Dietary protein level influences growth, adult emergence, and susceptibility to bacterial infection in Hermetia illucens L. (Diptera: Stratiomyidae) larvae

The larvae of the black soldier fly (BSFL), Hermetia illucens L. (Diptera: Stratiomyidae), are grown on diverse residual organic matter that differ in their protein content and contain a plethora of microorganisms. The effect of dietary protein level on the interaction of BSFL with entomopathogenic bacteria remains unexplored. In this study, we investigated the role of dietary protein level on BSFL growth, uric acid accumulation, and adult emergence and the outcome of the host-pathogen interaction between BSFL and the Gram-negative bacterium Pseudomonas protegens Pf-5. We formulated three experimental diets in which digestible carbohydrate content was maintained at 50% of dry matter (DM), and crude protein was included at either 10% (low protein), 22.5% (medium protein), or 35% (high protein) DM, respectively. The influence of these diets on larval biomass, accumulation of uric acid in the larval hindgut, and subsequent adult emergence were recorded. In addition, the survival of BSFL fed on these diets was monitored for 64 hours after injection with an LD50 dose (1 μl of 2.5 × 103 CFU/ml) of P. protegens Pf-5. The biomass of 5-day-old larvae grown on a low-protein diet was higher than that of larvae grown on a high-protein diet. However, no such difference in biomass was observed in 8-day-old larvae. The amount of uric acid in the hindgut of larvae fed on high protein was three-fold and two-fold higher than larvae fed on low- and medium-protein diets, respectively. Adult emergence on a high-protein diet was significantly reduced and delayed compared to low- and medium-protein diets. BSFL fed a high-protein diet displayed significantly lower survival after infection with P. protegens Pf-5 than those fed a low-protein diet. Overall, feeding a high-protein diet reduced adult emergence and combined with infection with a Gram-negative bacterium survival of BSF larvae was strongly reduced.