Abstract An innovative approach is developed to utilize the frass of mealworms (Tenebrio molitor Linnaeus 1758) for producing dye-removal biochar. Agricultural waste wheat straw (WS) was tested as feedstock versus wheat bran (WB) to rear mealworms for the production of insect biomass. Mealworms on WS grew at a slightly slower rate than those grown on WB for 32 days with ∼40% lignin removed at 25 °C. Biochars were generated using the frass of mealworms via pyrolysis. The best adsorption performance for three dyes, especially malachite green, a cationic dye, was the biochar of WS frass at 800 °C; it had better capacity (1738.6 mg/g) compared with the frass fed with bran, raw WS and raw WB as well as those reported in the literature. Adsorption of malachite green fits the Langmuir isotherm and the pseudo second order kinetic model, mainly due to chemisorption and electrostatic interaction. Our results demonstrated that the WS can be utilized to rear mealworms, and that frass of mealworms is an excellent raw material for generating high-efficiency bioadsorbents.