Natural control of EAB
In effort to protect Iowa's ash trees, the Iowa Department of Agriculture and Land Stewardship is releasing stingless, parasitic wasps as a means of biological control. These tiny wasps are natural enemies of EAB and are being used to help suppress rising EAB populations by reuniting them with their exotic host. Without a natural enemy, EAB has thrived and the consequences are quite evident. The goal of biological control is not to completely eradicate the pest, but rather begin transitioning the EAB population to a tolerable level where future generations of ash trees can be protected.
This long-term management strategy is part of a cooperative program that is being utilized in many states where EAB has already been detected. The parasitoids are produced and supplied by the USDA's Animal and Health Inspection Service (APHIS) EAB rearing facility in Brighton, Michigan. All of the wasp species, which are native to Asia, have undergone substantial screening to evaluate the risks and benefits they pose before being introduced into the U.S. These specialized wasps pose no threat to humans or pets.
Researchers in EAB's native homeland and the U.S. previously teamed together and have identified four different types of parasitic wasps which not only appear to be significant natural enemies of EAB in Asia, but are also host-specific. Although four types of parasitic wasps have been approved as part of a national biocontrol effort, only three of the four are being used in Iowa and other areas that experience colder winters.
The three species of parasitic wasps being used, attack EAB in their own unique ways. These biocontrol agents target either the egg or larval stage of EAB. Each of the wasp species are released at the appropriate time so that they coincide with the EAB life cycle they target.
Tetrastichus planipennisi target and lay eggs in EAB larvae found just beneath the bark of ash trees. As the immature stages of the wasps continue to develop within its host, the EAB larva eventually dies. Tetrastichus planipennisi complete several generations per year and overwinter as larvae within its host or host gallery. It has the ability to produce up to 130 adults from a single EAB larva. Parasitism rates up to 50% were found during initial assessments.
Oobius agrili are very small wasps that search the bark of ash trees for EAB eggs. Once found, they deposit a single egg inside an EAB egg. As the wasp hatches and grows within its host, the EAB egg is ultimately parasitized. Oobius agrili completes at least two generations per year and overwinter as larvae in EAB eggs. A female wasp can lay approximately 60 eggs and can reproduce without males (parthenogenesis). Parasitism rates up to 60% were found during initial assessments.
Spathius galinae also target EAB larvae, depositing eggs on the outside of their host by drilling through the bark where it is believed they detect vibrations from the EAB larvae feeding below. Offspring of the wasp will eventually consume its host. This species of wasp has the advantage of parasitizing its host in larger, thicker barked trees because of its long egg-laying organ (ovipositor). Spathius galinae complete approximately two generations per year and overwinter as cocoons. A female wasp deposits 5 to 15 eggs on its host. Parasitism rates up to 63% were found during initial assessments.
The EAB biocontrol effort began in Iowa in 2016 involving two locations - Whitham Woods (Jefferson County) and Mount Hosmer (Allamakee County). Since then three more locations were added including Millrace Flats Wildlife Management Area (Louisa County), Scott County Park (Scott County), and Lake Iowa Park (Iowa County). The biocontrol program is expected to grow and additional sites are being considered. Release of parasitoids is only permitted for public lands involving sites which have been approved by the USDA.
Parasitoids are introduced to a site either as an immature stage developing in its EAB host or as an adult wasp.
Immature stages of the wasps are shipped by way of ash bolts or oobinators. Ash bolts are small diameter sections of ash branches containing parasitized EAB larvae where the parasitoid Tetrastichus planipennisi emerges. Oobinators are pill vials containing coffee filters where Oobius agrili emerge through a mesh screen from the parasitized EAB eggs. Both ash bolts and oobinators are hung in ash trees. Adults arrive in enclosed plastic cups.
Selected release site will be monitored in attempt to recover parasitoids during 2019. The recovery phase of the program utilizes optional methods to recover the parasitoids of EAB to see if they are establishing.