Sterols are crucial membrane components in all eukaryotic cells. Cholesterol is the most common sterol in animals, including insects, and it is the required precursor of the insect steroid hormone that regulates molting and developmental processes. However, insects are sterol auxotrophs and require a dietary sterol source. Herbivorous insects obtain phytosterols to meet their needs, but only metabolize certain types of phytosterols into cholesterol. To explore whether this sterol metabolic constraint in insects can be exploited to control insect herbivore pests, I manipulated phytosterol profile and investigated its effects on two different insect herbivores: a chewing insect (Plutella xylostella) and a sucking insect (Myzus persicae). In this study I modified the sterol profile of Arabidopsis thaliana by silencing the HYD1 gene, which encodes a ?^8,7 -sterol isomerase that converts ?^8 sterols to ?7 sterols. Arabidopsis lines with > 95% transcript reduction exhibited normal growth, but contained less sterol, half of which showed a ?^8 configuration uniquely detected in the RNAi lines. Notably, ?^8 sterols were not detected in the phloem even in RNAi lines. I then examined growth and reproduction for P. xylostella (caterpillar) and M. persicae (aphid). Both insects displayed reduced survival, growth and reproduction when reared on plants with > 95% transcript reduction, and this pattern was consistent over successive generations. Caterpillars reared on Arabidopsis lines with > 95% transcript reduction showed reduced cholesterol and high levels of ?^8 sterols compared to caterpillars from control plants. Aphids on Arabidopsis lines with > 95% transcript reduction showed reduced sterol content (> 60%) compared to aphids on control lines. I then compared aphid feeding behavior across the lines but found no difference. Finally, I used our collective data to estimate insect population growth on different Arabidopsis lines. Compared to controls, caterpillar populations on lines with > 95% transcript reduction were 70% reduced after 3 generations, while aphid populations were 60% reduced after 4 generations. I also investigated M. persicae performance of various sterol-modified plants targeting different steps of plant sterol biosynthesis. Aphids reared on lines that accumulated atypical sterols (hyd2, ste1 and cas1-1, which exhibit ?^8,^14, ?^7 sterols, and 2,3-oxidosqualene, respectively) showed significantly reduced body mass and reproduction compared to M. persicae reared on control lines. Additionally, M. persicae on lines that accumulated campesterol instead of ?-sitosterol were also negatively affected. However, aphids reared on lines that displayed high levels of suitable sterols, but with normal sterol profiles, were similar to control aphids. Insects with poor performance consistently showed reduced sterols in their body. Collectively, these results demonstrate the potential of using sterol-modified plants as a novel strategy for controlling insect herbivore pests.
