Mineral requirements of Tenebrio molitor larvae: magnesium, potassium and phosphorus needs

Minerals represent a minor proportion of livestock diets; however, they are indispensable for optimal livestock health and performance. Deficiencies in mineral intake may result in impaired growth and reproductive issues, consequently, the mineral requirements of livestock are well documented. In contemporary legal contexts, insects are categorized as farmed animals. However, their rearing practices typically involve the utilization of substrates designed for other species, such as chicken feed, or circular substrates composed of by-products that are not balanced in their nutritional composition. There is limited knowledge about the nutritional requirements of insects, and improvements can be made to their substrates to maximize their performance. It has been demonstrated that, when sufficient protein is provided, the amino acid concentration of the substrates had a limited influence on the growth performance of mealworms (Tenebrio molitor) (MW), but their mineral requirements have not yet been investigated. Therefore, the present trial aimed to analyze the mineral needs of MW. The focus was on the requirements of magnesium (Mg), potassium (K) and phosphorus (P), based on the mineral content of wheat bran, the gold standard for rearing MW. Semi-artificial diets based on at least 25% wheat bran supplemented with sucrose, casein and inorganic minerals were tested. The diets were isoenergetic and isoproteinous. Each mineral was administered in four different doses per experiment (from 25% to 100% of the mineral content of wheat bran) and the other minerals were given in inorganic form at a concentration of 100%. The growth performance of the MW larvae was measured using two key metrics: weight gain and survival. The collected data were then analyzed in SPSS using a univariate ANOVA for final total weight and survivability, and a linear mixed effects model for individual larval weight measured over time. The groups demonstrated a comparable survivability rate (>90%), with the exception of the Mg 50% and Mg 75% groups, which exhibited a numerically lower survivability rate of 89.25%. Considering the average larval weight, differences have been observed between the different mineral groups. Specifically, the K 25% group demonstrated a lower mean individual weight in comparison with the Mg 50%, Mg 75%, P 50%, P 75% and P 100% groups. K 75% larvae showed a lower individual weight in comparison with Mg 75%. The differences observed were partially confirmed in the final total larval weight. Indeed, the K 25% group showed a significantly lower value in comparison with most of the other groups. These results indicate that several minerals are supplied in excess to MW using traditional substrates, and that a quantity higher than 25% of the K present in wheat bran seems to be crucial for MW growth. However, further studies are needed to understand the observed growth differences and to design a substrate that completely fulfils the nutritional requirements of MW.