| The cotton bollworm, Helicoverpa armigera (Hubner) (Lepidoptera:Noctuidae), is one of the most important lepidopteran pests on a wide range of crops in the north of China. By now, serious resistance of H. armigera to conventional insecticides has resulted in many problems in controlling of this pest. Spinosad is a naturally derived fermentation product of soil actinomycete Saccharopolyspora spinosa. Since it was registered as an insecticide in 1997, spinosad has been used on over 150 various crops in 37 countries for controlling the pest especially lepidoptera larvae. Because of its unique action mechanism, popular spinosad become the potential insecticide for management of resistance. In order to clarify the dynamic development of spinosad resistance, the state of cross-resistance and the resistant mechanism, we selected a spinosad-resistant strain of H. armigera with topical application in the laboratory and documented a series of related studies. The results are as follows:1. The selection of spinosad-resistant strain of H. armigera and the research of cross-resistanceThe parental strain was collected from Tai'an and selected with topical application in the laboratory. The development of resistance was slow during the early selection process (G1-G7), which was just about 2.73-fold increase in resistance compared with the parental strain. Exponential increase of resistance was found from the ninth generation. After 15 generations of selection, the resistance level to spinosad in the selected strain was up to 24.1-fold compared with the parental strain. After selected with spinosad, the susceptibilities of this moderate resistance strain against tested insectcides (chlorpyrifos, methomyl, fenvalerate, avermectin and chlorfenapyr) did not changed significantly, indicating that there was no cross-resistance existed between spinosad and tested five pesticides. Comparing the resistant with the susceptible strain, we found the survival of the resistant strain was half compared with susceptible strain, the development of resistant H. armigera larvae was prolonged about 4-5 days, the number of eggs laid per resistant female was 359, which was half of the susceptible female's, Net replacement rate (R0) and the intrinsic rate of population increase (rm) from the resistant strain were 25.35% and 65% compared with susceptible strain.Our results showed, even though the development of resistance is slow, H. armigera still has the potential to development moderate resistance. Cross-resistance of spinosad with other pesticides is not a significant factor that could prevent the effective use of spinosad against H. armigera. Spinosad should be recommended for rotational use to control the resistant H. armigera population. Spinsoad conferred the fitness cost in H. armigera, suggesting relaxation of selection pressure was likely to favor reversion to susceptibility for the H. armigera population. Therefore, the development of resistance to spinosad in H. armigera should be delayed by rational resistance management measures such as pesticide rotations.2. The study of resistant mechanism of H. armigera to spinosadFrom the studies of synergism, enzyme and gene, we found PBO and TPP could decrease significantly the toxicity of spinosad against H. armigera, and the resistant ratio reduced to 31.8% and 68.0% respectively. However, DEM exhibited no significant effects on toxicity of spinosad; Compared with CarE or GST activity, increasing activity of ODM was found in the resistant strain with difference at significant level, which was 8.26 times of that in the parental strain; CYP6AE14 from the resistant strain was overexpressed about 7.73-fold than that of the susceptible strain. CYP9A12, CYP9A18, CYP6B2 and CYP6B7 were ranged from 2.45-3.53-fold, but CYP9A14 and CYP6B6 were about 0.87- and 0.70- fold compared with the susceptible strain respectively. Our results indicated that resistance to spinosad in the cotton bollworm might be associated with an increase in cytochrome P450 monooxygenase. In addition, CYP6AE14 was the key gene during the development of spinsoad resistance. CYP9A12, CYP9A18, CYP6B2 and CYP6B7 could be enhancing factors to promote the resistant development. Based on our results, rational applications of PBO might increase the efficacy of spinosad in control of this pest and prolong the useful life of spinosad.3. Effects of spinosad on biological characteristics, detoxifying enzymes and detoxifying genes of H. armigera after treatment with spinosadBy clarification of changes about biological characteristics, detoxifying enzymes and detoxifying genes after treatment with spinosad, we could understand betterly the development of resistant cost, the reason of resistance and the development of resistance.Our found that spinosad not only has lethal effects (decrease in survival of eggs, larvae and pupae) but also has sublethal effects (decrease in weights of larvae and pupae; prolong periods of larvae, prepupae and pupae; shorten the adult longevity; decrease in the number of eggs laid per female). By the lethal and sublethal effects, spinosad changed the density and development of population.Spinosad at the tested concentration could induce ODM activity significantly. The induction was in a time-, dose- and population-specific manner. Greatest induction was found to be 5.8-fold in the Taian population after exposure to spinosad for 48 h. However, exposures to spinosad did not affect significantly CarE and GST activities. After treatment, the greatest induction was found in the expression of CYP6AE14 among all tested gene, which was 4.15-fold compared to the control. CYP6B2,CYP6B7,CYP9A12 and. CYP9A18 were induced to 3.32-,2.79-,2.60-,2.07- fold respectively, CYP9A14 and CYP6B6 were suppressed.Our results showed, there are the same tendency to biological characteristics, detoxifying enzymes and detoxifying genes between single induction and multiple selection, indicating insecticide induction may play an important role during the development of spinosad resistance. Based on our data, development of resistance is not due to simple addition of every single inductions. In the absence of spinosad, the induction may favor reversion to original level, which indicates the development of resistance to spinosad in H.armigera is slow.4. The effects of xenobiotics on spinosad-resistant costIn the tested concentrations, xenobiotics (gossypol, tannin, quercetin) and Bt did not change significantly effects of spinosad on biological characteristics of H.armigera (e.g. decrease in pupation ratio and pupal weight; prolong periods of larvae and pupae; decrease in fertility). These results indicated H.armigera has strong adaptability to the host. From the sight of biology, xenobiotics of hosts have little effect on spinsoad. However, we could not neglect the exist of xenobiotics because some researches about effects of xenobiotics on insecticide toxicity have been reported.
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