Effect of different ratios of “good” and “bad” sterols on the performance of Helicoverpa zea

Insects, like all other organisms, require sterols, most notably as structural elements in biomembranes and as essential precursors to steroid hormone. In contrast to most organisms, though, plant-feeding insects lack the ability to biosynthesize cholesterol. But cholesterol is rarely found in plants above trace levels, so plant-feeding insects must produce the cholesterol they need by metabolizing the sterols found in the plants they eat. Plant-feeding insects are, however, often limited in terms of which sterols can be converted to cholesterol, and this metabolic constraint could be exploited for use against pest insects of agricultural plants. Here we conducted three sets of experiments in which newly hatched Helicoverpa zea (Lepidoptera: Noctuidae) neonates were reared on synthetic diets that contained different dietary sterols for two generations. Results demonstrated that cholesterol and stigmasterol supported good growth and development and fecundity, while cholestan-one and cholestan-3-one did not. We mixed stigmasterol (a “good” sterol) with different amounts and ratios of cholestanol and/or cholestan-one (“bad” sterols) to determine what amounts of “bad” sterol is required in the diet to negatively affect caterpillar performance. Results suggest that the ratio needed to negatively affect caterpillars varies depending upon the structural properties of the “bad” sterol. In almost all instances, performance in the second generation was significantly reduced relative to the first generation, suggesting dietary sterols exhibit strong paternal effects. The findings from this study suggest that modification of plant sterol profile may be a novel and effective approach used to manage populations of economically important caterpillar species.