changes in insect herbivore performance under elevated atmosphere carbon dioxide concentrations e[co 2 ] are often driven by changes in the nutritional and defensive chemistry of their host plants. Studies addressing how the prolific pest cotton bollworm (Helicoverpa armigera) responds to e[co 2 ] show that performance usually declines, often associated with lower nutritional (e.g. nitrogen (N) concentrations) quality of host plants under e[CO 2 ]. We investigated the impacts of e[co 2 ] on nutritional quality and anti-herbivore (jasmonate) defensive signalling in lucerne (Medicago sativa) when challenged by H. armigera. While foliar n decreased under e[co 2 ], other aspects of nutritional quality (soluble protein, amino acids, foliar C:N) were largely unaffected, potentially due to increased root nodulation under e[co 2 ]. in contrast, e[co 2 ] greatly reduced jasmonate signalling in M. sativa following H. armigera attack; jasmonic acid concentrations were ca. 56% lower in attacked plants grown under e[co 2 ]. concurrent with this, relative growth rates of H. armigera were ca. 66% higher when feeding on e[co 2 ]-grown plants. in contrast with previous reports, which we meta-analytically summarise, we provide the first evidence that H. armigera performance can increase under e[co 2 ]. this may occur in plants, such as M. sativa, where e[co 2 ] has limited impacts on nutritional quality yet reduces jasmonate defence signalling. With global populations expected to reach 11.2 billion by 2,100 there is an urgent need to ensure future food security, a challenge which is complicated by global climate change 1. Invertebrate pests destroy enough food to feed a billion people a year 2 which has, in part, fuelled interest in understanding which pests may become more problematic under predicted changes in the Earth's climate 3. In particular, unprecedented increases in atmospheric carbon dioxide (CO 2) have the capacity to change plant chemistry which affect the susceptibility of crops to insect herbivores 4-6. The effects of elevated atmospheric CO 2 concentration (e[CO 2 ]) on plant nutritional and defensive chemistry, and their consequent effects on invertebrate herbivores, have received extensive attention 7-9. In terms of nutritional quality, nitrogen availability is considered to be the limiting factor in insect herbivore diets 10. Broadly speaking, e[CO 2 ] causes foliar nitrogen (N) concentrations to decrease due to one or more processes, including dilution effects (i.e. relative to increased carbohydrate concentrations), reduced N uptake by roots, increased NH 3 volatilization and reduced investment in the N-rich enzyme RUBISCO 11-14. The impacts of e[CO 2 ] on plant defences are less easily predicted, but some trends are emerging 5. Plant defences against herbivorous arthropods are regulated by several phytohormonal pathways, including the jasmonic acid (JA), salicylic acid (SA) and ethylene signalling pathways 15,16. Of these, the JA pathway is regarded as the master regulator of plant resistance to arthropod her...