Response of native arthropods to ash tree decline and mortality due to the exotic emerald ash borer

Background/Question/Methods

Emerald ash borer (Agrilus planipennis Fairmaire) (EAB), an Asian woodboring beetle, is causing large-scale decline and mortality of ash (Fraxinus spp.) trees in eastern North America.  We determined which arthropod species that are associated with ash may become endangered and co-extinct with the decline of ash.  Also, we assessed the short-term effects of ash mortality on ground beetle assemblages in EAB-invaded landscapes. Exhaustive literature searches were conducted on host association and trophic-levels of native arthropods on ash trees.  Pitfall traps were used in 2006-2007 to determine changes in ground beetle assemblages in black (F. nigra Marsh.) or hydric, green (F. pennsylvanica Marsh.) or mesic, and white (F. americana L.) or xeric ash stands in southeastern Michigan. 

Results/Conclusions

Literature searches revealed that a total of 282 native and exotic arthropod species in eight orders are associated with 16 North American ash species.  There were 44, 17, 13, and 208 arthropods, respectively in the high (monophagous), high-moderate (biphagous), moderate (triphagous), and low (polyphagous) endangerment risk rankings.  In the monophagous category, most of the ash arthropods are gall-formers followed by folivores, subcortical phloem/xylem feeders, sap feeders, and seed predators.

In the southeastern Michigan stands, mean mortality of ash trees (>5 cm DBH) increased from 76 % to 93% from 2006 to 2007.  Trapping results indicated that there was a negative relationship between ground beetle catches and percent ash mortality in 2006.  However, this relationship was absent in 2007, suggesting short-term impacts on trap catches.  Mesic and xeric sites in 2007 had slightly lower species diversity than in 2006.  Cluster analysis indicated that hydric stands were distinct, and that assemblages in hydric and mesic stands diverged in their composition from 2006 to 2007.  Overall, our results from both the literature and trapping studies suggested that the decline of North American ash species due to EAB invasion may lead to ecological impacts on native arthropods in these landscapes.