Frass from black soldier flies as a valuable fertilizer and biopesticide for crops

Abstract

Recently, insect farming has gained recognition globally. This is particularly true for the black soldier fly, Hermetia illucens. However, the byproduct of insect feeding, a combination of food residue and frass left by the insects after harvesting, has not been given much attention and even fewer studies focus on the use of frass as a bio-fertilizer. In sub-Saharan Africa, human populations are expected to double by 2050, expensive fertilizers and increased food prices will exacerbate conditions further. This study is divided into three data chapters. Firstly, we conducted a systematic review of published literature on BSF (black soldier fly) larvae frass. In this review we collected literature from across the world, and we compared the research done in Africa with that of the rest of the world. The results show that more studies have been done in developed countries (82 %) as compared to developing countries (18 %). Topics explored around BSF frass included waste disposal, organic fertilizer, bioconversion, chemical composition, and economics. The results from all these studies show that frass can be used as a complementary organic fertilizer or independently. However, none of the papers have explored all the benefits of frass and the impact of different waste streams on frass quality. A better understanding of pH and electrical conductivity (EC) impact on frass as fertilizer are required. Secondly, the data chapter presents the results from a greenhouse pot trial where we assessed the effectiveness of frass as a potential biofertilizer to improve plant growth. Frass was compared to commercial fertilizer and a control in a greenhouse at the University of Venda. Amaranthus hybridus was used in the pot trial, and growth parameters that were measured included number of leaves, plant height, dry biomass, and chlorophyll. The results indicated that dry biomass is highest in the NPK (Nitrogen, Phosphorus, and Potassium) treatment, with the control treatment being the lowest. Frass application at the lowest level did not significantly differ from the control, however, the biomass increased with in response to increased BSFL (black soldier fly larvae) frass addition and commercial fertilizer. The addition of 20g of frass improved all the parameters, and they performed similarly with commercial fertilizers. Frass maturity was performed through phytotoxicity test, and further evaluated the efficiency of frass tea as a biopesticide for crop pests through foliar spray technique. The results further revealed that frass used in the study was not matured enough iii as the relative root growth rate was below 50 %, hence it was phytotoxic. Less leaf damage was observed in brewed frass tea compared to boiled frass tea as it was utilized as a foliar spray. Lastly, ultra-high-performance liquid chromatography- quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS) and some chemometric software including XC-MS and R-Software were utilized to characterize different metabolites found in Amaranthus hybridus. A total of 30 metabolites have been identified including derivatives of coumaric acid, caffeic acid, ferulic acid, and flavonoids. These metabolites serve different functions in plants including plant defence against pests and cardiovascular diseases in humans. Different frass treatment levels affected the distribution of metabolites. In conclusion, frass remains a promising option to substitute inorganic fertilizers, thus reducing global warming challenges and alleviating poverty in developing countries.