Edge Density Key to Controlliing Gypsy Moth Spread

Controlling population peaks on the edges of the gypsy moth range may help to slow their invasion into virgin territory, according to a team of researchers.

“Slowing the spread of the gypsy moth is a priority in forest management in the United States,” says Ottar Bjornstad, associate professor of entomology and biology in Penn State’s College of Agricultural Sciences. “Understanding the underlying patterns in the spread of invasive species is important for successful management.”

The accidental release of the gypsy moth in 1869 in Massachusetts has led to an infestation covering more than 386,000 square miles in the Northeast. Native to Europe and Asia, gypsy moths are currently found from Maine to North Carolina and west into Wisconsin, where they defoliate trees and occasionally cause extensive damage to northern deciduous forests.

“We analyzed historical data on the spread of the gypsy moth in the United States and found that its invasion has been characterized by regular periods of rapid spread interspersed between periods of little expansion,” says Bjornstad. “This is the first identification of pulsed invasions for an invading species.”

The research team—which also included Derek Johnson of the University of Louisiana and Andrew Liebhold and Patrick Tobin of the U.S. Forest Service—used historical, county-level quarantine records as well as forest service data from more than 100,000 pheromone traps set along the expanding gypsy moth population front for their theoretical model. The pheromone trap data were collected from 1988 to 2004.

They used a theoretical model to show how the invasion pulses can be explained by an interaction between negative population growth at low densities—the “Allee effect”—and the existence of a few satellite seed colonies created by human transfer of the insects over long distances. The research was published in a recent issue of the journal, Nature.

The gypsy moth adult female is flightless and usually only spreads a short distance beyond infestation boundaries. External colonies occur when moths hitch a ride on vehicles or other items relocated by people. Without an Allee effect, these colonies would establish, but because gypsy moths exhibit an Allee effect, the low populations are insufficient for establishment of permanent populations.

This is also true at the edges of the population area. If the population density is low, the Allee effect prevents growth across the boundaries. The model showed that no pulsed expansion exists for populations unaffected by the Allee effect. However, when it is a factor, not only does pulsed expansion occur, but it mimics the historic pulses of the gypsy moth population from 1960 to 2002 found in the quarantine records.

Currently, the containment program for gypsy moths aims at controlling outbreaks outside the current population boundaries. The researchers suggest that “the invasion might also be slowed by suppressing outbreaks near the invasion front (within the populated area), to reduce the number of dispersers to below the donor threshold.” This would decrease edge populations and prevent the periodic surges of growth that expand the territory.

Other invading species may also exhibit pulsed spreading. If researchers can determine that the Allee effect is in place, then this same plan of containment might aid in controlling a variety of pests.

The National Research Initiative of the USDA Cooperative State Research, Education and Extension Service supported this research.

—A'ndrea Elyse Messer