Overwintering survival and emergence phenology of two introduced Spathius species for emerald ash borer biological control across a latitudinal gradient in the U.S

Spathius agrili Yang and S. galinae Belokobylskij & Strazanac are two larval parasitoids introduced as classical biological control agents against the emerald ash borer (EAB), Agrilus planipennis Fairmare, in the United States. While S. galinae has established and contributed to suppression of EAB populations in the northern United States, S. agrili has thus far failed to establish. 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 at three field sites (Delaware, Georgia, and Massachusetts) in late fall of 2023 and monitored adult emergence throughout 2024. We found that S. agrili emerged earlier than S. galinae in Delaware and that Georgia had the earliest emergence date. However, overwintering mortality of S. agrili (N = 560) was 18.1 – 40.1% greater than S. galinae (N = 661) in all states. 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 can emerge much earlier than previously documented. Additionally, high S. agrili mortality may be due to a portion of individuals waiting to emerge later in the season but failing to do so. Finally, individuals remaining in diapause through spring and summer was likely due to poor climatic matches.