Insecticidal activities of a Diospyros kaki root-isolated constituent and its derivatives against Nilaparvata lugens and Laodelphax striatellus

Diospyros kaki root-derived materials were examined for insecticidal properties against Nilaparvata lugens and Laodelphax striatellus. Based on the LD(50) values, the chloroform fraction of D. kaki extracts showed the most activity against N. lugens (3.78 mu g/female) and L striatellus (7.32 mu g/female). The active constituent of the chloroform fraction was isolated by various chromatographic methods and was identified as 5-hydroxy-2-methyl-1,4-naphthoquinone by spectroscopic analyses. To establish the structure-activity relationships, the insecticidal effects of 5-hydroxy-2-methyl-1,4-naphthoquinone and its derivatives against N. lugens and L. striatellus were determined using micro-topical application bioassays. On the basis of LD(50) values, 5-hydroxy-1,4-naphthoquinone was the most effective against N. lugens (0.072 mu g/female) and L. striatellus (0.183 mu g/female). 2-Bromo-1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, and 5-hydroxy-2-methyl-1,4-naphthoquinone also had potent insecticidal activities against N. lugens and L striatellus. In contrast, no insecticidal activity was observed with 2-methoxy-1,4-naphthoquinone or 2-methyl-1,4-naphthoquinone. These results indicate that the functional group (bromo- and hydroxyl-) at the C-2 position of the 1,4-naphthoquinone skeleton and the change in position of the hydroxyl group play important roles in insecticidal activity. Therefore, naturally occurring D. kaki root-derived 5-hydroxy-2-methyl-1,4-naphthoquinone and its derivatives may be suitable as insecticides. (C) Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society, 2011. Published by Elsevier B.V. All rights reserved.