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Forest Service Scientists Discover 'Pulses' of Gypsy Moth Invasion

Morgantown, WV, November 30, 2006 - Just what is the most effective way to control the spread of non-native insect pests --attack the heavily populated source or go after the far-flung outliers? U.S. Forest Service scientists Andrew M. Liebhold and Patrick C. Tobin and cooperators have recently confirmed that focusing control efforts at the leading edge can indeed slow and perhaps contain the gypsy moth. This finding offers hope and guidance for those working against a broader array of invasive insects, including more recent invasive pests such as the hemlock woolly adelgid and emerald ash borer. 

In a paper published in the November 16, 2006 issue of the journal Nature, Liebhold, Tobin, Ottar Bjornstad of Pennsylvania State University, and Derek M. Johnson of the University of Louisiana described how non-native species spread following establishment. These scientists analyzed historical data on the spread of the gypsy moth, including data from more than 100,000 pheremone-baited traps deployed by the Forest Service and state cooperators. The research team found that the gypsy moth invasion has been characterized by regular periods of rapid spread interspersed with periods of little expansion, a phenomenon termed pulsed invasions.  

The mechanism at the heart of this phenomenon is referred to as “the Allee effect,” which states that a population may decline toward extinction simply because its numbers are too low. It has been observed for several decades by scientists studying the extinction of rare species and its importance during biological invasions has been realized only recently.  In the model developed for the gypsy moth, populations at the expanding population front must grow to sufficient levels to overcome the Allee effect. When this occurs, populations surge forward into new habitats, but then the population front remains static for several years. 

Dr. Kenneth F. Raffa, forest entomologist at the University of Wisconsin, describes this study as taking a basic ecological concept and applying it to an important invasive pest management problem. “Historically, this has been a difficult link to make and this study realizes the value of the US Forest Service's sustained investment in research on invasives.  This is the result of combining innovative conceptual thinking with a strong long-term, geographically broad data base," Raffa said. 

This validation of the importance of the Allee effect in the spread of invasion fronts gives forest health specialists a method to slow the spread of a pest and potentially allow natural enemies to catch up. The gypsy moth has been slowly and inexorably chomping its way through eastern U.S. deciduous forests since its escape from captivity in Boston in 1869. It is now established in North Carolina to the south and Wisconsin to the west and is currently spreading at a rate of about 7 km/year along this front.  The historical (1965-1989) average rate of spread was 21 km/year. Since the 1990s, the spread rate has been significantly reduced, due in large part to the Slow the Spread Program, a cooperative project of states and the USDA’s Forest Service and Animal and Plant Health Inspection Service. 

The U.S. Forest Service is an agency of the U.S. Department of Agriculture, a mission of sustaining the health, diversity and productivity of the nation’s forests and grasslands to meet the needs of present and future generations. The mission of the Forest Service's Northern Research Station is to improve people’s lives and help sustain the natural resources in the Northeast and Midwest through leading-edge science and effective information delivery.

The U.S. Forest Service manages 193 million acres of public land, provides assistance to state and private landowners, and maintains the largest forestry research organization in the world. Public lands the Forest Service manages contribute more than $13 billion to the economy each year through visitor spending alone. Those same lands provide 20 percent of the nation’s clean water supply, a value estimated at $7.2 billion per year. The agency has either a direct or indirect role in stewardship of about 80 percent of the 850 million forested acres within the U.S., of which 100 million acres are urban forests where most Americans live. For more information, visit


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Last modified: November 30, 2006