Substrate and moisture content effects on pupation of the black soldier fly (Diptera: Stratiomyidae)

The black soldier fly (BSF) Hermetia illucens (L.) (Diptera: Stratiomyidae) is a beneficial insect species of great environmental and economic potential. Previous studies showed that providing a suitable pupation substrate can accelerate BSF pupation and increase pupation rate. However, the effect of substrate moisture content and the possible interactions between substrate type and moisture content on BSF pupation have not been investigated. In this study, we tested the effects of two pupation substrates (vermiculite and wood chips) and nine moisture levels (dry, 18.8, 37.5, 56.3, 75.0, 93.8, 112.5, 131.3, and 150.0%) on BSF pupation rate and depth. Substrate type and moisture content both affected BSF pupation rate and depth. Compared with the ‘no substrate’ treatment, neither substrate reduced BSF prepupal mortality without adding water, but the presence of dry substrate increased pupation rate by more than 8%. Within the range used in this study, moisturising substrates reduced BSF prepupal mortality by up to 88% for vermiculite and 95% for wood chips, and increased pupation rate by up to 9% for vermiculite and 6% for wood chips. In all substrate treatments more than 99% of the pupae were found in the top 4 cm, and more than 85% were found in the top 2 cm depth. The tendency of BSF to pupate deeper than 2 cm depended on substrate type and moisture content, and the effect of moisture content on pupation depth followed a parabolic trend. Based on our data, the proportions of BSF pupae presenting deeper than 2 cm would be the highest when the moisture content is around 20% for vermiculite and 70% for wood chips. Our results provide clues for selecting suitable substrate, moisture content, and substrate depth to facilitate BSF pupation.