Abstract: 【Aim】 At present, chemical control is the main measure in controlling Bactrocera dorsalis, a major pest of fruit trees. This study aims to determine the resistance of the adults of  the beta-cypermethrin-, trichlorfon- and abamectin-resistant strains of B. dorsalis to three insecticides, namely beta-cypermethrin, trichlorfon and abamectin, their cross-resistance to the other two insecticides and the genetic patterns of their resistance, so as to provide the theoretical support for the chemical rotation and resistance management in orchards. 【Methods】 B. dorsalis larvae were collected from the fields and reared in the laboratory for more than 52 generations. The sensitivity of adults to insecticides was measured by the drug film method. The population screening method was used in the resistance selection for each generation up to 52 generations to establish the resistant strains of B. dorsalis. The drug film method was used to determine the cross-resistance of adults of the beta-cypermethrin-, trichlorfon- and abamectin-resistant strains, respectively, to the other two insecticides. Pairwise reciprocal crosses were conducted among adults of the beta-cypermethrin-, betatrichlorfon- and abamectin-resistant strains, and the bioassay against the adults of the F₁ generations from the above crossing treatments was conducted to reveal the genetic patterns of the resistance. 【Results】 It was found that the adults of the beta-cypermethrin-, trichlorfon- and abamectin-resistant strains of B. dorsalis all exhibited some degree of cross-resistance to the other two insecticides. Among them, the adults of the beta-cypermethrin-resistant strain had a moderate level of cross-resistance to trichlorfon, with the resistance multiple (R_m) of 15.61, and showed a low-level cross-resistance to abamectin, with the Rm of 6.67. The median lethal concentration (LC₅₀) values of trichlorfon and abamectin to the beta-cypermethrin-resistant strain were 54.58 and 25.30 mg/L, respectively. The adults of the trichlorfon-resistant strain had low levels of cross-resistance to both beta-cypermethrin and abamectin, with the Rms of 9.15 and 6.18, respectively, and the LC₅₀ values of beta-cypermethrin and abamectin to adults of the trichlorfon-resistant strain were 43.12 and 23.35 mg/L, respectively. The adults of the abamectin-resistant strain also had low levels of cross-resistance to beta-cypermethrin and trichlorfon, with the Rms of 8.74 and 3.58, respectively, and the LC₅₀ values of beta-cypermethrin and trichlorfon to adults of the abamectin-resistant strain were 41.18 and 12.52 mg/L, respectively. When reciprocal crosses were performed between the beta-cypermethrin-resistant strain and the trichlorfon-resistant strain, the declining rates of resistance of adults of their F₁ generations were 33.52% and 56.42%, respectively, and when reciprocal crosses were performed between the beta.cypermethrin-resistant strain and the abamectin-resistant strain, the declining rates of resistance of adults of their F₁ generations were 8.49% and 84.25%, respectively. When reciprocal crosses were performed between the trichlorfon-resistant strain and the cypermethrin.resistant strain, the declining rates of resistance of adults of their F₁ generations were 21.41% and 81.47%, respectively. And when reciprocal crosses occurred between the trichlorfon-resistant strain and the abamectin-resistant strain, the declining rates of resistance of adults of their F₁ generations were 38.00% and 79.00%, respectively. When quadrature of the abamectin-resistant strain with the beta-cypermethrin- and trichlorfon-resistant strains occurred, the declining rates of resistance in adults of their F₁ generations were 3.62% and 12.77%, respectively, showing a little change as compared to that in the parents. When backcrossing occurred between the abamectin-resistant strain and the beta-cypermethrin- and trichlorfon-resistant strains, the declining rates of resistance of adults of their F₁ generations were 65.45% and 62.29%, respectively. 【Conclusion】 The beta-cypermethrin-resistant strain of B. dorsalis had a moderate level of cross-resistance to trichlorfon, and a low-level cross-resistance to abamectin. The trichlorfonresistant strain exhibited low levels of cross-resistance to both beta-cypermethrin and abamectin. The abamectin-resistant strain also showed low levels of cross-resistance to beta-cype-rmethrin and trichlorfon. The inheritance of the resistance of B. dorsalis to beta-cypermethrin, trichlorfon and abamectin was significantly biased toward the maternal genetic rule. In this study, the resistance levels of the beta-cypermethrin-, trichlorfon- and abamectin-resistant strains of B. dorsalis to beta-cypermethrin, trichlorfon and abamectin, their cross-resistance levels to the other two insecticides and the genetic pattern of resistance had been clarified, providing a theoretical basis for delaying the resistance of B. dorsalis and guiding the use of insecticides in the fields, and having a significant guidance in the resistance management of B. dorsalis.

Key words: Bactrocera dorsalis, beta-cypermethrin, trichlorfon, abamectin, cross-resistance