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Palomena prasina (L.) (Hemiptera: Pentatomidae) and Gonocerus acuteangulatus (Goeze) (Hemiptera: Coreidae) are new pests in French hazel orchards (Corylus avellana L., Corylaceae). Knowing the symptoms in growing hazelnuts resulting from pests feeding on them is important for risk assessment and risk management. Insects were isolated in net sleeves on hazelnuts of the Pauetet cultivar at various phenological stages for 1 week, and symptoms resulting from the insects' feeding were observed at harvest. From pre‐shell expansion to kernel expansion and from kernel expansion to kernel maturation, feeding mainly results in blank nuts and necrotic spots, respectively. The feeding of adult insects, as well as third‐ and fourth‐instars, resulted in blank nuts. When adults and fifth‐instars fed on the kernel, necrotic spots resulted. The stylet penetration potential of adult insects and late‐instars is higher, allowing them to reach the kernel and cause abortion or necrotic spotting. Blank nuts are the main damage observed, and their intensity is highly related to changes in both hazelnut and bug phenology. Therefore, symptoms should be discussed in light of the cultivar and bug occurrence in the field. This study contributes to the development of decision tools for integrated pest management of true bug pests that feed on hazelnuts. [ABSTRACT FROM AUTHOR]