Resource regulation of an invasive tree by a classical biological control agent

• We evaluated the Resource Regulation Hypothesis in a weed biological control project.
• Feeding by Oxyops vitiosa increases the amount of resources for subsequent generations.
• A positive feedback loop was induced by O. vitiosa and mediated by water.
• O vitiosa appears to be controlling Melaleuca quinquenervia resources.

The invasive tree Melaleuca quinquenervia experienced substantial declines in growth and reproduction in response to chronic herbivory by the defoliating weevil Oxyops vitiosa. Plants subjected to unrestricted defoliation replaced leaves that were more suitable for feeding by the next generation, a process envisioned by the Resource Regulation Hypothesis which posits that attack by one generation increases the amount of the preferred host resources for the next, resulting in a positive feedback loop for the herbivore. The production of juvenile replacement leaves stimulated additional bouts of oviposition and feeding by O. vitiosa, which ultimately produced positive effects for the herbivore with negative consequences for the plant. The addition of water resources to the plant prolonged the positive feedback loop such that more than twice as many insects were produced on irrigated versus non-irrigated trees. In a more simple, reassembled food web on M. quinquenervia, the lack of biotic constraints like parasitoids may have prevented the earlier termination of the feedback loop and thus increased the impact of the biological control agent on the target. The overall effectiveness of this classical biological control program can be attributed, in part, to the phenomenon of the target plant’s induced susceptible response to a herbivore.

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