The three species of parasitoid wasps released in Minnesota (Tetrastichus planipennisiSpathius 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 40 release sites in EAB infested areas of Minnesota. Releases at new and select existing sites will continue. View locations of EAB finds and biocontrol agent releases.

Biological Control Releases in Minnesota

Biocontrol agent 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 All
Tetrastichus planipennisi 2,154 19,480 19,822 45,579 34,434 151,022 45,288 43,245 27,310 34,607 4,062 3,030 9,039 435,914
Oobius agrili 0 3,641 10,241 8,597 12,062 31,490 42,600 23,100 12,600 8,400 8,320 2,200 5,400 168,351
Spathius galinae 0 0 0 0 0 0 1,613 2,151 4,636 4,609 4,438 2,809 5,894 21,283
Spathius agrili 1,172 7,596 15,258 0 0 0 0 0 0 0 0 0 0 27,177
Totals 3,326 30,717 45,321 51,176 46,496 182,512 89,501 68,496 44,546 47,616 16,820 8,039 20,333 655,468


How Biological Control is Implemented

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 percent ash of varying size class. Ideally, the site would be greater than 25 percent ash and connected to other woodlots.

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.