U.S. DEPARTMENT OF AGRICULTURE

Questions and Answers:

Biological Control for 

Emerald Ash Borer

For several years, the U.S. Department of Agriculture’s 

Animal and Plant Health Inspection Service (APHIS) has 

used the emerald ash borer’s (EAB) natural enemies—

tiny stingless wasps known as parasitoids—as biological 

control agents to help manage infestations. The goal of 

APHIS’ EAB program is to help maintain ash trees as 

part of the North American landscape and biological 

control significantly contributes to this goal.

What is biological control?

Biological control, or biocontrol, uses natural enemies 

such as parasitoids, predators, pathogens, or 

antagonists to reduce plant pests or diseases. Biological 

control can be a practical and an environmentally sound 

method for pest control. The biological control agents 

used against EAB include four tiny stingless wasp 

species—the largest is about the size of a typical 

mosquito. 

Why is biological control used against EAB?

Soon after the EAB was first detected in the United 

States, APHIS, the Agricultural Research Service, and 

the U.S. Forest Service began looking for natural 

enemies to fight the infestation. We conducted research 

in EAB’s native range in China and found three 

biological control agents—Spathius agrili, Tetrastichus 

planipennisi, and Oobius agrili. Several years later we 

also found Spathius galinae in Russia. These four wasp 

species specifically hunt and kill EAB, which helps reduce 

the number of EAB infesting and killing ash trees. Three 

species attack EAB larvae and one targets EAB eggs.

Animal and Plant Health Inspection Service 

Plant Protection and Quarantine

• Spathius agrili has a long egg-laying organ or 

ovipositor that helps it attack EAB larvae in many 

different sized ash trees. This wasp penetrates 

through tree bark and lays up to 20 eggs on an EAB 

larva. The hatching wasp larvae feed and develop on 

the EAB larva, resulting in its death.

• Spathius galinae also attacks EAB larvae like Spathius 

agrili does, but what makes this wasp different is that it 

can tolerate colder climates, which makes it an optimal 

biological control agent for releasing in the northern 

states.

• Tetrastichus planipennisi also attacks EAB larvae, but 

the female lays eggs inside EAB larvae where the wasp 

larvae grow and eventually kill their host. Because of its 

shorter ovipositor, Tetrastichus targets larvae in ash 

trees with a diameter of five inches or less.

• Oobius agrili is the smallest of the wasps and targets 

EAB eggs. When this wasp locates an EAB egg in tree 

bark, it injects an egg inside the host egg, where it will 

hatch, grow, and kill the EAB egg.

Do the wasps bother people, pets, or other creatures?

The wasps are attracted to EAB, not people or pets. The 

wasps may incidentally attack other Agrilus species that 

are similar to EAB, but such incidental attacks are very 

rare and will not disrupt their populations.

SPATHIUS GALINAE SPATHIUS AGRILI

OOBIUS AGRILI TETRASTICHUS PLANIPENNISI

SPATHIUS AGRILI SPATHIUS GALINAE OOBIUS AGRILI
U.S. DEPARTMENT OF AGRICULTURE

When and where have wasps been released in the 

United States?

APHIS first released wasps in Michigan in 2007. Since 

then, we have worked with biological control partners to 

release more than eight million wasps in 30 states and 

the District of Columbia—and recovered their offspring in 

22 states. This means the wasps are establishing, 

reproducing, and more importantly, attacking and killing 

EAB. You can see where wasps have been released by 

visiting www.aphis.usda.gov/plant-health/eab.

How effective are the wasps at killing EAB?

The most recent study that was conducted in Michigan 

and several northeastern states showed that the wasps 

are killing 20-80% of EAB in ash trees up to eight inches 

in diameter. The study documented that more EAB are 

being attacked by the wasps which is resulting in less 

EAB attacking ash trees, and that ash trees are 

regenerating in these areas because wasps are being 

released. You can read the study online by visiting 

https://www.fs.fed.us/foresthealth/technology/pdfs/FHAA

ST-2017-02_Biocontrol_role_EAB_regeneration.pdf

Will biological control eventually eradicate EAB?

The wasps alone will not eradicate EAB. They can be 

used in an integrated pest management plan to help 

control the pest and benefit our landscapes.

How does APHIS determine where to release wasps?

Each year, APHIS estimates wasp production to 

determine the number of release sites it can support. 

Then release sites are selected based on biological 

control criteria and EAB program priorities outlined in the 

EAB Biocontrol Release Guidelines. You can read the 

guidelines online by visiting 

https://www.aphis.usda.gov/plant_health/plant_pest_info

/emerald_ash_b/downloads/EAB-FieldReleaseGuidelines.pdf

How are wasps released?

Once the release areas are identified, APHIS ships the 

wasps to our biological control partners who release 

them in the preapproved and selected area(s). Wasps 

are released according to the Biological Control Release 

and Recovery Guidelines, see link included above. 

APHIS begins shipping wasps in the spring and 

continues until early fall. 

Is it safe to release wasps since they are non-native 

insects?

Before the wasps were released, research in China and 

in the United States revealed that the wasps prefer EAB 

over other insects. No adverse effects were found or 

raised through the environmental assessment process. 

You can read the documents and public comments by 

visiting https://www.regulations.gov/docket?D=APHIS2014-0094.

Can native insects be used to fight EAB instead?

While there are EAB parasitoids and predators in the

United States, such as the native stingless wasps 

Atanycolus and Cerceris fumipennis, and woodpeckers, 

these native enemies do not attack EAB at levels high 

enough to make a difference on infestations.

How can I become a biological control partner?

If you are a public landowner or land manager in an area 

where EAB is infesting ash trees, you can send email

containing information about the property where you 

would like to release wasps. If the property is selected, 

we will contact you by email. Please send the email to 

EAB.Biocontrol.Program@USDA.gov and include the 

following information in the body of the email:

1. Property owner/manager name;

2. Property owner/manager email address;

3. MapBio number issued through 

www.mapbiocontrol.org;

4. Name of the county where the property is located;

5. And the latitude and longitude of the property.

Can the stingless wasps be purchased?

No. The EAB program is providing stingless wasps to 

states and selected partners to release in EAB-infested 

public areas. Additionally, the stingless wasps are 

difficult to produce which limits their availability.

Where can I find more information on the EAB 

program and the use of biological control?

More information is available online, please visit 

www.aphis.usda.gov/plant-health/eab.

Photo Credits: First column left to right: USDA photo by Rhonda Santos, USDA photo by R. 

Anson Eaglin, USDA photo by Rhonda Santos; Second column left to right: Bugwood photo by 

David Cappaert, Bugwood photo by Jennifer Ayer, USDA photo by Debbie Miller, Bugwood 

photo by David Cappaert.

Issued December 2020

USDA is an equal opportunity provider, 

employer, and lender.