Studies On Control Techniques For First Generation Of Chilo Suppressalis In Single Cropping Rice Region

The rice stem borer, Chilo suppressalis (Walker), is one of major insect pests of rice in China. With cropping system reform, rice varieties substitution and insecticide resistance development, population of this pest and its damage to rice increased dramatically, severely threatening the production of rice. In Jiangsu and some other areas, postponing sowing and isolation culture of seedlings with covering membrane were employed to improve the management of this pest. However, due to the limited growing season for rice in the single cropping rice areas, these countermeasures could be not maintained longer enough to keep the moths of all the first generation from contacting the seedlings. So long persistence insecticides are required to handle rice seedlings before transplant. Otherwise, it is not clear if the moths in early spring could breed populations normally by using host plants other than rice. In order to evaluate and strengthen the strategy of "Taking it as the key for the management of borers to control their 1st generation by postponing rice sowing and transplanting date appropriately, and using effective insecticides", some related work has been done and the main results are summarized as follows:1. Toxicity of various insecticides on Chilo suppressalisThe toxicities of abamectin 1.8% EC, chlorantraniliprole 20% SC, hexaflumuron 5% EC, ethofenprox 10% EC, chlorpyrifos 40% EC, phenthoate 50% EC, triazophos 15% ME, monosultap 90% WP and profenofos 40% EC against the neonates and the forth-instar larvae of C. suppressalis were tested by Residues on Artificial Diet and Potter Tower Spraying, respectively. Similar results were obtained, which both showed that abamectinn and chlorantraniliprole were higher toxic, and followed by hexaflumuron, ethofenprox, chlorpyrifos and phenthoate, but monosultap, triazophos and profenofos were lower toxic to this pest. These results provide a direct and comprehensive reference basis for chemical control of C. suppressalis. 2. Selection of efficient mixtures of insecticides against Chilo suppressalThe toxicities of 13 different combinations of chlorantraniliprole, abamectin, ethofenprox and chlorpyrifos against the neonates and the forth-instar larvae of C. suppressalis were tested by Residues on Artificial Diet and Potter Tower Spraying, respectively. Similar results were obtained, which proved that the mixtures chlorantraniliprole with abamectin and chlorpyrifos with triazophos showed obvious synergism; chlorpyrifos with phenthoate, chlorpyrifos with profenofos, abamectin with triazophos, chlorantraniliprole with ethofenprox, chlorantraniliprole with triazophos and chlorantraniliprole with profenofos were additive, and three mixtures, ethofenprox with profenofos, chlorantraniliprole with hexaflumuron and chlorantraniliprole with phenthoate were antagonistic. Ethofenprox mixed with phenthoate and ethofenprox mixed with chlorpyrifos showed, respectively, additive and synergism when tested with Potter Tower Spraying. However, they were antagonistic and additive when tested by Residues on Artificial Diet. chlorpyrifos mixed with triazophos has been extensively used in practice. However, chlorantraniliprole mixed with abamectin was the synergistic combination newly discovered in this study. Further tests with mixtures of abamectin and chlorantraniliprole of different ratios found that their co-toxicity coefficient (CTC) values were as fellows:CTC2:1>CTC1:1>CTC4:1> CTC1:4>CTC1:2. The mixture with the ratio of 2 to 1 showed highest synergism(CTC 347.18). The mixture with the ratio of 1 to 1 and 4 to 1 were also synergistic (CTC 166.08 and 147.83). But all the other mixtures of different ratio were not synergistic with their CTC less than 100. Obviously, a new efficient insecticide formulation could be developed against C. suppressal in practice, which might delay the development of resistance to chlorantraniliprole and abamectin and reduce the control cost. The study also suggested those mixtures showed antagonistic should not be used in practice。3. Comparison of the persistence of insecticide representatives with indoor trialsPositive correlation between pesticide concentration and persistence of control effect was first proved by the trials controlling the inoculated neonates of C. suppressalis with rice seedlings treated with different doses of profenofos. Then, chlorantraniliprole, abamectin and profenofos were compared for their persistence of control effect under the prerequisite for no phytotoxicity. The results showed that treatment with 5000 time dilution of 20% chlorantraniliprole SC could keep the rice seedlings from infestation for 14 days, and high doses (1000 time dilution) could keep for 28 days. One thousand time dilution of 1.8% abamectin EC can keep no infestation for only 7 days, which could be extended to 14 days when 500 time dilution was used. The persistent control effect of 40% chlorprifos EC was poorest of the 3 insecticides. The control effect of only 84.3% in 7 days and 57.2% in 14 days was observed when 1000 time dilution was applied. However, the corresponding control effect could be enhanced to 100%,89.5% by using 300 time dilution. Obviously, insecticides varied in their persistent effect, which was not always consistent with their toxicity. It was concluded that chlorantraniliprole and abamectin have good persistent control effect, which could be further enhanced by use of high doses. Discussion came to the conclusion that with higher dose of long persistence insecticides, the first generation of C. suppressalis in a year could be effectively controlled by handling rice seedlings before transplant under the prerequisite for no phytotoxicity. Thus, the population density of the following generations in the whole year could be depressed, and also the early insecticide spray after transplant could be saved which is helpful for nature enemies to set up their population in rice field.4. The oviposition preference and population developing of Chilo suppressalis on different host plantsThe oviposition preference and survival rate of the neonates of C. suppressalis on different host plants was studied at laboratory condition. The result showed that the accumulated borer's eggs distributed on wildrice, rice, wheat, sugarcane and maize with the corresponding portion as 48.03%,44.16%,3.38%,2.39% and 2.04%. There was no significant difference between wildrice and rice, but the egg number was significantly higher on these two hosts than on other plants tested. The survival rate of neonates of C. suppressalis on wildrice, rice, wheat, sugarcane and maize was 10.75%,51.22%,44.11%, 1.45%,7.15%, respectively. No difference on rice and wheat was found, but the survival rate on these two was significantly higher than that on wildrice, maize and sugarcane. These results demonstrated that traditionally named host plants might be definitely different in maintaining borer's population. Only rice is thought to be suitable for breeding of this borer, when oviposition preference and survival rate were both taken into consideration.In this paper, the toxicities of 9 commonly used insecticides against C. suppressalis were tested, an efficient mixture was selected, and it was also proved that efficient and persistent control could be achieved by handling rice seedlings with high doses of insecticides under the prerequisite for no phytotoxicity. The results could be used to improve the management of rice stem borers. Otherwise, different host plants were evaluated for their role in maintaining borer's population, and evidences were provided for population depressing effect of the strategy to keep the borer moths of first generation from contacting with rice seedlings in single cropping rice areas.