The emerald ash borer (Agrilus planipennis) is an insect from Asia that has plagued our native ash trees in Illinois since 2006. This pest was first introduced in 2002 around the Detroit area and has rapidly spread across Michigan and Indiana to infect most of Illinois today. Sadly, the emerald ash borer will eventually wipe out our native ash species in Illinois as we know them, leaving a major void in our urban forests and natural areas since ash is currently so prevalent.
The age old phrase that “history repeats itself’ certainly holds true with exotic pests and disease in North America and expiration of individual trees species. Two such past incidences have cause the virtual elimination of American elm (Ulmus americana) and American chestnut (Castanea dentate) trees across our continent.
The impact of chestnut blight was so extensive it is hard to understand today. The American chestnut, whose native range spans more than 20 states in the eastern US, from Maine to Georgia, accounted for about 50% of the eastern deciduous forest. It was a highly prized timber and wildlife tree, producing chestnuts for wildlife (and humans) as well as lumber of superior quality. The wood has excellent resistance to rot with fiber as strong as many of the oak species we covet today. It is just really hard to imagine how the disappearance of chestnut impacted life in the eastern United States, changing everything from timber products to the diversity of biota in eastern forests.
Chestnut blight does differs from emerald ash borer because it is fungal pathogen (Cryphonectria parasitica) of chestnut as opposed to an insect pest. The fungus was first identified in 1913 in its native land of China where it is hardly a pathogen of any significant threat, typically infecting dying twigs and bark. However, since our native chestnuts don’t have any coevolutionary history with this pathogen they have little resistance. Once established, the pathogen readily spread from tree to tree by wind dispersion creating cankers that rapidly grow and girdle stems.
Infected chestnut trees were first observed in New York City in 1904. By 1940, chestnuts were wiped out as a commercial species and active component of its original ecosystem. To this day, chestnut does still exist in its home range because the roots and root collar are resistant allowing sprouts from old root systems to grow before the pathogen attacks and kills the above ground portion of the plant.
The fate of American elm was determined by combination of fungal pathogen (Ophiostoma ulmi) and elm bark beetles (Hylurgopinus rifipes), collectively referred to as Dutch Elm Disease. Very similar to the emerald ash borer, larvae of the elm bark beetle tunnel into the wood of elm trees. As the larvae feed, they are exposed to fungal spores that are then dispersed when the beetles emerge as adults and fly off to feed on other elm trees. Once introduced, the fungus grows into conductive tissues in the tree. As the tree’s defenses respond, the conductive tissue is actually clogged, stopping transport of water and nutrients, while the pathogen persists. It is an interesting relationship between insect and fungi, that is somewhat limited by the dispersion rate of beetles, spreading slower than wind-dispersed pathogens like Chestnut Blight.
Dutch Elm Disease was introduced near Cleveland in the 1930’s, reaching Chicago in the 1960’s and the western limits of American elm’s natural range by the 1970’s. Although American elm did not have the timber value of chestnut, it had a much more extensive range, extending from the east cost to the Dakotas and down to central Texas. It was a large component of eastern forests, occupying a wide range of environmental conditions. This adaptability made the elm an excellent urban tree which was widely planted, creating a huge impact to urban forests as the disease spread.
Emerald ash borer is fatal to trees because the larvae actually consume the conductive tissue within branches and trunks, often causing death within 2-5 years of initial infection. The beetle does fly and disperse on its own, but human movement of firewood has rapidly advanced its spread. Ash trees are important timber species, as well as urban trees, comprising up to 50% of the urban forest in some cities. Interestingly, ash has been noted to replace elm in many ecosystems where it was removed by Dutch Elm Disease. It was often one of the primary replacement species planted in urban areas following the death of elms.
As an arborist it has been very sad to observe the spread of emerald ash borer as it follows in the footsteps of past introduced outbreaks. The major lesson to learn from history is that we should all carefully consider the tree species we choose to replace ash and focus on planting a wide variety of native species to create a diverse and resilient urban forest. Inevitably, history will repeat itself and the strength of our urban tree populations will lie in the diversity of species that comprise them.