Buying Time: Preliminary Assessment of Biocontrol in the Recovery of Native Forest Vegetation in the Aftermath of the Invasive Emerald Ash Borer

Emerald Ash Borer Management and Research: Decades of Damage and Still Expanding

Since the discovery of the ash tree (Fraxinus spp.) killer emerald ash borer (EAB; Agrilus planipennis) in the United States in 2002 and Moscow, Russia in 2003, substantial detection and management efforts have been applied to contain and monitor its spread and mitigate impacts. Despite these efforts, the pest continues to spread within North America. It has spread to European Russia and Ukraine and is causing sporadic outbreaks in its native range in China. The dynamics of EAB's range expansion events appear to be linked to the lack of resistant ash trees in invaded ranges, facilitated by the abundance of native or planted North American susceptible ash species. We review recently gained knowledge of the range expansion of EAB; its ecological, economic, and social impacts; and past management efforts with their successes and limitations. We also highlight advances in biological control, mechanisms of ash resistance, and new detection and management approaches under development, with the aim of guiding more effective management.

Postrelease assessment of Oobius agrili (Hymenoptera: Encyrtidae) establishment and persistence in Michigan and the Northeastern United States

The emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is an invasive woodboring pest of ash trees (Fraxinus sp.) in North America. Among the Asiatic parasitoids being released for the management of EAB in North America, Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae) is the only EAB egg parasitoid. To date, more than 2.5 million O. agrili have been released in North America; however, few studies have examined its success as a biological control agent of EAB. We conducted studies to assess O. agrili establishment, persistence, spread, and EAB egg parasitism rates in Michigan at the earliest release sites (2007-2010), as well as at more recent release sites (2015-2016) in 3 Northeastern United States (Connecticut, Massachusetts, New York). In both regions, we documented successful O. agrili establishment at all but one release site. In Michigan, O. agrili has persisted at release sites for over a decade and spread to all control sites located 0.6-3.8 km from release sites. Overall, EAB egg parasitism ranged from 1.5% to 51.2% (mean of 21.4%) during 2016-2020 in Michigan and from 2.6% to 29.2% (mean of 16.1%) during 2018-2020 in the Northeastern states. Future research efforts should focus on factors affecting the spatiotemporal variation in EAB egg parasitism rates by O. agrili, as well as its potential range in North America.

Comparing Methods for Monitoring Establishment of the Emerald Ash Borer (Agrilus planipennis, Coleoptera: Buprestidae) Egg Parasitoid Oobius agrili (Hymenoptera: Encyrtidae) in Maryland, USA

The emerald ash borer, Agrilus planipennis Fairmaire (EAB), is an invasive beetle that has caused widespread mortality of ash trees in North America. To date, four parasitoids have been introduced in North America for EAB biological control, including the egg parasitoid Oobius agrili Zhang & Huang (Hymenoptera: Encyrtidae). Monitoring EAB egg parasitism is challenging because female beetles oviposit in bark crevices and EAB eggs and O. agrili are small (<1 mm in diameter). Consequently, multiple methods have been developed to recover this parasitoid. Here we compared two methods, visual surveys and bark sifting, used to monitor establishment of O. agrili in Maryland, USA. From 2009 to 2015, a total of 56,176 O. agrili were released at 32 sites across the state. In 2016, we surveyed nine of the study sites for O. agrili establishment using both methods. We compared the amount of time spent searching for eggs separately in each method, and also analyzed the effects of years-post release, total number of parasitoids released, and median month of release, on percent parasitism of EAB eggs, and the percentage of trees per site with parasitized EAB eggs. We found that visually surveying ash trees for EAB eggs was more efficient than bark sifting; the percent parasitism observed using the two methods was similar, but visually surveying trees was more time-efficient. Both methods indicate that O. agrili can successfully establish populations in Maryland, and June may be the best month to release O. agrili in the state. Future research should investigate EAB phenology in the state to help optimize parasitoid release strategies.