Abstract: 【Aim】 This study aims to clarify the difference in basic biological characteristics and population parameters like relative fitness and intrinsic rate of increase, between different trichlorfon-resistant strains and susceptible strain of the oriental fruit fly, Bactrocera dorsalis (Hendel). 【Methods】 The high- and medium-level trichlorfon-resistant strains of B. dorsalis were selected in the laboratory. Biological characteristics and parameters were investigated and analyzed between the resistant strains and the susceptible strain. 【Results】 There were no significant differences in the durations of egg, larva and pupa, pupation rate, pre-oviposition period, longevity of male and female adults, average longevity and female rate of B. dorsalis. Egg hatching rate (68.33%) and adult emergence rate (93.73%) of the medium-level trichlorfon resistant strain (Tri-M) was significantly lower than those of the susceptible strain (SS), which were 88.33% and 97.93%, respectively, and those of high-level trichlorfon resistant strain (Tri-H), which were 86.67% and 98.21%, respectively. Compared with the fecundity of SS (864.61 eggs laid per female) and Tri-H (750.70 eggs laid per female), the fecundity of Tri-M decreased to 630.87 eggs laid per female. All strains had coincidently daily fecundity dynamics of females, indicating that the oviposition peak periods come in a short time after sexual maturity of females. The oviposition peaks of the resistant strains were observed earlier than that of the susceptible strain. After the oviposition peak, fecundity decreased in a fluctuation. All the c values of equations to simulate adult survival rate dynamics of different strains were higher than 1, demonstrating that the equations fit to the I-type survival model. Adults of all strains could survive until the average longevity was reached. SS had the highest index of population trend (I =339.41), while Tri-H had a moderate (I =307.82) and Tri-M had the lowest index of population trend (I =175.79), indicating that the ascending of trichlorfon resistance of B. dorsalis populations was suppressed, and for Tri-M the suppression was stronger. The net reproductive rate (R₀) of Tri-M (217.49) was significantly lower than those of SS (327.89) and Tri-H (284.29), respectively. The intrinsic rate of increase (r_m) and finite rate of increase (λ) of the two resistant strains had no significant difference, but were much higher than those of SS, and the average generation period (T ) of the two resistant strains was shorter than that of SS. The relative fitness of the high-and medium-level resistant strains was 0.8670 and 0.6633, respectively, significantly lower than that of SS. 【Conclusion】 Under selection pressure of trichlorfon, there is significant decrease in the average generation period, egg hatching rate and pupation rate of Tri-M, while most of biological characteristics of the resistant strains of B. dorsalis have no significant change. Reproduction and development of populations of the trichlorfon-resistant strains are disadvantaged, while Tri-M is much more disadvantaged. Nevertheless, the resistant strains show better population growth potentials.

Key words: Bactrocera dorsalis, insecticide resistance, trichlorfon, biology, life table