This study investigates how the maize stem borer (Sesamia nonagrioides) and the fungus Fusarium verticillioides jointly influence carbon metabolism in different maize genotypes. Eight genetically distinct lines were exposed to various treatments, including individual and combined infestations. The results revealed that insect attack significantly reduces δ¹³C values in the stem, but this effect is partially mitigated when the fungus is already present, suggesting a complex interaction in the plant’s defense response. These findings provide valuable insights into how maize responds to simultaneous biotic stresses and could inform future crop protection strategies.