Chromolaena odorata is a herb or subshrub native to the Americas from southern USA to northern Argentina, and is considered one of the worst terrestrial invasive plants in the humid (sub)tropics of the Old World. In introduced ranges, C. odorata may have (1) evolved increased resistance to generalists in response to the novel enemy regime (presence of generalists but absence of specialists), and (2) maintained or even increased tolerance to damage due to generalists and other causes such as mowing.
To test the two hypotheses mentioned above, Dr. LIAO Zhiyong and his colleagues of Xishuangbanna Tropical Botanical Garden (XTBG) carried out a common garden experiment comparing 15 invasive populations with 13 native populations of Chromolaena odorata, including putative source populations identified with molecular methods and binary choice feeding experiments using three generalist herbivores. They also considered growth costs of resistance to generalists and tolerance to simulated herbivory (shoot removal).
The common garden experiment was carried out in Xishuangbanna Tropical Botanical Garden (21°56′N, 101°15′E, 600 m a.s.l.). Seeds of C. odorata were collected in 2009 from 15 populations in its invasive ranges and 13 populations in its native ranges. Within each population, seeds were collected from ten to 15 plants chosen randomly, and mixed together.
They found that C. odorata plants from invasive populations had higher resistance to generalist herbivores, resistance traits, and tolerance to simulated damage than plants from native populations. Some of those differences could be attributed to post-introduction evolution because the source populations of the invader were known. Their results indicated that invasive plants may evolve to increase both resistance to generalists and potential tolerance to damage in introduced ranges, especially when the defense traits have low or no fitness costs.