THE ROLE OF BIOCONTROL OF 

EMERALD ASH BORER

 IN PROTECTING ASH REGENERATION 

 AFTER INVASION

Jian J. Duan1

Roy G. Van Driesche2

Leah S. Bauer3

Richard Reardon4

Juli Gould5 

Joseph S. Elkinton2

1

USDA, Agricultural Research Service, Newark, DE 2

Dept. Environmental Conservation, Univ. Massachusetts, Amherst, MA 3

USDA Forest Service, Lansing, MI 4

USDA Forest Service, Morgantown, WV 5

USDA, Animal and Plant Health Inspection Service, Buzzards Bay, MA

FHAAST-2017-02

November 2017

Forest Health Assessment and 

Applied Sciences Team
This publication is available online at: 

https://www.fs.fed.us/foresthealth/

technology/pdfs/FHAAST-2017-02_

Biocontrol_role_EAB_regeneration.pdf

The Forest Health Technology Enterprise Team (FHTET) 

was created in 1995 by the Deputy Chief for State and 

Private Forestry, USDA Forest Service, to develop and 

deliver technologies to protect and improve the health of 

American forests. 

FHTET became Forest Health Assessment and Applied 

Sciences Team (FHAAST) in 2016. This booklet was 

published by FHAAST as part of the technology transfer

series.

http://www.fs.fed.us/foresthealth/technology/
KEY FINDINGS

The goal of the EAB biocontrol program

is to protect ash regeneration

and restore canopy cover over time

Long-term monitoring in Michigan and several 

northeastern states has documented increasing 

parasitism and reduced EAB attack rates.

Ash regeneration is currently benefiting from 

releases of introduced parasitoids, which now 

cause 20-80% parasitism of EAB larvae in ash 

saplings (1-2 inch dia.) and young trees (5-8 

inch dia.).

Spathius galinae, the most recently released 

EAB biocontrol agent, appears to be establishing 

in the northeast United States and Michigan, 

based on 2017 recoveries. Only 5% of ash 

are too big for S. galinae to attack EAB larvae 

through the bark of the main trunk. We believe 

that S. galinae, where released and established, 

will begin protecting most large ash trees over 

the next five years and may provide continuity of 

protection for young ash trees as they mature, 

allowing ash regeneration in EAB-damaged 

forests.

Emerald ash borer, Agrilus planipennis (Photo: Pennsylvania 

Department of Conservation and Natural Resources – Forestry Archive)
2

BACKGROUND 

INFORMATION

Emerald ash borer (EAB) is an invasive 

Asian beetle that is destroying ash in forests over 

much of eastern North America because of the 

high susceptibility of our native ash and a lack 

of effective natural enemies (Fig. 1). To increase 

mortality of EAB larvae and eggs, the USDA (FS, 

ARS, APHIS) is carrying out a biological control 

program based on importation of parasitoids from 

the beetle’s native range in northeastern China 

and the Russian Far East. Four such parasitoids 

have been approved for release since 2007 (Figs. 

2-5), and these species are being reared at the 

USDA APHIS EAB Biocontrol Facility in Brighton, 

Michigan, for release in the United States. Three 

of these natural enemies are now established 

at various locations and are helping lower EAB 

densities. 

Detailed information on EAB, biocontrol, 

release site selection, and how to request 

parasitoids is in the “EAB Biocontrol Release 

and Recovery Guidelines,” available at 

www.MapBiocontrol.org.

Fig. 1. Typical damage to native ash trees caused by an infestation 

of the invasive Asian beetle, Emerald Ash Borer (Photo: 

USDA FS)
3

Fig. 2. Egg parasitoid Oobius agrili (Photo: USDA ARS)

Fig. 3. Larval parasitoid Tetrastichus planipennisi (Photo: USDA ARS)

Fig. 4. Larval parasitoid Spathius agrili (Photo: USDA ARS)

Fig. 5. Larval parasitoid Spathius galinae (Photo: 

USDA ARS)
4

How Will Asian Parasitoids 

Help Protect Ash?

We know that in Asia, 

parasitoids cause high 

mortality of EAB, 

while in North 

America our 

native parasitoids 

only sometimes 

do so. While 

North American 

ash are less 

resistant than 

Asian ash, 

American ash 

can tolerate some 

level of EAB attack. 

Conversely, woodpeckers, 

which are scarce in northeastern

Asia, are common in North America and kill 

about 30-50% of surviving large EAB larvae. 

The goal of the EAB biocontrol program is 

to lower survival of EAB eggs and larvae by 

establishing more effective Asian parasitoids. 

The combination of these introduced parasitoids, 

together with moderate levels of tree resistance, 

high woodpecker predation, and some attack by 

native parasitoids, will reduce EAB densities and 

lower the number of attacks per tree to levels 

that ash trees can survive. This lower attack 

rate is already happening at Michigan study 

sites and is allowing ash saplings (1-2 inch dia.) 

and young ash (5-8 inch dia.) to survive and 

produce seed for ash regeneration. We have 

made considerable progress over the last ten 

years documenting the impacts of these factors, 

as described below. Moreover, in the aftermath 

of EAB in southeast Michigan forests where T. 

planipennisi is protecting young ash, recruitment 

and growth of native hardwoods are favored over 

invasive species.
5

What Gains Has the EAB

Biocontrol Program Made?

