Long-term monitoring of the introduced emerald ash borer (Coleoptera: Buprestidae) egg parasitoid,Oobius agrili (Hymenoptera: Encyrtidae), in Michigan,USA and evaluation of a newly developed monitoring technique

Emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a serious invasive pest of ash trees (Fraxinus spp.) in North America from China. The egg parasitoid Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae) was introduced from China as a biological control agent for this pest in Michigan and throughout the infested area of the United States. A critical component of any biological control program is post-release monitoring and evaluation; however, because of the small size and cryptic nature of O. agrili, evaluation of its impact is difficult. We compared two methods for measuring parasitism of emerald ash borer eggs: (1) timed visual searches of bark on standing ash trees and (2) bark collection, sifting, and sorting. Both methods were carried out in paired parasitoid-release and control plots, the visual search method over a six-year period (2008–2013) and the more recently developed bark-collection and sifting method for 2 years (2012–2013). The visual search method found parasitism in release plots remained low (0.7–4.2%) in samples taken from 2008 to 2012 and reached 10.6% in 2013. In comparison, the bark-sifting method found that rates of egg parasitism were considerably higher in release plots, 21.8% and 18.9% for samples taken in 2012 and 2013, respectively. These findings indicate that the population-level impact of O. agrili is increasing and may be an important source of mortality for EAB populations. We recommend the bark-collection and sifting method as the more effective method to recover parasitoids and estimate parasitism rates of O. agrili.