Effects of Elevated CO₂ and O₃ on Aboveground Brassicaceous Plant-Insect Interactions

Atmospheric gases, such as carbon dioxide (CO₂) and ozone (O₃), influence plant-insect interactions, with variable effects. The few studies that have investigated the direct effects of elevated CO₂ (eCO₂; 750-900 ppm) or elevated O₃ (eO₃; 60-200 ppb) on insects have shown mixed results. Instead, most research has focused on the indirect effects through changes in the host plant. In general, the lower nitrogen levels in C3 brassicaceous plants grown at eCO₂ negatively affect insects and may result in compensatory feeding. Phytohormones involved in plant resistance may be altered by eCO₂ or eO₃. For example, stress-related jasmonate levels, which lead to induced resistance against chewing herbivores, are weakened at eCO₂. In general, eCO₂ does not affect herbivore-induced plant volatiles, which remain attractive to natural enemies. However, floral volatiles and herbivore-induced plant volatiles may be degraded by O₃, affecting pollination and foraging natural enemy behavior. Thus, eCO₂ and eO₃ alter plant-insect interactions; however, many aspects remain poorly understood.