Susceptibility and resistance profiles of field and laboratory strains of Trogoderma granarium Everts to pirimiphos-methyl, alpha-cypermethrin and spinetoram.

The khapra beetle, Trogoderma granarium Everts, 1898, is a significant pest of stored commodities worldwide. Insecticides are heavily relied upon to manage T. granarium. However, the long-term usage of insecticides has led to the development of resistance to insecticides, reducing their effectiveness against T. granarium. This study investigated variations in susceptibility to pirimiphos-methyl, alpha-cypermethrin and spinetoram in a laboratory and seven field strains of T. granarium, using dose-mortality bioassays. Metabolic resistance mechanisms were investigated through synergism studies using piperonyl butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF). Susceptibility of the laboratory strain was the highest to all insecticides compared to the field strains. For field strains, the LD50 values ranged from 25.9 to 49.6 mg/kg grain for pirimiphos-methyl, 15.0 to 40.5 mg/kg grain for alpha-cypermethrin, and 2.2 to 6.0 mg/kg grain for spinetoram. Compared to the laboratory strain, field strains of T. granarium exhibited significant resistance ratios ranging from: 19.9 to 38.1 fold against pirimiphos-methyl, 12.3 to 45.0 fold against alpha-cypermethrin, and 7.3 to 20.0 against spinetoram. In synergism bioassays, there was a significant effect of enzyme inhibitors on enhancing suceptibility to pirimiphos-methyl only in field strains, suggesting P450 monooxygenase and esterases may contribute to pirimiphos-methyl resistance. In conclusion, variable susceptibility to insecticides was observed across different strains of T. granarium. Reduced susceptibility to insecticides in field strains compared to the laboratory strain poses challenges for effective control of T. granarium.