Organisms (animals, plants, or microbes) living outside their historical natural ranges are termed “exotic”, and only rarely is this applied as a positive attribute. But importing exotic species is as old as human travel. Most gardens are teeming with exotic plants, and at least half of all woody plant species offered in U.S. wholesale grower catalogs are exotic to North America. Many exotic species that have become naturalized to North America have enhanced, not degraded, our environment.
But not all exotics are equal; contrasts range from seed of non-shattering rye to rats bearing fleas that in turn bear the bacterium that causes Black Death. Some exotics brought here by design or by accident are inherently highly adaptable and have a high reproductive ability. They have no natural enemies here in their adopted land, having evolved elsewhere, and they flourish, displacing or parasitizing our native species as their populations explode, unchecked. These are not merely exotic species; they are exotic, invasive species. And they pose a real threat to native North American fauna and flora and, ultimately, us.
The list of these invasive species is long, and many are familiar. Take garlic mustard. A native of Europe, first reported in North America in 1868, it’s now naturalized to most U.S states and Canadian provinces. It out competes and displaces native plants and threatens the animals that depend on the plant communities wherever it now grows. You may not know the fungus Cryphonectria parasitica, which is thought to have been brought to the USA from China on lumber around 1900, but you know Chestnut Blight, the disease it causes that essentially wiped out American chestnut (arguably, at that time, the most important North American tree) in its range, from Maine to Georgia and west to Ohio and Tennessee, all in about 40 years.
As travel and commerce have become more global, the importation of exotic, invasive species has become more inevitable. A recent addition to our list is a small, rather lovely, metallic, blue–green beetle called emerald ash borer (EAB, Agrilus planipennis), also thought to have been introduced on wood from China. It burrows under the bark of ash trees (Fraxinus spp.) and feeds on the phloem, cambium, and xylem tissues, girdling and ultimately killing the tree. EAB was first identified as the cause of ash tree decline and death in the Detroit area in 2002, but it had probably been there since the mid-1990s. EAB has since killed as many as 50 million North American ash trees and has spread to at least 13 U.S. states and parts of eastern Canada (see 2011 infestation map).
Ash is an important tree in North American mixed deciduous woodland habitats. According to the USDA–Natural Resources Conservation Service, it is present in most Canadian provinces and every U.S. state except Idaho and Alaska. The fruit is eaten by birds and small mammals; it is browsed by deer. White ash trunks form cavities that are used and improved by various woodpeckers and once the woodpeckers move out, by such creatures as nuthatch, squirrel and owl. There are 16 ash tree species and some 7 billion individual trees in North America. The wood is hard, strong and flexible and has many commercial uses. It’s also an excellent firewood and a major landscape tree, in both residential and urban settings.
Early detection of any infestation or disease is critical. With EAB, this is tricky because symptoms are not distinct and are so similar to those caused by many other pests. Initially, Federal and State agencies scrambled to develop strategies to cope with EAB. Early on, they recognized that while EAB adults fly well, the pest’s most efficient means of travel is on ash nursery stock, unprocessed logs, firewood and other ash products such as crates or pallets. Federal and State quarantines were thus established (and are enforced to the degree that is possible). These quarantines remain one of the most effective tools we have.
In Asia, where EAB is endemic, pest population outbreaks are rare and seem to be related to environmental stresses such as drought that weaken ash trees and predispose them to EAB infestation. When outbreaks do occur, the damage to ash is much more limited than here in North America. The reasons for this are unclear. Almost certainly, though, Asian ash trees, having coevolved with EAB, have a degree of resistance to it that our trees lack. In addition, there are EAB predators in Asia that are not present here and that help keep EAB populations under control. USDA has identified and cultured several small, wasp-like insects from Asia that parasitize EAB larvae. Two have been released in nature, and they have successfully reproduced and survived the U.S. winter. Biological control of such invasive insect and weed pests as gypsy moth, Japanese beetle and purple loosestrife has been utilized with some success in the USA for almost 100 years. So this is certainly promising, if only preliminary.
Less promising is the reality that urban trees in most North American cities are typically limited to a few species; this lack of arboreal diversity leaves us vulnerable. Recall that at one time, American elm was a dominant tree in North American cities and towns. Images of its cathedral-like boughs sheltering American streets are an almost iconic reminder of a bygone era before Dutch elm disease (caused by another exotic and invasive fungus), in little more than 50 years, killed essentially all American elm. The loss was so devastating and pronounced because we had overplanted American elm, but we repeated our mistake and filled the void with mostly maple, ash, honeylocust and basswood. With EAB already across our threshold (and other pests such as the Asian longhorned beetle, a potential threat to maple and several other common trees, at the door), this has the makings of a disaster. For the future, a principle we should adopt is to plant no more than 5% of one species, 10% of one genus, and 20% of one family. This applies to urban as well as residential landscapes.
From a forestry point of view (and this includes urban forestry), a tree infested with EAB is a dead tree. For residential trees, if infestations are caught early, there are systemic, EPA-registered insecticides that have varying degrees of effectiveness against EAB; consult an arborist or county agricultural and extension agents for more information.
Will ash trees in North America go the way of the American chestnut and the American elm? That’s an open question. It’s true that we are far more sophisticated than we once were and that we have many more resources and strategies for dealing with a pest like EAB. Still we cannot eradicate it, and effectively trees that are attacked are killed. Currently, the best we can do is adapt tactics that slow or contain EAB’s spread, control its population density, and reduce its impact on ash trees.