Abstract: The synergism of Bacillus thuringiensis (Bt) pretreatment to organophosphate and carbamate insecticides in the larvae of the diamondback moth, Plutella xylostella was studied. In 24 hours after pretreatment with LC₅ of Bt, the synergisms to methamidophos, isocarbophos and carbofuran were 6.74, 8.83 and 8.50 folds as high as that without Bt pretreatment in the R strain,while it was 2.96, 1.69 and 3.88 folds as high as that without Bt pretreatment in the S strain respectively. The K_m and V_max of acetylcholinesterase (AChE) in R strain without Bt pretreatment were 1.86 and 13.56 folds respectively as high as that with Bt treatment.The bimolecular rate constants (K_I)　of AChE to methanidophos, isocarbophos, carbofuran, dichlorpos, methomyl and isofenphos methyl in R strain with Bt pretreatment were about 1.80～2.66 folds as high as that without Bt pretreatment.There was no obvious change of the K_m and K_I of carboxylesterase(CarE), but a strong inhibition of CarE activity after using Bt pretreatment in R strain. The V_max of CarE without Bt preteratment was 2.02 folds as high as that with Bt pretreatment. In addition, the strong inhibition was found both in the activity of GSTs and the content of glutathion (GSH) in R strain with Bt pretreatment.The activity of GSTs and the amount of GSH without Bt pretreatment was 1.73 and 1.66 folds respectively as high as that with Bt preteratment. There was no great change in three respects: K_m, V_max and K_I of AChE, K_m and K_I of CarE and the content of GSH after Bt pretreatment in S strain. However, obvious inhibition could be found on the activity of CarE and GSTs. The CarE and GSTs without Bt pretreatment were 1.54 and 1.64 folds as high as that with Bt pretreatment respectively in S strain. The results suggested that synergism of Bt pretreatment to Ops and carbamates in resistant DBM might be greatly related to inhibitions of AChE, CarE, GSTs and GSH, as well as the increment of AChE sensitivity.

Key words: Plutella xylostella, Bacillus thuringiensis, insecticides, acetylcholinesterase, detoxication enzymes