Introduced species often quickly change in terms of genetic variation and phenotypic traits due to adaptive evolution. However, the time scale responding to the trait change after invasion is poorly explored. The recent expansion of an invasive herbivorous insect Corythuca marmorata of the exotic plant Solidago altissima in Japan provides an opportunity to test the potential of the selective impacts of this herbivore on plant defensive traits. Because the invasion process of C. marmorata differs geographically in Japan, the defensive traits of S. altissima may have locally evolved.

We focused on the geographic variation of the insect species, abundance, and genetic and phenotypic differences among S. altissima populations. The averaged damage by C. marmorata was lower in populations in the area where C. marmorata was initially introduced than the populations in the recent expanded range. Moreover, the S. altissima populations were exclusively hexaploid, although diploid, tetraploid, and hexaploid are reported in its native range.

These findings suggest that the defensive traits may have been subjected to natural selection differently in local populations in response to local biotic environments in a short time scale, and provide a new insight to understanding the establishment of S.altissima.