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Home > Plants, Pests & Pest Control > Pest Management > Emerald Ash Borer Program > EAB Biocontrol > Releases

Releases of EAB biological control agents in Minnesota


The three species of parasitoid wasps released in Minnesota (Tetrastichus planipennisi, Spathius galinae and Oobius agrili) are reared in a specialized USDA facility in Michigan. They are provided at no charge to state cooperators such as the Minnesota Department of Agriculture to release. A fourth species, Spathius agrili, was released in Minnesota from 2010 – 2012. Releases of S. agrili were discontinued for northern regions because this species was unlikely to establish.

We initiated releases in 2010 in southeastern Minnesota with adult parasitoids. To increase the efficiency of parasitoid rearing we began receiving and releasing immature stages of the parasitoids in 2013. They are in protective structures that are placed in the field then the adult wasps emerge within the next few weeks. There are now over 30 release sites in the Twin Cities and southeastern Minnesota. Releases at new and select existing sites will continue. View locations of EAB finds and biocontrol agent releases.


After a viable biological control site is identified, coordination by the MDA with local natural resource managers, property owners and the USDA EAB Parasitoid Rearing Facility are necessary. At each site one has to obtain permission, guarantee access and ensure other management objectives won’t interfere with implementation. Special permits may be necessary depending on the ownership and designation of land. Long term site access is important for follow-up monitoring of ash health and documenting parasitoid establishment.

Site Suitability:

Not all sites fit the criteria for biological control. Once an EAB infestation is identified, several steps need to be completed to determine if biological control is viable:

  1. Perform a delimit survey of the infestation to identify the perimeter of where symptoms are visible.
  2. Gauge the intensity or pest pressure in the area based on severity of EAB symptoms throughout the identified visibly infested area. Low to moderate EAB densities are recommended for potential sites.
  3. Identify forested areas on public or private land within the visibly infested area where removal and/or treatment of infested ash trees will not be feasible. Size and composition of forest should be at least 40 acres and at a minimum include 20% ash of varying size class. Ideally, the site would be greater than 25% ash and connected to other woodlots.
Biocontrol agent 2010 2011 2012 2013 2014 2015 2016 2017
Tetrastichus planipennisi 2,154  19,480  19,822  45,579  34,434 151,022  45,288 43,245  358,024
Oobius agrili  0  3,641  10,241  8,597  12,062 31,490  42,600 23,100  131,731
Spathius galinae 0 0 0 0 0 0 1,613 2,151  3,764
Spathius agrili 1,172  7,596  15,258  0  0 0  24,026
 Totals  3,326  30,717  45,321  51,176  46,496 182,512  89,501 68,496  517,545
Yellow starthistle seedlings
Adult Tetrastichus planipennisi ready for release
Yellow starthistle plants
Tetrastichus planipennisi emerge from the small log attached to the tree when they are fully developed
Yellow starthistle seedhead
Releasing Tetrastichus planipennisi at a biocontrol site in Winona, MN
Yellow starthistle rosette
Oobius agrili will complete their development then emerge from containers attached to ash trees
Yellow starthistle flower and seedheads
Underside of the Oobius agrili release container that is attached to ash trees


MDA Contact

Chris Mallet, EAB Biological Control Coordinator

Jonathan Osthus, Project Manager

Plant Protection Division