Effects of latitude and soil microbes on the resistance of invasive Solidago canadensis to its co-evolved insect herbivore Corythucha marmorata

There is an increasing likelihood that invasive plants are again exposed to their co-evolved specialist herbivores in the non-native range. However, whether there is a latitudinal pattern associated with the resistance of an invasive plant to its co-evolved herbivores and how soil microbes affect resistance has been little explored. We hypothesized that the resistance of invasive Solidago canadensis to its co-evolved insect herbivore Corythucha marmorata could increase with latitude, and that local rhizosphere microbes could facilitate invasive plants to become resistant to their co-evolved herbivores. We conducted a field survey and a greenhouse experiment to examine whether there was a latitudinal pattern in the abundance of C. marmorata and in the damage it caused to S. canadensis in China. We tested whether local rhizosphere microbes of invasive plants can promote the resistance of S. canadensis to C. marmorata herbivory. In the field survey, both density of C. marmorata and damage level of S. canadensis were positively correlated with latitude, and with S. canadensis plant growth, indicating a latitudinal pattern in the resistance of S. canadensis to C. marmorata. However, in the greenhouse experiment, S. canadensis from different latitudes did not suffer significantly from different levels of damage from C. marmorata. Additionally, the damage level of S. canadensis was lower when rhizosphere soil and rhizomes originated from field S. canadensis with same damage level than with different damage levels. This result indicates that local rhizosphere soil microbes promote the adaptation of S. canadensis to resistance of C. marmorata.