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A SPECIES鈥� survival normally depends on its genetic diversity. The Argentine
ant, however, is conquering the world with the opposite strategy. This invasive
creature has established a homogeneous 鈥渟upercolony鈥� in the US capable of
dominating all local ants, say researchers in California.

The Argentine ant (Linepithema humile) arrived in the US in the 19th
century, in shipments of South American coffee delivered to New Orleans. Its
descendants have since spread as far as California, displacing local species
throughout the state. The ants have also made it to Europe, South Africa and
Australia.

New infestations usually grow from a few closely related individuals. This
forms a genetic bottleneck in a population鈥檚 history, and increases the chances
that a single disease or environmental change could wipe it out. Yet Argentine
ants have thrived in the US, and now biologist Neil Tsutsui and colleagues at
the University of California in San Diego have found that the bottleneck has
actually helped them.

California鈥檚 Argentine ants, unlike most species native to the US, don鈥檛
fight with ants from other nests, and will even tend each other鈥檚 broods. The
scenario in Argentina is very different. 鈥淚f you take an ant from San Diego and
drop it in a nest in San Francisco, it will walk in as if it鈥檚 right at home,鈥�
he says. 鈥淲hereas in Argentina, you pick up an ant, walk 50 metres and put it
into another colony and it gets torn to pieces.鈥�

Tsutsui thought this might be because the US ants are more alike and so are
less likely to view each other as enemies. Looking at ants from dozens of nests
in California and Argentina, the team sampled seven genes, each of which had a
number of different versions, or alleles. Whereas nests in California shared
between 75 to 100 per cent of their alleles, nests in Argentina rarely shared
more than 40 per cent. Furthermore, the greater the genetic difference, the
more aggressively the ants behaved toward one another when placed together in a
jar. The least related pairs of ants typically charged and tried to bite, but
closer kin ignored each other.

The conclusion is that California hosts what could be the biggest ant colony
in the world, since all the ants behave as if they were from one nest. Since
they don鈥檛 waste energy on aggression, they can spend it all on finding food.
Local ants can鈥檛 compete.

One way to bring the Argentine ant under control might be to introduce
genetically distant colonies, Tsutsui suggests. 鈥淚t鈥檚 a fight-fire-with-fire
strategy.鈥� But there are risks, he points out. Other control methods that take
advantage of their genetic similarity, such as introducing a parasite, might
then fail.

Kenneth Ross, an expert on fire ants at the University of Georgia, points out
another problem. The bottleneck probably left behind many detrimental genes that
have remained because of a lack of alternatives. Many fire ants, for example,
are sterile. Boosting variability might make Argentine ants even more
invasive.

But Ross believes the Argentine ants may soon meet their match. The much
fiercer fire ant has already established a beachhead in southern California. 鈥淚
predict the fire ants are going to completely overwhelm the Argentines,鈥� he
says.