Data from traps show Allee effect may prevent spread of gypsy moth

Controlling population peaks on the edges of the gypsy moth range may help to slow their invasion, said Ottar Bjornstad, associate professor of entomology and biology at Penn State.

“Slowing the spread of the gypsy moth is a priority in forest management in the U.S.,” he said. “Understanding the underlying patterns in the spread of invasive species is important for successful management.”

Bjornstad and his team found the gypsy moth’s invasion in the United States has been characterized by regular periods of rapid spread interspersed between periods of little expansion.

“This is the first identification of pulsed invasions for an invading species,” he said.

The team 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 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 over long distances, explain the invasion pulses. The gypsy moth adult 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. 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.

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For more: Ottar Bjornstad, Penn State University, (814) 863-2983; onb1@psu.edu.