Impact of nitrogenous-fertilization on the population dynamics and naturalcontrol of rice leaffolders (Lep.: Pyralidae)

The effect of nitrogenous-fertilization on the population dynamics and natu ral control of rice leaffolders was studied in an irrigated rice area in th e Philippines. Nitrogen was applied at three levels (0, 75 and 150 kg N ha( -1)), and its impact on crop growth and yield, arthropod abundance, and rat es of leaffolder parasitism and survival was assessed with weekly samples. Rice plants were taller and had a higher leaf nitrogen content with increas ing levels of nitrogenous-fertilization, but grain yield was highest at the medium nitrogen level. Herbivores, predators, and parasitoids increased in abundance with nitrogenous-fertilization level. The average density of ric e leaffolder larvae at the highest nitrogen level was eight times the densi ty at zero nitrogen level, and the peak percentage injured leaves increased from 5 to 35%. The strong increase in larval density was due to the positi ve effect of nitrogenous-fertilization on egg recruitment and survival of m edium-sized larvae. The percentage parasitism of eggs and larvae was not af fected by nitrogenous-fertilization. The increase in survival of medium-siz ed larvae with nitrogen levels was associated with lower predator to leaffo lder ratios. The strong effect of nitrogenous-fertilization in the present small-scale experiment was attributed mainly to allowing the moths an ovipo sition choice between plots with different application levels of nitrogen. Therefore it is hypothesized that the effect of increasing nitrogenous-fert ilization level on leaffolder larval densities will be less pronounced when implemented over a large area.