Cuticular penetration of β-cypermethrin in insecticide-susceptible and resistant strains of Bactrocera dorsalis

Semi-thin sectioning and transmission electron microscope techniques were employed to investigate the cuticle thickness, integument structure, and fat body of larvae from susceptible and resistant strains of Bactrocera dorsalis. The results showed that the cuticle of β-cypermethrin-resistant strains (25.96 ± 1.00 μm) was thicker than that of susceptible strains (19.36 ± 0.82 μm). The number of chitin layers in the endocuticle of β-cypermethrin-resistant strains (98.00 ± 3.61 layers) was more than that in susceptible strains (75.67 ± 2.40 layers). Compared with susceptible strains, the laminated structure of the chitin layers in the endocuticle of resistant strains revealed higher density and more distinctive structure, and the interspace of epidermal cells was thicker. Fat body in the resistant insects contained more fat granules than those in susceptible insects. Moreover, HPLC analysis showed that the cuticular penetration of β-cypermethrin into larvae of resistant strains was slower than that of susceptible strains. In addition, the metabolism of β-cypermethrin in resistant strains was faster than that in susceptible strains, indicating that the resistant strains could enhance detoxification metabolism. These results indicated that cuticle thickness, fat body, laminated structure of the chitin layers, and interspace of epidermal cells might be correlated with cuticular penetration between susceptible and resistant strains, suggesting that the resistant strains could decrease the rate of penetration of insecticide into the internal cavity.

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► Cuticular penetration of β-cypermethrin was slower in resistant strain.
► Cuticle of resistant strains was thicker than that of susceptible strains.
► Chitin layers in resistant strain showed higher density structure.
► Fat bodies in resistant insects contained more fat granules.