The increase in population leads to a rise in consumption and, consequently, a higher generation of wastes. As a result, managing solid wastes has become a major environmental challenge for governments and communities. To tackle this issue, various techniques have been developed worldwide. Insect-based bioconversion is a promising alternative to traditional waste management. The black soldier fly Hermetia illucens is widely used for this purpose. However, organic wastes may contain contaminants, such as phenolics, especially L-DOPA, that could impact the bioconversion efficiency by imposing physiological costs. Therefore, the current study aimed to assess the impact of L-DOPA on H. illucens larvae by exposing them to different concentrations of L-DOPA (0-150 mg/mL). The fifth instar H. illucens larvae's gut tissues exhibited a variety of oxidative stress parameters such as oxidants (superoxide anion radical (O₂^•-)), antioxidants (2,2-diphenyl-1-picrylhydrazyl (DPPH), phosphomolybdenum ability, reducing power ability, or total antioxidant assay), oxidative damage products (protein carbonyls), and detoxification enzymes (α-esterase, acetylcholine esterase (AChE)) were measured. The highest levels of O₂^•-, protein carbonyls, GR, PPO, DPPH, phosphomolybdenum, total antioxidant ability, reducing power, α-esterase, and AChE occurred at 150 mg/mL L-DOPA treatment with the fold of 0.7, 0.6, 2, 0.6, 1.3, 3.9, 0.2, 18.8, 2.8, 0.6, respectively, compared to control levels. Moreover, all oxidative stress parameters and detoxification enzymes were significantly affected by L-DOPA except APOX activity. The results emphasized the toxicity of L-DOPA on H. illucens.