Synergism of enzyme inhibitors and mechanisms of insecticide resistance in Bemisia tabaci (Gennadius) (Hom., Aleyrodidae)

Abstract

Abstract: Synergism of enzyme inhibitors and mechanisms of insecticide resistance in Bemisia tabaci (Gennadius) (Hom., Aleyrodidae), collected from the commercial cruciferous (Brassica oleracea var. italica L.) vegetable fields in Shangjie, Minhou County, Fujian, China (Sj), were studied by using the dry film method and biochemical analysis. In comparison with the insecticide‐susceptible insectarium population of B. tabaci, the resistance ratios in the field population were determined to be 29.3 for methamidophos, 21.8 for chlorpyrifos, 2.2 for phoxim, 72.4 for fenvalerate, 9.4 for avermectin, 5.5 for emamectin benzoate, 1.8 for spinosad, 11.6 for fipronil and 8.0 for imidacloprid. The field population of B. tabaci displayed significantly low acetylcholinesterase (AChE) activity, but high AChE insensitivity to methamidophos and dichlorvos, as well as high carboxylesterase (CarE) activity. Piperonyl butoxide (PB), triphenyl phosphate (TPP) and diethyl maleate (DEM) showed low synergistic effects on the susceptibility to the nine insecticides in the insetarium population of B. tabaci. On the other hand, in the field population of B. tabaci, high synergism ratios to the nine insecticides with PB, to methamidophos, chlorpyrifos, phoxim, fenvalerate and imidacloprid with TPP, and to methamidophos and avermectin with DEM were found. Significant inhibition of AChE activity by PB and of glutathione‐S‐transferase (GST) activity by DEM in the insectarium and field population, of CarE activity by PB, TPP and DEM in the insectarium populations, and of CarE activity by PB and DEM in the field population were found in vivo. The results suggest that the resistance to organophosphates in B. tabaci was associated partly with AChE insensitivity. Oxidative degradation was believed to be the major mechanism of insecticide resistance in B. tabaci found in Sj. Hydrolytic reactions might also be partially involved in the resistance in some cases. These findings indicate that more than one target besides the traditional ones for PB, TPP or DEM might exist in B. tabaci. This might provide a plausible explanation for the improvement in insecticide toxicity by the three enzyme inhibitors. Especially, the highest synergism of PB on the nine insecticides could now be explained by PB's multiple attack on the activity of Car or AChE in B. tabaci.