Elevated CO2 and aboveground–belowground herbivory by the clover root weevil

Predicted increases in atmospheric carbon dioxide (CO2) concentrations are expected to increase primary productivity in many terrestrial ecosystems, which could lead to plants becoming N limited. Studies suggest that legumes may partially overcome this by increasing biological nitrogen fixation. However, these studies have not yet considered how these changes may be affected by the altered dynamics of insect herbivores feeding on the plant. This study investigated how elevated CO2 (700 μl l−1) affected the clover root weevil (Sitona lepidus), a significant pest of white clover (Trifolium repens). Adults feed on leaves aboveground where they lay eggs; soil-dwelling larvae initially feed on root nodules that house N2-fixing bacteria. Foliar C:N ratios rose by 9% at elevated CO2, but the biggest responses were observed belowground, with increases in root mass (85% greater) and nodule abundance (220% more abundant). Root C:N ratios increased significantly from 10.95 to 11.60 under elevated CO2, which increased even further to 13.13 when nodules were attacked by larval S. lepidus. Adult S. lepidus consumed significantly more leaf tissue at elevated CO2 (0.47 cm2 day−1) compared with ambient CO2 (0.35 cm2 day−1), suggesting compensatory feeding, but laid 23% fewer eggs at elevated CO2. Even though fewer eggs were laid at elevated CO2, 38% more larvae were recovered suggesting that larval survival was much better under elevated CO2. Increased larval abundance and performance at elevated CO2 were positively correlated with the number of nodules available. In conclusion, reduced foliar quality at elevated CO2 was generally disadvantageous for adult S. lepidus living aboveground, but extremely beneficial for S. lepidus larvae living belowground, due to the enhanced nodulation. Climate change may, therefore, enhance biological nitrogen fixation by T. repens, but potential benefits (e.g. provision of N without chemical fertilizers) may be undermined by larger populations of S. lepidus larvae belowground.