The rearing substrate can influence the survival rate and antimicrobial activity in BSF larvae challenged with bacteria

The larvae of black soldier fly (BSF, Hermetia illucens, L.) are nowadays gaining increasing attention for their bioconversion capability and the possibility to exploit their biomass for multiple biotechnological applications. The substrates used for larval rearing needs to be carefully evaluated as it can significantly influence the insect growth. The literature reports data on the nutritional needs of the larvae and proper chemical composition of the rearing substrate to obtain optimal growth performance, but more in-depth evaluations on how diet composition affects the immune system of the larvae are needed. This research aimed at evaluating if three diets with different composition (i.e., Plant-based by-products (P-BPs): 100% fruit-vegetable; Meat-based by-products (M-BPs): 100% meat; Mix by-products (Mix-BPs): 50% Plant-based by-products and 50% meat-based by-products) influenced the antimicrobial activity of BSF larvae. Last instar larvae were injected with 5 µL of a mix of Gram-positive and Gram-negative bacteria (Escherichia coli and Micrococcus luteus) at concentrations ranging from 104 to 108 CFU/mL and larval mortality was monitored every 24 hours for 3 days. Larvae reared on P-BPs diet showed the lowest resistance to bacterial injection compared to those reared on M-BPs and Mix-BPs. Moreover, larvae reared on the three diets and injected with 5 µL of E. coli or M. luteus at concentration of 105 CFU/mL showed a different antimicrobial activity. In particular, the total bacterial load in the hemolymph 6, 14, and 24 h post-infection was lower in larvae grown on M-BPs and Mix-BPs, confirming that these two substrates were able to confer higher resistance against bacterial infection in BSF larvae. The approach herein presented represents a novel, promising method for screening diets that may guarantee appropriate growth performance of the larvae improving, at the same time, their capability to counteract pathogens.