Trehalose is an important carbohydrate substance in insect hemolymph. Chitin is the main component of cuticle and peritrophic matrix in insects. Trehalase (Tre) catalyzes the decomposition of trehalose. Few studies of trehalase in lepidopteran insects have been conducted. Here, the functions of soluble Tre (Tre1) and membrane-bound Tre (Tre2) in the growth and development of Mythimna separata were investigated. We cloned and identified Tre1 and Tre2 cDNA sequences in M. separata. Analysis expression revealed that MsTre1 and MsTre2 were highly expressed in midgut and integument, respectively. The expression of MsTre1 and MsTre2 was highest in the pupal stage. We used RNA interference (RNAi) to inhibit Tre expression in M. separata larvae. Injection of dsMsTre1 or dsMsTre2 resulted in abnormal phenotypes and impeded normal molting. Silencing of MsTre1 and MsTre2 resulted in significant changes in the expression of genes in the trehalose and chitin metabolism pathways, significantly increased the trehalose and glycogen content, and significantly decreased MsTre1 and MsTre2 activity, the glucose content, and the chitin content in midgut and integument. Silencing of MsTre1 slowed larval molting, and the new cuticle was significantly thinner. These results indicate that RNAi of Tre may be useful for control strategies against M. separata.

Population-density-dependent polymorphism is important in the biology of some agricultural pests. The oriental armyworm (Mythimna separata) is a lepidopteran pest (family Noctuidae). As the population density increases, its body color becomes darker, and the insect eats more and causes greater damage to crops. The molecular mechanisms underlying this phase change are not fully clear. Here, we used transcriptomic and metabolomic methods to study the effect of population density on the differentiation of second-day sixth instar M. separata larvae. The transcriptomic analysis identified 1148 differentially expressed genes (DEGs) in gregarious-type (i.e., high-population-density) armyworms compared with solitary-type (low-population-density) armyworms; 481 and 667 genes were up- and downregulated, respectively. The metabolomic analysis identified 137 differentially accumulated metabolites (DAMs), including 59 upregulated and 78 downregulated. The analysis of DEGs and DAMs showed that activation of the insulin-like signaling pathway promotes the melanization of gregarious armyworms and accelerates the decomposition of saccharides, which promotes the gregarious type to take in more food. The gregarious type is more capable of digesting and absorbing proteins and decreases energy consumption by inhibiting transcription and translation processes. The phase change traits of the armyworm are thus attributable to plasticity of its energy metabolism. These data broaden our understanding of the molecular mechanisms of insect-density-dependent polymorphism.