Ecotoxicological Effects of the Herbicide Metribuzin on Tenebrio molitor Hemocytes

Herbicides are synthetic chemicals that are extensively employed in agricultural practices with the objective of enhancing crop yield and quality. Despite their selectivity for plant systems and being generally regarded as non-toxic to animals, there is a paucity of understanding surrounding the sublethal effects on non-target organisms, including animals. This gap underscores the necessity for ecotoxicological research that prioritizes the identification of suitable models and develops reliable biomarkers for the early assessment of environmental impact. In this context, hemocytes—circulating immune cells found in invertebrates—have been identified as a crucial system for assessing sublethal toxicological effects, given their role in immune defense and overall organism health. Tenebrio molitor, a beetle pest of stored grain, was used as a model for the assessment of the effects of a metribuzin-based herbicide (MTB, Feinzin DF 70, 70% metribuzin, 0.25 kg ha−1). Following a 96 h exposure to MTB, the males (7–10 days post-eclosion) were examined for multiple biomarkers in their hemocytes, including cell density, phagocytic activity, lysosomal membrane stability, and cytological changes. Although no mortality was observed, exposure to MTB resulted in a reduction in the phagocytic index and an increase in blast-like cells, indicating the potential for immunotoxicity. Lysosomal membrane stability was reduced, though no significant changes in hemocyte density or nuclear morphology were observed. These responses indicate potential immune system impairment, which could affect the beetle’s fitness and reproductive potential. This study highlights the potential of hemocytes for assessing sublethal herbicide effects, raising concerns about the ecological impact of herbicides in agroecosystems and their potential risks to both wildlife and human health.