The role of detoxifying enzymes in the resistance of the cowpea aphid (Aphis craccivora Koch) to thiamethoxam

The cowpea aphid (Aphis craccivora Koch) is considered a serious insect pest attacking several crops. We carried out biochemical studies to elucidate the role of the metabolising enzymes in conferring resistance to thiamethoxam, in two strains (resistant and susceptible) of the cowpea aphid. Bioassay experiments showed that the thiamethoxam selected strain developed a 48 fold resistance after consecutive selection with thiamethoxam for 12 generations. This resistant strain also exhibited cross-resistance to the tested carbamates; pirimicarb and carbosulfan, organophosphorus (malathion, fenitrothion, and chlorpyrifos-methyl), and the neonicotinoid (acetamiprid). Synergism studies have indicated that S,S,S-tributyl phosphorotrithioate (DEF), a known inhibitor for esterases, increased thiamethoxam toxicity 5.58 times in the resistant strain compared with the susceptible strain. Moreover, the biochemical determination revealed that carboxylestersae activity was 30 times greater in the resistant strain than in the susceptible strain. In addition, the enzyme activity of glutathione S-transferase (GST) and mixed function oxidases (mfo) increased only in the resistant strain 3.7 and 2.7 times, respectively, in relation to the susceptible (the control). Generally, our results suggest that the higher activity of the detoxifying enzymes, particularly carboxylesterase, in the resistant strain of the cowpea aphid, apparently have a significant role in endowing resistance to thiamethoxam, although additional mechanisms may contribute.