Originally posted in February, 2015
In 2014, the Journal of Economic Entomology published a paper confirming the emerald ash borer has successfully attacked fringetree (Chionanthus virginicus) in Ohio.While this was the first incidence of EAB attacking anything other than an ash, this is not a cause for alarm. The beetle has not switched hosts, and it has not adapted to the fringetree after consuming all the available ash in an area. Rather, it is simply pre-adapted to feed on fringetree in the same way it is pre-adapted to feed on North American ash trees, even though it has never encountered any of these species before.
Ash and fringe trees are in the same family, Oleaceae family, and are the most Phyllogenetically closely related genera on the Oleacea family tree. The ash genus (Fraxinus) is most closely related to the fringetree (Chionanthus), with ash being used as rootstock for grafting fringetree. So it’s not surprising that fringetree physiology and chemistry are similar enough that the insect is able to reproduce on fringetree.
Fig. 1 Summary of the molecular phylogeny of Oleaceae ( Wallander and Albert, 2000)
Insects do not eat Latin binomials. This is a man-made classification used to categorize organisms. They eat plant material that contains the right mixture of feeding stimulants and nutrition, as well as feeding inhibitors, deterrents, and plant defenses. EAB co-evolved with its host ash species in Asia. Over millennia the Asian ash evolved chemicals and other tree responses to ward off attacks by EAB. But when the trees are weakened and stressed they can’t produce the chemical deterrents or compartmentalize the damaged tissue. EAB is able to overwhelm the tree and kill it.
The beetle arrived in North America pre-adapted to select and attack ash trees. The problem for the trees is they have not evolved the defense like their relatives in Asia. The beetle attacks and overwhelms the tree’s defenses, and kills it. In North America, the tree defenses are inadequate to protect it from EAB, and the beetle is able to attack and kill apparently healthy trees. In its evolution to attack Asian ash, EAB developed host selection behaviour and larval feeding processes based on its ability to find trees that smell, look, taste, grow, and defend themselves like Asian ash . Given their evolution, trees in the olive family (North American ash, fringe tree, lilac, etc.) are likely to have some chemistry and physiology in common with Asian ash. So it’s no surprise EAB attacks North American ash. It’s also not surprising that there might be a few species like the fringe tree which it may also attack. The Fringe tree is very closely related to ash. Ash rootstock is used for nursery production of fringe trees, so the trees must have similar physiology.
Fig. 2 EAB damage on white fringetree,
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Moreover, earlier bioassays by Deborah McCullough at Michigan State University showed that EAB will colonize lilac, which is also in the olive family with ash, but in her studies EAB larvae do not survive to pupation. McCullough could not get them feed on privet, which is also in the olive family.
The situation in Ohio is not a case of EAB moving to the fringe tree after killing all the ash. It is feeding on the fringe tree while there is still lots of ash around. Its survival doesn’t appear to be as good as on ash, so fringe tree doesn’t appear to be an optimal host. It’s simply feeding on a tree species is that is a lot like ash in its chemistry, physiology, and tree defenses.
By Taylor Scarr
Director Integrated Pest Management
Great Lakes Forestry Centre