Trichlorfon resistance: its stability and impacts on biological parameters of Bactrocera zonata (Diptera: Tephritidae)

The peach fruit fly, Bactrocera zonata (Saunders) (Diptera: Tephritidae), is a serious multivoltine and polyphagous pest of fruits and vegetables. Trichlorfon, an organophosphate insecticide, is widely used to control fruit flies. To determine the fitness cost, stability, and dominance of trichlorfon resistance, a population of B. zonata was collected from the vegetable and fruit market and reared in the laboratory. After eleven rounds of continuous selection with trichlorfon, the LC 50 of the selected strain (TRI-SEL) was significantly higher than that of the susceptible and unselected (UNSEL) strains. Dominance values of 0.62 and 0.61 for Cross 1 (15 TRI-SEL♀ × 15 UNSEL♂) and Cross 2 (15 TRI-SEL♂ × 15 UNSEL♀), respectively, suggested the incomplete dominance of resistance. Trichlorfon resistance was associated with a fitness cost; relative fitness value for TRI-SEL strain was 0.52, with disadvantageous traits like larval duration, pupal weight, fecundity, and reproductive rate. The TRI-SEL displayed an unstable level of resistance after six generations without selection. Our results indicate that incompletely dominant, unstable, and autosomally inherited trichlorfon resistance has negative effects on the fitness of the TRI-SEL strain. Fitness costs and the unstable nature of trichlorfon resistance suggest that the efficacy of this chemical can be enhanced by alternating it with a different mode of action insecticides.