Insecticidal constituents from rhizomes of Zingiber cassumunar and Kaempferia rotunda

Abstract

Rhizomes from 18 different species of the Zingiberaceae were screened for insecticidal constituents against neonate larvae of the pest insect, Spodoptera littoralis. Extracts from rhizomes of Kaempferia rotunda and Zingiber cassumunar, when incorporated into artificial diets, displayed significant insecticidal activity in chronic feeding bioassays at concentrations of 2500 ppm and 1250 ppm, respectively. Bioassay-guided isolation afforded two phenylbutanoids from rhizomes of Z. cassumunar which had LC50 values against neonate larvae of 121 and 127 ppm, respectively, in the chronic feeding bioassay. Both compounds were also active in the residue-contact bioassay (LC50 values of 0.5 and 3.6 μg cm−2, respectively). The presence of oxygenated substituents (−OH or −OAc groups) in the side-chain nullified insecticidal activity. Rhizomes of K. rotunda yielded two active metabolites: benzyl benzoate and the cyclohexane derivative, crotepoxide. Compared to the bioactive phenylbutanoids from Z. cassumunar, crotepoxide was less active in the chronic feeding bioassay (LC50, 1450 ppm) and was inactive in the residue-contact bioassay. Benzyl benzoate, however, exhibited insecticidal activity only when applied topically (LC50, 5.6 μg cm−2) suggesting detoxification in the larval gut when applied orally.