Invasive woody plants are a major problem in forests worldwide. Invasive trees and shrubs are often fast-growing with light, wind-dispersed seed or bird-carried fruit. They are able to quickly colonize disturbed sites and may prevent native species from germinating or growing. Here in central Kentucky, almost every forest patch near urban areas is so choked with bush honeysuckle that native species are not able to regenerate.

Buy why are invasive species so prevalent?  Conventional wisdom is that North America was a biological island before European settlement, and species introduced from Europe and Asia are able to establish themselves here and create conditions that allow them to perpetuate themselves.

An alternative view is that human activities do two things – bring new species into an area, and create the ecological conditions for those species to thrive. Sometimes this is easy to see. Royal paulownia (Paulownia tomentosa) and tree-of-heaven (Ailanthus altissima) are invasive Asian trees in eastern North America, but they are almost never found in mature forests. Instead, they quickly colonize areas that are disturbed by fire, plowing, or other human activities. 

Joshua King and Walter Tschinkel of Florida State University have done experiments with the exotic fire ant, Solenopsis invicta, comparing the success of this invasive species in disturbed (mowed or plowed) and undisturbed forest habitat. They found that plowing reduced the abundance and diversity of native ants and allowed fire ants to become established. In the undisturbed forest, fire ants were unable to invade the habitat of native ants.

These experiments show that fire ants are ‘passengers’ rather than ‘drivers’ of ecological change. King and Tschinkel say that fire ants and other invasive species should be described as ‘disturbance specialists’. 

If their results can be generalized, land managers may want to pay more attention to disturbance than to invasive species. Invasive species have proved difficult or impossible to control in many habitats. It may be better for land managers to focus on minimizing the kinds of disturbances that favor disturbance specialists.

Pictures:

• Paulownia tomentosa, an invasive tree, Pennsylvania Department of Conservation and Natural Resources – Forestry Archive, , Bugwood.org
• Red imported fire ant, Solenopsis invicta, Russ Ottens, University of Georgia, Bugwood.org

Articles

• Fire-Ant Invasions Are Ecological Karma, Brandon Keim, Wired Science
• King, JR, WR Tschinkel 2008 Experimental evidence that human impacts drive fire ant invasions and ecological change. PNAS DOI: 10.1073/pnas.0809423105

  

The Big Five: emerald ash borer, Asian longhorned beetle, Sirex wood wasp, sudden oak death, hemlock woolly adelgid

Forests in eastern North America are beset with a number of exotic pests and pathogens. Several of these pests and pathogens collectively could have a larger impact on eastern forests than chestnut blight, which killed billions of trees in the first half of the 20th century.  This article is a brief introduction to the Big Five.  Expanded articles will follow over the next month.

There are 4 insects and 1 disease that, in my opinion, represent the biggest threat to eastern forests.  Although there are other major pests and pathogens, both native and exotic, these five either threaten a large number of important tree species or threaten a single very important tree species.  ‘Important’ refers to commercial value of the species, ecological importance or both.

The Big Five (each link is to a longer article which will be updated frequently. NOTE: The linked articles will be available on November 27):

1. Sudden Oak Death (SOD). Caused by the fungus Phytophthora ramorum. A major disease in California, SOD threatens the dominant oak forests of the eastern US.  Transmission is mainly through shipment of stock from infected nurseries in the west. A vigorous inspection and sanitation program in nurseries is slowing the spread of the disease. 
2. Emerald ash borer (EAB). The emerald ash borer is a small, brilliant green beetle native to Asia. It was first found in North America feeding on ash trees in southeastern Michigan in 2002, and is now in 10 US states and Ontario. The beetle kills all ash (Fraxinus spp.), but white, green and black ash are particularly susceptible.
3. Asian longhorned beetle (ALB). The Asian longhorned beetle first appeared in Brooklyn NY in 1996, probably hitching a ride on pallets and other wood containers from China. ALB outbreaks are primarily urban. If the insect spreads into eastern forests, many tree species are threatened. Hosts include maples, buckeyes, willows, elm, birch and sycamore, though other species may be susceptible if ALB reaches high populations.
4. Hemlock woolly adelgid (HWA). The hemlock woolly adelgid, a sucking insect native to Asia, has been in the US sine 1924, slowly spreading to encompass much of the range of eastern hemlock. Although it is spreading slowly, it has the potential to wipe out eastern and Carolina hemlocks throughout their native range.
5. Sirex wood wasp.  The Sirex wood wasp is a major pest of pine trees throughout Europe and Asia. The female wasp lays eggs in pine bark and injects both a toxic mucus and a fungus. The toxin kills tree cells, the fungus grows on the dead cells and the wasp larvae feed on the fungus. Further outbreaks of the wasp are expected, as it hitches rides in wood packing materials. Sirex wood wasps have been found in 15 New York counties, several areas in Ontario, and in Pennsylvania and Vermont. The greatest economic threat is to the southern pine industry, but Sirex wood wasps have not yet been found outside of the northeastern US.

Each of these pests and pathogens poses a threat to particular tree species. Collectively, they threaten the entire eastern forest region.  Emerald ash borer is completely beyond the ability of sanitation and quarantine protocols to control its spread, and it is only a matter of time before the beetle is found throughout the range of ash trees.  Hemlock woolly adelgid is also beyond our capacity to control.  Whether sudden oak death, Asian longhorned beetles and the Sirex wood wasp will reach epidemic proportions is impossible to predict. Vigorous surveillance is so far holding SOD at bay in the east, while Asian longhorned beetles are gaining ground in Massachusetts.

The impact of these pests and pathogens on our forests is closely tied to the impacts of global warming. As the earth warms, habitat conditions for tree species will shift northward. The ability of trees to shift ranges in response to global warming depends on the existence of suitable habitat. The Big Five, as well as other insect and disease problems, may fundamentally alter the structure and function of eastern forests well before the major impacts of global warming occur, and may limit the ability of trees to migrate in response to a warming climate. For example, if warmer conditions allow the hemlock woolly adelgid to become well established at the northern limit of the current range of eastern hemlock, there may be no northward migration of hemlock because trees on the northern fringe will be killed.

Please check back for frequent updates on these pests and pathogens. Although there are many web sites describing each of these individually, there does not seem to be an effort to track information collectively about all these problems.  If any reader knows of such an effort, please post a comment.

Picture credits (all images used by permission):

• emerald ash borer, David Cappaert, Michigan State University, Bugwood.org
• Asian longhorned beetle, Pennsylvania Department of Conservation and Natural Resources – Forestry Archive, , Bugwood.org
• Sirex wood wasp, David R. Lance, USDA APHIS PPQ, Bugwood.org
• sudden oak death (canker on tanoak), Joseph O’Brien, USDA Forest Service, Bugwood.org
• hemlock woolly adelgid on eastern hemlock, North Carolina, picture by Tom Kimmerer.