Elucidation of detoxication mechanisms involved in resistance to insecticides in the third instar larvae of a field-selected strain of Helicoverpa armigera with the use of synergists

Synergism studies showed that whereas piperonyl butoxide (PB) synergism was either absent with cis-cypermethrin or slight with trans-cypermethrin and fenvalerate in the third instar larvae of a susceptible strain of Helicoverpa armigera, it was very high in a resistant Thailand strain where it was recorded 11-fold for cis-cypermethrin, 9-fold for trans-cypermethrin, and 139-fold for fenvalerate at the LD 50 level. After synergism with PB, resistance factors for the three pyrethroids declined to the same value of around 3.5 at LD 50. This indicates a vital role of monooxygenases in the detoxication of pyrethroids in the Thai strain. The synergism of pyrethroids by DEF, either alone or with PB, was much lower in the resistant strain than in the susceptible strain. This implies that esterases may not be important in imparting resistance to the Thailand strain. FDMC did not synergise DDT in the susceptible strain but it produced a very high synergism in the resistant strain, presumably due to the inhibition of DDT-dehydrochlorinase. Carbaryl was considerably synergised by both PB and DEF in the susceptible strain but mildly in the resistant strain, thus suggesting that esteratic and oxidative enzymic mechanisms may not be involved in resistance to carbaryl in the resistant Thai strain.