Diagnostic Studies on the Resistance to Pyrethoroid Insecticides in American Bollworm (Lepidoptera, Noctuidae) Strain from Western Thailand

Abstract

Populations of Helicoverpa armigera collected from cotton fields in Kanchanaburi province were reared at LENI-CIRAD in Montpellier-France. To assess their level of resistance to synthetic pyrethroid insecticides, in comparison with LENI susceptible strain, the method of topical applications on larvae was used and data analysed by using CIRAD’s designed “LD50” software. Resistance ratios of Thai populations to esfenvalerate varied from 14.5 to 36.5. Twenty percent of the caterpillars belonging to two replications survived the application of doses higher than 60 g/g, which corresponds to 200 times the LD50 of the reference susceptible strain. A comparison between topical applications on larvae and vial tests on adults was also made. The results indicated that the vial method could be used to determine the frequency of the resistance phenomenon and develop an on-farm diagnosis kit.