Susceptibility Of Sesamia Infernes (Walker) Of Field Populations To Various Insecticides And Resistance Mechanisms

Pink stem borer, Sesamia inferens (Walker), is one of the most important insect pests of rice. However, this pest has a broad range of host plants, and usually occurred at out part of paddy field and damaged less serious than Chilo suppressalis (Walker) and Tryporyza incertulas (Walker). In recent years, with the changes in cultivation and diversification of crops, the damage caused by this pest becomes serious year by year. In some areas, pink stem borer even becomes the dominant species of rice borers. But people is still used to controlling rice borers with the insecticides targeting at C. suppressalis and T. incertulas. Little attention has been put on the control methods against S. inferens. In order to select available insecticides against S. inferens and meet the demands for rice pest control in the new situation, this study surveyed on insecticide susceptibility with borers collected from both Nanjing, Jiangsu Province and Wenling, Zhejiang Province in two consecutive years (2009-2010) with a developed bioassay. Resistance mechanisms of S. inferens to several insecticides were also analysized, and the mixtures given best synergism were also selected with Phenthoate and Emamectin benzoate or Hexaflumuron. 1. Susceptibility of S. inferens of field populations to various insecticidesIn this study, S. inferens in Nanjing, Jiangsu Province and Wenling, Zhejiang Province had been collected and tested for their susceptibility to various insecticides in two consecutive years (2009-2010), by using newly developed artificial diet residue bioassay method. Results showed that S. inferens had not developed obvious resistance to most insecticides except for Monosultap, Triazophos and few others which had been used extensively for long time. Little change in sensitivity to various insecticides was found between different geographical population and different years. The toxicity of 14 insecticides to S. inferens was tested and ordered as:Emamectin benzoate>> Abamectin>> Phenthoate≈Chlorantraniliprole>> Hexaflumuron≈Tebufenozide> Indoxacarb> Phenothrin> Fenitrothion≈Profenofos≈Triazophos≈Butene-fipronil≈Chlorpyrifos>> Monosultap. Comparison analysis revealed that S. inferens and C. suppressalis had different sensitivity to various insecticides. S. inferens was found much more sensitive to Phenthoate and Profenofos than C. suppressalis. But to Abamectin, Chlorantraniliprole and Chlorpyrifos, S. inferens was much more tolerant. Finally, it was thought that extensive use of Abamectin and Chlorpyrifos for control of rice borers and planthoppers in recent years was not conducive to the control of S. inferens. For the control targeting at S. inferens, cheap organophosphorus pesticides with low toxic side effect, such as Phenthoate and Profenofos, could play full roles.2. Mechanisms for insecticide resistance in S. inferensThe synergisms of piperony butoxide (PBO), diethylmeleate (DEM) and triphenyl phosphate (TPP) on Monosultap, Emamectin benzoate, Phenothrin, Triazophos and Profenofos in S. inferens of Jiangsu and Zhejiang populations and a strain reared for one year in laboratory were tested together with the activity of three detoxification enzymes in their body. The results showed that all three synergists had little effects on Monosultap. TPP exited some synergistic effects on Emamectin benzoate and Triazophos, PBO showed some synergistic effects on Profenofos; Both PBO and DEM exhibited some synergistic effects on Phenothrin. Enzyme activity tests showed that the activity of MFO changed significantly among the three populations and was positively related with their insecticide tolerance. Esterase activity is similar between Nanjing population and its recovering laboratory strain, but significantly higher in more tolerant Zhejiang population. The CDNB activity of Glutathione S-transferase was significantly higher in the field populations than in the laboratory recovering strain. Thus, it was though that the enhancement of three detoxification enzymes activity in borers might result in the increase in their resistance level. Resistance mechanisms always varied with insecticides and resistance level. 3. Selection of synergistic insecticide mixturesThe mixtures of Emamectin benzoate and Phenthoate with different ratios were tested, and their co-toxicity coefficient (CTC) values were found as fellows:CTC1:2>CTC1:4> CTC1:1>CTC2:1>CTC4:1. Obviously, the mixture with the ratio of 1 to 2 showed highest synergism (CTC 200.79). The mixture with the ratio of 1 to 4 was also synergistic (CTC 135.27). But all the other mixtures tested with different ratio were not synergistic with their CTC less than 100. When Hexaflumuron mixed with Phenthoate, the mixtures with different ratios were found with their CTC values as fellows:CTC1:1>CTC1:2>CTC2:1 >CTC1:4>CTC4:1. Of which, the value of CTC1:1 was 227.99, which means the mixture with ratio 1 to 1 had the highest synergism. The values of CTC1:2, CTCl:4 and CTC2:1 were 118.01,94.03 and 99.55 respectively, which indicated additive effects. The value of CTC4:1 was only 68.18, which implied antagonistic effect. These results indicated that two different combinations of insecticides with high synergism had been selected. Further research might develop new and effective insecticides against S. inferens.4. Comparison of different indoor methods for rearing S. inferensFresh water-oat, artifical diet, and rice seedlings were, respectively, used for rearing larvae of S. inferens by inoculation of newly hatched neonates, and larval survival rate to 3rd instar, pupation rate and pupa body weight were checked and compared. The results showed that the larval survival rate to 3rd instar was high (79%and 74%) when fed on fresh water-oat or artifical diet, and no significant difference was found between these two rearing methods. However, pupation rate and pupa body weight varied significantly with rearing methods. Feeding on water-oat resulted in significantly higher pupation rate (47%) and heavier pupae (195.04 mg) than rearing with artificial diet (34%and 147.21 mg). When reared with rice seedlings, the larval survival rate to 3rd instar was very low (less than 10%), and no larvae could grow to pupation. These results indicated that rice seedling method, the very successful method for rearing C. suppressalis in laboratory was not appropriate for S. inferens. Fresh water-oat feeding method is the best one while artificial diet feeding method should be further improved.The suitable method for rearing S. inferens indoor and insecticide toxicity bioassay with chemical residues on artificial diet have been selected and established in this study, which provides new practical techniques for the study of S. inferens. Survey on insecticide resistance, and selection of efficient insecticides and their combinations with high synergism provide the basis for reasonable use of insecticides and development of new efficient insecticide formulations for control of S. inferens. The paralellel analysis of the synergism of different synergist on different insecticides, resistance and detoxification enzyme activity in different populations provide ideas for further research pursuing mechanisms and the onset mode for the resistance of S. inferens to different insecticides.