Potential Impacts of Emerald Ash Borer Biocontrol on Ash Health and Recovery in Southern Michigan

Tree species richness and ash density have variable effects on emerald ash borer biological control by woodpeckers and parasitoid wasps in post-invasion white ash stands

Emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae) is the most destructive insect to invade North American forests. Identifying habitat features that support EAB natural enemies is necessary to enhance EAB biological control. In many forest ecosystems, tree species diversity has been linked with reduced pest abundance and increases in natural enemy abundance. We assessed the influence of tree species richness, ash density, and proportion of total ash basal area on ash canopy condition, EAB larval densities, and biocontrol by woodpeckers and parasitoids in pairs of healthy and declining overstory (DBH > 10 cm) and recruit-sized ash (DBH 2-10 cm) in 4 post-invasion forests in Michigan, USA. Tree species richness and ash density were not significantly associated with EAB larval densities, ash canopy dieback and transparency, and woodpecker predation of EAB larvae. In declining and healthy overstory ash, woodpeckers killed 38.5 ± 3.9% and 13.2 ± 3.7% of larvae, respectively, while the native parasitoid Phasgonophora sulcata Westwood killed 15.8 ± 3.8% and 8.3 ± 3.0% and the introduced parasitoid Spathius galinae Belokobylskij & Strazanac killed 10.8 ± 2.5% and 5.0 ± 2.6% of EAB larvae. Parasitism by P. sulcata was inversely related to ash density while parasitism by S. galinae was positively associated with ash density. Ash density, but not tree diversity, appears to differentially influence biological control of EAB by parasitoids, but this effect is not associated with reduced EAB densities or improved canopy condition.

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.

Mortality of Emerald Ash Borer Larvae in Small Regenerating Ash in New York Forests

Despite a robust biocontrol program against emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), parasitoid populations cannot build quickly enough to save mature ash trees. The future of ash as a viable component of North American forests depends on survival of immature ash that were too small to be attacked during the initial outbreak. This study was designed to quantify impacts of established introduced parasitoids and native woodpeckers on the population growth of emerald ash borer infesting regenerating ash saplings in white ash forests. Most emerald ash borer larvae were killed by Tetrastichus planipennisi Yang and woodpeckers in the fourth instar. Life-table analyses revealed that the estimated net reproductive rate of emerald ash borer was R0 = 4.2 in eastern New York and R0 = 0.0 in western New York. Without mortality from parasitoids and woodpeckers, R0 values would have been 16.4 and 7.9 in eastern and western New York, respectively. We monitored the health of mature and sapling ash trees from 2012 to 2017 and found that large trees were significantly more likely to be infested with emerald ash borer and to die than smaller trees. Fifty-four percent to 81% of ash saplings contained no living emerald ash borer, and the density of emerald ash borer at all sites was very low (< 4/m2). This study adds to the growing body of evidence that emerald ash borer biocontrol is contributing to population control and can help promote survival of young ash trees as forests recover from emerald ash borer outbreaks.

Effects of Extreme Low Winter Temperatures on the Overwintering Survival of the Introduced Larval Parasitoids Spathius galinae and Tetrastichus planipennisi: Implications for Biological Control of Emerald Ash Borer in North America

Climate change has been linked to shifts in the distribution and phenology of species although little is known about the potential effects that extreme low winter temperatures may have on insect host-parasitoid interactions. In late January 2019, northern regions of the United States experienced a severe cold wave caused by a weakened jet stream, destabilizing the Arctic polar vortex. Approximately 3 mo later at six study sites in southern Michigan and three in southern Connecticut, we sampled the overwintering larvae of the emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), and two larval parasitoids, Spathius galinae (Hymenoptera: Braconidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae), that are being introduced as emerald ash borer biocontrol agents in North America. At these nine study sites, emerald ash borer-infested ash trees and/or saplings were debarked and each overwintering emerald ash borer and parasitoid larva was then examined for cold-induced mortality, as indicated by a brown coloration, flaccid, and watery consistency. In early spring in Michigan, we found 4.5-26% of emerald ash borer larvae, 18-50% of S. galinae larvae, and 8-35% of T. planipennisi larvae were killed by cold. In Connecticut where temperatures were more moderate than in Michigan during the 2019 cold wave, <2% of the larval hosts and parasitoids died from cold injury. Our findings revealed that cold-induced mortality of overwintering larvae of emerald ash borer and its larval parasitoids varied by location and species, with higher mortality of parasitoid larvae in most Michigan sites compared to host larvae. The potential impacts of our findings on the management of emerald ash borer using biocontrol are discussed.

Establishment and Early Impact of Spathius galinae (Hymenoptera: Braconidae) on Emerald Ash Borer (Coleoptera: Buprestidae) in the Northeastern United States

The emerald ash borer (EAB), Agrilus planipennis Fairmaire, a buprestid beetle native to Asia, has become a serious pest of ash trees (Fraxinus spp.) in North America since the early 2000s. Due to the impracticality of applying insecticides in natural forests, biocontrol is the most viable method to manage EAB in natural ecosystems. Here, we report the first evidence for the establishment and impact of Spathius galinae Belokobylskij & Strazenac, a larval parasitoid first released in North America in 2016 and 2017 at six mixed-hardwood forest sites, in Connecticut, New York, and Massachusetts. We also report current levels of abundance and parasitism of another introduced larval EAB parasitoid, Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae), released in 2015 and 2016 in these same sites. Spathius galinae was recovered at all release sites in 2018, and its density in sampled trees had increased 1.5- to 20-fold (relative to the first postrelease sample year), reaching a final density of 2.3-14.3 broods/m2 of phloem area and causing 13.1-49.2% marginal rate of parasitism at four of the six sites. In contrast, T. planipennisi was only recovered in 2018 at four of the six release sites, and both its density (0.1-2.3 broods/m2 of phloem area) and parasitism (0.1-5.6%) were lower than that of S. galinae throughout the study at the four sites where recoveries were made. Our data fill a critical gap in the development of a biocontrol-based EAB management plan to protect surviving ash trees capable of reaching maturity and producing replacement trees.