Quantifying parasitoid and predator controls on rice hopper eggs using a dynamic stage-structured model and field data.
Rice hoppers can cause devastation to rice crops in SE Asia via disease spread and direct feeding. Current regulation is through pesticides; however, a more sustainable approach is to use the natural enemies of the hoppers. Here, we show how the combination of a process-based model and field data can give insights into natural pest control, beyond those obtained through direct analysis of the field data. We use a stage-structured host-parasitoid model based on time-delay ordinary differential equations (using the R package "stagePop"; Kettle & Nutter, 2015; Methods in Ecology and Evolution 6:1484-1490) to investigate the interactions of four parasitoid genera and an egg predator on the eggs of two hopper hosts: the brown plant hopper (Nilaparvata lugens (Stål)) and the green leaf hopper (Nephottetix spp.). Model parameters are derived from 62 (×3) datasets from experiments in the Philippines. In these experiments, hopper eggs on rice plants were exposed in the field for 3 days and then taken back to the laboratory. The number of emerging hopper nymphs and parasitoid adults were then recorded. Single-host and mixed host experiments were performed. The field data are not sufficient to fully constrain the model parameters so our results are given in terms of the value of an unknown parameter (γ; the number of hopper eggs killed per successful parasitism) which may be ascertained via future experiments. Synthesis and applications. Our model experiments suggest that although parasitoids are most likely the main control on hopper egg abundance, and despite the fact that the egg predator also consumes parasitoid eggs, the two-enemy system (parasitoids and predators) is more effective at hopper egg control than if either enemy is present alone. Our results provide evidence in support of ecological engineering approaches that promote both parasitoids and egg predators.