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Plant-feeding arthropods, in the course of evolution have adapted differentially to their plant host and thereby gained access to the available resource. Plants responded by developing chemicals and morphological defense to arthropods. Some secondary metabolic substances in plants have been assumed to be toxic or deter feeding. Aconitic acid in the barnyard-grass subspecies, Echinochloa oryzycola Vasing has been reported to be an active component against brown rice planthopper Nilapalvata lugens (Stal) (Kim et al. 1976). Aphids, Ropalosiphum padi. L., R. maidis (F.), Schizaphis graminum (Rondani)and Sitobion akeviae (Shinji), are important pests of barley (Hordeum vulgare L.) and wheat (Triticum aesstivum L.). On barley plants, more than seven aphids per stem causes yield loss (Hansen 1990). R. maidis sometimes severely infested corn plants (Zea mays L.), the leaves being wet by honeydew. Melanaphis sacchari (Zehntner) caused brownish decay of leaves of a susceptible line・ of sorghum Sorghum bicolor Moench. We examined the relation between aconitic acid contents and aphid densities in wheat, barley, corn, sorghum, barnyard grass and rice (Rustamani et al. 1992a). The plants containing larger amounts of aconitic acid showed a moderate degree of resistance to aphids. The findings obtained on additional lines in 1991, and on other components, such as DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), are reported herein.
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