The susceptibilities of six clones of the cotton aphid, Aphis gossypii GLOVER (five resistant and one susceptible strain) were tested by topical application to organophosphates (malathion, fenitrothion, diazinon and dichlorvos), carbamates (carbaryl, methomyl and pirimicarb), pyrethroids (permethrin and fenvalerate) and an organochlorine (DDT), with and without esterase inhibitors (K-2 and DEF) and mixed-function oxidase inhibitors (PB and S-421). Compared with the susceptible clone, all the resistant clones showed moderate resistance to each organophosphate (max. 45 fold) and carbamates (max. 70 fold) except pirimicarb to which extremly-high resistance was observed (>1,600 fold). Cross-resistance was demonstrated in organophosphates. A resitant clone showed very high levels of resistance to pyrethroids, permethrin (ca. 4,600 fold) and fenvalerate (ca. 16,000 fold). The results suggest that resistance to organophosphates may not be cross-resistant with pyrethroids. Low susceptibility to DDT was observed in all tested clones. The highest co-toxicity coefficient in combination with esterase and oxidase inhibitors was 250 and 94 for organophosphates, 430 and 914 for carbamates (carbaryl and methomyl), and 120 and 199 for pyrethroids. The aliesterase activity of each clone appeared to correlate with the degree of resistance to organophosphates but not to pyrethroids. The results suggest that esterase plays a major role in the mechanism of resistance to organophosphates, and both esterase and mixed-function oxidase play a major role in resistance to carbamates (methomyl and carbaryl). However, neither enzyme may be responsible for resistance to pyrethroids.
