The Effect Of Magnesium Element On Development And Fecundity Of The Rice Leaffolder, Cnaphalocrocis Medinalis Guenee (Lepidoptera:Pyralidae)

The rice leaffolder, Cnaphalocrocis medinalis Guenee (Lepidoptera:Pyralidae) is leaf-feeding pest of rice. C. medinalis larvae tie the edges of the tips of rice leaves together with silken threads, thereby forming a protective cavity within which they feed by scraping mesophyll cells and epidermal tissue from leaf blades.Therefore, the nutrient source for the feeding insect is mesophyll cells.Mesophyll cells are mainly composed of chloroplasts, which are largely made up of elemental magnesium (Mg).An investigation into the effect of Mg on these life-history traits of C. medinalis is important for understanding the population growth and resistance mechanisms of rice. We measured the effect on chemical elements of rice leaves and examined changes in the survival rates of larvae and pupae, pupal weight and the number of eggs laid by adult females developed from two varieties in hydroponic solutions containing different concentrations of magnesium (Mg). In natural populations, the level of nitrogen fertilizer influences the occurrence of C. medinalis.To approach realistic cause of nitrogen fertilizer-induced outbreak of the leaf-feeding insect, the present experiments investigated changes in development, fecundity and biochemistry in the pest under magnesium (Mg) foliar sprays on hydroponic culture rice and under the combinations of nitrogen fertilizer levels and Mg spray on soil culture rice. The results are as follows:1.Effects on chemical elements of rice leaves and changes in the survival rate and fecundity of the rice leaffolder at different magnesium concentrations in hydroponic cultureThe results showed that Mg content in rice leaves increased with increasing Mg concentration in the hydroponic solution. The contents of Cu, Fe, Mn, Zn on the resistant variety and the content of Zn on the susceptible decreased with increasing Mg concentration. By the way, the concentration of Mg concentration in the hydroponic solution significantly affected the survival rates of the larvae and pupae, the pupal weights and the numbers of eggs laid. Larval survival rates on the resistant variety were significantly higher for fourth instars exposed to60and80ppm Mg and pupae exposed to20,40,60and80ppm Mg than those for the control treatment (without Mg). For the susceptible variety, survival rates of first,second and third instars were significantly higher at high concentrations of Mg than those under control conditions. In addition, the number of eggs laid by adult females increased with increasing Mg concentrations, though there was a slight decrease at80ppm compared to60ppm. These results indicate that Mg is beneficial to the development, survival and fecundity of C. medinalis. They further suggest that high levels of nitrogen fertilizer in rice fields favour outbreak s of this pest because application of nitrogen elevates the chlorophyll content and Mg levels in the rice leaf.2.Effects of Mg foliar spray and the combinations of Mg spray and nitrogen fertilizer levels on several life-history traits and biochemistry of the rice leaffolder under hydroponic culture and soil cultureThe results showed that foliar sprays of different Mg concentrations in hydroponic culture rice significantly decreased developmental duration (days)(DD) of larvae and pupae, but did not influence pupation rate and adult emergence rate. The number of eggs laying by adult females, contents of protein and total sugar in larvae and pupae significantly increased with increasing Mg concentrations. For soil culture, DD of larvae and the number of eggs laying by adult females significantly increased with increasing nitrogen fertilizer levels, but fertilizer did not influence DD of pupae, pupation rate and adult emergence rate. In contrast, DD of adults decreased with an increase of fertilizer levels. In addition, contents of protein and total sugar in larvae and pupae significantly increased with increasing nitrogen fertilizer levels, but there was a decrease trend at the highest fertilizer level (12.5g/L). Mg foliar sprays and fertilizer significantly elevated Mg content in rice leaves. The present findings demonstrated that elevation of nitrogen fertilizer levels resulted in an increase of Mg content in leave, which is in turn suitable for development and fecundity of C. medinalis.