Community-wide impact of an exotic aphid on introduced tall goldenrod

1. The aphid Uroleucon nigrotuberculatum Olive, which is specialised to the tall goldenrod, Solidago altissima L., in its native range, has become a dominant species on the introduced tall goldenrod in Japan. How this exotic aphid influenced arthropod communities on the introduced tall goldenrod in aphid-present (spring) and aphid-absent (autumn) seasons was examined, using an aphid removal experiment. 2. In spring, aphid presence increased ant abundance because aphid honeydew attracted foraging ant workers. A significant negative correlation was found between the numbers of ants and herbivorous insects other than aphids on the aphid-exposed plants, but no significant correlation was detected on the aphid-free plants. Thus, the aphid presence was likely to decrease the abundance of co-occurring herbivorous insects through removal behaviour of the aphid-tending ants. There were no significant differences in plant traits between the aphid-exposed and aphid-free plants. 3. In autumn, the numbers of lateral shoots and leaves, and the leaf nitrogen content were increased in response to the aphid infestation in spring. Because of the improvement of plant traits by aphid feeding, the abundance of leaf chewers increased on aphid-exposed plants. In contrast, the abundance of sap feeders decreased on the aphid-exposed plants. In particular, the dominant scale insect among sap feeders, Parasaissetia nigra Nietner, decreased, followed by a decrease in the abundance of ants attending P. nigra. Thus, aphid feeding may have attenuated the negative impacts of the tending ants on leaf chewers. 4. Aphid presence did not change herbivore species richness but changed the relative density of dominant herbivores, resulting in community-wide effects on co-occurring herbivores through ant-mediated indirect effects, and on temporally separated herbivores through plant-and ant-mediated indirect effects. The aphid also altered predator community composition by increasing and decreasing the relative abundance of aphid-tending ants in the spring and autumn, respectively.