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<i>Spodoptera exigua</i> (Hübner) (Lepidoptera: Noctuidae) is a highly dispersive, polyphagous insect pest that severely defoliates crops. Excessive reliance on synthetic insecticides leads to ecological pollution and resistance development, urging scientists to probe eco-friendly biopesticides. Here, we explore the virulence of an entomopathogenic fungus, <i>Beauveria bassiana</i>, against <i>S. exigua</i>, resulting in 88% larval mortality. Using an age–stage, two-sex life table, we evaluated the lethal and sublethal effects of <i>B. bassiana</i> on the demographic parameters of <i>S. exigua</i>, including survival, development, and reproduction. Sublethal (LC₂₀) and lethal concentrations (LC₅₀) of <i>B. bassiana</i> impacted the parental generation (F₀), with these effects further influencing the demographic parameters of the first filial generation (F₁). The infected F₁ offsprings showed a reduced intrinsic rate of increase (<i>r</i>), mean generation time (<i>T</i>), and net reproduction rate (<i>R</i>₀). Larval developmental duration varied significantly between the control (10.98 d) and treated groups (LC₂₀: 10.42; LC₅₀: 9.37 d). Adults in the treated groups had significantly reduced lifespans (M: 8.22; F: 7.32 d) than the control (M: 10.00; F: 8.22 d). Reduced fecundity was observed in the <i>B. bassiana</i>-infected groups (LC₂₀: 313.45; LC₅₀: 223.92 eggs/female) compared to the control (359.55 eggs/female). A biochemical assay revealed elevated levels of detoxification enzymes (esterases, glutathione S-transferases, and acetylcholinesterase) in the F₀ generation after <i>B. bassiana</i> infection. However, the enzymatic activity remained non-significant in the F₁ generation likely due to the lack of direct fungal exposure. Our findings highlight the enduring effects of <i>B. bassiana</i> on the biological parameters and population dynamics of <i>S. exigua</i>, stressing its use in eco-friendly management programs.