Use of agro-industrial by-products enriched with Selenium and Zinc for the rearing of Hermetia illucens: effects on growth and bioaccumulation

It is well known that insects are natural bioaccumulators of various nutrients. Preliminary studies have demonstrated the ability of Hermetia illucens (Diptera: Stratiomyidae)to bioaccumulate selenium, transforming it from its inorganic to organic form. Selenium (Se) is a trace element essential for the activity of numerous enzymes in both humans and animals, as is zinc (Zn), which has recently been banned in the European Union in its inorganic form of zinc oxide as a component in piglet feed. Since several insect species are currently approved as feed for various livestock animals, the possibility of using insects enriched with Se and Zn as dietary supplements is being considered. The objective of this study was to evaluate the effects of including Se or Zn and their combination (Se/Zn) in the rearing substrate for H. illucens larvae. Specifically, trials were set up with: I) a control diet (Gainsville diet); II) a mixed substrate (50:50) consisting of two agro-industrial by-products, okara and potato waste (OP); III) OP enriched with 0.3 mg/kg of Se; IV) OP enriched with 150 mg/kg of Zn; V) OP enriched with 150 mg/kg of Zn + 0.3 mg/kg of Se. The doses of both trace elements were chosen based on other feed formulations for livestock. For each experimental group, five replicates were created, each containing 500 newly hatched larvae. Several parameters were monitored, including growth, bioconversion efficiency, and final biomass yield. Additionally, chemical analyses were performed on both the substrates and larvae to assess the accumulation of Se and Zn. The mean larval weight and larval survival were not affected by the treatment (P>0.05). The only difference was found in the larval biomass produced, which was significantly higher in the control diet (P<0.05). The nutritional composition of the larvae was not influenced by the inclusion of Se and Zn, but only by the substrate used. In contrast, the addition of Se and Zn to the substrate affected their content in the produced larval biomass; in particular, the total Se concentration was higher in the Se-only treatment, while the Zn concentration was higher in the SeZn treatment, highlighting an interaction between the two elements. The combined presence of both elements appears to have interfered with Se absorption. These results suggest the possibility of enriching growth substrates with these two trace elements to produce biofortified larvae intended for quality feed. However, further research is needed to determine whether the larvae underwent an organic transformation of zinc and selenium and to evaluate their influence on the reproductive performance of Hermetia illucens. The project was funded by the Agritech National Research Center and received funds from the European Union through NextGenerationEU (NATIONAL RECOVERY AND RESILIENCE PLAN (PNRR) – MISSION 4 COMPONENT 2, INVESTMENT 1.4 – D.D. 1032 17/06/2022, CN00000022).