Potential of black soldier fly larvae frass (BSFL) as a novel fertilizer: impacts on tomato growth, nutrient uptake, and mycorrhizal formation

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Abstract Background and Aims The growing rates of production of edible insects is leading to an increase in the availability of insect frass, comprising mostly the solid excretions of larvae and undigested substrate. Insect frass is considered a novel organic fertilizer, rich in nutrients and believed to further boost plant growth through its high content of substances like chitin. This study investigated the fertiliser potential of black soldier fly larvae (BSFL) frass, its ecotoxicity, and its interactions with arbuscular mycorrhizal fungi (AMF). Methods Two commercial BSFL frass products were analysed for plant nutrient concentration and effects on seed germination under laboratory conditions. In a greenhouse bioassay, using only one of the products, its impact on tomato biomass production and AMF colonisation was evaluated. Results Chemical analysis of the frass products revealed more macro-nutrients than typically found in composts with between 1.3% to 1.6% total phosphorus and 3.6% and 3.9% total nitrogen (N). 98% of the mineral N was in the form of ammonium. Total carbon was between 44 and 45% for both products. Micronutrient concentrations varied between the products, with iron reaching up to 1236 mg kg−1 or zinc up to 206 mg kg−1. Variability was also observed in seed germination inhibition, with one product demonstrating stronger inhibitory effects than the other. The greenhouse bioassay revealed issues around ammonia toxicity at higher application rates of 150 to 250 kg N ha−1 and an almost complete inhibition of arbuscular mycorrhizal root colonization between 100 and 250 kg N ha−1. Biomass production increased rapidly with higher frass application rates and plateaued between 150 and 250 kg N ha−1. Conclusion Insect frass shows significant potential as a nutrient-rich organic fertiliser, with some imbalances that, if improved, could further strengthen its efficiency as a fertiliser.