The goldspotted oak borer, Agrilus coxalis Waterhouse (Coleoptera:Buprestidae), is adding to the woes of California’s magnificent oak forests. 

While sudden oak death is killing oak and other trees in the northern part of the state, the oak forests of the southern part of the state have escaped damage due to the dry conditions that prevent spread of the disease. 

Now, the goldspotted oak borer is causing oak mortality in the southern part of the state.  Like other Agrilus beetles, such as the emerald ash borer and twolined chestnut borer, the goldspotted oak borer kills trees when the larvae feed on the phloem and cambium.

The borer, first identified during oak mortality surveys in 2004 in San Diego County, has been found attacking coast live oak, Quercus agrifolia, canyon live oak, Q. chrysolepis, and California black oak, Q. kelloggii, on the Cleveland National Forest.  It is now considered a major cause of oak mortality in Southern California.

The goldspotted oak borer is native to parts of southern Arizona, Mexico and Guatemala. It has not previously been considered a major pest in those areas.  While the borer may have extended its range naturally, is it likely that it was introduced into southern California by movement of firewood, as with the emerald ash borer.

It remains to be seen how significant a pest this will turn out to be.  Major mortality was not seen until 2008 in Cleveland National Forest, and it will take some years to assess the extent of the outbreak.  A risk assessment by the US Forest Service (click map for details) shows that oak forests throughout California are at risk from this pest.

Additional information:  USDA Forest Service Pest Alert: Goldspotted Oak Borer. (pdf, will open in new window)

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