Significant suppression of an invasive forest pest by Spathius galinae, an introduced larval parasitoid: potential for North American ash recovery

Authors: Duan, Jian; VAN DRIESCHE, ROY - University Of Massachusetts, Amherst; Schmude, Jonathan; CRANDALL, RYAN - University Of Massachusetts, Amherst; RUTLEGE, CLAIRE - Agricultural Experiment Station, Connecticut; QUINN, NICOLE - University Of Massachusetts, Amherst; SLAGER, BENJAMIN - Animal And Plant Health Inspection Service (APHIS); GOULD, JULI - Animal And Plant Health Inspection Service (APHIS); ELKINTON, JOE - University Of Massachusetts, Amherst

Submitted to: Journal of Pest Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2021
Publication Date: 11/13/2021
Citation: Duan, J.J., Van Driesche, R.G., Schmude, J.M., Crandall, R., Rutlege, C., Quinn, N., Slager, B.H., Gould, J.R., Elkinton, J. 2021. Significant suppression of an invasive forest pest by Spathius galinae, an introduced larval parasitoid: potential for North American ash recovery. Journal of Pest Science. 2021: 1-9. https://doi.org/10.1007/s10340-021-01441-9.
DOI: https://doi.org/10.1007/s10340-021-01441-9

Interpretive Summary: The emerald ash borer (EAB) is a serious invasive forest pest that has killed millions of ash trees in the U.S. Two parasitic wasps attacking EAB larvae in Asia were introduced to the U.S. between 2007 and 2015 for biocontrol of the pest. We conducted a six-year study (2015 to 2020) of the impact of these parasitoids on EAB populations in ash-dominated hardwood forests in northeastern U.S states. We found that introduction of the two parasitic wasps, along with local natural enemies, successfully suppressed the invasive EAB populations to low densities that may allow surviving ash trees to recover.

Technical Abstract: Successful management of invasive forest pests with sustainable approaches such as biological control is critical to the restoration of the affected or damaged forest ecosystems. The emerald ash borer (EAB), Agrilus planipennis Fairmaire, native to Asia and first detected in South Michigan in 2002, has now become the most destructive invasive pest of ash (Fraxinus spp.) trees in North America. Several natural enemies introduced from Northeast Asia were released from 2015 to 2017 in several northeastern U.S. states for biocontrol of this invasive pest. Here, we report results of a six-year study (2015 to 2020) aimed at assessing the impact of two introduced larval parasitoids, Spathius galinae and Tetrastichus planipennisi, on EAB population dynamics in five ash-dominated hardwood forests in three northeastern U.S states. Immediately following the initial parasitoid releases in 2015, every autumn we sampled larval stages of EAB larvae and associated parasitoids by debarking infested ash trees from each of the forests, and then constructed lifetables to estimate EAB population growth rates. Results from this six year field study showed that average density of EAB populations in these five recently invaded forests in the northeastern U.S. decreased approximately 76% to a very low density (<7 live larvae per m2 of tree phloem) from 2015 to 2020, and this reduction in pest density was driven primarily by the significant increase in larval parasitism rates (from 35 to 78%) by the introduced biocontrol agent S. galinae, along with low-to-moderate levels of mortality from local generalist natural enemies such as woodpeckers. Life table analyses further showed that S. galinae alone caused a 31 to 57% reduction in the net EAB population growth rate from 2018 to 2020. These findings demonstrate that in the recently invaded ash forests in the northeast United States, timely introduction of specialized natural enemies such as S. galinae, along with local generalist natural enemies, can suppress the invasive EAB populations to low densities (<7 live larvae per m2 of tree phloem in this study), that may allow surviving ash trees to recover.