Abstract
The study of plant-mediated herbivore-herbivore interactions is a central theme in community ecology and has lately undergone great development. However, there are many unresolved patterns, the factors determining the patterns are unclear, and indirect effects that need to be taken into account for biodiversity conservation and pest management receive less attention. Focusing on these issues, the present study reviews five case studies of plant-mediated herbivore-herbivore interactions. In the first, leaf traits and gall-feeding insects are found to be important factors affecting the plant-species use of the Phyllocolpa sawfly, which induces leaf-roll galls on Salix plants. The second suggests that, since the gall-parasitic flea weevil Orchestes hustachei feeds obligately on leaf pouch galls induced by aphids on Zelkova serrata trees, and since the weevil may conceivably have evolved as a gall-feeder (cecidophage) from a leafminer, gall formation may have affected the evolution of feeding habits in this insect. The third proposes that, as bacterium galls on wisteria trunks are used by a wide array of arthropods, and as gall size correlates positively with the number of arthropod species and individuals, the plant-pathogenic bacterium increases arthropod diversity via gall induction. The fourth finds that, although the leafminer Ectoedemia sp. induces early leaf abscission in the evergreen oak in early spring and can then complete its development in the fallen leaves, the sika deer preying on the mined leaves on the ground results in considerable mortality for the leafminer. The fifth finds that, after the cicada Cryptotympana facialis emerges in the urban parks of Osaka, Japan, in midsummer, many flower chafers, ants, and wasps feed, possibly for intake of water and minerals, on the xylem sap exuding from the cicada feeding sites. These studies show that herbivores with different feeding habits - gallers, leafminers and sap-suckers - induce different trait changes in trees, resulting in different plant-mediated herbivore-herbivore interactions. Study thereof provides a range of pointers for pest management and biodiversity conservation.
