Euschistus heros (F.) (Hemiptera: Pentatomidae) is a soybean pest in Brazil, controlled with synthetic chemical insecticides, which may be harmful to the environment and humans, as well as to select pest resistant strains. The research for new pest control strategies such as the use of plant essential oils has been increased due to the selectivity and biodegradation of these molecules. The objective was to evaluate the cytological changes in the salivary glands, fat body and midgut of E. heros exposed to different concentrations of essential oil of Piper aduncum L. (Piperales: Piperaceae), which the main compounds were identified as myristicin 30.03%, aromadendrene 9.20%, dillapiole 8.43%, α-serinene 7.31%, tridecane 6.26%, γ-elemene 4.58% and o-cymene 4.20%. The essential oil of P. aduncum was toxic for E. heros with LD = 36.23 mg per insect and LD = 50.42 mg per insect. Cytological changes such as tissue disruption, increase in mitochondria population, and glycogen and lipid depletion occur in the fat body cells, whereas salivary glands and midgut are not affected by this essential oil. Results suggest that P. aduncum essential oil causes fat body cellular stress, which may compromise some physiological processes for the insect survival.