Evolved resistance to specialist and generalist herbivory as revealed by quantitative and qualitative defence in the invasive tallow tree

Background/Question/Methods

To explain the invasion success of exotic plants, several hypotheses, e.g., the Enemy Release, Evolving Increased Competitive Ability (EICA), Biotic Resistance and Novel Weapons have been proposed. Although numerous studies have recently examined these hypotheses, the debate is still intensive over the relative importance of various factors such as herbivores and secondary chemical compounds of invasive plants. Given that invasive plants are escaped from suppressions by co-evolved specialists but attacked by generalists in introduced range, and that specialists and generalists may select for different defence mechanisms (quantitative defence vs. qualitative defence), examining the variations in defence to herbivory by specialists and generalists and variations in secondary chemical compounds between native and invasive plant populations are critical in understanding the plant invasion success and for biological control. Using the tallow tree (Triadica sebifera), a native plant in China but invasive in the US, as a model species, we investigate the biogeographic variations in resistance to herbivory by specialist and generalist caterpillars. We conducted a lab bioassay to examine the insect development on native and invasive plants. We compared the plant quantitative (mainly tannins) and qualitative (mainly flavonoids) chemical compounds in leaves from the invasive and native populations.  

Results/Conclusions

Relative to the China plants, the US plants show reduced resistance to herbivory by specialists, but no difference for generalists. Chemical analysis shows that the US populations had low tannins contents but high flavonoids contents, suggesting potential trade-offs between quantitative and qualitative defence in the invasive tallow, in response to the differing insect communities. The low resistance to herbivory by specialists helps predict a fast population build-up of biocontrol agents on the US populations which was confirmed by a field common garden experiment. Though our lab bioassay did not show the greater impact of the high flavonoids contents on generalists in the US populations than the China populations, we argue that some generalist populations in the field may be negatively affected through a long term exposure (relative to the short lab tests) of the high contents of flavonoids in the US populations. Our study shows that invasive plants may employ novel defense strategies by possessing novel biochemical traits to cope with the differing herbivore communities, thus affecting biological control and food web.