Inhibition of acetylcholinesterase activity of individual homogenates of Callosobruchus species

The insensitivity of Callosobruchus species to malaoxon inhibition was explored in order to explain the differences observed in the in vivo survival after treatment with malathion. Acetylcholinesterase activity of this genus was investigated in the absence and in the presence of malaoxon, using an in vitro microtitre-plate assay. Different species, geographic strains and food lines were tested. The food lines were the bruchid populations raised separately on cowpea and mungbean. Individual insects were homogenized and tested for sensitivity to malaoxon, a specific inhibitor of acetylcholinesterase using the artificial substrate, acetylthiocholine iodide. There were significant differences (p < 0.05) in the activity and malaoxon inhibition of acetylcholinesterase among the species, strains and food lines except in the esterase activity between the food lines of C. maculatus. This shows that there are differences in the sensitivity of populations in this genus to malaoxon inhibition. Likewise, food type plays an important role on the activity of acetylcholinesterase in bruchids. Generally, Pearson correlation analysis revealed none significant positive correlation between esterase activity and % inhibition in the populations. Hence, this insensitive target site mechanism might not be involved in the tolerance of Callosobruchus to malathion. Key words: Acetylcholinesterase; inhibition; Callosobruchus; esterase activity