UC Davis

Humanity faces two colossal and interwoven challenges: 1) By 2050, the world population may exceed 9 billion people, thereby requiring more food production, and 2) Increased food production will generate more agricultural byproducts from downstream food processing (i.e. stems, seeds, pulp), posing a significant environmental safety and management concern due to associated greenhouse gas emissions. As detritivores and herbivores, the diversity of insect species includes groups highly specialized in their ability to thrive on unique and unbalanced organic substrates, such as, agricultural by-products. Examples of agricultural by-products with relevance to the Davis area include tomato waste (skins, extracted pulp), wine waste (skins, stems, seeds), and almond hulls. In this project and in a rapidly growing body of research literature, two insect species, mealworms (Tenebrio molitor) and black soldier flies (Hermetia illucens) were studied as potential bioconverters of such locally available agricultural by-products. Insects feeding on agricultural by-products are harvested for protein and fats which may be reintroduced into the food system as animal feed. In addition, insects may be used to produce additional commodities, such as chitinous products, pharmaceuticals, biofuels, lubricants, and fertilizer from their excrement. Consequently, the services rendered from insect-based bioconversion provide marketable solutions for reducing food byproducts that are fiscally manageable, modest both in space and energy requirements, environmentally sustainable, while yielding higher feed conversion ratios than conventional livestock. As such, insect bioconversion is gaining traction both as a research topic and as a business opportunity. This dissertation addresses questions covering multiple stages across the bioconversion process with the expressed intent on exploring strategies to improve the bioconversion of agricultural by-products into value-added products. Specifically, I ask how variable is individual insects bioconversion efficiency, how does the microbial community influence performance, and how may insect-based products be reincorporated into agriculture? Chapter 1 is a review of the use and potential of using insects as a tool for food waste management. Chapter 2 is a review on the targeted breeding of insects, diversity of insect bioconverters, and research into insect-gut microbial complexes. Chapter 3 provides a framework to characterize intraspecific phenotypic variation in mass-reared insect populations. Tenebrio molitor larvae are compared in three bioassays evaluating variation in feeding efficiency on a novel diet (polystyrene), and diets representative of those used in the insect bioconversion industry. This chapter presents linear regressions of ranked trait responses as a useful tool to quantify and compare intraspecific variation both within and across populations of mealworms. Bioassays reveal that addition of polystyrene in T. molitor diets increases larval weight and overall diet consumption. However, feed conversion of larvae is lower and less variable on the polystyrene amended diets than on a standard diet. Chapter 4 describes effects of different rearing environments and their influence on black soldier fly, Hermetia illucens, production efficiency on agricultural by-products. This chapter also describes effects of microbial inocula on production efficiency. Main conclusions drawn from chapter 4 are that different rearing conditions favored bioconversion of particular agricultural byproducts, but only have a small effect on microbiota. In contrast, addition of microbial inocula did not significantly improve performance, nor does it alter intestinal and residue microbiota. Chapter 5 describes applications of insect-derived value-added by-products in an agricultural setting. More specifically, legume seeds were treated with chitinous products (raw insect derived chitin, pure chitin, pure chitosan) to determine their effects on mold growth, seed germination, and seedling vigor (biomass). Key findings from chapter 5 were that application of chitinous products significantly suppress mold growth. At higher dosages (5% and 10% by weight), raw insect-derived chitin adversely affects germination of soybeans, but not of fava beans and black-eyed peas. However, chitinous products do not significantly affect seedling weight. Considering the world-wide availability, low economical cost, and superior efficacy as a mold growth inhibitor, results from this study highlight raw insect-derived chitin as a promising novel fungicide when applied at a low dosage (2.5% by weight).