Reduced fitness associated with spinosad resistance in Helicoverpa armigera

The fitness cost of spinosad resistance was investigated in the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Laboratory experiments were conducted to compare relative fitness of H. armigera between the spinosad-susceptible and -resistant strains. During the experiments, the average development periods of the resistant strain were lengthened by 4–5 days, reflected in a prolongation of egg, larval and pupal periods. Furthermore, pupal survival, pupal weight, the mean life span of emerged adults, eggs laid and hatched decreased greatly in the resistant strain in comparison with the susceptible strain. Other life-cycle parameters such as larval survival, larval wet weights, prepupal periods, pupation ratio, and sex ratio did not change significantly. As a result, both net replacement rate (R ₀) and intrinsic rate of increase (r _m) were reduced for the resistant strain. Our results clearly indicated that relative fitness of resistant individuals was reduced in the absence of spinosad. Rational measures including pesticide rotations should be expected to delay development of resistance to spinosad in H. armigera field populations from China.