Changes in levels of juvenile hormone and molting hormone in larvae and adult females of Chilo suppressalis (Lepidoptera: Pyralidae) after imidacloprid applications to rice.

Insect hormones regulate growth and development and fecundity of insects. The current study investigated changes in juvenile hormone (JH) and molting hormone (MH) levels in fourth instars and adult females of Chilo suppressalis (Walker) (Lepidoptera: Pyralidae) after imidacloprid application to rice, Oryza sativa L. The results showed that JH level in fourth instars that developed feeding on Fengyouxiangzhan rice plants sprayed with 15, 30, and 60 ppm imidacloprid was significantly higher than that of larvae that developed on control plants, increasing by 5.04, 6.39, and 4.89 times, respectively. The relationships between JH level and imidacloprid concentrations showed a significant negative correlation. In contrast, molting hormone (MH) level in larvae fed on control plants was significantly higher than that on treated plants. JH:MH values in fourth instars developed from larvae feeding on rice plants treated with 15, 30, 60, 80, and 100 ppm imidacloprid increased by 49.17, 39.43, 13.48, 15.80, and 0.2 times, respectively, compared with control. JH and JH:MH ratio in larvae fed on Wujing 15 plants treated with imidacloprid were significantly lower than those fed on Fengyouxiangzhan under the same treatments. JH level in adult females that developed from larvae feeding on rice plants sprayed with imidacloprid significantly decreased with increase in imidacloprid concentration, but it increased compared with control. JH level in adult females was associated with times of imidacloprid application. JH level in adult females developed from larvae feeding on rice plants after double spray with 30 ppm imidacloprid was significantly higher than control, increasing by 61.6 and 116.5%, respectively, compared with a single spray and the control. Moreover, hormone levels in the larvae were related to the application method of imidacloprid. JH level in fourth instars after root application and topical application of imidacloprid was significantly lower than in control. Thus, the dynamics of JH and MH in insects after insecticide applications are an extremely interesting problem, because hormones are related to insect growth and development.

Distribution of imidacloprid residues in different parts of rice plants and its effect on larvae and adult females of Chilo suppressalis (Lepidoptera: Pyralidae).

The distribution of imidacloprid residue in different parts of rice, Oryza sativa L. (Graminales: Poaceae), plants was investigated for three rice varieties. Changes in larval hormones in Chilo suppressalis (Walker) (Lepidoptera: Pyralidae) that fed on imidacloprid-treated plant sources and the fecundity of resultant adult females (moths) also were determined. No significant differences in imidacloprid residue were detected in rice stem, leaves, and the unexpanded new leaf 7 d after treatment except in the rice variety Fengyouxiangzhan, in which residue content in leaves and the unexpanded new leaf was significantly lower than that in the stem. The percentage of reduction of imidacloprid residue after treatment varied with rice variety and plant parts. The greatest reduction in the three plant parts was shown in Wujing 15 compared with the other two varieties. The decomposition rate of imidacloprid residue in Wujing 15 was greater in leaves and unexpanded new leaf than in the stem, whereas in Wuyunjing 7 it was lower in leaves than in the unexpanded new leaf and stem. In Fengyouxiangzhan, the decomposition rate of imidacloprid in the stem increased 14 d after treatment. Juvenile hormone III and molting hormone titers in fourth instar larvae that had fed on rice plants treated with imidacloprid since the neonate stage were significantly higher than those in larvae fed on untreated plants (control). Juvenile hormone titer in larvae that had fed on Zizamia lalifolia Turcz galls treated with imidacloprid and the fecundity of the female moths that developed from such larvae were also significantly higher than that on the control counterparts. These results are discussed in relation to the impact of imidacloprid on nontarget insects and pest management.

Selective insecticide-induced stimulation on fecundity and biochemical changes in Tryporyza incertulas (Lepidoptera: Pyralidae).

The use of selective insecticides in rice, Oryza sativa L., fields often causes resurgence of nontarget pest insects. This study was conducted to investigate the effect of two selective insecticides, buprofezin and imidacloprid, on Tryporyza incertulas (Walker), a nontarget pest. After larval feeding on rice plants treated with each insecticide, fecundity, ovary protein content, and titer of juvenile hormone III (JHIII) in the resulting female moths were determined with 'Xiushui 63' rice susceptible to T. incertulas and 'Zhendao 2' moderately resistant to T. incertulas. The fecundity of females developed from larvae that fed on the insecticide-treated Xiushui 63 plants was stimulated compared with that of moths from larvae that fed on rice plants that were not treated with either insecticide. There was no stimulating effect in females from larvae that fed on insecticide-treated Zhendao 2 plants. The weight of fourth instars (final instars) that fed on the insecticide-treated Xiushui 63 rice plants was significantly greater than that of control, increasing by 50.3 and 46.7% for 60 and 112.5 g (AI) ha(-1) buprofezin, and by 23.7 and 19.5% for 15 and 37.5 g (AI) ha(-1) imidacloprid treatments, respectively. Ovary protein content in adult females developed from larvae that fed on the rice treated with the high dose of buprofezin was significantly higher than that in control. For the high and low doses of imidacloprid during the second instar, and the low dose of imidacloprid during the fourth instar, JHIII titers in female adults were also significantly higher than that in control, increasing by 152.81, 90.52, and 114.19%, respectively.