The emerald ash borer (Agrilus planipennis) is an invasive beetle that has killed hundreds of millions of ash trees in North America since its discovery in 2002. Many regions of the United States are still in the early stages of EAB infestation. An infestation can expand by roughly 15 to 20 miles per year, with untreated populations growing and small infestations combining to make ever-larger beetle populations.
Control efforts
Shortly after the discovery of EAB in Michigan in 2002, federal and state foresters attempted eradication through mechanical control. This involved locating infested trees, cutting them down and chipping them. States responded by implementing regulatory controls to mitigate the spread of EAB, such as a moratorium on moving logs, nursery stock and lumber. They also halted the movement of firewood to and from campgrounds. Unfortunately, these efforts did little to stop the spread of EAB, and it quickly moved across half of the country (Herms et al., 2014). Following this, federal and state EAB control-program objectives shifted away from eradication to containment. Fortunately, researchers at the U.S. Forest Service were already looking into chemical and biological pest-management options.
Chemical control, primarily through trunk-injected insecticides, can be an effective treatment for individual trees, but this method is typically cost prohibitive, labor intensive and not an option for treating an entire forest. However, emamectin benzoate, the systemic insecticide, was found to provide effective control of EAB (Poland et al., 2016). Other insecticides, such as imidacloprid and azadirachtin, also were reported to control EAB.
Genetics are another area researchers are exploring to save ash trees. Scientists have identified candidate genes in the ash tree that may confer resistance to EAB (Kelly et al., 2020). This may eventually lead to breeding new ash trees with this genetic trait to increase resistance.
Biological control
Biological control (also referred to as biocontrol) is defined as the reduction of pest populations by natural enemies, and is a component of an integrated pest management (IPM) strategy. IPM strategies typically involve an active human role. In the case of EAB, biocontrol is the practice of importing and releasing natural enemies from EAB’s native range (in Asia) to control the invasive populations of EAB in areas of infestation. Biocontrol has been used successfully for more than a century to control invasive plant and insect pests such as gypsy moths and borers.
How did the USDA search for a classical biocontrol for EAB? Scientists sought natural, parasitoid-insect enemies of EAB in China, Russia and Korea, and then considered factors such as sustainability, risks and disadvantages for each potential biocontrol agent, as well as climate appropriateness (Liu et al., 2003; Bauer et al., 2008; Crosthwaite et al., 2011; DeSantis et al., 2013).
Many questions need to be answered before an ideal biocontrol program is initiated. Are there possible non-target effects associated with enemy insects such as parasitoid wasps? Could this negatively impact humans? Is it feasible to rear the insects? What are the timing and up-front costs? With the understanding that the biocontrol agent will likely not eliminate the pest, will there still be a high probability of successful establishment of the agent and reduction of the pest?
Four candidates
By 2007, USDA scientists identified several parasitoid wasps and selected four – Tetrastichus planipennisi, Oobius agrili, Spathius galinae and Spathius agrili – for release in the U.S. (Liu et al., 2007). Below, we explore key findings for each of these insects.
T. planipennisi (family Eulophidae) is a tiny parasitic wasp native to northern Asia. It is a larval endoparasitoid whose female deposits its eggs inside EAB (host) larvae, where the wasp larvae grow and eventually kill the host. It is a gregarious parasitoid in that it will lay multiple eggs in one individual host. One EAB larva can sustain the development of more than 100 T. planipennisi larvae. T. planipennisi is more successful in parasitizing EAB larvae in young ash trees less than 6 inches in diameter due to its short ovipositor (Duan et al., 2018). To be successful, the wasps must have EAB larvae to parasitize when they emerge in the spring. As part of the biocontrol program, they are preferentially released in northern areas of the U.S., where EAB are likely to have a two-year lifecycle. T. planipennisi are relatively easy to rear in the laboratory compared with the other parasitoids, with about one million reared for release annually (Wu et al., 2007).
Spathius agrili (family Braconidae) is another parasitic wasp that was found parasitizing EAB larvae in ash trees east of Beijing, China (Yang et al., 2005). The climate in Beijing is warm and mild, thus making the wasp a good candidate for release in the southern U.S. This wasp is an ectoparasite, meaning it lays eggs on the outside of EAB larvae. Once mature, they spin cocoons inside the EAB gallery. While also gregarious like Tetrastichus, S. agrili produce only eight to 10 cocoons per larva, but have several generations each year.
S. galinae has a similar biology to that of S. agrili but is found in far-eastern Russia. The climate there is similar to the northern regions of North America, so S. galinae is more likely to establish itself in northern states where EAB larvae are available for the newly emerged adult wasps to parasitize in early spring. S. galinae has a longer ovipositor than Tetrastichus, which allows it to parasitize EAB larvae in large-diameter ash trees. S. galinae is also a gregarious ectoparasitoid of EAB larvae, with all life stages living on the outside of the host. S. galinae requires a period of chill (such as winter) to break diapause to emerge as an adult (Belokobylskij et al., 2012).
Oobius agrili (family Encyrtidae) is an egg-parasitoid wasp native to northern Asia. Oobius spend the winter as larvae inside EAB eggs and emerge as adults the following spring. Each Oobius adult can parasitize up to 80 EAB eggs during its lifetime. In 2007, permits were issued for its release for EAB biocontrol in 29 states and three Canadian provinces (USDA–APHIS, 2020).
