Overwintering survival and spring emergence phenology of two introduced Spathius species for emerald ash borer biological control across different geographic ranges in the U.S.

Authors: GREEN, MITCHELL - University Of Massachusetts; Duan, Jian; BOLADO, WHIT - University Of Georgia; BARNES, BRITTANY - University Of Georgia; GANDHI, KAMAL - University Of Georgia; DODGE, CHRISTINE - Animal And Plant Health Inspection Service (APHIS); ANDERSEN, JEREMY - University Of Massachusetts

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/29/2026
Publication Date: 5/30/2026
Citation: Green, M.A., Duan, J.J., Bolado, W., Barnes, B.F., Gandhi, K.J., Dodge, C., Andersen, J.C. 2026. Overwintering survival and spring emergence phenology of two introduced Spathius species for emerald ash borer biological control across different geographic ranges in the U.S.. Biological Control. https://doi.org/10.1016/j.biocontrol.2026.106072.
DOI: https://doi.org/10.1016/j.biocontrol.2026.106072

Interpretive Summary: The emerald ash borer (EAB) is the most destructive invasive pest of North American ash trees in the U.S., causing more than $1 billion annually in costs for tree removal, replacement, and management. Two parasitic wasps (Spathius agrili and S. galinae) were introduced from Asia to help control EAB populations. While S. galinae has established and contributed to EAB suppression in northern states, S. agrili has not, and its potential for establishment in the southern U.S. is under investigation. Our study evaluated overwintering survival and spring emergence of both species in Massachusetts, Delaware, and Georgia, and found significant differences among regions. These results indicate that biological control effectiveness varies by location, so release strategies must be adapted to regional conditions—helping scientists improve pest management, communities save trees, and reduce economic losses nationwide.

Technical Abstract: Spathius agrili and S. galinae are two larval parasitoids introduced as classical biological control agents against the emerald ash borer (EAB), Agrilus planipennis, in the United States. While S. galinae appears to have established and contributed to suppression of EAB populations in the northern United States, S. agrili has failed to establish but is currently being investigated for establishment in the southern United States. Here we conducted a semi-field study to evaluate the overwintering survival and phenology of both species across a latitudinal gradient to examine how climate may have influenced their differential success. We placed logs containing late instars of S. agrili and S. galinae larvae into three field sites (Delaware, Georgia, and Massachusetts) in late fall of 2023 and monitored the adult emergence the following spring and summer. We found that S. agrili emerged earlier than S. galinae in Delaware. We also found that overwintering mortality of S. agrili was greater than S. galinae in all states. Additionally, the probability of S. agrili remaining alive and in diapause through spring and summer was greatest in Massachusetts while the probability of S. galinae remaining in diapause was greatest in Georgia. Our results suggest that S. agrili is capable of emerging earlier than previously documented. Additionally, high S. agrili mortality may have been due to a portion of individuals waiting to emerge later in the season but failing to. Finally, individuals remaining in diapause through spring and summer may suggest evidence of a “bet-hedging” strategy resulting from poor climatic matches.