Mechanism of resistance to pirimicarb in the cowpea aphid Aphis craccivora

A resistant strain of Aphis craccivora Koch was selected with pirimicarb for 12 generations. The selected strain exhibited 47-fold resistance compared to the susceptible laboratory strain. The resistant strain also showed cross-resistance to carbosulfan, malathion, chlorpyrifos methyl and thiamethoxam. Low resistance to fenitrothion and acetamiprid was also observed. Bioassays in combination with biochemical synergist studies revealed that the higher inhibition of carboxylesterase using the synergist tributyl phosphorotrithioate (DEF) was associated with increased toxicity of pirimicarb in the resistant strain. On the other hand, piperonyl butoxide (PBO) and diethyl maleate (DEM) had less inhibitory effect on mixed function oxidases (MFO) and glutathione-S-transferase (GST) in the resistant strain. The activity of carboxylesterase was 29-fold greater in the resistant strain, whereas the activity of MFO and GST in the resistant strain was only 5.5 and 1.7- fold greater, respectively. The activity of acetylcholine esterase (AChE) in the resistant strain was 2 fold higher than in the susceptible strain. The I50 (the concentration of pirimicarb that inhibit 50% of the enzyme activity) ratio of R-strain to S-strain was 12.6. Molecular studies using real-time quantitative PCR showed that the transcription level of Ace2 gene in the resistant strain was 3.4-fold higher than that in the susceptible strain. In conclusion, mechanism of resistance in the pirimicarb resistant strain of A. craccivora may include overexpression of Ace2 gene and higher activity of the detoxification enzymes esterases and partly MFO.