Fungi associated with the emerald ash borer on urban green ash

Abstract

The emerald ash borer (Agrilus planipennis) is an invasive beetle recently introduced from Asia. It has quickly spread throughout much of eastern North America causing death of millions of ash (Fraxinus spp.) in both natural forests and in urban landscapes. Since 2016, this insect has been found in Thunder Bay, where approximately 25% of the urban street trees are ash. The larvae produce serpentine galleries beneath the bark which girdle the tree within a few years. It has always been assumed that death of the ash is due to this girdling and no studies to date have investigated fungi associated with the emerald ash borer. This study attempts to investigate whether such fungal associations exist, and whether the fungi, if present, are involved in the decline of the trees. During the autumn a total of three green ash (Fraxinus pennsylvanica) infested with emerald ash borer were removed from a local park in Thunder Bay and cut into small logs, bagged and brought to Lakehead University. Isolations were made directly from galleries, and from larval frass found in these galleries. In addition, washings were made from larvae when these were found. A total of 178 fungal isolates were obtained, including species of Cytospora (26.9%), species of Phoma (10.6%), and species of Cladosporium (9.0%). All fungi found were either typical inhabitants of woody plants or else are cosmopolitan in their substrate preference. None of the species found are thought to be closely associated with the emerald ash borer, but rather have an accidental association with the insect. Further studies are needed to verify this finding.