332 Martinson et al.: Water Stress and Geographic Origin Influence Borer Infestations in Ash Arboriculture & Urban Forestry 2014. 40(6): 332–344 Tree Water Stress and Insect Geographic Origin Influence Patterns of Herbivory by Borers in Green (Fraxinus pennsylvanica) and Manchurian (F. mandshurica) Ash Holly M. Martinson, Chris Sargent, and Michael J. Raupp Abstract. Emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), is a devastating buprestid beetle introduced to North America from Asia. Asian ash trees exhibit resistance to EAB, likely due to a shared co-evolutionary history. Resistance to one pest, however, does not necessarily confer resistance to others. Should Asian ash be highly susceptible to North American herbi- vores, the utility of such species for hybridization, breeding programs, and establishment in managed landscapes could be compro- mised. Common urban problems, such as drought stress, can increase borer attack and survival and may further complicate the search for resistant plant material. The objectives of this study were to examine the relative susceptibility of green (Fraxinus pennsyl- vanica) and Manchurian (F. mandshurica) ash to EAB and indigenous borers and whether that susceptibility changed with drought stress. In a common garden experiment, EAB occurred more frequently and reached higher abundances in green than Manchurian ash. The frequency and abundance of bark beetles (Curculionidae), North American native clearwing borers (Sesiidae), and longhorn beetles (Cerambycidae) were similar in the two ash species. Generation time of EAB was uniformly one year and did not depend on ash species or water stress. Although borers increased as expected in stressed trees, the relative susceptibility of green and Manchurian ash to borers did not change. The findings suggest Manchurian ash may be resistant to several classes of borers, regardless of insect geographic origin, although these conclusions should be viewed with some caution until the results can be verified in larger trees. Key Words. Agrilus planipennis; Ash Trees; Defense Free Space; Emerald Ash Borer; Fraxinus; Green Ash; Invasive Species; Manchurian Ash; Voltinism; Water Stress. A key component of integrated pest management programs for woody plants is the deployment of resis- tant plant material (Raupp et al. 1992; Herms 2002). Traditional plant breeding and genetic engineer- ing can be effective at creating resistance, but many years may be required to develop resistant materials (Herms 2002). On the other hand, existing cultivars and species can be screened for resistance to key pests and deployed much more rapidly in the face of lethal pest outbreaks (Raupp et al. 1992; Herms 2002). Emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), is the most serious threat to North American ash (Fraxinus spp.) since its discovery in Michigan in 2002 (Cappaert et al. 2005; Haack 2006). To date, in North America, EAB has killed millions of trees and spread to twenty- four states plus the District of Columbia (United ©2014 International Society of Arboriculture States) and two provinces (Canada) (USDA For- est Service and Michigan State University 2014), and threatens approximately eight billion ash trees in continental U.S. forestlands (Nowak 2003). Ash is also a major component of urban forests (Mac- Farlane and Meyer 2005; Cumming et al. 2006; Raupp et al. 2006). Kovacs et al. (2010) estimated that 17 million ash trees on developed lands are at risk from EAB, with USD $10.7 billion in pro- jected management costs over a 10-year horizon. North American ash species are susceptible to EAB but Asian ash species appear to be resistant. In a common garden experiment, Rebek et al. (2008) found that EAB caused substantially higher mortality for the North American species, green ash (F. penn- sylvanica) and white ash (F. americana), than for the Asian species, Manchurian ash (F. mandshurica). In
November 2014
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