Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2010
Publication Date: 4/1/2011
Citation: Wang, Y., Huang, W., Siemann, E., Wheeler, G.S., Carrillo, J., Ding, J. 2011. Lower herbivore resistance and higher tolerance in invasive plants: biological control agents reach high densities but exert weak control of their host plants. Journal of Applied Ecology. 21(3):729-738. Interpretive Summary: Success in biological control of invasive plants depends upon the ability of host-specific natural enemies to suppress the growth and reproduction of their host plant. Understanding the interactions of insect biological control agents and their host plant is critical for forecasting the control before the insects are introduced. The results of this analysis illustrate the significance of the evolution of a weed after many generations of growth in a new geographic range. In the case of Chinese tallow, a species introduced into North America in the late 1700’s, these trees grow bigger than those grown in China. Additionally, the Chinese insects being developed for biological control of this weed grow bigger when fed the North American varieties of Chinese tallow plants. However, biological control of these trees in North America may be complicated by the more rapid growth and tolerance of the weed to insect damage. These results predict that the impact of these insects may be less than desired however additional studies need to be conducted in open-field conditions to confirm the significance of these findings.
Technical Abstract: 1. Invasive plants often have novel biotic interactions in their introduced ranges. Changes in herbivore defense traits are important for the effectiveness of biological control, because insect natural enemies may rapidly build up their populations on invasive plants due to decreased resistance but exert weak control due to increased tolerance. Moreover, resource availability may affect the efficacy of biological control agents. 2. We tested these predictions using Chinese tallow tree (Triadica sebifera) and two specialist herbivores (Heterapoderopsis bicallosicollis and Gadirtha inexacta) that are candidates for biological control. We performed a pair of field experiments in China in which Triadica seedlings from the native or introduced range were grown in low or high light conditions and subjected to different levels of herbivory by each herbivore in a factorial design. 3. We found that Heterapoderopsis achieved greater population densities when insects fed on tallow trees from the introduced range or when they were in high light conditions. When Gadirtha was raised in the lab on tallow tree foliage we also found that it performed better (larger pupal size) when fed foliage from introduced populations. 4. However, introduced populations had greater herbivore tolerance such that the impact of each agent on plant performance was lower than on native populations despite higher herbivore loads on those plants. Tallow trees grew more slowly and achieved smaller sizes in lower light levels but the impact of biological control agents were comparable at different light levels. Plants from introduced populations grew larger than those from native populations in all conditions. 5. Synthesis and applications. Our results suggest that reduced resistance and increased tolerance to herbivory in introduced populations may impede success of biological control although the insects can reach high density. Ineffective biocontrol agents may cause unexpected non-target effects. Thus, biological control practitioners should include plants from the introduced range in the pre-release evaluation and examine novel interactions of insects and invasive populations, which will help improve the prediction of insect impact and reduce potential risk to native ecosystem.