Effects of replacing protein with non-protein nitrogen sources on growth, development and composition of Tenebrio molitor larvae

Abstract

It is suggested that Tenebrio molitor larvae (yellow mealworms) have the capacity to fix non-protein nitrogen (NPN) to produce amino acids. To test this, mealworms were fed an alpha-cellulose-based synthetic feed (Cell+) containing sources of energy (lipid and glucose), minerals and multivitamins, at concentrations predicted to be like wheat bran (WB), the same feed with casein added (13.8 g/100 g) (casein+), or WB. Following 24 days on the Cell+ diet mealworms did not grow nor pupate, whilst on the Casein+ these were significantly greater, but did not quite reach the values for WB. When casein was replaced (equivalent nitrogen content) with various dietary NPN sources, including urea (Urea+), potassium nitrate (V) (Nitrate+) or ammonium bicarbonate (Amm+), growth and pupation was the same as Cell+, with the death rate being lower than Casein+ or WB. Mealworms fed Urea+ or Amm+ had no difference in crude protein composition (per unit DM) compared to WB, whilst Nitrate+ fed mealworms were significantly lower than all non-WB diets. Apart from phenylalanine, there were no changes in indispensable amino acid composition, but there were significant changes in several dispensable amino acids. The crude fat content of mealworms fed NPN was significantly lower than those fed Casein+ or WB. Of the major fatty acids making up crude fat (palmitic (>11%), oleic (>25%) and linoleic (>24%)), only oleic acid was different, with diets containing protein having the highest concentration. In conclusion, this study demonstrates that mealworms are unable to utilise NPN as the sole nitrogen source for either growth or pupation.