How can elevated CO2 influence plant-herbivore insect interactions?

Abstract Elevated carbon dioxide ( e CO 2 ) induces various plant physiological, biochemical and morphological modifications by providing extra carbon and altering tissue nutrient composition and biomass allocation, which can influence ecological processes such as plant-insect interactions. In this study, a literature review was conducted to examine how e CO 2 could potentially influence plant-herbivore insect interactions. Studies suggested that e CO 2 decreases foliar nitrogen and increases leaf toughness, leaf biomass, and the carbon to-nitrogen ratio in plants. This in turn alters foliar chemistry and can significantly decrease the relative growth rate and consequently pupal or nymphal weight, which could exert an influence on the adult. Hence, the herbivore performance can be decreased. The changes in the plant nutritional quality can also result in compensatory feeding as a result of the reduction of food quality. This consequently leads to an increased defoliation rate, which results in altered leaf chemical composition with enhanced secondary metabolites, including defense compounds and toxins. This increases the risk for larvae in their early instars, whose growth and development are more sensitive to the disruptive effects of toxins, compared to the late-instar larvae. Altogether, these effects may modify the overall abundance of the herbivorous species. An emerging hypothesis is that e CO 2 downgrades the nutritional quality more in C3 than in C4 plant species, facilitating herbivorous insect species to escalate their feeding stress on C3 plants by increasing food intake which is considered a behavioral adaptation to compensate for the reduced leaf quality (and protein content) relative to C4 species. However, this hypothesis warrants confirmation and cannot be generalized at present.