Biodiversity relevant to pest management of tropical irrigated rice pests is discussed in terms of variation within rice plants, rice fields, groups of rice fields and rice associated ecosystems. It is concluded that, in the unique cropping conditions and stable water supply of tropical irrigated rice, the manipulation of a relatively few manageable components of diversity can confer stability such that pests are mostly kept at levels which do not justify the use of insecticides. Durable rice plant resistance, including moderate resistance, together with ability to compensate for damage by certain key pests, are regarded as fundamental to successful biological control by the natural enemy complex. Reliable natural enemy action is also considered to depend on all-year-round continuity of prey or hosts made possible by the relatively short fallow periods between staggered two to three rice crops per year and by proximity of certain non-rice habitats, notably the vegetation-covered bunds (levees) surrounding each field. In contrast, synchronous cropping could upset stability by destroying the continuity needed for natural enemy success. Such conclusions are supported by the experience of farmers who use little or no insecticide. Much evidence on destruction of natural enemies by certain insecticides supports the contention that insecticide use, especially early in the crop season, upsets natural enemy control of insects such as Nilaparvata lugens (Stal) (Hemiptera: Delphacidae) and also creates heavy selection pressure for strains of pests that can overcome previously resistant rice cultivars. Such circumstances create outbreaks of secondary pests and impair biological control of some key primary pests such as stem borers. It is concluded that pest management of much tropical irrigated rice must be based on natural controls rarely supplemented by insecticides. The success of this approach depends in particular on further research on dynamics of natural enemy and pest communities in rice ecosystems, especially where climatic conditions and water supply are marginally stable. Much more needs to be known about the nature and utilization of rice plant compensation for damage, particularly by defoliators and stem borers. The justification for, and supplementary use of, insecticides needs to be radically reassessed. There is no evidence that a natural control-based approach, as recommended in this review, is incompatible with farmer practicability or with future developments in rice production Correspondence: Professor Michael J. Way, technology, except perhaps the possible mechanization-driven increase in field size which would decrease bund area. In contrast, the insecticide-based approach is not only harmful to natural controls but is costly and mostly demands impracticable decision making by farmers on need-based use.

Reduction of noncrop habitats, intensive use of pesticides and high levels of disturbance associated with intensive crop production simplify the farming landscape and bring about a sharp decline of biodiversity. This, in turn, weakens the biological control ecosystem service provided by arthropod natural enemies. Strategic use of flowering plants to enhance plant biodiversity in a well-targeted manner can provide natural enemies with food sources and shelter to improve biological control and reduce dependence on chemical pesticides. This article reviews the nutritional value of various types of plant-derived food for natural enemies, possible adverse effects on pest management, and the practical application of flowering plants in orchards, vegetables and field crops, agricultural systems where most research has taken place. Prospects for more effective use of flowering plants to maximize biological control of insect pests in agroecosystem are good but depend up on selection of optimal plant species based on information on the ecological mechanisms by which natural enemies are selectively favored over pest species.

Many species of insects associated with cultivated rice do not over-winter in Korea and Japan, but migrate into these areas each year. To understand better the origins of these immigrations as well as the geographic structure of rice pests in Asian rice growing regions, intraspecific variation in two species of delphacid planthoppers, Nilaparvata lugens (Stål) and Sogatella furcifera Horvath, was examined. An 850 base pair region of mitochondrial DNA cytochrome oxidase-I (CO-I) was sequenced from a total of 71 individuals collected from 11 localities in seven countries: Korea, Philippines, China, Bangladesh, Malaysia, Vietnam and Thailand. In N. lugens, three haplotypes were found and all populations sampled shared a dominant haplotype. Localities in Korea contained two haplotypes and localities in China and the Philippines contained three. However, in samples from the Indochina peninsula no variation was detected either within or between populations, consistent with a hypothesis of regular migration and gene flow. These populations did not contain some haplotypes found in Korea, suggesting they were not the source of yearly immigration into Korea and, by extension, Japan. Populations from China did share haplotypes with Korea, which was consistent with the hypothesis that China was the source for yearly immigration into Korea. There was insufficient resolution to distinguish among populations in China. For N. lugens, the data suggested that populations south of the Red River Valley in Vietnam experienced regular mixing and were distinct from populations to the north which contributed to yearly immigrations. In S. furcifera, there was less differentiation among populations. Two haplotypes were found in all populations except Malaysia. The results for both species were consistent with seasonal weather data and indicated that more detailed analysis of DNA sequence data will be fruitful.

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The brown planthopper (BPH) Nilaparvata lugens, whitebacked planthopper (WBPH) Sogatella furcifera and smaller BPH Laodelphax striatellus increasingly exhibit resistance to insecticides and adaptation to resistant varieties, so they threaten food security. This review draws together, for the first time, information on the parasitoids of planthopper pests of rice from the non-English literature published in Asia. This is integrated with the English language literature to provide a comprehensive analysis. Planthopper pests of rice are attacked by a large range of parasitoids from Strepsiptera, Diptera and, especially, Hymenoptera. Levels of field parasitism vary widely between parasitoid species and locations. For many taxa, especially within Mymaridae, there is evidence that non-crop habitats are important as overwintering habitat in which alternative hosts are available. These source habitats may promote early season parasitism of pest Hemiptera in rice crops, and their movement into crops could be manipulated with applications of herbivoreinduced plant volatiles. Non-crop plants can also provide nectar to improve parasitoid longevity and fecundity. Despite evidence for the importance of environmental factors affecting parasitoids of rice pests, the use of habitat manipulation to enhance biological control in the world's most important crop is surprisingly underrepresented in the literature. Current research in China, Vietnam and Thailand on ecological engineering, carefully selected vegetation diversity introduced without disrupting profitable farming, is briefly reported. Although the most important pest, BPH (N. lugens), is a migratory species, maintaining local communities of parasitoids and other natural enemies offers scope to prevent even r-selected pests from reaching damaging population densities.