Host range expansion may provide enemy free space for the highly 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.

Lessons from the Frontline: Exploring How Stakeholders May Respond to Emerald Ash Borer Management in Europe

The emerald ash borer (EAB) has caused extensive damage and high mortality to native ash trees (Fraxinus; sp.) in North America. As European countries battle with the deadly pathogen Hymenoscyphus fraxineus (ash dieback) affecting European ash (Fraxinus excelsior), there is concern that the arrival of EAB will signal the demise of this much-loved tree. While Europe prepares for EAB it is vital that we understand the social dimensions that will likely influence the social acceptability of potential management measures, and experiences from the USA can potentially guide this. We draw on differing sources including a literature review, documentary analysis, and consultation with key informants from Chicago and the Twin Cities of Minneapolis and St. Paul. In this paper, we focus on EAB management responses that involve chemical applications, tree felling and replanting, and biological control, and assess their likely social acceptability to stakeholders based on the perceived risks and benefits. Benefits involve protecting specific ash trees and slowing the spread of EAB across the landscape. Risks include collateral harm from insecticide use on human and environmental health, financial costs and liabilities, and the effectiveness of each approach. Biological control and replacing ash with other species are likely to be largely acceptable across contexts and stakeholder groups but pre-emptive felling and insecticide application could be more problematic if seeking widespread social acceptance. Based on our observations from the evidence collected we offer suggestions for approaching EAB management in Europe with a focus on improving prospects of social acceptability. Strong engagement will be necessary to establish the relevance and reason for using different management approaches and to build awareness and trust.

The Fate of Ornamental White Fringetree Through the Invasion Wave of Emerald Ash Borer and Implications for Novel Host Use by This Beetle

Emerald ash borer (EAB, Agrilus planipennis Fairmaire [Coleoptera: Buprestidae]) is a wood boring beetle that is an invasive pest of ash trees (Fraxinus spp.) in North America. In 2014, it was reported that EAB had infested white fringetree (Chionanthus virginicus L. [Lamiales: Oleaceae]) in Ohio and was since found to have infested this species across its invasive range. In 2018, we reexamined 166 white fringetrees in Illinois, Indiana, Ohio, and Pennsylvania that had been previously examined for EAB attack in 2015 to determine their fate. We assessed tree health and EAB infestation in each tree, assigned an infestation status of newly, continuously, not reinfested, or never infested, and compared the trees' current status to their 2015 status. This assessment was done to determine whether their health and infestation status had changed through the EAB invasion wave. We found that attack rates declined: 26% of trees were infested in 2015 whereas only 13% were in 2018, likely coinciding with declining beetle populations in the area. Overall tree health improved for trees that were not reinfested by EAB after a record of attack in 2015, suggesting that they can survive and recover from EAB attack. Conversely, health declined for newly and continuously infested trees, indicating that they became stressed from EAB attack. Although the majority of the trees survived the invasion wave, several were removed from various sites due to EAB attack suggesting that white fringetree varies in its resistance and tolerance to attack. As beetle populations continue to expand geographically, infestation rates will likely increase and health of white fringetrees will decrease with the EAB attack wave, especially as EAB reaches denser populations of fringetrees.

Larval Performance of a Major Forest Pest on Novel Hosts and the Effect of Stressors

Novel hosts lacking a coevolutionary history with herbivores can often support improved larval performance over historic hosts; e.g., emerald ash borer [Agrilus planipennis (Fairmaire) Coleoptera: Buprestidae] on North American ash (Fraxinus spp.) trees. Whether trees are novel or ancestral, stress on plants increases emerald ash borer preference and performance. White fringetree [Chionanthus virginicus (L.) Lamiales: Oleaceae] and olive [Olea europaea (L.) Lamiales: Oleaceae] are closely related non-ash hosts that support development of emerald ash borer to adulthood, but their relative suitability as hosts and the impact of plant stress on larval success has not been well studied. In a series of experiments, survival and growth of emerald ash borer larvae on these novel hosts were examined along with the impact of stress. In the first experiment, larvae grew more slowly in cut stems of olive than in green ash [Fraxinus pennsylvanica (Marshall) Lamiales: Oleaceae] and several adults successfully emerged from larger olive stems. In two experiments on young potted olive with photosynthesizing bark, larvae died within a week, but mechanical girdling increased the rate of gallery establishment. The final two experiments on field-grown fringetrees found increased larval survivorship and growth in previously emerald ash borer attacked and mechanically girdled plants than in healthy stems or stems treated with the defense hormone, methyl jasmonate. Our results demonstrate that these non-ash hosts are less suitable for emerald ash borer than preferred ash hosts, but previous emerald ash borer attack or girdling led to better survival and growth demonstrating the importance of stress for larval success. In potted olive, high mortality could be due to higher loads of toxic compounds or the presence of chlorophyllous tissue.