Testing an insect-plant circular system based on black soldier fly and duckweed

Abstract Optimizing the circularity of insect farming by reusing and repurposing its byproducts is a priority to reach global sustainable goals that benefit both nature and people. The frass of the black soldier fly (BSF) has been used as fertilizer for various crops, but its potential in promoting the growth of aquatic plants with high-protein content has not been investigated. Here, we assess the potential of using BSF frass to produce duckweed ( Lemna minor , a fast-growing aquatic plant), and then use duckweed as a nutritional supplement for BSF larvae. First, we performed a set of laboratory experiments assessing duckweed growth under different concentrations of frass (with and without the presence of algae). Then, we quantified various life history traits (survival rate, larval growth rate, adult body mass and life span) of BSF across different treatments with increasing levels of duckweed supplementation (fresh and dry) in the standard BSF diet (0%, 25%, 50%, 75% and 100%). We found that the optimal concentration of frass that provided the fastest duckweed growth rates was between 4 and 6 g/L, exceeding that recorded for the traditional Hoagland’s E media. BSF larvae growing in a diet supplemented by fresh and especially dry duckweed did not improve growth rate, survival rate, body size, nor lifespan. Higher duckweed contents tended to reduce the performance of BSF. Our results suggest that BSF frass could be used as a cheap fertilizer to produce duckweed, but the latter is not an optimal supplement for BSF, and thus should be used for other applications.