Effects of Low Benzoic Acid Concentrations on Growth and Substrate Utilization in Black Soldier Fly Larvae

Black soldier fly (BSF) larvae are among the most widely mass-reared insects and develop in moist feed substrates where larvae and microorganisms jointly degrade organic matter but also compete for nutrients. Microbial activity introduces variability and often decreases substrate conversion efficiency (SCE), defined as the ratio of larval biomass produced to substrate consumed. Supplementing feed substrates with antimicrobial agents may suppress microbial activities and thereby enhance the SCE. In this study, BSF larvae were reared on chicken feed supplemented with 0-0.2% benzoic acid at either initial pH = 7.6 or pH ≤ 4, under varying larval densities. Larval weights and CO₂ production from both larvae and substrates were measured periodically. At low pH, benzoic acid lowered the CO₂ evolution from the feed substrate while the substrate reduction rate (SRR) diminished dose dependently, indicating suppressed microbial activity. Despite the lower SRR, larval biomass yield remained unchanged, resulting in a dose-dependent increase in SCE. The effect was most pronounced in feed-sufficient larvae. Benzoic acid had no effect on larval performances in terms of maximal larval weight, specific growth rate, or mortality. Their overall net growth efficiency (larval weight gain relative to assimilated substrate) even increased dose-dependently. However, the low pH needed for benzoic acid to be active did have minor negative effects on larval performances. These findings demonstrate that microbial activity influences SCE during productions of BSF larvae and that substrate conversion efficiency can be manipulated and potentially optimized without harming the larvae through the inclusion of antimicrobial agents such as benzoic acid in their feed substrates.