Essential oils and their isolated constituents are constantly being studied for the control of insect pests. In this context, the present research reports the chemical composition of Piper marginatum (Jacq.) oil aiming to: 1) establish lethal concentrations LC30 and LC50 for this oil and the compound geraniol, 2) histologically examine the embryonic development of Spodoptera frugiperda (J.E. Smith) through light microscopy and scanning electron microscopy (SEM), as well as 3) compare the efficacy of the P. marginatum oil with that of the botanical insecticide azadirachtin, the synthetic insecticide deltamethrin, and acetone as a negative control. Semithin sections of S. frugiperda eggs revealed that the oil, geraniol, azadirachtin, and deltamethrin affected embryonic development at both concentrations. However, geraniol and the oil were more efficient because they caused more significant damage, even at lower concentrations. SEM revealed that all products altered the morphology of the eggs, modifying the structure of the chorion and making the eggs nonviable. Thus, this work demonstrates that P. marginatum oil is effective in the control of S. frugiperda because it results in embryonic damage even at the lowest concentrations.