Inheritance Mode And Mechanisms Of Phoxim Resistance In Cotton Bollworm, Helicoverpa Armigera (Hübner)

The cotton bollworm, Helicoverpa armigera (Hubner), is one of the most destructive pests of cotton and threats the economic production of cotton seriously. Since 1996, transgenic Bt cotton has been expanded rapidly in China. However, chemical insecticides are still essential for the control of H. armigera on Bt cotton due to lower expression of Bt toxin in late cotton season. Phoxim is one of the most important insecticides for H. armigera control in China. In this paper, the inheritance mode of resistance to phoxim in the PCP strain of H. armigera was investigated. A near isogenic phoxim-resistant strain was selected by recurrent backcross of the PCP strain to the susceptible France strain. The cross-resistance patterns and mechanisms of the near isogenic phoxim-resistant strain were also studied.1. Genetic analysis of phoxim resistance in the PCP strain of H. armigeraThe mode of inheritance of phoxim resistance in H. armigera was evaluated from log dosage-probit mortality curves constructed from the response of third-instar larvae to phoxim treatment. The resistant PCP strain was selected in laboratory with phoxim and shown 129.9-fold resistance to phoxim compared with the susceptible France strain. Genetic analysis indicated that phoxim resistance in the PCP strain is controlled by one incompletely dominant gene on an autosomal chromosome.2. Selection for a near-isogenic phoxim-resistant strain of H. armigeraThe phoxim-resistant PCP strain was backcrossed to a laboratory susceptible France strain. Progenies from each backcross were selected with a discriminating dose of phoxim that killed all susceptible homozygostes. After four generations of recurrent backcross the progenies of the last backcross (BC4) were further selected for six generations with phoxim to generate a near-isogenic phoxim-resistant strain (BC4F6). This near-isogenic resistant strain has developed 141.9-fold resistance to phoxim compared with the susceptible France strain.3. Cross-resistance of the near-isogenic phoxim-resistant strain of H. armigeraCompared with the France strain, the near-isogenic strain resistant to phoxim had high level cross resistance to deltamethrin (252.1-fold) and fenvalerate (189.9-fold), middle level resistance to cypermethrin (20.6-fold), methomyl (29.0-fold) and endosulfan (11.4-fold) and remains susceptible to profenofos (3.43-fold).4. Synergism studies in vivoPiperonyl butoxide (PBO) increased phoxim toxicity to the near-isogenic phoxim-resistant strain by 1.70-fold, deltamethrin by 19.57-fold, methomyl by 2.22-fold and endosulfan by 1.06-fold. However, S, S, S-tributylphosphoro-trithioate (DEF) increased phoxim toxicity by 7.03-fold, deltamethrin by 1.53-fold, methomyl by 2.02-fold and endosulfan by 1.90-fold. These results suggest that both esterase and cytochrome P450 monooxygenases might be partly responsible for resistance to phoxim and cross-resistance to other insecticides in the near-isogenic phoxim-resistant strain of H. armigera.5. Biochemical characterization of the near isogenic phoxim-resistant strain of H. armigeraThe activities of cytochrome P450 monooxygenases from midguts of sixth-instar larvae to p-nitroanisole, ethoxycoumarin and methoxyresorufin were 2.27-, 2.86- and 2.31-fold respectively in the near-isogenic resistant strain compared with the susceptible strain. The activities of glutathione S-transferases from midguts of sixth-instar larvae to DCNB and CDNB in the resistant strain were 1.21- and 1.41-fold compared with the susceptible strain. For esterase activity from midguts of sixth-instar larvae, no difference was found between the near isogenic phoxim-resistant strain and susceptible strain. However, 2.82, 2.55 and 1.07-fold higher general esterase, carboxylesterase and alkaline phosphatase activities were observed in the resistant strain than in the susceptible strain when third-instar larvae were used for enzyme preparation. Acetylcholinesterase from the resistant strain was 5.83-fold less sensitive to phoxim-oxon than that from the susceptible strain. Kinetics studies showed that th...