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Native Winter Ant Resists
There’s a lot we don’t know about Argentine ants--including their natural history in their lands of origin. In Argentina and Brazil, they are hard to find and little studied. Yet they do seem to be very territorial, forming discrete colonies with limited range. Intraspecific aggression helps them keep these lines drawn.

In California, in contrast, “Argentine ants from different nests don’t fight with each other much,” Gordon said. “It was thought that that meant they were all genetically related and the whole coast of California was essentially one big supercolony.” Through genetic and behavioral study, Gordon’s team has refined that understanding, observing that they maintain dispersed nests seasonally: during the wet months they live in one large colony, all together, and in the dry season they fan out and establish multiple nests, often with multiple queens. “We don’t know how they decide who ends up in which nest,” said Gordon. It does appear that even when they’ve established satellite nests, they are not strongly territorial. Ants still move around, contributing to colony maintenance. And they still get along just fine.

Gordon has a theory for the Californian population’s easygoing ways. When ants eat, they incorporate hydrocarbons into their bodies, chemicals that are then used to signal colony odor. In their South American homeland, each colony may have a discrete diet, and so each colony will signal “difference” through its smell. In California, Gordon said, this sensitivity to smell seems to be diminished. “That may have something to do with diet,” Gordon said--specifically, “the extent to which they are all eating McDonalds out of garbage cans. Maybe their diet is so homogeneous in the urban places where they are living because our diet is so homogeneous.”

One native species on the Jasper Ridge preserve, however, does seem to be withstanding the foreign onslaught. The winter ant (Prenolepis imparis) looks a lot like the Argentine ant, but it’s a little bigger. Like the Argentine ant, it is a dietary generalist, and it also tends aphids. (The aphids suck sugars out of the plant and then secrete “honeydew,” which the ants eat; in return, the ants defend the aphids from potential predators.) “It may be,” said Gordon, “that the winter ant can resist the Argentine ant. What we see is places at the edge that blink back and forth from year to year, where the Argentine ants move in and then get pushed back. By looking over the climate data for the last 14 years, it looks like wet years are better for Argentine ants.” Last year was dry, and sure enough, the winter ant’s range has expanded. Gordon’s team is watching this rivalry with interest, hoping to learn more about interspecific competition and the invasive-native balance of species.

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