Microbial interactions in mass-reared insects to promote host growth and health

Introduction: Microorganisms have established complex interactions with their insect hosts exerting a plethora of crucial functions for host biology, survival and immunity. In the recent years, insect farming has been promoted as an environmentally-friendly way to bioconvert agro-industrial by-products and to replace/complement traditional protein-based feed/food. Objective: We aim to investigate the microbiota associated with mass-reared insects, i.e. the yellow mealworm (YM) Tenebrio molitor L. (Coleoptera: Tenebrionidae) and the black soldier fly (BSF) Hermetia illucens (L.) (Diptera: Stratiomyidae), to promote host growth and health, particularly when reared on different agro-industrial by-products. Method: Besides insect microbiota characterization by 16S rRNA gene high-throughput sequencing, this multidisciplinary work evaluates insect performances on various byproducts, with different nutrient compositions and under both healthy and diseased conditions. Findings: Both species were able to grow on different locally available by-products (i.e. okara, brewer’s spent grains, potato peels and potato selection waste for BSF, or mixes of wheat bran with brewer’s spent grain or tomato peels for YM), though with varying degrees of efficiency. We then characterized BSF and YM gut microbiota through DNAbased techniques and cultivation approaches to gain an in-deep knowledge of the gut microbiota and to select potential bacterial probiotics for further insect dietary supplementation. Considering the possible involvement in immunity response and thus insect health, we characterized the hemolymph microbiota in YM to unveil interactions with the host and the hemocytes. Conclusion: A better knowledge of microbial dynamics in mass-reared insects can promote the development of healthy production of insects as feed and food.