Biological Control of the Emerald Ash Borer
In 2002, the emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), an Asian beetle that feeds on ash trees (Fraxinus spp.), was discovered as the cause of widespread ash tree mortality in southeast Michigan and nearby Ontario. The results of subsequent studies showed that EAB was inadvertently introduced near Detroit, Michigan during the 1990s from northeast China, probably in EAB-infested solid-wood packing materials used in international trade. Despite federal and state quarantines that restricts the movement of ash out of infested areas, EAB continues spreading in the U.S. and eastern Canada. Although this beetle spreads naturally by flying short distances, long-distance spread is caused by people moving EAB-infested ash firewood, nursery stock, and timber. The rapid expansion of the EAB infestation across a wide range of climate zones suggests that this invasive beetle will continue spreading throughout the continent. (See USDA APHIS Cooperative Emerald Ash Borer Project Map of Initial county EAB detections in North America.)
The spread of EAB in North America puts our 16 native ash species and the organisms dependent on these trees at risk. Ash species in our forests already devastated by the EAB invasion are: white ash (F. americana), green ash (F. pennsylvanica), black ash (F. nigra), blue ash (F. quadrangulata), pumpkin ash (F. profunda), and Carolina ash (F. caroliniana). Each species is adapted to different sites in our forests,with many adapted to sensitive wetland and riparian habitats. Before the arrival of EAB, ash trees were also important economically with 8 billion ash trees on U.S. timberlands valued at $282.25 billion. Sustainable methods to manage EAB in forested ecosystems generally involve the use of classical biological control or biocontrol.
Biocontrol is a long-term management strategy used throughout the world for sustained control of invasive pests. This approach is generally limited to such pests that have been established for more than five years, cannot be eradicated, and cause significant ecological, environmental, or economical damage. Biocontrol involves studying the biology of the pest, determining where it is native, and exploring those countries for natural enemies. These natural enemies are typically insect parasitoids or predators that coevolved with the target pest and specifically attack it. In the U.S., permits for release of highly host-specific natural enemies or “biocontrol agents” may be granted by USDA APHIS PPQ (APHIS) after completion of extensive research on 1) the biology and host range of each natural enemy in U.S. quarantine laboratories and in countries of origin; 2) risk benefit analyses including potential nontarget impacts; 3) consensus by North American experts; 4) posting of an Environmental Assessment on the Federal Register for public comment; and 5) state concurrence.
EAB Research and the EAB Biocontrol Program
Biocontrol of EAB began in the U.S. in 2007 when APHIS issued permits for the environmental release of three hymenopteran parasitoid species of EAB from China to EAB-infested ash stands in southern Michigan. The EAB biocontrol agents are: an egg parasitoid, Oobius agrili (Encyrtidae) and two larval parasitoids, Tetrastichus planipennisi (Eulophidae) and Spathius agrili (Braconidae). A third EAB larval parasitoid, Spathius galinae from Russia, was approved for release in 2015. USDA initiated the EAB Biocontrol Program in 2009 following field recovery of the parasitoids at least one year after their initial release. This program includes 1) the APHIS EAB Biocontrol-Rearing Facility where parasitoids are mass-reared, stored, packaged, and shipped for release at approved sites; 2) federal state, local, university researchers, land managers, and citizens in North America studying and implementing EAB management with biocontrol; 3) the "EAB Biocontrol Release and Recovery Guidelines" that provides the “how to”; and 4) www.mapbiocontrol.org, the online database for the release and recovery of biocontrol agents. In collaboration with research partners, ongoing studies include ongoing foreign exploration for new natrual enemies; evaluations of EAB population dynamics in response to parasitoid release; parasitoid taxonomy, genetics, biology, establishment, prevalence, and interactions; impacts of EAB biocontrol on the long-term survival, growth, and regeneration of ash.
