Arboriculture & Urban Forestry 41(3): May 2015 Arboriculture & Urban Forestry 2015. 41(3): 125–135 125 A Borer-Specific Assessment Scheme for Identifying Sentinel Trees to Delimit Invasive Borers in Urban Forests Gabriel P. Hughes, Clifford S. Sadof, and Matthew D. Ginzel Abstract. Aggressive insects like the emerald ash borer [Agrilus planipennis (Fairmaire)] (EAB) increasingly threaten the health of the urban forest. Early detection of exotic wood-boring pests is critical for rapid response efforts, and allows for effective manage- ment while populations are relatively low. During street tree inventories, arborists record subjective rankings of tree condition and pest incidence; however, the extent of insect attack is rarely quantified. Moreover, it is unknown how the assessment of tree vigor pro- vided by city foresters during these inventories relates to the likelihood of wood-borer infestation. In this study, researchers developed a borer-specific tree vigor assessment scheme to rapidly evaluate street trees, and to identify when EAB populations begin to grow exponentially based on the health of the forest. This scheme incorporates common indicators of EAB attack, including canopy thin- ning and epicormic sprouts, as well as attack by common native wood-boring insects. This scheme was used to track the health of ash trees from 2010 to 2013 in one urban forest with advanced symptoms of EAB decline and one without (Indianapolis and Lafay- ette, Indiana, U.S., respectively). Trees declined more rapidly in Indianapolis where emergence holes from native borers were pos- itively correlated with EAB in infested areas. Over the course of the study, first detections of EAB occurred on progressively weaker trees at both sites, suggesting that early detection of incipient EAB populations can be improved by surveying apparently healthy trees. Key Words. Agrilus planipennis; Early Detection-Rapid Response; Emerald Ash Borer; Surveys; Tree Inventory; Tree Vigor; Wood Borers. The introduction of aggressive exotic wood-boring pests into the United States is forcing urban for- estry managers to consider innovative approaches for maintaining forest health. On a small scale, en- demic woodborers killing already weakened trees can be managed through a combination of insec- ticides and cultural control methods to improve tree vigor (Herms et al. 2014). Alternatively, weak- ened trees containing borers can be removed and replaced with less susceptible tree species. Yet, the invasion of more aggressive exotic borers, like the Asian longhorned beetle [Anoplophora glabripen- nis (Motchulsky) (Coleoptera: Cerambycidae)] and the emerald ash borer [Agrilus planipennis (Fairmaire) (Coleoptera: Buprestidae)], threaten a substantially larger part of the urban canopy than ever before. In fact, recent surveys of 12 eastern North American cities indicate that urban forests could lose between 30% and 70% of their standing trees to these destructive pests (Raupp et al. 2006). The emerald ash borer (EAB) originated from Asia, where it preferentially attacks stressed trees in the genus Fraxinus (Yu 1992). In North Amer- ica, however, it is capable of killing even apparently healthy ash trees in as little as three years aſter initial colonization (Poland and McCullough 2006; Knight et al. 2013). Adults emerge May–July and feed on leaves in the canopy before mating and laying eggs (Cappaert et al. 2005; Lyons and Jones 2005; Liu et al. 2007; Wang et al. 2010). Since its detection in North America in 2002, EAB has killed millions of green, white, and black ash trees, and is expected to cause more than USD $10 billion in damage in urban areas over the next decade (Kovacs et al. 2010). Human-assisted spread of EAB has compounded the problem, as people unwittingly transport infested logs or ash nursery stock to new locations (Haack et al. 2002; Cappaert et al. 2005). Early in an invasion, emergent EAB populations go undetected because affected trees appear vigorous and show no ©2015 International Society of Arboriculture
May 2015
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