| The cotton bollworm, Helicoverpa armigera (Hubner), is one of the most serious agricultural pests and has developed resistance to most insecticides. For effective management of this insect to ensure cotton industry, the resistance and its management of cotton bollworm have been studied in many countries. But the resistance mechanism, especially to pyrethroid, is still not clear. For nerve insensitivity, there are several reports based on the neurophysiology evidence, but the target mutation has not been confirmed so far. As to metabolic mechanisms responsible for pyrethroid resistance, it is the subject of controversy regarding the relative role of oxidation by cytochrome P450 monooxygenases and hydrolysis by esterases. So the design and implementation of effective resistance management tactics still have some problem. In this study the metabolic resistance mechanism in five contemporary strains of the bollworm from China, Pakistan and India is investigated. This study provides the first evidence that enhanced oxidative metabolism by cytochrome P450 monooxygenases is a major mechanism responsible for pyrethroid resistance in H. armigera from Asia, with evidences from synergism study of PBO and DEF, determination of enzyme activity, in vitro metabolism of deltamethrin, and mRNA expression of relative P450 genes.1. The biochemical mechanism for pyrethroid resistance in H. armigeraCompared with a standard susceptible strain that originated from the Cote DTvoire in the 1970s ('SCD'), all the five contemporary strains of the cotton bollworm from China, Pakistan and India were found with high resistance to pyrethroids. Of which, two of theChinese strains ('YGF' and 'YGFP') were derived by laboratory selection from a field collected strain ('YG') by fenvalerate and a mixture of fenvalerate and piperonyl butoxide (PBO), respectively. The resistance ratios (RRs) for YGF, YGFP were 1690-, 540-, 73-fold and 2510-, 2920-, 286-fold to fenvalerate, cypermethrin and deltamethrin, respectively. YG had lower resistance and the corresponding RR was 7-, 14-, 21-fold. The RR in a Pakistani strain (PAK) was 2320, 4100 and 223-fold. IND strain was collected from India with more than 10000 fold resistance to cypermethrin.The synergisms of PBO and DEF on pyrethroids were tested in these strains. DEF showed lower synergism on pyrethroid (SR 2-4) and no difference was found between resistant and susceptible strains. On the other hand, PBO had a strong synergism. The SRs were from 11- to 950-fold in resistant strains and 1.3- to 5-fold in susceptible strain. In both YGF and PAK strains, resistance to fenvalerate dropped dramatically from 1690- and 2320-fold to 4- and 5-fold, implying that the resistance was almost completely suppressed by PBO.The activity of p-nitroanisole O-demethylation (PNOD), ethoxycoumarin O-deethylation (ECOD), methoxyresorufin O-demethylation (MROD) and aldrin epoxidation (AE) of cytochrome P450 monooxygenases from midguts of final instar larvae of the resistant strains were compared with those of the susceptible SCD strain. The PNOD were 11.7-, 6.5-, 2.1-, 15.3-and 13.5-fold, respectively, in YGF, YGFP, YG, IND and PAK. The ECOD were 2.9-, 5.2-, 1.1-, 26.6- and 8.1-fold and the MROD were 3.2-, 6.2-, 3.7-, 7.7- and 3.8-fold. The activity increase in these resistant strains was correlated with their pyrethroid resistance. This further implies that the resistant strain had a more active monooxygenase system. Survey of the activity distribution revealed that there were more individuals with high activity of heme-peroxidase in resistant strains than in the SCD strain, and the average activities increased to 1.1- or 2.4-fold. The increase of heme-peroxidation suggested the increase of total content of cytochrome P450. So, the specific monooxygenase isoenzymes associated with resistance could have more elevation.In the resistant strains, there were minor increases in glutathione S-transferase activity (to both substrates CDNB and DCNB), and in esterase activity (to the substrate a-naphthyl acetate). This suggested that GST an...
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