Supplementing Hermetia illucens diet with minerals: effects on production performance, proximate composition and nitrogen loss

Abstract Black soldier fly (BSF, Hermetia illucens ) farming is an emerging industry and the nutritional requirements of the larvae still remain unclear. In particular, the larval needs in micronutrients, such as minerals, are poorly understood although they play a critical role in insect physiology. Thus, we investigated whether supplementing BSF diet with polyhalite (K 2 MgCa 2 (SO 4 ) 4 ⋅ 2H 2 O), a pure mineral source, could improve the production performance traits of the larvae. Once hatched, the larvae were fed a single nursery diet until the fifth day post-hatch, then transferred to the experimental diets and harvested on the tenth day post-hatch. The control diet was made of brewery by-products, which are commonly used at industrial level in BSF farming, and rich in protein (38.1% of dry matter). On this basis, three others were designed to include 0.5, 1.0 and 1.5% of polyhalite. Larval survival, growth, feed conversion ratio (FCR) and yield were estimated, in addition to the feeding substrate reduction. Moreover, the larvae and diets had their nitrogen content determined to estimate the nitrogen loss for each diet. The proximate and amino-acid compositions of the larvae were also analysed. Larval survival was around 100% for all diets. For all the diets containing polyhalite, the growth, FCR, yield and substrate reduction were significantly improved compared to the control diet. However, 0.5% of polyhalite resulted in significantly better growth, FCR and yield than 1.5%, with 1.0% being intermediate. A decrease in nitrogen loss proportional to polyhalite inclusion was estimated, from 6.9% (0.5% of polyhalite) to a 27.4% decrease (1.5% of polyhalite). No clear variation appeared in the larval composition but for the ash content that increased with the inclusion rate of polyhalite. Mineral supplementation produced encouraging results in our study but requires further investigation, especially regarding its impact on the nitrogen gaseous emissions and larvae meal quality.