Release of four EAB parasitoids from Asia. 

Oobius agrili, egg parasitoid from China, 

released since 2007 

Tetrastichus planipennisi, internal larval 

parasitoid from China, since 2007 

Spathius agrili, external larval parasitoid from 

China, since 2007

Spathius galinae, external larval parasitoid 

from Russia, since 2015

Establishment of two parasitoid species 

confirmed, third likely. Oobius agrili and T. 

planipennisi are established in various areas 

and are being widely released. Spathius galinae 

is increasingly encountered during field surveys, 

but it is too early to confirm establishment.

Oobius agrili causes up to 22% parasitism of 

EAB eggs at monitored Michigan study sites 

and is spreading slowly.

Tetrastichus planipennisi is now widely 

established in the infested parts of North 

America and is spreading rapidly, causing ~20% 

parasitism in young ash trees (Fig. 6) and 50-

80% parasitism in ash saplings (Fig. 7).

90% Reduction in EAB attacks on surviving 

trees. In Michigan, EAB attack on surviving ash 

(larvae per m2

 of phloem area) is now <10% of 

the peak in 2009 (Fig. 8).

Avoidance of plant invasions. In southern 

Michigan, where forests are recovering from the 

loss of ash overstory trees, native hardwoods 

are favored over invasive plants where ash 

seedlings, saplings, and young trees are 

protected by T. planipennisi.
6Parasitism Rate (± SE)%

Fig. 6. (A). EAB larval parasitism by the introduced parasitoid 

Tetrastichus planipennisi in young ash trees (5-8 inch dia.) at release 

and non-release control plots, after parasitoid releases in monitored 

Michigan study sites starting in 2007 (arrows show number of adult 

parasitoids released by year) (Duan et al. 2015. J. Appl. Ecol. 52:1246-

1254); (B). T. planipennisi larvae developed inside the parasitized EAB 

larva (Inset photo: USDA ARS) Parasitism Rate (± SE)%

Fig. 7. EAB larval parasitism by Tetrastichus planipennisi in ash 

saplings (1-2 inch dia.) from 2013-2015 at the monitored Michigan 

study sites (Duan et al. 2017. For. Ecol. Manag. 394:64-72) EAB larvae (mean ± SE)/m2 phloem

Fig. 8. EAB larval densities declined in the surviving ash trees at 

Michigan study sites after death of the larger ash trees (2008-2009). 

The parasitoids, released from 2007-2010, became established in the 

surviving ash and EAB parasitism increased from 2012-2014 (see 

Fig. 6) (Duan et al. 2015. J. Appl. Ecol. 52:1246-1254)
7

What is Likely to Happen 

in the Next Few Years?

Based on (1) our work during the past 10 years in 

Michigan and 3-6 years in the northeastern United 

States, (2) the likely imminent spread of EAB to the 

South and West, and (3) the species of ash in those 

not-yet-invaded areas, we expect

That the newest larval parasitoid, S. galinae, 

recovered in 2017 in 11 of our 12 lifetable study 

plots in Michigan, Connecticut, Massachusetts, 

and New York, will establish and spread quickly 

in the northeastern and north central United 

States.

That small ash (1 to 8 inch dia.), now and in the 

near future, will be protected by O. agrili and 

T. planipennisi, allowing ash regeneration in the 

northeastern and central United States. 

That within 5-10 years, as current young ash 

reach mid-size (9-15 inch dia.), S. galinae may 

be abundant and able to protect these trees 

as they enter the mid-size cohort, allowing ash 

regeneration (Fig. 9) and regrowth to continue.

Fig. 9. Ash seedlings grow quickly after EAB kills the overstory ash trees, 

thereby favoring the regeneration and recruitment of native hardwoods over 

invasive species. (Photo: USDA FS, also used on booklet 

front cover)
8

What Do We Need for the 

Not-Yet-Invaded Regions 

in the South and West?

New regions may need different parasitoids. 

States just now being invaded in the South 

(Louisiana and nearby states) or likely to 

be invaded soon in the West (California 

through Oregon) and Southwest (Arizona and 

nearby states) may need DIFFERENT ASIAN 

PARASITOIDS (due to climatic differences 

between these states and northeastern China 

or the Russian Far East, the source of the 

parasitoids we are now releasing). Efforts to 

find and collect EAB parasitoids in southern and 

western China are needed now. 

South. In 2017, work began in southern China 

(at about the latitude of mid-Florida) in a newly 

located natural ash stand in an area potentially 

matching Louisiana.

West. In 2017, a new EAB population was found 

in western China in an area with a climate similar 

to that of the Interior Great Basin (Utah and 

surrounding states). This EAB population needs 

to be sampled for parasitoids.

Southwest. Several rare species of ash are 

native to Arizona and surrounding southwestern 

states. Conservation and restoration plans 

should be developed before EAB invades the 

region as these native ash species have small 

distributions and are expected to suffer high 

mortality from EAB.
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Booklet back cover image: Young ash trees and saplings shown 

here are protected by introduced EAB parasitoids, especially by the 

larval parasitoid Tetrastichus planipennisi, which attack about 20% 

of large EAB larvae in young trees (5-8 inch dia.) and 50-85% in 

saplings (1-2 inch dia.) (Photo: USDA FS)
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