The potential of natural enemies to suppress rice leaffolder populations

Rice leaffolders Cnaphalocrocis medinalis and Marasmia spp. (Lepidoptera: Pyralidae) are considered major pests in many Asian countries. Insecticide use against leaffolders is wide-spread, but may not be justified due to tolerance of the rice crop to leaffolder injury and a high level of natural biological control. This study was conducted to obtain more insight in the potential of indigenous natural enemies to suppress rice leaffolder populations and reduce the damage inflicted to the crop. The study started with a descriptive analysis of leaffolder population dynamics in Philippine rice fields, and then concentrated on experimental analysis of egg mortality and the impact of individual predator species. Models were used to integrate the experimental findings, to explain field observations, and to explore the consequences of varying biotic and abiotic conditions for leaffolder population dynamics and damage.Rice leaffolder populations in eight unsprayed rice crops were characterized by an egg peak at maximum tillering and a broad larval peak around the booting stage, with peak larval densities ranging from 0.2 to 2.0 per hill. Variation in survival from egg to larval stages between crops was not correlated with the level of egg parasitism, natural enemy abundance, or predator-prey ratios. High levels of N-fertilization resulted in a strong increase in leaffolder larval density and injury, due to a positive effect on egg recruitment and survival of medium-sized larvae. The increase in larval survival was associated with lower predator-prey ratios. Egg mortality in the field a veraged about 60%, and was mainly due to disappearance of eggs and to a lesser extent to parasitism by Trichogramma spp. Non-hatching was of minor importance. The level of egg disappearance was positively correlated with the densities of the predatory crickets Metioche vittaticollis and Anaxipha longipennis. Direct observations confirmed the major role of these crickets: in two crop seasons they were responsible for more than 90% of the observed egg predation. Minor predators were Micraspis sp., Ophionea nigrofasciata, and Conocephalus longipennis. The egg predation rate of the crickets in cages was described adequately with a linear functional response model, indicating that predation was limited only by the search rate. Increasing the predator density per cage led to a decrease in the egg predation rate per capita. Field testing of a model of predation of leaffolder eggs based on cage experiments showed that the observed trend in egg predation could be described as a function of cricket densities and crop leaf area. The evaluation also indicated that predator interference may limit the egg predation rate of the crickets, while the presence of alternative prey did not. A simulation study with a combined model of leaffolder population dynamics and rice crop growth highlighted the importance of natural enemies as well as crop growing conditions. The simulations indicated that larval densities as observed in the unsprayed fields would not cause significant yield loss in a wellfertilized crop. Yield losses simulated with an average leaffolder immigration pattern exceeded economic damage levels when no natural enemy action was included, while introduction of three field-observed natural mortality factors (egg predation, egg and larval parasitism) reduced losses to below these levels. Over their observed range in seasonal abundance, the predatory crickets could reduce leaffolder damage by 5 to 60% (average: 35%).The identification of the major egg predators and quantification of their impact can serve as a starting point for research on strategies to conserve natural enemies of rice leaffolders, and as inputs to IPM training programs to stimulate farmers to reduce insecticide sprays against rice leaffolder. The study also indicated the importance of optimization of nitrogen fertilization to avoid reliance on chemical control, by maximizing the positive effects on yield formation and tolerance to injury, while minimizing the leaffolder density response. For this purpose, a combined leaffolder-rice simulation model is a useful, integrative tool, to study how interaction between these mechanisms affects rice yield