| The striped stem borer, Chilo suppressalis (Walker) (Lepidoptera:Pyralidae), is an important pest in rice in southern China. In recent years, the pest occurs at a periodic character, which is caused by cropping system. Pesticides are main approaches to control the pest but their applications change the structure of pests and natural enemies. This paper investigated effects of selective insecticides imidacloprid and buprofezin residues on fecundity and hormones juvenile and molting hormones (JH and MH) of C. suppressalis, and on soluble sugar and free amino acid contents and chemical elements in rice plants. The results are as follows.1. Larvae of C. suppressalis were reared with rice plants and coba Zizania lalifolia (Turcz) treated with imidacloprid and buprofezin to examine effect of two insecticides on pupa weight and fecundity of female adult. The results showed that pupa weight showed a significant difference among insecticide treatments, but did not show significant differences compared to control. Number of laid eggs by female adult developed from which larvae fed on the coca treated with 10 and 15 mg/kg imidacloprid did not show significant difference compared to that on control coca, but that on the coca treated with 15 mg/kg imidacloprid did significant difference compared to that with 10 mg/kg imidacloprid. Number of laid eggs by female adult from on Wujing 15 plants treated with 30 mg/kg imidacloprid did not show significant difference compared to on control plants. However, number of laid eggs by female adult from on Fenyouxiangzhan plants treated with 15 mg/kg imidacloprid significantly decreased, with percent reduction of 37.82% compared to control, that with 30 mg/kg imidacloprid significantly higher than that with Wujing 15 plants treated with the same treatment.2. Effect of insecticides on physiology and biochemistry of rice plants can affect the pest feeding on rice. The study demonstrated hormone levels in C. suppressalis feeding on food treated with insecticides changed. JH level in larvae feeding on treated coca showed significant difference but MH did not compared to that on control coca. Ratio of JH to MH levels (JH/MH) increased with increase of imidacloprid concentrations. JH level in larvae varied with application methods of imidacloprid. JH level in larvae fed on Fenyouxiangzhan plants treated with 30 mg/kg imidacloprid and 10 mg/kg triazophos was significantly higher than that on control plants, increasing by 1429.31% and 1351.15%, respectively. While that fed on Wujing 15 with the same concentration treatment was not significantly higher than on control plants. For different application methods, JH level in larvae following topical application and foliar spray of insecticides showed significant decline. MH level in larvae fed on Fengyouxiangzhan treated with insecticides did not show significant difference compared to on control plants. That in larvae fed on Wujing 15 varied with insecticide concentrations and application methods. For example, MH level in larvae fed on rice plants treated with 30 mg/kg imidacloprid showed significant difference, that in larvae after topical application with the same concentration varied with concentrations. That in larvae after topical treatment with 80 mg/kg imidacloprid decreased. In additional, hormone level in larvae was related to rice variety. ANOVA showed that JH level in larvae showed significant difference between Fengyouxiangzhan and Wujing 15, but MH level did not show such difference. There were significant interaction effects for JH between rice variety and insecticide and no such effect for MH. Imidacloprid application disturbed JH/MH balance in larvae.JH level in adult developed from larvae feeding on the rice plants treated with imidacloprid did not show significant difference compared to control. Lineal regression relationship between JH level in adult developed from larvae feeding on Fengyouxiangzhan rice plants with different concentrations of imidacloprid treatments and concentrations could be constructed, with a negative lineal correlation between them. As the above-described, changes of insecticide-induced hormone level in larvae varied with insecticides, and application methods and rice varieties.3. This paper also investigated residue dynamics in different parts and developmental stages in both Fengyouxiangzhan and Wujing 15 after root zone application and foliar spray with 30 mg/kg imidacloprid and control efficacy of the residue concentration to Nilaparvata lugens. The results showed that imidacloprid residue concentration in Fengyouxiangzhan at 7 days after application (7DAA) was significantly lower than that in Wujing 15, in contrast, that at 15DAA was significantly higher than that in Wujing 15.Imidacloprid residue concentrations in plant parts at 7DAA after foliar spray at the tillering stage of two varieties were maximums. Residue test in different parts of rice plants indicated that imidacloprid concentration in leaf and unexpanded leaf for Fengyouxiangzhan at 14DAA were significantly lower than that in stem, decreasing by 47.52% and 26.63%, respectively, that in leaf at 28DAA decreased by 64.59% compared to that in stem. For Wujing 15, however, residue changes showed a different trend.At the booting stage, regardless of application methods, residue concentration of imidacloprid in leaf for Fengyouxiangzhan at the same DAA was higher than that in stem. Residue concentration in stem at 28DAA for root zone application and foliar spray reached a maximum. ANOVA for plant parts showed that residue concentration in leaf was significantly higher than that in stem for two application methods. ANOVA for days after treatment indicated that residue concentration in leaf showed a significant difference between 14DAA and 21DAA, but did not such difference among other DAAs.At the tillering stage, control efficacy of imidacloprid residue to N. lugens both at 1DAA and 15DAA showed significant difference for different treatments. For example, the control efficacy after root zone application for N. lugens release at 1DAA was significantly higher than that after foliar spray under the same imidacloprid concentration treatments; for N. lugens release at 1 DAA, a treatment combination of root zone application and high concentration showed a higher control efficacy, with 67.01% control efficacy to N. lugens. At the booting stage, for N. lugens release at 1DAA, control efficacy at 7 days after insect release showed a significant difference among treatments. As the above-described, changes of imidacloprid residue concentration in rice plants varied with rice varieties, plant parts and imidacloprid application methods. Insecticides impact physiology and biochemistry of rice, and the effect, in turn, affects pest population stayed at different positions of rice. Therefore, control efficacy of imidacloprid to N. lugens is associated with application methods.4. Alteration of plant nutrient under environmental stresses will result in beneficial or unbeneficial effects to insects of crop. The present investigation demonstrated that soluble sugar content in Fengyouxiangzhan after different imidacloprid concentrations showed significant difference compared to control. For example, the contents in leaf at 7 and 21 DAA after 20 mg/kg imidacloprid treatment and at 21DAA after 50 mg/kg imidacloprid treatment were significant lower than control; but that at 14DAA after 30 mg/kg imidacloprid was significantly higher than control; the contents in stem after 20, 30 and 50 mg/kg imidacloprid treatments were significantly lower than those in leaf. For Wujing 15, Soluble sugar content in leaf at 14 and 21 DAA after 30 mg/kg imidacloprid application was significantly lower than control, but that at 28DAA was significantly higher than control; that in stem at 14, 21 and 28DAA was significantly higher than control. Change of free amino acid content varied with rice varieties. For Fengyouxiangzhan, free amino acid content in rice plant after insecticide applicationwas significantly higher than control, in contrast, for Wujing 15, significantly lower than control. In additional, chemical elements contents in rice plants after insecticide treatment were affected, which varied with rice variety for the same element.
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