Rice by-product-based diets as rearing substrates for yellow mealworm: physical characteristics and nutritional content interplay on larval growth

Tenebrio molitor L. (TM) can be reared on a variety of substrates with a high DM content. However, information about the suitability of rice by-products as a rearing substrate is still very limited. This study aimed to evaluate the suitability of rice by-products-based diets and their physical characteristics in terms of growth performance and bioconversion efficiency of TM larvae. Four compound diets containing varying percentages of rice by-products inclusion were tested against a control compound diet with no rice by-products inclusion (five replicates/diet, 10 000 3-weeks-old larvae/replicate, 0.26 g/larva). A diet composed entirely of wheat bran was used as environmental control to monitor whether factors outside the diet affected the results. Diets were isonitrogenous, isolipidic and isoenergetic; they only differed in the number and nature of the substrates used, as well as the diet’s granulometry. Larvae were sampled every 7 days from the start of the experiment, and bioconversion parameters and larval nutritional profile were obtained at the end of the experiment. Data were analysed using RStudio software. Diet did not influence larval survival (79–83%) and development time (5 weeks), which were similar among the experimental treatments and the environmental control (P > 0.05). However, all the rice by-products-based diets led to higher final larval weights than the control diet (P < 0.05). The Feed Conversion Ratio (FCR) was comparable among rice-based diets, which performed comparably or, in some cases, better than the control. Regarding the nutritional profile, rice by-products-based diets diminished the fat content in the larvae when compared to the control (P < 0.001), with similar CP content in most diets (P > 0.05). In conclusion, rice by-products are able to improve growth performance and bioconversion efficiency of TM larvae with no negative effects on larval proximate composition. Although diet composition is an important factor, diet physical characteristics can have an enormous impact on larval development, and they should be taken into account.