Effect of Pirimiphos-Methyl on Esterases of Different Developmental Stages of Three Strains of Tribolium castaneum (Herbst.) - An Electrophoretic Study

The effect of sub-lethal doses of Pirimiphos-methyl was studied on esterases of three different strains of Tribolium castaneum viz., Pak, CTC-12 (resistant) and FSS-II (insecticide susceptible). The multiple forms of esterases were resolved by polyacrylamid gel electrophoresis (PAGE) and their activities analyzed by band patterns of different strains of T. castaneum. The control electrophoretic pattern of Pak and FSS-II strains showed seven distinct esterases bands. The 4th and 6th instar larvae of both the strains showed carboxylesterase (CE), cholinesterase (ChE), arylesterase (AE) and acetylcholinesterase (AChE) bands, whereas AChE (band No. 6) was only visible in the FSS-II strain. The newly emerged adults of FSS-II had all the esterases, while the newly emerged adults of Pak strain had low level of CE, ChE and AE activity, whereas AChE was absent. In 15 days old beetles of Pak and FSS-II strains all the seven esterases bands were prominent. After pirimiphos-methyl treatment the ChE, AE and AChE of all the developmental stages in Pak and FSS-II strains were completely inhibited. The higher molecular weight CE, however, was not much affected. The CTC-12 control pattern comprises eight esterase bands in newly emerged adults and 15 days old beetles. The 4th and 6th instar larvae had almost similar pattern except for the absence of AChE (band No. 8). The pirimiphos-methyl treatment generally reduced the intensity of all the esterases. The AE and AChE (bands 5, 7, 8) were completely inhibited in the newly emerged adults and 15 days old beetles after insecticide treatment. However, one band of AChE (band 6) remained unaffected. In CTC-12 strain esterase bands were decreased in intensity after treatment with pirimiphos-methyl, whereas in other strains these bands were completely inhibited in all the developmental stages. The variable thickness of the band in the gel indicated relative esterases induction that could be correlated with the development of resistance in T castaneum.