ARS | Publication request: Genetic variation in anti-herbivore chemical defenses in an invasive plant

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 20, 2012
Publication Date: April 23, 2012
Repository URL: http://doi: 10.1111/j.1365-2745.2012.01980.x
Citation: Wang, Y., Siemann, E., Wheeler, G.S., Zhu, L., Gu, X., Ding, J. 2012. Genetic variation in anti-herbivore chemical defenses in an invasive plant. Journal of Ecology. 100:894-904.

Interpretive Summary: Plants produce many different classes of defensive compounds that protect them from insect and disease attack. Some, like tannins, function quantitatively where their effectiveness depends greatly on concentration. Others, like flavonoids, are more qualitative where they function at relatively low concentrations. Invasive plants have evolved in their new adaptive range independent of their co-evolved herbivores that have selected for these defensive compounds. In our experiment, native Chinese tallow (Triadica sebifera) populations from China and invasive populations from the US were grown in a common garden in China where they were damaged by three insect species and by mechanical clipping. Two of the herbivores were generalists, the third was a specialist and a candidate for biological control of this invasive weed in the US. Many plants increase the level of defensive compounds in response to insect feeding. We measured the changes in flavonoid and tannin concentrations in these damaged native or invasive tallow plants. Additionally, we measured insect growth of the three caterpillar species when fed plants damaged by the caterpillars. We found invasive plants had higher flavonoid and lower tannin concentrations than native populations, especially in new leaves following damage. The caterpillar responses to these chemical changes varied in direction and strength where the two generalists species either had lower or no change in growth, whereas the specialist had greater growth on invasive plants. These results imply a trade-off in defensive chemistry in the invasive plants with greater qualitative (flavonoids) and lower quantitative (tannins) levels in the US where specialists insects are absent. Moreover they indicate that the specialist potential biological control agent will grow well when fed these US plants.

Technical Abstract: Plants produce a variety of secondary metabolites such as flavonoids (qualitative defense) or tannins (quantitative defense), that vary in effectiveness against different herbivores. Because invasive plants experience different herbivore interactions in their introduced versus native ranges, they may vary in defense chemical profiles. We subjected tallow tree (Triadica sebifera) seedlings from native (China) and invasive (US) populations to induction by leaf clipping or one of three Chinese caterpillars (two generalists and one specialist). We measured concentrations of five flavonoids and four tannins in leaves produced before or after damage. We measured growth of caterpillars fed these leaves from plants of each induction treatment or undamaged controls. Invasive populations had higher flavonoids and lower tannins than native populations, especially in new leaves following induction. Caterpillar responses to changing chemical concentrations varied in direction and strength so overall performance varied from significantly lower (generalist Grammodes geometrica), unchanged (generalist Cnidocampa flavescens), to significantly higher (specialist Gadirtha inexacta) on invasive populations. Together, such a trade-off in secondary metabolism in invasive plants and the effect on herbivores suggest divergent selection may favor greater qualitative but lower quantitative defense in the introduced range where coevolved natural enemies, especially specialists, are absent.