Resource allocation to defence and growth are driven by different responses to generalist and specialist herbivory in an invasive plant

Invasive plants often have novel biotic interactions in introduced ranges. These interactions, including less herbivore attacks, may convey a competitive advantage over native plants. Invasive plants may vary in defence strategies (resistance vs. tolerance) or in response to the type of herbivore (generalists vs. specialists). Here, we examined resistance and tolerance of Triadica sebifera populations from the introduced and native ranges to generalist (Cnidocampa flavescens) and specialist (Gadirtha inexacta). To test for differences in resistance, caterpillars of each species were raised on native and invasive populations. We found the specialist grew larger on and consumed more mass of invasive populations than native, while the generalist showed the same performance between them. The results were consistent with our lab bioassay using excised leaves. Chemical analyses showed that the invasive populations had lower tannin and higher ratio of carbohydrate to protein than native. To test for differences in tolerance, plants were first defoliated by specialist or generalist herbivory and then allowed to regrow for 100 days. We found that invasive populations had greater tolerance than native, especially for tolerance to generalist. They also grew more rapidly than native in the absence of herbivory. The results indicate that differences in selective pressures between ranges have caused dramatic reductions in resistance to specialist and those changes in secondary chemistry likely underlie these differences. The greater tolerance of invasive populations appears to at least partly reflect an increase in growth rate. The greater tolerance to generalist suggests the intriguing possibility of selection for traits that allow plants to tolerate generalist more than specialist.