The peritrophic matrix (PM) is a chitin and glycoprotein layer that lines the invertebrate midgut. Although structurally different, it is functionally similar to the mucous secretions of the vertebrate digestive tract. The PM is a physical barrier, protecting the midgut epithelium from abrasive food particles, digestive enzymes, and pathogens infectious per os. It is also a biochemical barrier, sequestering and, in some cases, inactivating ingested toxins. Finally, the PM compartmentalizes digestive processes, allowing for efficient nutrient acquisition and reuse of hydrolytic enzymes. The PM consists of an organized lattice of chitin fibrils held together by chitin binding proteins. Glycans fill the interstitial spaces, creating a molecular sieve, the properties of which are dependent on the immediate ion content and pH. In this review, we have integrated recent structural and functional information to create a holistic model for the PM. We also show how this information may generate novel technologies for use in insect pest management.

Lipids are the primary storage molecules and an essential source of energy in insects during reproduction, prolonged periods of flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. The fat body is primarily composed of adipocytes, which accumulate triacylglycerols in intracellular lipid droplets. Genomics and proteomics, together with functional analyses, such as RNA interference and CRISPR/Cas9-targeted genome editing, identified various genes involved in lipid metabolism and elucidated their functions. However, the endocrine control of insect lipid metabolism, in particular the roles of peptide hormones in lipogenesis and lipolysis are relatively less-known topics. In the current review, the neuropeptides that directly or indirectly affect insect lipid metabolism are introduced. The primary lipolytic and lipogenic peptide hormones are adipokinetic hormone and the brain insulin-like peptides (ILP2, ILP3, ILP5). Other neuropeptides, such as insulin-growth factor ILP6, neuropeptide F, allatostatin-A, corazonin, leucokinin, tachykinins and limostatin, might stimulate lipolysis, while diapause hormone-pheromone biosynthesis activating neuropeptide, short neuropeptide F, CCHamide-2, and the cytokines Unpaired 1 and Unpaired 2 might induce lipogenesis. Most of these peptides interact with one another, but mostly with insulin signaling, and therefore affect lipid metabolism indirectly. Peptide hormones are also involved in lipid metabolism during reproduction, flight, diapause, starvation, infections and immunity; these are also highlighted. The review concludes with a discussion of the potential of lipid metabolism-related peptide hormones in pest management.

To infect per os, baculovirus virions cross the peritrophic matrix (PM) to reach the midgut epithelium. Insect intestinal mucins (IIMs) are PM proteins that protect the PM and aid passage of the food bolus through the gut. Some baculoviruses, including Mamestra configurata nucleopolyhedrovirus (MacoNPV-A), encode metalloproteases, known as enhancins, that facilitate infection by degrading IIMs. We examined the interaction between MacoNPV-A enhancin and M. configurata IIMs both in vivo and in vitro. Per os inoculation of M. configurata larvae with MacoNPV-A occlusion bodies (OBs) resulted in the degradation of McIIM4 within 4 h of OB ingestion, while McIIM2 was unaffected. The PM recovered by 8 h post-inoculation. To investigate whether enhancin was responsible for the degradation of IIM, a recombinant Autographa californica multiple nucleopolyhedrovirus expressing MacoNPV enhancin (AcMNPV-enMP2) was constructed. Enhancin was found to be a component of occlusion-derived virions in AcMNPV-enMP2 and MacoNPV-A. In in vitro assays, McIIM4 was degraded after MacoNPV-A and AcMNPV-enMP2 treatments. Degradation of McIIM4 was inhibited by EDTA, a metalloprotease inhibitor, indicating that the degradation was due to enhancin activity. Thus, MacoNPV-A enhancin is able to degrade major structural PM proteins, but exhibits target substrate specificity.