A field population of the rice stem borer (Chilo suppressalis Walker) with 203.3-fold resistance to triazophos was collected. After 8-generation of continuous selection with triazophos in laboratory, resistance increased to 787.2-fold, and at the same time, the resistance to isocarbophos and methamidophos was also enhanced by 9- and 4-fold, respectively, implying some cross-resistance between triazophos and these two organophosphate insecticides. Resistance to abamectin was slightly enhanced by triazophos selection, and fipronil and methomyl decreased. Synergism experiments in vivo with TPP, PBO, and DEM were performed to gain a potential indication of roles of detoxicating enzymes in triazophos resistance. The synergism results revealed that TPP (SR, 92) and PBO (SR 63) had significant synergistic effects on triazophos in resistant rice borers. While DEM (SR 0.83) showed no effects. Assays of enzyme activity in vitro demonstrated that the resistant strain had higher activity of esterase and microsomal O-demethylase than the susceptible strain (20- and 30-fold, respectively). For glutathione S-transferase activity, no difference was found between the resistant and the susceptible strain when DCNB was used as substrate. However, 28-fold higher activity was observed in the resistant strain when CDNB was used. These results showed that esterase and microsomal-O-demethylase play some roles in the resistance. Some iso-enzyme of glutathione S-transferase may involve in the resistance to other insecticides, for this resistant strain was selected from a field population with multiple resistance background. Acetylcholinesterase as the triazophos target was also compared. The results revealed significant differences between the resistant and susceptible strain. The Vmax and Km of the enzyme in resistant strain was only 32 and 65% that in the susceptible strain, respectively. Inhibition tests in vitro showed that I50 of triazophos on AChE of the resistant strain was 2.52-fold higher. Therefore, insensitive AChE may also involved in triazophos resistance mechanism of rice stem borer.

The activity and electrophoretic mobility of nonspecific esterases, and the sensitivity of acetylcholinesterase (AChE) to organophosphate and carbamate insecticides, was determined for 21 strains or clones of Aphis gossypii of worldwide origin and related to patterns of insecticide resistance disclosed by diagnostic dose bioassays with demeton-S-methyl, pirimicarb, and permethrin. Results confirmed previous work showing the occurrence of two forms of mutant AChE, both conferring insensitivity to pirimicarb and representing the primary mechanism of resistance to this insecticide. One of these mutant AChE enzymes also caused resistance to demeton-S-methyl in its own right, whereas the other only conferred resistance in combination with moderate or high levels of nonspecific esterase activity. Lack of any correlation between permethrin resistance and quantitative or qualitative changes in esterases implied a distinct, as yet uncharacterised, mechanism of resistance to pyrethroids, possibly based on insensitivity in the target protein for these pesticides.

Dynamics of pyrethroid resistance in a field population of cotton bollworm (Helicoverpa armigera) was demonstrated by continuous monitoring with twin discriminating dosages, and the influencing factors were also experimentally analysed. Resistance in a field population in China increased rapidly in the 3rd and 4th generations when population density became higher and insecticides were applied repeatedly, then decreased suddenly during over-wlntering and slowly in the 1st and 2nd generations when insecticide spraying was suspended. Resistance increase could be countered by dilution as a result of immigration of susceptible moths from corn fields, which were found to be a natural refuge for this pest in China, The reduction of resistance during over-wintering and the 1st and 2nd generations was affected by the lower fitness of resistant cotton bollworms to low temperature and disadvantages in reproduction. The possibilities of managing the resistance in field populations on the basis of these observations are discussed.

Two acetylcholinesterase genes, Acel and Ace2, have been fully cloned and sequenced from both oranophoshate-resistant and susceptible clones of cotton aphid. Comparison of both nucleic acid and deduced amino acid sequences revealed considerable nucleotide polymorphisms. Further study found that two mutations occurred consistently in all resistant aphids. The mutation F139L in Ace2 corresponding to F115S in Drosophila acetylcholinesterase might reduce the enzyme sensitivity and result in insecticide resistance. The other mutation A320S in Acel abutting the conserved catalytic triad might affect the activity and insecticide sensitivity of the enzyme. Phylogenetic analysis showed that insect acetylcholinesterases fall into two subgroups, of which Acel is the paralogous gene whereas Ace2 is the orthologous gene of Drosophila AChE. Both subgroups contain resistance-associated AChE genes. To avoid confusion in the future work, a nomenclature of insect AChE is also suggested in the paper.

Insensitive acetylcholinesterase was identified as a esistance mechanism by comparing biochemical analysis with a laboratory selected monocrotophos resistant cotton bloodworm (RR: 200) and the susceptible strain The cDNA enCODING AChE was cloned by the method of RACE (rapid amplification of cDNA ends). The complete AChE gene deduced from the cDNA consisted of a putative signal peptide of 32 amino acid residues, a mature protein of 615 residues, 5' untranslated regions (UTR) of 315bp and 3' UTR of 324bp. The Coding sequence had a high degree of Homology to the AChE from other insect species reported in the GenBank. After comparing analysis of the entire AChE gene sequence from 5 resistant and 6 susceptible cotton bollworm individuals, nine mutations were identified. One of them, the Ala/Thr mutation, is likely to be responsible for the AChE insensitivity to monocrotophos.