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Elevated CO2 and increased N availability can alter a variety of plant physiological processes leading to changes in the nutritional quality of leaf tissue for herbivores. Numerous experiments have examined the responses of herbivores to environmental change; however the potential effects of simultaneous change in multiple factors on leaf-chewing insect herbivores are less well understood. The plant-mediated effects of elevated CO2 and high N on the performance of a generalist leaf-chewing insect herbivore, Trichoplusia ni, were investigated.Newly hatched T. ni larvae were introduced to Amaranthus viridis and Polygonum persicaria plants grown under ambient and elevated CO2 and low and high N conditions. Insect performance was assessed by measuring larvae weight after ten days of feeding. Plant photosynthesis, biomass, leaf area and specific leaf weight were measured to determine the effects of elevated CO2, N and insect feeding on plant performance.Elevated CO2 did not have strong effects on plant or insect performance, only affecting a few responses under low or high N conditions, but not both. Growth under high nitrogen improved almost all measures of plant performance. Trichoplusia ni performed significantly better on Amaranthus viridis (C4) compared to Polygonum persicaria (C3), despite similar leaf C:N ratios in both species. The performance of T. ni caterpillars was only improved by the high nitrogen treatment when they were feeding on P. persicaria, the host they performed poorly on. The only interactions between N and CO2 affecting plant performance were seen for leaf photosynthesis of P. persicaria and leaf area of A. viridis. Contrary to the predictions, there were no significant CO2 by N interactions affecting T. ni performance.