Prospects for integrating augmentative and conservation biological control of leaffolders and stemborers in rice.

Abstract

Possibilities to combine augmentative biological control using Trichogramma spp. egg parasitoids and conservation biological control through habitat manipulation, for the management of rice leaffolder and rice stemborer pests have received only cursory mention in the literature. We reviewed information on the use of Trichogramma releases and on habitat manipulation to manage leaffolders and stemborers in rice. Stemborers have become a priority for biological control since the 1990s with research focusing mainly on Chilo suppressalis in China and Iran, Scirpophaga incertulas in South and Southeast Asia, and Chilo agamemnon in Egypt. In most cases, 100 K wasps (T. japonicum or T. chilonis) released over 30-100 release points ha^-1 at least once during early crop stages, resulted in good control (>50% reduction in damage). Despite positive results accumulated over decades, larger scale releases in rice have only been conducted very recently. Research on conservation biological control of stemborers has focused on manipulating rice field habitat, particularly along rice bunds (levees). Several studies reported higher Trichogramma densities or greater egg parasitism in rice fields with flowering plants on bunds compared to control fields (without bund vegetation and usually with insecticides). These trends have mainly been attributed to nectar as a supplementary food for the adult wasps, although evidence for this mechanism is weak. Trap plants, such as vetiver grass (Chrysopogon zizanioides) attract ovipositing stemborers, but suppress larval development. Repellent and banker plants have not yet been identified for rice stemborers or leaffolders. We outline the opportunities and challenges for combining augmentative and conservation biological control of leaffolders and stemborers in rice.