Asymmetric priority effects influence the success of invasive forest insects

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1. Settlement timing is often an important factor in interspecific herbivore interactions, as early-arriving species may encounter higher resource availability and/or avoid induced defences. Despite the general importance of priority effects to the outcome of herbivore interactions, there has been little exploration of such interactions on woody host plants where their impact can only be measured over multiple years. 2. In the eastern U.S.A., two invasive species, the hemlock woolly adelgid Adelges tsugae and the elongate hemlock scale Fiorinia externa, share a native host, eastern hemlock Tsuga canadensis. Their interaction and its consequences were investigated for plant growth - hemlock saplings that had been inoculated with either A. tsugae or F. externa, starting in spring 2007, were cross-infested with the other insect in spring 2009. A set of uninfested trees was simultaneously infested with A. tsugae, F. externa, both, or neither insect (= control), and insect density and plant growth was assessed in all treatments. 3. Adelges tsugae settlement rates did not differ if it settled alone or simultaneously with F. externa, but were ∼45% lower on trees previously infested with F. externa. There was no difference in F. externa settlement rates, and plant growth did not differ substantively between any of the herbivore treatments. 4. At a temporal scale (i.e. multiple growing seasons) appropriate to interactions between woody plants and their herbivores, this work demonstrates that plant-mediated priority effects can substantially affect herbivore settlement and thus the outcome of interspecific competition. © 2012 The Authors Ecological Entomology © 2012 The Royal Entomological Society.

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Ecological Entomology