Influence of Cucurbitacins, Sugars, and Fatty Acids on Cucurbit Susceptibility to Spotted Cucumber Beetle1,2

Abstract

Abstract Seedlings of 18 plant types from 5 genera of Cucurbitaceae were analyzed for cucurbitacins, total sugars, and fatty acids and these were correlated with spotted cucumber beetle feeding. The species were: Citrullus colocynthis L. (Schrad.), C. lanatus (Mansf.) Matsumara: Cucumis anguria L., C. dipsaceus Ehrenb., C ficifolius Bouche’ C. longipes Hook f., C. melo L., C. myricarpus Naud., C. prophetarum L.: Cucurbita foetidissima H. B. K., C. pepo L.; Lagenaria siceraria (Mol.) Stamdl.; Luffa acutangula (L.) Roxb., and L. cylindrica Roem. Seedling injury ranged from 0 (none) to 3 (severe). Regression analysis indicated that cucurbitacins, total sugars, and the fatty acids (palmitic and linolenic) except linoleic contributed to insect feeding; correlation between concn and feeding was positive. Cucurbitacins, which cucumber beetles were able to locate without feeding, played the major role in seedling susceptibility; next in importance were palmitic acid, linolenic acid, and total sugars. All apparently related to the preference of beetles for specific strains or cultivars. In non-preferred seedlings, preference was induced by topical application of cucurbitacins A, B, C, D, E, and 1 and by the glycosides from C. foetidissimma roots. A barrier seemed to obstruct feeding of the beetles on the upper surface of the cotyledons, even when the attractant cucurbitacins were applied.