Nowadays, insects are reared for food and feed. This idea includes the rearing of yellow mealworm (Tenebrio molitor L.). The study aimed to assess the effect of pretreatment of lignocellulosic materials on the growth, survival, and chemical composition of mealworm larvae. The main factor in the experiment was the type of feed. The components of the experimental mixed diets were wheat bran (control feed), enzymatically hydrolysed wheat straw pretreated with steam explosion (WES), enzymatically hydrolysed wheat straw pretreated by the organosolv method (WEO), and enzymatically hydrolysed cup plant pretreated by the organosolv method (CEO) in different combinations with wheat bran. Larval development and survival were monitored and measured. In the final bioassay, larval growth on all feeds containing 10% of pretreated lignocellulosic feed was similar to that of insects reared on the control diet. The specific growth rate of larvae reared on the WEO10 diet was significantly the highest (10.1%). The diet used to feed the insects had a significant effect on the crude protein and crude fat content in their biomass. The highest protein content was found in insects fed wheat bran and fed the CEO10 diet. Protein digestibility averaged 40.7% and did not differ statistically among diets. In conclusion, a moderate inclusion of processed lignocellulosic biomass can be used as a feed component for insect diets. Moreover, insect rearing on such substrates not only enables the utilisation of agricultural residues but also converts them into high-quality protein and fat, which can find applications in the feed, cosmetic, or food industries.