Ecological variation and resistance levels to propoxur and chlorpyrifos in Anopheles stephensi (Diptera: Culicidae), a malaria mosquito from India

A total of 39 strains of Anopheles stephensi, an important urban malaria vector, were collected from various parts of India and maintained in the insectary for this study. Based on the egg-float ridge number, 19 strains were classified into ecological variants and 32 strains were exposed to chlorpyrifos and propoxur to investigate their resistance status. Filter paper containing freshly laid eggs was taken, the ridge numbers on the floats were counted under the microscope, and strains were classified into ecological variants. Of the 19 strains, 18 were of 'type form', with ridge numbers ranging from 15 to 21. The Papareddipalya (PRP) strain belonged to the 'intermediate form', with 14 to 17 ridge numbers. Larval bioassays were carried out according to the procedure of the WHO. For chlorpyrifos, the lowest LC50 value was 0.00107mg/l (Padmanabhanagar strain) and the highest value was 0.0403mg/l (GOA-A strain). Furthermore, the lowest LC90 value was 0.00368mg/l (Delhi strain) and the highest was 0.1746mg/l (GOA-A strain). For propoxur, the lowest LC50 value was 0.00029mg/l (Goraguntepalya strain) and the highest value was 0.0037 mg/l (JP Nagar strain). Moreover, the lowest LC90 value was 0.00094mg/l (Goraguntepalya strain) and the highest value was 0.0115mg/l (JP Nagar strain). The tolerance values ranged from 1.26 to 37.68 for chlorpyrifos and from 1.34 to 12.77 for propoxur. All the type forms were from urban and semi-urban locations, and the intermediate strain was from a semi-urban location. The bioassay results indicated that the strains of An. stephensi were more susceptible to propoxur than to chlorpyrifos.