Cucurbitacins: A Role in Cucumber Beetle Steroid Nutrition?

Abstract

The conditional role of cucurbitacins as phytosteroid supplements, cholesterol precursors, or ecdysteroid antagonists in the spotted cucumber beetle, Diabrotica undecimpunctata howardi, was investigated in two ways: by comparing larval survival and growth rate on cucurbitacin-rich and cucurbitacin-poor squash cultivars of Cucurbita pepo and by manipulating the presence of cholesterol, phytosteroids, and cucurbitacins in an artificial diet and examining the effects on adult survival and fecundity. Larvae that developed on cucurbitacin-rich roots grew significantly faster and survived as well as larvae on cucurbitacin-poor roots. There was no evidence, however, that adults could substitute cucurbitacins in vital phytosteroid roles. Beetles reared on a cucurbitacin-rich, phytosteroid-poor diet laid significantly fewer eggs and died significantly younger than beetles with a full complement of dietary phytosteroids and also laid fewer eggs than beetles with no access to phytosteroids in their adult diet. The data are consistent with the hypothesis that, when the side chain of dietary cucurbitacins can be successfully hydrogenated, these compounds play a nutritional role as substitutes or precursors for structural steroids. In contrast, when the carbon–carbon double bond cannot be hydrogenated, cucurbitacins may become antagonists at ecdysteroid receptors, negatively affecting beetle fitness.