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In a food preference chamber, fewer adults of red flour beetle (Tribolium castaneum Herbst) settled in rice grain treated with 100, 500, or 1,000 ppm of the oil of turmeric Curcuma longa (L.), sweetflag Acarus calamus (L.), or neem Azadirachta indica A. Juss, or "Margosan-O" (a commercial neem-based insecticide). Repellency increased with increasing concentrations of the oils and Margosan. In another choice test, filter paper strips treated with turmeric oil or sweetflag oil at 200, 400, or 800 JLg/cm 2 repelled insects during the first2 wk; thereafter, repellency decreased more rapidly than with neem oilor Margosan-O. Tribolium castaneum adults fed wheat flour, which had been treated with the test materials at 200 ppm, produced fewer and underweight larvae, pupae, and adults compared with the control.

Certain physicochemical stresses affect the susceptibility of plants to insects. The objective of this study was to determine the effects of different levels of Fe, Si, and Al on the establishment of Sogatella furcifera (Horvath) on resistant IR2035-117-3 (IR2035) and susceptible Taichung Native 1 (TN1) rice (Oryza saliva L.) cultivars under glasshouse conditions in natural daylight of 12 h, 29/21 °C (day/night), and minimum 70% relative humidity. Insect population increased on rice plants grown in culture solution containing low (0.02 mg L~') or high (40 mg L~') Fe than on plants grown in standard culture solution (Fe at 2 mg L~'). Increased Si (400 mg L-') reduced intake and assimilation of food, growth, adult longevity, fecundity, and population increase by 5. furcifera on susceptible TN1 plants.

Both in a choice and multi-choice laboratory tests, fewer adults of the banana root borer, Cosmopolites sordidus (Germar), settled under the corms of the susceptible banana "Nakyetengu" treated with 5% aqueous extract of neem seed powder or cake or 2.5 and 5% emulsified neem oil than on water-treated corms. Feeding damage by larvae on banana pseudostem discs treated with 5% extract of powdered neem seed, kernel, or cake, or 5% emulsified neem oil was significantly less than on untreated discs. The larvae took much longer to locate feeding sites, initiate feeding and bore into pseudostem discs treated with extract of powdered neem seed or kernel. Few larvae survived when confined for 14 d on neem-treated banana pseudostems; the survivors weighed two to four times less than the larvae developing on untreated pseudostems. Females deposited up to 75% fewer eggs on neem-treated corms. In addition, egg hatching was reduced on neem-treated corms. The higher the concentration of neem materials the more severe the effect.

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Resistant rice varieties have proved to be highly effective against the brown planthopper Nilaparvata lugens (Stal), but their long-term stability is threatened because of the evolution of prolific biotypes. At present, identification of biotypes is based principally on the differential reactions of host rice varieties to pest in greenhouse screenings and in field plantings of test nurseries at many locations. So far, three N. lugens biotypes that attack different rice varieties have been identified in the Philippines. Biotype 1 can survive on and damage only those varieties that do not carry any genes for resistance. Biotype 2 can thrive on varieties carrying Bph 1 resistance gene and on those susceptible to biotype 1. Biotype 3 can infest and destroy varieties having bph 2 resistance gene and those susceptible to biotype 1. However, none of these biotypes is capable of damaging varieties with Bph 3 and bph 4 genes for resistance, and the variety Ptb 33 carrying two unidentified genes is resistant to almost all known biotypes. Recently, a population of N. lugens was found thriving on the common weed grass Leersia hexandra Swartz on the IRRI farm and in some locations in Luzon, Philippines, This biotype does not survive on rice.Several varieties which are resistant in the Philippines are susceptible in India and Sri Lanka because the South Asian biotypes of N. lugens are more virulent than the Southeast Asian biotypes. To monitor the pest biotypes in different geographical regions and to identify new sources of resistance, an International Rice Brown Planthopper Nursery (IRBPHN) has been established in many cooperating Asian countries.Biotypes can also be differentiated by studying their behavioural and physiological responses to plants of known genotypes. Distinctive differences can be observed in insect feeding, growth, longevity, fecundity, oviposition and population increase on differential varieties and relative susceptibility or tolerance for hosts' allelochemics. Recent investigations of the existence of subtle morphological, cytological, and electrophoretic differences among N. lugens biotypes and results of hybridization experiments strongly complement other methods of biotype identification.The evolution of biotypes is a complex process that is governed by the interactions of the genetic and other biological characteristics of the pest populations and the genetic makeup of cultivated varieties. While the strategy of sequential release of varieties with major resistance genes has been fairly successful, the monogenic nature of those varieties makes them vulnerable to the development of virulent pest biotypes. Therefore, present breeding endeavors envisage use of both major and minor resistance genes for effective pest suppression.

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