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Native bark-foraging birds preferentially forage in infected ash (Fraxinus spp.) and prove effective predators of the invasive emerald ash borer (Agrilus planipennis Fairmaire)


Inadvertently introduced into North America in the 1990s, the invasive emerald ash borer (EAB, Agrilus planipennis) has been spreading across the Great Lakes Region resulting in widespread ash tree (Fraxinus spp.) mortality. Native woodpeckers and other bark-foraging insectivores represent one of the few potential natural predators of EAB in the U.S. In this study, we combined observational and destructive tree harvesting approaches to assess bark-foraging bird predation on EAB larvae in a deciduous forest of central Ohio. Results of our observational study show that in an EAB impacted forest, bark-foraging birds forage more heavily on ash trees than non-ash trees, and that they forage preferentially on ash trees that exhibit canopy decline symptoms relative those with healthy canopies. These patterns were further supported by the destructive sampling of 46 ash trees wherein predation by bark-foragers significantly reduced tree-level EAB densities by upwards of 85%. Bark-foraging predation intensity increased with increased EAB infestation levels, with bark-foragers harvesting ~45% of EAB in trees with thinning canopies compared to ~22% in ash trees with healthy canopies. Woodpeckers harvest EAB in a density-dependent pattern that could contribute to population control. Despite bark-forager predation, EAB had a high likelihood of successfully emerging from the heavily infested ash trees (~30% or 35 EAB per m2). Our results suggest that woodpeckers and other bark-foragers may use visual canopy decline, and perhaps other cues, to target ash trees with increased EAB densities. Moreover, our results provide insight into the indirect effects of invasive species on biotic interactions in forest ecosystems highlighting potential shifts in bark-foraging and other bird behaviors in response to a novel forest pest. Bark-foragers respond to EAB infestation and may thus potentially help regulate EAB populations and their spread in a mixed deciduous forest. We suggest that maintaining snags and nesting sites during and after forest pest outbreaks may enhance populations of bark-foraging bird species and, thus, their biological control of pest insects in temperate deciduous forests.