Abstract
The susceptibility of four clones of the cotton aphid, Aphis gossypii, which were separated from vivipara of field populations on the basis of aliesterase activity, to organophosphorus, carbamate and pyrethroid insecticides was determined by two insect body dip methods: the standard method (keeping insects in case used for dipping), and the transfer method (transfering dipped insects into clean case). The LC50 of each insecticide determined using the transfer method was more than ten times higher than that using the standard method. Although the susceptibility of a susceptible strain (S) differed between organophosphorus insecticides, it was especially susceptible to chlorpyrifos-methyl, phosalone, and profenofos. Resistant strains with high aliesterase activity exhibited up to ten times higher resistance to most organophosphorus insecticides. The resistance ratios were especially high for thiometon, oxydeprofos, and phosalone, but, not so high for chlorpyrifos-methyl and profenofos. Organophosphate-resistant strains also exhibited resistance to carbamate insecticides. Resistance ratios were high for ethiofencarb and pirimicarb, but moderate to carbaryl and methomyl. Some combinations of organophosphorus and carbamate insecticides, such as malathion plus carbaryl exhibited synergism for resistant strains, but not for S strains. All tested clones were susceptible to pyrethroid insecticides.
