How did the interaction of Solidago altissima and lacebugs change between the native and introduced range?
Studies of interactions among introduced plants and herbivores can provide insights into important ecological processes. Plants encounter geographically heterogeneous mosaics of selection due to abiotic and biotic factors that can result in local adaptation in plant defensive traits. Biological invasions are increasingly recognized as an opportunity to study local adaptation on a contemporary time scale. Solidago altissima is a North American perennial plant that has extensively invaded abandoned fields in Japan. Corythucha marmorata (lacebug) is an herbivorous insect that feed on S. altissima in North America and was introduced to Japan 15 years ago. We studied the geographic variation in interactions between S. altissima and lacebugs to infer how their interactions have changed during the invasion process. First, we investigated geographic differences in the abundance of lacebugs and other herbivorous insects in S. altissima populations in U.S. and Japan. Second, we conducted a reciprocal transplant experiment in gardens to test whether defensive traits of S. altissima and feeding traits of lacebugs differ between the native and introduced ranges. Finally, we conducted population genetic analysis using neutral genetic markers to elucidate the origin of the introduced S. altissima populations and to track the changes in plant traits.
The abundance of lacebugs was significantly higher in populations with warmer climates, and it was significantly higher in Japan than in the U.S. even when these climatic differences were considered. Other herbivorous insects were lower in abundance and diversity in Japan than in the U.S. The resistance of S. altissima to lacebugs was higher in plants from lacebug-abundant populations than in plants from lacebug-absent populations in both countries. Both S. altissima and lacebug growth were higher in the Japanese garden. Moreover, the lacebugs performed better in the high-temperature treatment, and the Japanese lacebugs showed shorter term for growth compared to the U.S. lacebugs. Genetic analysis revealed that the Japanese plant populations were most closely related to southeastern U.S. populations where lacebug abundances were high. Overall, our findings suggest that multiple factors may influence lacebug abundance including climate, competition with other herbivorous insects, plant resistance, and lacebug traits. Solidago altissima in Japan, which had initially escaped lacebug herbivory, is once again experiencing high lacebug densities and selection for resistance in the introduced range.