Effects of aphid herbivory on biomass and leaf-level physiology of Solanum dulcamara under elevated temperature and CO2

Forecasted increases in atmospheric CO2 and global mean temperature are likely to influence insect–plant interactions. Plant traits important to insect herbivores, such as nitrogen content, may be directly affected by elevated CO2 and temperature, while insect herbivores are likely to be directly affected only by temperature. This study investigates changes in the effects of herbivory by the aphid Macrosiphum euphorbiae (Homoptera: Aphididae) on the C3 perennial Solanum dulcamara under two conditions of atmospheric CO2 concentration (350/750 ppm) and three temperature treatments (20/15, 23/18, 26/21 °C; day/night temperatures). Plants were grown in glass-topped chambers and initially infested with three apterous, adult aphids. Aphid population size, leaf photosynthetic capacity, carbon to nitrogen (C:N) ratio, specific leaf area, plant height, and total plant biomass were measured after 3 weeks of infestation. Aphid herbivory reduced photosynthetic capacity under all conditions, and resulted in smaller leaf C:N ratios. Aphid populations did not change significantly under elevated CO2, but tended to increase slightly. Average aphid weight decreased at high temperatures. Plant height and biomass were not significantly affected by the CO2 treatment, but growth rates before infestation were enhanced by elevated CO2. These results indicate that the combined effects of both elevated CO2 and temperature may exacerbate aphid damage to certain plants, particularly to plants which respond weakly to increases in atmospheric CO2. Modifications of plant physiology under altered CO2 and temperature do not impair, and may slightly enhance aphid population growth.