In the summer of 2002, scientists realized that widespread damage to ash (Fraxinus) in southern Michigan was caused by an introduced insect, the emerald ash borer (Agrilus planipennis) (Federal Register, October 14, 2003, Volume 68, Number 198). The pest is thought to have been established in Michigan for at least 10 years by the time of its discovery (Siegert, 2006) and had already infested a large area in Michigan and adjacent Ontario. Due to a combination of factors, including both the biological and human dimensions of infestation and dispersal pathways, the pest spread into surrounding states and provinces despite many different attempts to contain it. As of 2021, EAB is found in 35 states and 5 Canadian provinces, and outbreaks sometimes emerge at sites hundreds of miles from the nearest known location (including Boulder Colorado and Winnipeg Manitoba) – indicating that infested wood continues to be moved despite state regulations, international regulations, and extensive outreach campaigns.
In Europe, EAB was first detected in Moscow, in European Russia in 2003. As of 2021, EAB is established in one province in Ukraine and 18 oblasts of European Russia, especially to the west of Moscow towards the borders with Belarus and Ukraine (Musolin et al. 2021). In Moscow, initial damage was greatest on the introduced North American species, green ash (Fraxinus pennsylvanica); however, by 2021 mass mortality was observed to European as (F. excelsior) as well (Volkovitsh, Bienkowski and Orlova-Bienkowskaja, 2021).
Two disjunct populations of EAB have been detected 400-500km from Moscow. The first detection, in 2019, was in southwest Russia and eastern Ukraine, which may be correlated with the planting of F. excelsior and F. pennsylvanica along roads, railways, field shelter belts, and urban forests (Musolin et al. 2021). The second disjunct population is near St. Petersburg, only 130km from the borders of Estonia and Finland; it was detected in September 2020.
New outbreaks and infestations of emerald ash borer are announced sporadically. To find current information on locations (and the status of each state, province, and county), it is best to go to Emeraldashborer.info or access their current map, which is typically updated on a monthly basis. Canadian provinces are also included on this location map, with confirmed infestations now ranging from New Brunswick to Manitoba. In general, EAB infestations in North America are slowly advancing westward.
EAB larvae feed in the phloem and outer sapwood, producing galleries that damage and eventually kill the host. Adult EAB feed on host foliage. In its native range in east Asia, the pest feeds on species of Fraxinus (ask), Ulmus (elm) and Juglandaceae (walnuts and hickories) (McCullough & Roberts, 2002a and 2002b). In North America, the borer feeds primarily on Fraxinus species, although it will also infest Chionanthus virginicus, fringtree. All North American ash trees (Fraxinus) are attacked by EAB; the insect shows a high degree of preference for green (F. pennsylvanica) and black (F. nigra) ash, lower preference for healthy blue ash (F. quadrangulata), and appears to have a relatively complex interaction with white ash (F. americana) (Robinette and McCullough, 2019).
USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pest & pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to North America. The analysis identified 15 priority taxonomic groups requiring immediate conservation intervention due to their exposure to an extrinsic threat, their sensitivity to that threat, and their ability to adapt to it. Each of these 15 most vulnerable species, as well as several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort.
Carolina ash (Fraxinus caroliniana) and pumpkin ash (F. profunda) are among six species that face severe pest threats but have a high capacity to adapt (according to CAPTURE project). For this reason, conservation and the facilitation of resistance through breeding are high priorities. Several other ash species that have not yet been infested by EAB may eventually experience extensive mortality, and because they tend to be rare, CAPTURE ranked them high for conservation and the facilitation of resistance. Such species include Texas ash (Fraxinus albicans), velvet ash (F. velutina), Chihuahua ash (F. papillosa), fragrant ash (F. cuspidata), Berlandier ash (F. berlandieriana), and Gregg ash (F. greggii).
EAB is also known to attack another, related, genus of trees: fringetrees (Cionanthus spp.). EAB attacks on fringetrees were first noticed by Dr. Don Cipollini of Wright State University in Ohio in 2014 (Hannah, 2014). Initially, it was uncertain whether EAB could establish in the Cionanthus genus. However, by summer 2015, Dr. Cipollini demonstrated that EAB attacked fringetrees across a wide area – including much of Ohio and parts of Illinois. EAB attacks on fringetrees appear to occur when EAB populations are high (Entomology Today, 2015). This could have implications for the white fringetree (C. virinicus) which is native to North America. Wild populations of C. virinicus grow from New Jersey south to Florida and west to Texas; it has also grown in popularity as an ornamental plant in a wider area of the country. In laboratory tests, a few EAB larvae also survived in the more distantly related devilwood tree (Osmanthus americanus). EAB did not, however, survive in the Chinese fringetree (Chionanthus retusus). Since Chinese fringetree is native to the same parts of China as is EAB, it may have evolved chemicals that protect it from the insect (Entomology Today, 2015). While the newly discovered ability of EAB to infest fringetrees may not extend the geographic area that is vulnerable to invasion, it will increase the amount of trees killed as well as the associated economic and ecological damage. For example, in Missouri and Arkansas, white fringetree is one of the few trees or shrubs that grows on bald knobs and limestone/dolomite glades (LeDoux, 2014).
The emerald ash borer is indigenous to Asia and known to occur in China, Korea, Japan, Mongolia, the Russian Far East, and Taiwan. It was likely introduced to North America in wood packaging; USDA APHIS has intercepted the insect 36 times at ports in eleven eastern states. Those shipments originated from at least eleven countries (Federal Register: October 14, 2003 (Volume 68, Number 198)). In recent years, no emerald ash borers have been intercepted; their family (Buprestidae) is currently detected considerably less frequently in wood packaging than are the longhorned beetles (Cerambycidae) (Wu et al. 2017).
