Increased insect herbivore performance under elevated CO₂ is associated with lower plant defence signalling and...

Changes in insect herbivore performance under elevated atmosphere carbon dioxide concentrations e[CO₂] 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[CO₂] show that performance usually declines, often associated with lower nutritional (e.g. nitrogen (N) concentrations) quality of host plants under e[CO₂]. We investigated the impacts of e[CO₂] on nutritional quality and anti-herbivore (jasmonate) defensive signalling in lucerne (Medicago sativa) when challenged by H. armigera. While foliar N decreased under e[CO₂], other aspects of nutritional quality (soluble protein, amino acids, foliar C:N) were largely unaffected, potentially due to increased root nodulation under e[CO₂]. In contrast, e[CO₂] greatly reduced jasmonate signalling in M. sativa following H. armigera attack; jasmonic acid concentrations were ca. 56% lower in attacked plants grown under e[CO₂]. Concurrent with this, relative growth rates of H. armigera were ca. 66% higher when feeding on e[CO₂]-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[CO₂]. This may occur in plants, such as M. sativa, where e[CO₂] has limited impacts on nutritional quality yet reduces jasmonate defence signalling.