Due to the large number of ash species in North America and the long life cycle of trees, it will be many years before we know if biocontrol can protect ash species against EAB. But there are some encouraging research results from study sites in Michigan where parasitoid releases began in 2007. These results show the establishment and spread of T. planipennisi and O. agrili attacking EAB in surviving green, white, and black ash saplings and trees. Moreover, reductions in EAB densities following parasitoid release were correlated with increased parasitism, first by two native larval parasitoids when EAB densities were high, then by the introduced parasitoid T. planipennisi when EAB densities are low. The combined mortality of EAB caused by woodpeckers, native and introduced parasitoids, intraspecific competition, disease, innate tree defenses, and reduced ash abundance contributed to the collapse of EAB populations. Tree inventories at Michigan study sites find low numbers of large ash trees survived, while numbers of ash sprouts, saplings, and small to medium trees increased. Growth, reproduction, and survival of large- diameter ash, however, will require EAB parasitoids capable of parasitizing EAB larvae feeding under thick bark. Although there are two such parasitoid species approved for release in North America, S. agrili and S. galinae, their sustained establishment in North America has yet to be confirmed.
As EAB spreads throughout North America, researchers remain optimistic that native and introduced natural enemies will help suppress EAB densities below a damage threshold for the long-term survival and reproduction of EAB-tolerant ash genotypes in North America.
Bauer, Leah S.; Duan, Jian J.; Gould, Juli R.; Van Driesche, Roy. 2015. Progress in the classical biological control of Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) in North America. The Canadian Entomologist. 147(3): 300-317.
Duan, Jian J.; Bauer, Leah S.; Abell, Kristopher J.; Ulyshen, Michael D.; Van Driesche, Roy G. 2015. Population dynamics of an invasive forest insect and associated natural enemies in the aftermath of invasion: implications for biological control. Journal of Applied Ecology. 52(5): 1246-1254.
Abell, Kristopher J.; Bauer, Leah S.; Duan, Jian J.; Van Driesche, Roy. 2014. Long-term monitoring of the introduced emerald ash borer (Coleoptera: Buprestidae) egg parasitoid, Oobius agrili(Hymenoptera: Encyrtidae), in Michigan, USA and evaluation of a newly developed monitoring technique. Biological Control. 79: 36-42.
Duan, Jian J.; Bauer, Leah S.; Abell, Kristopher J.; Lelito, Jonathan P.; Van Driesche, Roy. 2013. Establishment and abundance of Tetrastichus planipennisi (Hymenoptera: Eulophidae) in Michigan: potential for success in classical biocontrol of the invasive emerald ash borer (Coleoptera: Buprestidae). Journal of Economic Entomology. 106: 1145-1154.
Liu HP, LS Bauer, DL Miller, TH Zhao, RT Gao, LW Song, QS Luan, RZ Jin, CQ Gao. 2007. Seasonal abundance of Agrilus planipennis (Coleoptera: Buprestidae) and its natural enemies Oobius agrili (Hymenoptera: Encyrtidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae) in China. Biological Control 42: 61-71.
Zhang YZ, DW Huang, TH Zhao, HP Liu, LS Bauer. 2005. Two new species of egg parasitoids (Hymenoptera: Encyrtidae) of wood-boring beetle pests from China. Phytoparasitica 53: 253-260.
Liu HP, LS Bauer, RT Gao, TH Zhao, TR Petrice, RA Haack. 2003. Exploratory survey for the emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), and its natural enemies in China. Great Lakes Entomologist 36: 191-204.
EAB Biocontrol Methods
Bauer, Leah; Hansen, Jason; Gould, Juli. 2016. Yellow Pan Traps: A Simple Method for Trapping Larval Parasitoids Released for Biological Control of the Emerald Ash Borer. 3 p. (pdf - You may obtain a free PDF reader from Adobe.)
USDA–APHIS/ARS/FS. 2016. Emerald Ash Borer Biological Control Release and Recovery Guidelines. USDA–APHIS–ARS-FS, Riverdale, Maryland. 57 p.
- Leah S. Bauer, US Forest Service, Northern Research Station Research Entomologist
- Jian Duan, USDA ARS BIIRU
- Juli Gould, USDA APHIS PPQ S&T
- Roy Van Driesche, University of Massachusetts
- Dan Kashian, Wayne State University
- Xioa-Yi Wang, Zhong-Qi Yang, Chinese Academy of Forestry, Beijing
- Song Yang, Southwest Forestry University, Kunming
- Mike Gates, Jason Mottern, USDA ARS SEL
- Therese Poland, Deborah Miller, Toby Petrice, USDA Forest Servise Northern Research Station
- Bill Ravlin, Michigan State University
Last Modified: 02/21/2017