Eradication is no longer feasible for the emerald ash borer in North America. In January 2021, USDA APHIS terminated the domestic regulatory program it had implemented since 2003. At that time, 1,198 counties in 35 US states were released from the federal EAB regulation (EAB Manual 2020). The 2021 domestic deregulation does not affect the international movement of materials; federal regulations put in place in 2008 continue to restrict importation of ash wood and nursery stock from Canada.
Current programs by states and provinces focus on curtailing human facilitated spread of the insect by regulating the movement of infested materials. These state-based programs are complemented by federal research programs focused on long-term control measures, including biological control.
Within North America, the continued geographic spread of the now widely established EAB has been aided by movement of nursery stock and firewood. Firewood has been implicated as the source of dozens of infestations, especially those found in or near campgrounds and homes heated with wood; these include the initial infestations identified in Missouri, Indiana, Colorado and West Virginia. Shipments of infested nursery stock caused the Maryland infestation as well as several others in the upper Midwest.
The Future of North American Ash Trees
Ash trees are important members of deciduous forests, riparian and wetland vegetation across North America and are co-dominants (for example with maples [Acer] and beeches [Fagus]) in some ecological communities. There are seventeen ash species in North America north of Mexico (Kartesz, 1994), and it is possible the emerald ash borer will attack them all, although susceptibility apparently varies (Haack et al. 2004; McCullough, 2004). Ashes are particularly important components of rich, mesic woodlands, cove forests, swamps, floodplain and bottomland forests (Wagner, 2007)- all habitats harboring exceptional biodiversity. Wagner (2007) lists 21 species of North American butterflies and moths (lepidopterans) believed to be specialists or largely dependent on ash that he fears might be extirpated if the emerald ash borer kills all ash on the North American continent. He expresses concern for additional species that feed, in part, on other genera. Ash trees that dominate riparian forests on the Pacific slope as well as the southwestern deserts could also suffer high mortality rates. Wagner (2007) notes that little is known about the lepidopteran associates of these western ash.
Ongoing federal efforts to strengthen the resiliency of ash across North America now focus on biocontrol and breeding. Ash species that co-evolved with EAB in its native range (e.g., F. chinensis and F. mandshurica) are naturally more resilient to the pest and thus may provide a source of genes for resistance breeding (Herms and McCullough, 2014). A recent study by Kelly et al. (2020) sought to determine the genes involved in EAB resistance across the Fraxinus genus and searched for evidence of co-evolution by assessing resistance in 26 Fraxinus species. They found six resistant taxa (all native to Asia), several of which are close enough in relation to the three most vulnerable North American species (F. pennsylvanica, F. americana and F. nigra) to allow the validation of any dandidate genes identified (Kelly et al. 2020). Kelly et al. (2020) also searched for, and successfully identified EAB resistant variants in a large pool of genetically diverse F. excelsior; this means that researchers may be able to use marker assisted selection to accelerate breeding and match variants with different resistant genes, thus improving resistance in the progeny. Efforts are now underway to find resistant variants in a diverse population of the North American native, F. nigra (Dr. Jennifer Koch, personal communication September 2021).
In fact, attempts to amplify resistance in multiple species of native North American ash are well under way. While monitoring ash die offs, Knight et al. (2012) found a small percent of ash populations survived heavy EAB infestation. It was hypothesized that these remaining or “lingering” ash trees likely survive, at least in part, due to a natural genetic advantage resulting in a higher tolerance or resistance to EAB infestation (Koch et al. 2015). Through their efforts to preserve and study lingering ash, Koch and colleagues (2015) found that lingering green ash were less preferred by adult EAB feeding and harbored less larval survival than susceptible controls.
Recent research has confirmed the efficacy of breeding for higher resistance by using 42 selected lingering ash and grafted replicates of a lingering ash progeny; the crossing demonstrated the genetic heritability of EAB resistance in green ash, since lingering ash parents produced a higher frequency of resistant progeny – with some progeny expressing stronger resistance than the parents (Stanley et al., 2021). Further, partial resistance has been identified at higher frequencies in blue and black ash, which means researchers may forego waiting for large die-offs to reveal lingering ash in these species and, instead, can select candidates by using open-pollinated seedlings (Dr. Jennifer Koch, personal communication Sept. 2021). Cumulatively, research suggests that more than one mechanism is likely responsible for increased EAB resistance, and selective breeding to enhance resistance shows promise as a powerful conservation strategy for native ash (Koch et al. 2015; Romero-Severson and Koch, 2017; Stanley et al., 2021). The precedence for success is evident in similar breeding programs to improve resistance against vectors that cause pathoses including fusiform rust and blister rust diseases, Port-Orford-cedar root disease, and beech bark disease (Pike et al. 2021).
Ultimately, a combination of biological and cultural control techniques will provide the best defense against invasive pests that are established in North America, including EAB. Long term monitoring of host and pest populations along with diligent preventative measures to limit new infestations are crucial components of the integrated pest management framework needed for the EAB problem in North America and beyond (Knight et al. 2020).
For more information on this pest, please consult:
- USDA APHIS Plant Health Emerald Ash Borer Program Page
- Emerald Ash Borer Manual, USDA APHIS 2020
- Federal Deregulation of Emerald Ash Borer, blog 2020
- Natural Resources Canada Emerald Ash Borer Page
- Multi-state Emerald Ash Borer Insecticide Fact Sheet
- Present State, Use and Perspectives of Gensus Fraxinus in Mexico (2010)
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