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Carbon dioxide (CO 2 ) emissions are central to the climate crisis and their consequences indiscriminately affect natural and anthropogenic ecosystems. Among ecological interactions, those between plants and insects are among the most impacted by the elevation in CO 2 concentration (eCO 2 ). We selected a plant and an herbivore species of worldwide relevance and tested the hypothesis that eCO 2 affects leaf quality and defences of sunflower, Helianthus annuus L. (Asteraceae), and negatively impacts the larval preference and performance of the important pest Helicoverpa armigera H€ ubner (Lepidoptera: Noctuidae). Plants and insects developed inside open-top chambers under ambient CO 2 (ca. 400 ppm) and eCO 2 (ca. 800 ppm). Sunflowers under eCO 2 grew more (e.g., increased height and had more leaves) but were of lower nutritional quality at an early developmental stage (e.g., lower nitrogen content, greater leaf thickness, and higher flavonoids content). Despite showing no preference for either treatment, H. armigera larvae performed better when fed with leaves from eCO 2 plants. We argue this was observed because larvae under eCO 2 sustained a greater leaf consumption, even when sunflower leaf quality became similar between treatments (by the 11th week after germination). Besides, they overcompensated a more deficient diet during early developmental stages and presented a higher growth rate; ca. 2.59 more individuals reached the pupal stage, and 49 more individuals became adults. The improvement in H. armigera larval performance under eCO 2 indicates a worrisome scenario in which a species that already exerts a significant impact on ecosystems would increase its consumption, develop faster, and support a larger population size.

This study evaluated the photochemical responses of photosystem II and growth of Hymenaea stigonocarpa under CO 2-enriched conditions with exposure to simulated herbivory events. After herbivory simulation in two distinct parts of the stem of plants (apex and base), chlorophyll a fluorescence, chlorophyll index, growth, extrafloral nectary density, leaf mineral nutrition, and biomass production were evaluated. Plants of H. stigonocarpa grown under high [CO 2 ] after simulated herbivory in the apical part of the stem had higher electron transport rate, effective quantum yield of photosystem II, and chlorophyll contents. However, simulated herbivory in the basal portion of plants grown under high [CO 2 ] increased plant height, branch and root length, leaf number, leaf area, node number, and leaf expansion rate. In conclusion simulated herbivory at the basal portion and high [CO 2 ] induce positive responses in H. stigonocarpa, leading to the allocation of biomass to vegetative parts related to the capture of resources such as water and light. Apical leaves could compensate for the elimination of part of their leaf blades by increasing their photosynthetic yield. Thus, the increase of [CO 2 ] attenuated the adverse effects of leaf removal on H. stigonocarpa plants by inducing photosynthetic improvement and growth after the loss of leaf tissue.

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