Changes in the survival rate and fecundity of the rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), at different magnesium concentrations in hydroponic culture

Abstract

The rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), is a leaf-feeding pest and the physical and chemical nature of the rice leaf affects its feeding, survival and fecundity. We examined changes in the survival rates of larvae and pupae, pupal weight and the number of eggs laid by adult females developed from a resistant variety (Huaidao 9) and a susceptible variety (Yangjing 9538) in hydroponic solutions containing different concentrations of magnesium (Mg). The concentration of Mg 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 to 60 and 80 ppm Mg and pupae exposed to 20, 40, 60 and 80 ppm 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 at 80 ppm compared to 60 ppm. 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 outbreaks of this pest because application of nitrogen elevates the chlorophyll content and Mg levels in the rice leaf.