Hope for North American Ash Trees?
Top: Female Tetrastichus wasps have a long ovipositor to drill into trees and lay eggs on emerald ash borer larvae.
Bottom: Tetrastichus larvae under tree bark.
The exotic invasive beetle, Emerald ash borer (EAB), is now known to occur in 25 states including the newest detection in Louisiana. The Winter 2015 Utah Pests News contains an article discussing the threat to all North American ash trees, signs and symptoms of EAB, and management options.
This issue will discuss the biological control of EAB. Classical biological control (or biocontrol) is the practice of importing and releasing host-specific enemies from a pest’s native range to control populations in the area of introduction. Biocontrol has been used for over 100 years in the U.S. and has successfully controlled invasive insect and weed pests such as gypsy moth, eucalyptus long horned borer, and Klamath weed. At present, the most promising long-term approach for reducing EAB populations and conserving ash in forested areas of North America is biological control.
Two USDA agencies—the Animal and Plant Health Inspection Service (APHIS) and the Forest Service—initiated a biological control effort shortly after EAB was detected in Michigan in 2002. USDA research in the beetle’s native range of China identified three potential biological control agents for EAB—Spathius agrili, Tetrastichus planipennisi, and Oobius agrili.
These biocontrol agents are very tiny wasps— the largest one is about the size of a typical mosquito. Female wasps use an organ that looks like a stinger (called an ovipositor) to lay their eggs. Because they must complete a part of their life cycle inside another organism, they are called parasitoid wasps. Two of the species attack EAB larvae, and one targets EAB eggs.
Following host range and specificity testing, USDA prepared an environmental assessment that outlined the risks and benefits of releasing the wasps. With approval from the State of Michigan, USDA began to release the wasps in July 2007. The USDA APHIS PPQ Biological Control Production Facility in Brighton, MI became operational in January 2009. It was designed to produce the EAB parasitoids for field release.
Over 720,000 individual wasps have now been released in 19 States: Colorado, Connecticut, Illinois, Indiana, Kentucky, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New York, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and Wisconsin.
Spathius agrili parasitizes up to 90 percent of EAB larvae in ash trees in China. It has a long ovipositor that enables it to attack larvae in ash trees of various sizes. Adult female Spathius target EAB larvae by drilling through the bark and laying up to 20 eggs on the surface of the host. The wasp larvae feed and develop on the EAB, resulting in its death. The cycle is repeated 3 to 4 times each summer and fall. Spathius overwinter as pupae inside cocoons under the bark of ash trees and emerge as adults in the summer.
Tetrastichus planipennisi also attacks EAB larvae. In China, it can kill up to 50% of the EAB larvae. The life cycle of Tetrastichus is similar to that of Spathius; however, the female lays about 100 eggs inside EAB larvae where the wasp larvae grow, eventually killing their host. Because of its shorter ovipositor, Tetrastichus targets larvae in ash trees with a diameter of 5 inches or less. Tetrastichus completes at least four generations each year. They survive the winter as larvae inside their host or host gallery under the bark of ash trees.
Oobius agrili is the smallest of the biocontrol wasps and targets EAB eggs. When Oobius locates an EAB egg in bark crevices, the wasp injects its own egg inside the host egg, where it will hatch, grow, and kill the EAB egg. In China, they can kill up to 60 percent of EAB eggs each season. At least two generations can occur during the EAB egg-laying season. Each Oobius adult can parasitize up to 62 EAB eggs during its life time. Oobius spends the winter as larvae inside EAB eggs and emerge the following spring as adults.
USDA and State officials, along with national EAB Program management, collectively determine release sites for EAB biocontrol. State cooperators secure permits and agree to follow standardized release and monitoring protocols. Utah has been issued a permit for all three species and will request agents for release when EAB is found here.
These releases will continue while scientists continue to study the establishment, dispersal, and impact these natural enemies have on suppressing EAB populations and the recovery of ash trees. Scientists will also continue to explore the U.S. and Asia for additional EAB natural enemies for possible use in the EAB biological control program.
Although it is premature to talk about the wasps’ effect on EAB populations, a recent study reported that parasitoid populations in Michigan are increasing and spreading into adjacent areas. In addition, cooperators in 10 states (Indiana, Illinois, Maryland, Michigan, Minnesota, New York, Ohio, Pennsylvania, Tennessee, and Wisconsin) have successfully recovered the offspring from one or more wasp species. We anticipate more evidence of progress as release sites are continually monitored.
These parasitoid wasps will not eradicate EAB. However, they can be used in an integrated pest management plan to help control the pest and benefit our urban, suburban, and rural landscapes. Continued scientific advances in the fields of forest health, pest management, and entomology offer promise that more effective treatments and tools will eventually be available to fight EAB.
Top: Spathius agrili larvae consuming EAB host.
Bottom: Oobius agrili adult parasitizing EAB egg.
Resources (orange text links to web)
Emerald Ash Borer Biological Control Release and Recovery Guidelines, USDA and US Forest Service.
View a short film about the emerald ash borer infestation and the Michigan EAB Biocontrol Production Facility
Pre-recorded webinars on emerald ash borer biocontrol