Emerald Ash Borer: Invasion of the Urban Forest and the Threat to North America’s Ash Resource

entomology & pathology ABSTRACT Therese M. Poland and Deborah G. McCullough The emerald ash borer (EAB), a phloem-feeding beetle native to Asia, was discovered killing ash trees in southeastern Michigan and Windsor, Ontario, in 2002. Like several other invasive forest pests, the EAB likely was introduced and became established in a highly urbanized setting, facilitated by international trade and abundant hosts. Up to 15 million ash trees in urban and forested settings have been killed by the EAB. Quarantines in the United States and Canada restrict the movement of ash trees, logs, and firewood to prevent new introductions. Research studies are underway to assist managers leading eradication and containment efforts. Long-term efforts will be needed to protect ash in urban and forested settings across North America. Keywords: Agrilus sp., Fraxinus, invasive pest, quarantine T he emerald ash borer (EAB), Agrilus planipennis Fairmaire, a phloemfeeding beetle (Coleoptera: Buprestidae) native to Asia, was determined to be the cause of widespread decline and mortality of ash (Fraxinus sp.) in Detroit, Michigan, and nearby Windsor, Ontario, in July 2002. Results from initial delimitation surveys in Michigan in 2002 showed that the EAB population densities and tree mortality were highest in the greater Detroit area. This evidence, along with recent dendrochronological data used to determine the year of EAB attack or tree mortality across the infested area, indicates that the EAB initially was introduced, became established, and developed into an invasive pest in the highly urbanized area of Detroit. At first glance, Detroit may seem an unlikely locale for an exotic forest pest problem. Several features of urban forests, however, are particularly conducive for invasive forest pest introductions and establishment. Nonindigenous organisms likely arrive more 118 Journal of Forestry • April/May 2006 often in cities than in rural or natural settings because of the ever-increasing volume of international commerce and trade at ports of entry. Historically, imported nursery stock was an important source of nonindigenous forest insects and plant pathogens (Niemelä and Mattson 1996, National Research Council [NRC] 2002). More recently, solid wood packing material, including crating and pallets that often accompany commodities shipped to the United States, has emerged as a major source of potentially invasive forest pests (USDA Animal and Plant Health Inspection Service and Forest Service 2000). In addition to the EAB, at least 10 nonindigenous forest insects associated with solid wood packing material have been discovered in the United States or Canada since 1990 (Haack 2005). Nonindigenous organisms that arrive in a new habitat must find suitable hosts to become established (NRC 2002). Forest insects and pathogens that originate in regions of Europe and Asia often have a remarkably good chance of encountering North American tree species of the same genus or family as their native hosts (Niemelä and Mattson 1996, NRC 2002). For instance, in nine US cities, 12– 61% of the urban trees were preferred hosts of the Asian longhorned beetle (Anoplophora glabripennis; Nowak et al. 2001). Ash trees that can serve as hosts to the EAB are among the most common fastgrowing woodland trees in the northeastern states and have been widely planted as a popular shade tree in urban areas. Nonindigenous ornamental plants that have been naturalized are common in urban landscapes and also may serve as hosts to nonindigenous organisms. In addition, urban trees frequently are planted in unfavorable sites such as parking lots or other areas where they experience stress from pollution, soil compaction, or damage from human activities. Such stressful conditions may predispose trees to insect or pathogen attack, increasing the likelihood that nonindigenous forest pests will successfully establish and increase in density. Large residential or business developments or roadside plantings in urban areas often are composed of a single shade tree species. When an invasive pest becomes established in a monoculture planting, the impacts can be devastating, as evidenced by the rapid spread and impact Dutch Elm disease (Ophiostoma ulmi and O. novo-ulmi) on American elm (Ulmus americana; Karnosk 1979). Ironically, in several northeastern and midwestern cities, many dead elms were replaced with maple (Acer spp.) or ash trees, Emerald Ash Borer: Invasion of the Urban Forest and the Threat to North America’s Ash Resource Biology The life cycle of the EAB in Michigan generally appears similar to that described by Chinese scientists (Chinese Academy of Science 1986, Yu 1992). In spring, adult beetles chew their way out of the tree, leaving D-shaped emergence holes approximately 3– 4 mm in width. In southeast Michigan in 2003 and 2004, adults first emerged in midMay at roughly 230 –260 degree days, using a base 10° C threshold (Brown-Rytlewski and Wilson 2005), and adult activity peaked from late June to early July (Cappaert et al. 2005). Beetles feed on ash foliage (Figure 1), causing superficial aesthetic damage that is not very evident until it is quite extensive. Adults feed for 5–7 days before mating begins and female beetles feed for an additional 5–7 days before beginning to lay eggs. Each female beetle can lay 50 –90 eggs during her lifetime. Beetles continue to feed and mate during the remainder of their lifespan, which can last from 3 to 6 weeks (Bauer et al. 2004, Lyons et al. 2004). Eggs, laid in bark crevices, hatch within 2 weeks. Larvae feed Figure 1. Emerald ash adult feeding on ash leaf. (Photographer: David Cappaert, Michigan State University.) in the phloem and cambium from July through autumn, excavating serpentineshaped galleries packed with frass. Extensive larval feeding disrupts translocation, girdling the tree and ultimately results in tree death within 1–3 years. Larvae pass through four instars (Cappaert et al. 2005) and most larvae complete feeding in October or November. Prepupae overwinter in cells about 0.5 in. deep in the sapwood or outer bark. Pupation begins in mid-April and continues into May, followed by adult emergence roughly 3 weeks later. Some EABs, however, overwinter as young larvae rather than as prepupae, and then require a second year of development before emerging as adults (Cappaert et al. 2005, Siegert et al. 2005). Although the cause of multiyear development still is not known, it appears to be most common in low-density populations and may reflect a combination of factors such as host resistance, host quality, or weather. Multiyear development clearly has major implications for EAB population dynamics and for trapping, survey protocols, and other aspects of the operational program. For instance, multiyear larval development slows EAB population growth but delays the onset of external symptoms on infested trees, reducing the efficacy of visual surveys. Applications of (...truncated)