Efficacy of Commonly Used Insecticides to Aedes aegypti in Southern Taiwan

Wei-Tai Hsia, Ho-Sheng Wu, Yi-Chieh Yang, Cheo Lin

2011 Vol.27 NO.3

Correspondence Author： Wei-Tai Hsia

Abstract：

The dengue endemic usually began with scattered cases in a small geographic area to a village-wide cluster infection, and finally became a city-wide or country-wide large-scale outbreak. Therefore, the concentration and dosage of insecticides, frequency of insecticide usage, and the development of mosquito resistance would change accordingly at different stages of the endemic. This study was conducted to evaluate the efficacy of insecticides commonly used in southern Taiwan for dengue control against various strains of Aedes aegypti from different geographic areas. A net cage test with bioassay method was used. The insecticides were diluted into a series of concentrations by the label instructions, and then were applied to each geographic strain of A. aegypti. The mortality rate of A. aegypti after 24 hours of insecticide exposure was used as an indicator of insecticides efficacy. Based on World Health Organization and researchers recommendations, plus Taiwan CDC’s insecticide spraying experiences, when mortality rate of wild strains and susceptible strains of A. aegypti after 24 hours of insecticide exposure both reach 95% or more, the insecticide is regarded effective. Concerning about public perception of disease control, we also included the knockdown rate (80% or more after 30 minutes of exposure) of targeted mosquitoes as an indicator of insecticides selection. This study showed that the susceptibility of A. aegypti to the insecticides changed with various strains from different geographic areas as well as previous local insecticide spraying history. In addition, the active ingredients and concentration of insecticides were the major factors affecting the insecticides efficacy. We suggest establishing a “Standard Operating Procedure of Biological Experiment on Dengue Vectors” as a guideline for each division to conduct bioassay test in high risk areas. With constant monitoring the susceptibility of each strain of dengue mosquitoes in different geographic areas to various insecticides, we can find out the most effective active ingredients, minimal concentration and dosage of insecticides. Along with previous insecticide spraying history and frequency, we can use them as a reference to purchase insecticides during usual time or acute outbreak. To avoid the emergence of insecticide resistance, we suggest choosing insecticides with multiple active ingredients or with both knockdown and killing effects, and meanwhile, establish an alternative insecticides usage system. With all these efforts, we can delay the emergence of insecticide-resistant mosquitoes, use insecticides efficiently at the early stage of outbreak, terminate the epidemic rapidly, and consequently improve disease control.