Arboriculture & Urban Forestry 40(6): November 2014 its native range, EAB rarely damages its native hosts and is considered a secondary pest (Liu et al. 2007). In the introduced range, even healthy trees can be attacked and killed within a few years (Cappaert et al. 2005; Poland and McCullough 2006; Klooster et al. 2014). The putative mechanisms of resistance for F. mandshurica include a rapid rate of wound browning, constitutive and induced phenolic com- pounds, lignans and their derivatives, and a unique proteome signature (Eyles et al. 2007; Cipollini et al. 2011; Whitehill et al. 2011; Whitehill et al. 2012). The resistance of Asian ash species to EAB likely results from their long, shared evolutionary history, which North American ash clearly lack (Whitehill et al. 2012; Herms and McCullough 2014). Man- churian ash is therefore being considered in tra- ditional and hybrid breeding programs, as well as for genetic engineering (Whitehill et al. 2012; Herms and McCullough 2014). Although Asian ash species such as Manchurian ash have effec- tive defenses against EAB, resistance to one pest does not necessarily confer resistance to others (Herms 2002). North American ash are attacked by a suite of other borers, including the banded ash clearwing borer, Podosesia aureocincta, the ash/ lilac clearwing borer, P. syringae, the redheaded ash borer, Neoclytus acuminatus, and bark beetles (family Curculionidae) (Johnson and Lyon 1994; Solomon 1995; Herms 2007). Should Manchurian ash lack effective defenses against such indigenous North American insects, its utility as a source of resistant plant material may be compromised. Common urban problems, such as tree water stress, can increase borer attack and survival and may further complicate the search for resistant plant material (Raupp et al. 2010). Typically, tree water stress increases borer oviposition, survival, and rela- tive growth rates in part due to the disruption of defensive chemical transport (Koricheva et al. 1998; Hanks et al. 1999; Huberty and Denno 2004). EAB has been reported to attack Manchurian ash under stressful conditions, such as drought and fire dam- age (Baranchikov et al. 2008), and green ash are known to be more susceptible to clearwing borers under stressful conditions (Cregg and Dix 2001). The objectives of this study were to examine the relative susceptibility of green and Manchurian ash to EAB and indigenous North American bor- ers and to determine whether that susceptibility 333 might change with experimentally imposed drought stress. Additionally, reports indicate that the gen- eration time of EAB may depend on environmental conditions (Duan et al. 2010; Siegert et al. 2010; Tluczek et al. 2011). Therefore, a third goal of this study was to determine the generation time of EAB and whether it changes with tree stress. To achieve these goals, potted green and Manchurian ash were exposed to a heavy infestation of EAB and indige- nous North American borers, and the development of EAB and abundance of all borers were assessed for two years as a function of tree drought stress. MATERIALS AND METHODS Ash Tree Exposure to EAB and Indig- enous Borers Forty-eight individuals each of Manchurian ash (F. mandshurica ‘Mancana’) and green ash (F. pennsyl- vanica ‘Marshall’s Seedless’) were acquired as bare root stock from a nursery outside the known range of EAB (Bailey Nurseries, St. Paul, Minnesota, U.S.) on April 6, 2009. All trees had an initial DBH ≈ 5 cm. Trees were planted in 56.8 L pots with bark-based soilless potting media (Sunshine PX3 Series, Sun Gro Horticulture, Agawam, Massachusetts, U.S.), fertilized (16-4-8 Vigoro Tree, Shrub, and Evergreen Food, Spectrum Brands, Inc., Middleton, Wisconsin, U.S.) in early spring and early autumn, and watered daily until well established. No chemical pesticides were applied at any time. Trees were maintained in a cold frame until May 13, 2009, and then moved to a forested site in Alexandria, Virginia, U.S. (N 38°44’38”, W 77°10’12.2”) where high numbers of EAB were detected in early spring. Trees were exposed to natural colonization by EAB and indigenous North American borers from nearby (<30 m) ash trees heavily infested with EAB and other borers. Heavy EAB flight was confirmed through the use of an attractant-baited, three-sided purple prism trap. From May 13 through June 30, all trees were watered daily to allow for root establish- ment before beginning watering treatments in July. Three watering treatments were established to create a gradient in drought stress. Sixteen trees of each ash species were assigned randomly to one of three watering treatments: full, half, and low water availability. Trees assigned to the full water- ing treatment received ambient rainfall plus hand ©2014 International Society of Arboriculture
November 2014
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