Novel changes in host plant sterol chemistry negatively affect generalist caterpillars

Insects, like all other organisms, require sterols in many physiological processes, most notably as structural elements in biomembranes and as essential precursors to steroid hormone. In contrast to most organisms, though, insects lack the ability to biosynthesize cholesterol. Cholesterol is rarely found in plants above trace levels, so plant-feeding insects must produce it 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. In this study a transgenic tobacco plant containing a high proportion of 3-ketosteroids and stanols (which are novel sterols for insect herbivores) was fed to three economically important caterpillar species over two generations, and various performance measures, including reproductive output, were recorded. For all three species, performance on the modified tobacco plant was in some way reduced compared to the caterpillars reared on control plants. In the second generation, these negative effects were stronger, and more pronounced. Mating was only successful for two species, and in the first generation there was no effect of dietary sterols on egg production or viability. In the second generation, though, novel sterols negatively affected reproduction in a substantial fashion. These results, when extrapolated to the population level, suggest that there is real potential for modifying plant sterol profiles to manage plant-feeding insect pests in agricultural systems.