Background: The fall armyworm, Spodoptera frugiperda, poses a significant threat to over 90 plant species, causing extensive damage by consuming fresh leaves, hindering growth, and ultimately reducing crop yields. Integrated pest management strategies emphasize eco-friendly approaches, minimizing harm to non-target organisms. Among these, biological control methods, including the use of plant extracts and specific microorganisms, have shown considerable efficacy in controlling the fall armyworm. Methods: This study assessed the impact of neem seed oil extract and CuO & ZnO nanoparticles on the biological aspects of Spodoptera frugiperda. Fourth-instar larvae were exposed to fresh castor leaves treated with neem seed oil extract, obtained through Soxhlet extraction from neem seeds, and nanoparticles prepared via direct precipitation. Results: The determined LC50 values of neem seed oil extract, ZnO, and CuO NPs against the 4th instar were 500, 520, and 440 ppm, respectively. All treatments had an impact on the durations of larval and pupal stages, resulting in increases of 66% and 15% with neem seed extract, 11.5% and 15% with ZnO, and 21.7% and 23% with CuO. Additionally, pupation rates were reduced by 50% following exposure to neem seed extract and ZnO, and by 45% with CuO nanoparticles. It is also worth noting that the digestive enzymatic activities in the insects were significantly affected following treatment with the LC50 values of neem seed extracts and nanoparticles. Conclusion: The study demonstrates the potent larvicidal properties of both neem seed extract and nanoparticles against Spodoptera frugiperda. All treatments effectively prolonged larval and pupal stages and reduced successful adult emergence at low concentrations. All treatments also showed a negative effect on the enzymatic activity of insects, which indicates their potential as natural insecticides in combating fall armyworm.