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Herbivory and novel weapons: no evidence for enhanced competitive ability or allelopathy induction of Centaurea diffusa by biological controls

Abstract

Biological control of weeds by arthropod herbivores is thought to work by reducing the competitive ability of the weed relative to the surrounding vegetation. However, the assumption that herbivory reduces plant competitive ability has not been tested in most biological control systems, and counter to expectation, recent research on the impact of biological control agents on invasive Centaurea species suggests that this genus may respond to herbivory by increased competitive ability through enhanced plant re-growth and/or by inducing increased production of phytotoxic allelochemicals. We examined the impact of two biological control agents of the invasive plant diffuse knapweed (C. diffusa) to see if feeding by either of these insects would enhance the plant’s competitive ability or allelochemical output. Sub-lethal herbivory by either of the biological control agents significantly reduced knapweed performance when the plant was grown in competition with either of two native species. Competition with knapweed significantly reduced the performance of both native species (Artemisia frigida and Bouteloua gracilis), and herbivory by one of the biocontrol agents resulted in a small but significant increase in both native species’ performance. Diffuse knapweed’s putative allelochemical 8-hydroxyquinoline was not detected in experimental or field collected soils from knapweed-infested sites. In contrast to other studies on the impacts of biological control on other Centaurea species, these data support the premise that biological control agents may reduce invading plant competitive ability. We find no evidence for diffuse knapweed allelopathy mediated by 8-hydroxyquinoline or enhanced allelopathy in response to herbivory by biological control agents.

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Acknowledgments

We would like to thank Heather Batts, Emily Sadler, Todd Green, and Nathan Kirkley for assistance in plant harvests. This research was greatly improved through continual group discussions with Cynthia Brown, Helen Rowe, Erin Bard, Hilary Drucker, Ruth Hufbauer, Steve Rauth, Dan McKinnon, Robin Marrs, and Dan Cariveau. The manuscript was greatly improved by thoughtful comments from three anonymous reviewers. This research was funded by a Colorado State University Agricultural Experiment Station grant to APN. All experiments and analyses conducted comply with current US laws.

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Correspondence to Andrew P. Norton.

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Norton, A.P., Blair, A.C., Hardin, J.G. et al. Herbivory and novel weapons: no evidence for enhanced competitive ability or allelopathy induction of Centaurea diffusa by biological controls. Biol Invasions 10, 79 (2008). https://doi.org/10.1007/s10530-007-9111-3

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  • DOI: https://doi.org/10.1007/s10530-007-9111-3

Keywords

  • Novel weapons
  • Interspecific competition
  • 8-Hydroxyquinoline
  • Centaurea diffusa
  • Bouteloua gracilis
  • Artemisia frigida
  • Indirect effects
  • Non-target impacts
  • Allelochemical induction