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KNOXVILLE — Cold weather may not freeze the spread of fire ants into northern states, contrary to earlier expectations, University of Tennessee researchers have found.

UT entomologists have discovered that a fire ant variety crossbred between imported red and black species can survive much colder temperatures than either of the parent species.

Shannon James, a UT graduate research assistant, found that 80 percent of the hybrid ants can survive in their burrows for a week at 32 degrees Fahrenheit, a temperature that kills the red and black varieties of the destructive pest.

“What James found was that the hybrid ant is able to survive these long-term cold temperatures a little bit longer than the parent species,” said Dr. Roberto Pereira, assistant professor of entomology and plant pathology who supervised the research.

“In terms of predicting how well the ants will do here in the United States, most of the models are based on data that was obtained with the red imported fire ant only.”

Both black and red imported fire ants were accidentally introduced from South America in 1918 and have spread steadily across the South. They are destructive to native ecological systems, have painful, stinging bites and cause problems for farmers. The hybrid is the result of crossbreeding in the wild.

James’ research was presented at a recent meeting of the Entomological Society of America in Montreal and was featured on the Web site of Nature, an international science journal.

Pereira, James and Dr. Karen Vail, another UT entomologist, are taking part in an 11-state project to find organic ways to control the pest, including an introduced protozoan and a fly that preys on only fire ants.