Document Type

Peer-Review Article


Three native species of tree-infesting Agrilus have regularly reached outbreak levels in the Lake States (Michigan, Minnesota, and Wisconsin), including A. anxius Gory (bronze birch borer), A. bilineatus (Weber) (twolined chestnut borer), and A. granulatus liragus Barter & Brown (bronze poplar borer). The main host trees for these Agrilus are species of Betula for A. anxius, Castanea and Quercus for A. bilineatus, and Populus for A. granulatus liragus. Based on 197 annual forest health reports for Michigan (1950–2017, 66 years), Minnesota (1950–2017, 64 years), and Wisconsin (1951–2017, 67 years), A. bilineatus was the most often reported Agrilus species in all three states (mentioned in 90 annual reports), with A. anxius second (71 reports) and A. granulatus liragus third (21 reports). Drought was the most commonly reported inciting factor for outbreaks of all three Agrilus species, with defoliation events ranking second. The top two defoliators reported as inciting outbreaks of each species were, in decreasing order, Fenusa pumila Leach (Hymenoptera: Tenthredinidae; birch leafminer) tied with Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae; forest tent caterpillar) for A. anxius; M. disstria and Alsophila pometaria (Harris) (Lepidoptera: Geometridae; fall cankerworm) for A. bilineatus; and M. disstria and Choristoneura conflictana (Walker) (Lepidoptera: Tortricidae; large aspen tortrix) for A. granulatus liragus. Other environmental factors occasionally listed as inciting Agrilus outbreaks included late spring frosts, ice storms, and strong wind events.



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