N availability does not modify plant-mediated responses of Trichoplusia ni to elevated CO_(2)

Aims Elevated CO_(2) 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 CO_(2) and high N on the performance of a generalist leaf-chewing insect herbivore,Trichoplusia ni,were investigated.Methods Newly hatched T.ni larvae were introduced to Amaranthus viridis and Polygonum persicaria plants grown under ambient and elevated CO_(2) 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 CO_(2),N and insect feeding on plant performance.Important Findings Elevated CO_(2) 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 CO_(2) 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 CO_(2) by N interactions affecting T.ni performance.