Spectral characterization of cucurbitacins in a bitter mutant of Hawkesbury watermelon (Citrullus vulgaris Schrad) that elicit a feeding response to diabroticite beetles (Coleoptera: Chrysomelidae)

Abstract

Fractions obtained by open-column flash chromatography of a crude methanolic extract of the rind of a bitter mutant of Hawkesbury watermelon, Citrullus vulgaris Schrad, were further purified by preparative high-performance liquid chromatography (HPLC) to isolate chemical component(s) that elicit a visitation/feeding stimulancy response to the southern corn rootworm, Diabrotica undecimpunctata howardi Barber. Activity of chromatographic fractions were followed with a laboratory bioassay involving total insect-response counts. The chemical structure of the most active component in C. vulgaris was confirmed by chemical ionization mass spectrometry and proton nuclear magnetic resonance spectroscopy to be cucurbitacin-E glycoside. Two other cucurbitacin-like compounds were isolated and structures for them postulated. A procedure to prepare a crude, biologically active, extract of C. vulgaris is reported. Dose-response data for the crude extract in laboratory tests against two diabroticite beetles D. undecimpunctata howardi and D. virgifera virgifera LeConte are also reported.