Progress toward successful biological control of the invasive emerald ash borer in the United States

Abstract

Emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), was first detected infesting ash (Fraxinus spp.) trees in southeastern Michigan and nearby Ontario in 2002. Shortly after the discovery of EAB in North America, researchers began foreign exploration for natural enemies of this destructive beetle in northeastern Asia, where it is native. This activity led to development of a classical biological control program in 2003. In 2007, North American regulatory agencies approved the first environmental releases of three EAB hymenopteran parasitoid species from China to EAB-infested forests in southeastern Michigan: Oobius agrili (Encyrtidae), Tetrastichus planipennisi (Eulophidae), and Spathius agrili (Braconidae). Oobius agrili parasitizes EAB eggs, whereas the two other species parasitize EAB larvae. In 2015, a fourth EAB parasitoid, Spathius galinae (Braconidae) from the Russian Far East, was approved for release in northern states. To date, one or more of these introduced parasitoid species has been released in over 350 counties in 30 EAB-infested states and Washington, D.C. in the United States, and four provinces of Canada. Recent studies in Michigan and several northeastern states, where parasitoids were released five or more years ago, indicate that two larval parasitoids (T. planipennisi and S. galinae) have established co-existing populations in EAB infesting different size-class ash trees; there they complement each other by partitioning host resources, and they play a significant role in suppressing EAB populations. The egg parasitoid O. agrili has also established and spread in EAB-infested forests. It is still too early to determine if biological control will result in significant improvement in ash recovery and regeneration. However, the results from long-term field studies in Michigan and several states in the northeastern United States reveal that ash saplings and pole-size ash trees now experience low EAB densities, which are regulated by established populations of the three introduced biocontrol agents. Researchers and land managers are hopeful that surviving North American ash trees will be protected from EAB and ultimately become overstory trees after successful areawide suppression of EAB populations by these introduced natural enemies. Monitoring over the next 10–20 years is required to evaluate survival of the different ash tree species impacted by EAB.

Citation

Duan, Jian J.; Gould, Juli R;. Slager, Ben H.; Quinn, Nicole F.; Petrice, Toby R.; Poland, Therese M.; Bauer, Leah S.; Rutledge, Claire E.; Elkinton, Joseph S.; Van Driesche, Roy. 2022. Progress toward successful biological control of the invasive emerald ash borer in the United States. In: Van Driesche, R.G.; Winston, R.L.; Perring, T.M.; Lopez, V.M., editors. Contributions of classical biological control to the U.S. food security, forestry, and biodiversity. FHAAST-2019-05. Morgantown, WV: U.S. Department of Agriculture, Forest Service: 232-250.