern of influence differed considerably among the four larval instar stages (i.e., L1 – L4). On the other hand, however, duration of pupal development was not significantly affected (range = 1.84±0.04 days in 0.18g/l to 2.02±0.05 days in 0.72g/l). While, Total Immature Survival (TIS) was only significantly reduced in mosquitoes exposed to 0.72g/l herbicide (i.e., 51.16±2.13%), the larval stage survivorship was inversely related to herbicide concentration (range = 67.56±14.60 to 38.06±2.92%). The findings of this study revealed that application of 2,4-DAA herbicide in rice-farming can be modified and used as an integral tool for reducing mosquito production from wetland rice-fields.