Invasive interactions: can Argentine ants indirectly increase the reproductive output of a weed?
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The direct and indirect interactions of invasive ants with plants, insect herbivores, and Hemiptera are complex. While ant and Hemiptera interactions with native plants have been well studied, the effects of invasive ant–scale insect mutualisms on the reproductive output of invasive weeds have not. The study system consisted of Argentine ants (Linepithema humile), boneseed (Chrysanthemoides monilifera monilifera), and sap-sucking scale insects (Hemiptera: Saissetia oleae and Parasaissetia nigra), all of which are invasive in New Zealand. We examined the direct and indirect effects of Argentine ants on scale insects and other invertebrates (especially herbivores) and on plant reproductive output. Argentine ants spent one-third of their time specifically associated with scale insects in tending behaviours. The invertebrate community was significantly different between uninfested and infested plants, with fewer predators and herbivores on ant-infested plants. Herbivore damage was significantly reduced on plants with Argentine ants, but sooty mould colonisation was greater where ants were present. Herbivore damage increased when ants were excluded from plants. Boneseed plants infested with Argentine ants produced significantly more fruits than plants without ants. The increase in reproductive output in the presence of ants may be due to increased pollination as the result of pollinators being forced to relocate frequently to avoid attack by ants, resulting in an increase in pollen transfer and higher fruit/seed set. The consequences of Argentine ant invasion can be varied; not only does their invasion have consequences for maintaining biodiversity, ant invasion may also affect weed and pest management strategies.
KeywordsLinepithema humile Boneseed Indirect effects Mutualism Scale insect
We thank the Auckland City Council for permission to work in Te Whau Point reserve. We also thank Jo Rees for data checking, Rosa Henderson for identifying the scale insects, and Stephen Thorpe for identifying other invertebrates. Greg Arnold and Guy Forrester helped with statistical design and analyses. Richard Toft, Shaun Forgie and two anonymous reviewers improved earlier versions of the manuscript. This work was funded by the University of Auckland summer student scholarship programme (to L. Phillips, H. Nathan, J. Galbraith, S. Knight) and the New Zealand Foundation for Research, Science and Technology (C09X0211) and complies with New Zealand laws.
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