Biological Load and Tray Position Shape Larval Yield and Frass Production in a Stacked Hermetia illucens Rearing System

Industrial black soldier fly (Hermetia illucens) production commonly relies on stacked tray systems, yet little is known about how tray position and biological load influence production heterogeneity. This study evaluated whether different initial biological loads, imposed by varying the initial larval mass while maintaining a constant numerical density, together with tray position, influence larval biomass and dry frass production. Same-age third-instar larvae were seeded at a constant density of 9000 larvae per tray under five biological-load treatments (29, 54, 64, 97 and 200 g tray⁻¹, fresh weight basis). Initial biological load affected wet larval biomass, dry larval biomass, and dry frass production. However, higher initial biological load did not produce proportional gains in final biomass. The lowest initial biological load resulted in the greatest final wet larval biomass (3.54 kg tray⁻¹) whereas the highest initial biological load resulted in only 1.75 kg tray⁻¹ ($p=0.025$). Intermediate tray positions produced significantly greater wet larval biomass than the bottom tray. Mean substrate temperature did not differ among tray positions, whereas moisture declined over time and varied by position. These findings demonstrate that production consistency in stacked H. illucens rearing systems depends not only on biological load but also on tray-level microenvironmental heterogeneity, providing practical guidance for the design and management of industrial stacked-rearing systems.