A BIOCHEMICAL-MECHANISM OF RESISTANCE TO PIRIMICARB IN 2 GLASSHOUSE CLONES OF APHIS-GOSSYPII

The susceptibility of three clones of Aphis gossypii Glover to 15 inse cticides was established by bioassay. A high level of resistance towar ds pirimicarb was confirmed for a clone from Holland (Dutch R) and a c lone from Japan (Jap R), while the susceptible clone (S) was killed by very low doses of the insecticide. However, only limited cross-resist ance was shown towards other carbamates and organophosphates, and no m arked resistance to the pyrethroids tested. The acetylcholinesterase ( AChE) of both resistant clones hydrolysed acetylcholine faster than th at of susceptible aphids, with greatest enzyme activity shown by the D utch R clone. Inter-clone differences in these rates were consistent w ith differences in catalytic centre activities. Inhibition (I-50) of A ChE by pirimicarb was approximately 900-fold higher for the resistant clones than for the S clone. First-order kinetics revealed that resist ance to pirimicarb in Dutch R and Jap R involved a modified AChE which had a reduced (approximately 350-fold) affinity (K-d) for pirimicarb. The marked change in AChE affinity for pirimicarb was not repeated wi th the other carbamates tested, ethiofencarb and aldicarb. It was cons idered that the resistant aphids would not require mechanisms in addit ion to insensitive AChE in order to show the high level of resistance to pirimicarb shown in the bioassay.