Once wasps are released, how do we know parasitism is working? EAB density peaked in 2009, but by 2014 it had experienced a five-fold decline following the release of parasitic wasps. In 2007, a marked increase was observed in both the parasitoid T. planipennisi and woodpeckers, as reported by Jennings et al. (2013). Since then, researchers have found T. planipennisi to be the dominant EAB parasitoid (Duan et al., 2017; USDA-APHIS, 2020). T. planipennisi and O. agrili were observed dispersing from their release sites and were presumably successful at parasitizing EAB larvae. The biological-control impacts of O. agrili will take longer to evaluate than the other EAB parasitoids, due to its slower spread and lower rate of reproduction (USDA-APHIS, 2020).
In the U.S., S. agrili has been released and subsequently detected in six southern states, but has failed to establish itself in northern states. This demonstrates the importance of climate matching in evaluating parasitoids, as this species was not able to reproduce in colder climates (Duan et al., 2018).
S. galinae, which originated in Russia,was first released in the United States in 2015. As a result of climate matching, it was successfully released in Connecticut, Massachusetts, Michigan, New York, Colorado, Illinois, Maryland and Delaware. A second Oobius species that hails from northern Asia is being tested for possible release in the Canadian provinces.
USDA studies in Colorado, New York and Illinois saw T. planipennisi spread over two kilometers by 2017 (Duan et al., 2018; USDA-APHIS, 2020). By 2018, EAB density in the study area was significantly reduced, allowing insecticide treatment to be discontinued. Due to the success of the program, the study is being extended to natural forests
(USDA-APHIS, 2020).
The parasitoid release program, which is intended to protect the next generation of ash trees, has been quite successful to date (Bauer et al., 2015; USDA-APHIS, 2020). Where researchers have continued to monitor the health of young ash trees, 64% of saplings had no EAB, 17% showed signs of infestation (old galleries) and recovered and only 18% had EAB living in them. Of significance, woodpecker predation has been found to be a significant cause of EAB mortality, sometimes reducing populations by up to 40%.It appears that T. planipennisi, along with woodpeckers, have successfully killed most EAB in young trees within the biocontrol-treatment areas (Flower et al., 2014; Jennings et al., 2013).
Coming to an ash near you
Are you considering recommending a site near you for parasitoid release and study? As arborists and land managers, you can report EAB you encounter and suggest a site for release and study by emailing USDA at EAB.Biocontrol.Program@usda.gov. Managers considering EAB biocontrol need to look at growing-degree days to insure a good parasitoid match for their area. Wooded areas comprised of at least 25% ash trees and at least 40 acres in size are preferred for parasitoid-release sites (USDA-APHIS, 2020). Smaller release sites (less than 40 acres) require higher ash-tree densities and ash-tree corridors connecting the sites to other wooded areas. Examples of ash corridors are rivers, ditches, highways and fence rows lined with a concentration of ash trees. Use of these criteria will facilitate parasitoid reproduction, establishment and dispersal.
Although older and highly stressed ash are less likely to benefit from EAB biological control, they can provide a high density of EAB eggs and larvae to serve as a food source for the wasps and increase the probability of parasitoid reproduction at the site. Smaller trees, saplings and seedlings provide potential for regeneration of ash trees and will support EAB and their natural enemies following the loss of larger trees in the stand.
The parasitoid agents you identify, produce and deploy are an important part of IPM, and a smart and sustainable way to control pests in our forest and community ecosystems. It is encouraging to observe the success of the USDA biocontrol program and consider how IPM practices can help address future invasive-pest issues.
Resources
U.S. Department of Agriculture emerald ash borer beetle website:
https://www.aphis.usda.gov/aphis/resources/pests-diseases/hungry-pests/the-threat/emerald-ash-borer/emerald-ash-borer-beetle
Emerald Ash Borer Information Network:
http://www.emeraldashborer.info/
Emerald Ash Borer: A guide to identification and comparison to similar species: http://www.emeraldashborer.info/documents/eab_id_guide.pdf
U.S. Forest Service biological control of the emerald ash borer: https://www.nrs.fs.fed.us/disturbance/invasive_species/eab/control_management/biological_control/
University of Massachusetts progress toward controlling the emerald ash borer with biological control (video): https://ag.umass.edu/landscape/videos/progress-towards-controlling-emerald-ash-borer-with-biological-control
EPA’s Integrated Pest Management website:
https://www.epa.gov/ipm
Marcia L. Anderson, Ph.D., is a New Jersey licensed tree expert and an environmental protection specialist with the U.S. Environmental Protection Agency – Center for IPM, based in Arlington, Virginia.
Join the EPA June 8, 2021, (2-3:30 p.m. EST) for a free webinar that will provide information on the identification, IPM-based treatment and biological controls for the emerald ash borer. Thirty states and the ISA have pre-approved CEUs for webinar attendees. Registration link: https://register.gotowebinar.com/register/6544725730006522383.
References
Marcia L. Anderson, Ph.D., is a New Jersey licensed tree expert and an environmental protection specialist with the U.S. Environmental Protection Agency – Center for IPM, based in Arlington, Virginia.
Join the EPA June 8, 2021, (2-3:30 p.m. EST) for a free webinar that will provide information on the identification, IPM-based treatment and biological controls for the emerald ash borer. Thirty states and the ISA have pre-approved CEUs for webinar attendees. Registration link: https://register.gotowebinar.com/register/6544725730006522383.
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