In 1918 a few stowaways from South America arrived on North American shores, dug in and made themselves at home. Since then, the descendants of these imported black fire ants (Solenopsis richteri), and their red cousins (Solenopsis  invicta) that followed, have advanced across the southern
United States, rooting out other insects and stinging humans and livestock in their paths.

Now new findings hint that the temperature barrier keeping the northward spread of these destructive fire ants in check may crumble, if a hybrid of the two species proves better than either parent at bearing the cold.

Old models of fire-ant expansion, predicting that Tennessee's cold winters would freeze the ants out, may be faulty, according to a team from the University of Tennessee, Knoxville. Shannon James, Roberto Pereira and Karen Vail announced their findings last week at a meeting of the
Entomological Society of America in Montreal, Canada.

In the wild, ants take shelter in burrows, the researchers argue, so the kind of below-zero temperatures that early models used would be rare.

Farmers, ranchers and environmentalists are alarmed by the kind of 'scorched earth policy' operated by the imported fire ants, that leaves few native insects alive and harms workers and cattle. This testy creature not only bites when disturbed, but also circles its tightly clamped jaws, stinging its victim in a burning, blistered ring.

James tested the long-term survival of the ants at temperatures near freezing and found that a hybrid of the two species lives longer at cold temperatures than either of its parent species. In the lab, 80% of hybrid ants survived for more than seven days at zero degrees centigrade, compared with only 20-40% of the parent species.

"I think they're really on to something," says US Department of Agriculture entomologist Sanford Porter, who studies the control of fire ants with parasitic flies. More research like this will be key to marking out the northern limits of the fire ants' range, he believes.

Distribution maps show that the hybrid lives in the overlap directly between the ranges of the red and black ants. Cold tolerance could help hybrids shift farther north.

But cold tolerance also depends on behaviours such as moving deeper underground, Pereira points out. Just as wearing warm clothes has extended our cold-weather tolerance to match that of Arctic mammals, behavioural
factors may mean that all three varieties could already be at their northern limits.

Other ants have staged similar, far-reaching invasions. The pharaoh ant (Monomorium pharaonis), which originated in India or Africa, became a common nuisance in European hospitals 100-150 years ago. With a taste for blood products, pharaoh ants find their way into dressings and intravenous
lines, spreading bacteria.

"They'll traipse around in cesspools and garbage and then go climb around on your toothbrush," adds David Williams, an ant-invasions expert at the US Department of Agriculture in Gainesville, Florida.

The populations of introduced ant species like this often explode, he says, because they leave behind their natural enemies: competitors or diseases. But cold has usually blocked their spread.

  1. James, S., Pereira, R., Vail, K. Cold tolerance
in imported fire ants. Poster presented at the annual meeting of the Entomological Society of America, Montreal, Canada (2000). Fire Ant Research - Ames Plantation
http://www.amesplantation.org/FireAntResearch/fireantres.htm
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