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Balanced nutrition is fundamental to health and immunity. For herbivorous insects, nutrient-compositional shifts in host plants due to elevated atmospheric CO₂ concentrations and temperature may compromise this balance. Therefore, understanding their immune responses to such shifts is vital if we are to predict the outcomes of climate change for plant–herbivore–parasitoid and pathogen interactions. We tested the immune response of Paropsis atomaria Olivier (Coleoptera: Chrysomelidae) feeding on Eucalyptus tereticornis Sm. seedlings exposed to elevated CO₂ (640 μmol mol⁻¹; C_E) and temperature (ambient plus 4 °C; T_E). Larvae were immune-challenged with a nylon monofilament in order to simulate parasitoid or pathogen attack without other effects of actual parasitism or pathology. The cellular (in vivo melanisation) and humoral (in vitro phenoloxidase PO activity) immune responses were assessed, and …