The relationship between the emerald ash borer (Agrilus planipennis) and ash (Fraxinus spp.) tree decline: Using visual canopy condition assessments and leaf isotope measurements to assess pest damage

• We assessed the mechanism by which the emerald ash borer causes tree mortality.
• EAB larvae cause tree mortality through their girdling behavior.
• EAB larval gallery cover was positively related to the leaf C isotopic composition.
• EAB gallery cover exhibited a significant relationship with tree canopy decline.
• Canopy condition can be used to evaluate tree health and guide management decisions.

Ash trees (Fraxinus spp.) in North America are being severely impacted by the invasive emerald ash borer (Agrilus planipennis Fairmaire) which was inadvertently introduced to the US in the 1990s from Asia. The emerald ash borer (EAB) is a phloem boring beetle which relies exclusively on ash trees to complete its life cycle. Larvae feed in the cambial tissue forming serpentine galleries that may girdle the tree, causing mortality in as little as two years. Although larval feeding is thought to be the cause of rapid tree mortality, the relationship between tree-level water stress and EAB larval activity has never been quantified. Identifying symptoms of an emerald ash borer outbreak at an early stage can facilitate informed management decisions. Although a user-friendly system of ash canopy condition rating has been used extensively to study EAB impacts, the mechanistic relationship between canopy ratings and EAB larval activity has not been quantified.The objective of this research was to use the stable carbon isotopic composition of canopy leaf tissue (foliar δ13C, a proxy of tree level water stress) to quantify the mechanism by which EAB causes tree mortality and to relate this mechanism to the ash canopy condition rating system. We found that as the canopy condition was rated as less healthy, EAB density and gallery cover increased, and foliar δ13C became more enriched as well. The rating system was able to identify trees in early stages of EAB infestation with relatively low levels of EAB (<20% gallery cover or < 40 EAB/m2). We also found that foliar δ13C and EAB larval gallery cover exhibited a significant positive correlation. These results suggest that as EAB larval feeding occurs, the tree canopy exhibits thinning, and as feeding continues the tree experiences chronic water stress and canopy dieback occurs. This study highlights the usefulness of the ash canopy condition rating system as a proxy of emerald ash borer densities at the tree level.