Home & Garden Columns

Wild Neighbors: Running on Honeydew: Diet Secrets of the Argentine Ant

By Joe Eaton
Tuesday January 29, 2008

Not that I miss them, but I haven’t found any Argentine ants in the house this winter. I hesitate to consider this a permanent victory, though. They’re out there somewhere, biding their time. 

Linepithema humile has been called, against stiff competition, one of the world’s worst invasive species. Argentine ants don’t sting or bite, like that scourge of the South the red imported fire ant. What they do is more insidious: they disassemble whole ecosystems. They kill or drive out native ant colonies and eat their way through the local arthropod prey base. (They can handle much larger native ant species; photographs of a half-dozen of them dragging down a huge, as ants go, harvester ant are reminiscent of that Planet Earth footage of a pride of lions tackling an elephant.)  

Ant-eating reptiles like the coast horned lizard can’t stomach them, and horned lizard populations have declined by up to 50 percent in invaded areas. Plants that depend on native ants to transport their seeds are left partnerless. The Argentines use exotic plantings like iceplant as staging areas for colonizing native plant communities. 

Once they arrive in a new locale, Argentine ants form supercolonies containing millions of individual workers. Back home in South America, neighboring colonies live in a constant state of mutual hostility. But that isn’t true in California and other Linepithema beachheads in Mediterranean Europe, Asia, Australia, southern Africa, and the Pacific islands.  

According to research by Andrew Suarez, now at the University of Illinois, there is in effect one great big Argentine ant colony in California that stretches from San Diego to Ukiah. Normally, introducing a worker ant to a foreign colony is a death sentence. But you can drop an Argentine ant from Lompoc into a colony in Milpitas and she’ll receive a sisterly welcome, and be put right to work. 

That’s because she’ll have the correct colony smell. South American colonies are genetically varied, and each one has its distinctive odor which serves as a badge of membership. But the Argentine ants in the great California supercolony, descendants of a small founder population, all smell alike. They haven’t had time to re-evolve the variation. Although genetic bottlenecks are supposed to be a bad thing, reducing a population’s resistance to disease and other stressors, these ants seem to benefit from their genetic uniformity. 

Despite that, you would think that Argentine ant booms would eventually go bust, since eating everything in sight is not a sustainable foraging strategy. They don’t, though. These ants have another trick up their sleeves. 

According to a recent study by David Holway at UC San Diego, who collaborated with Suarez, Argentine ants do start out as generalist predators of other insects. At some point, however, they switch to a high-carb diet of the honeydew that aphids and scale insects excrete. “Honeydew nectar is essentially digested plant sap,” Holway says. “If you’ve ever parked your car under a tree and found your windshield covered with sticky stuff, that’s honeydew from aphids and scales.” Think, for example, of the tulip trees on University Avenue. It’s honeydew that fuels the growth of the supercolonies. 

A mutualist relationship with honeydew producers is not rare among ants. What’s unusual is the change from predation to nectar-sipping. It’s as if a band of human hunter-gatherers moved into a new hunting territory, killed off almost everything edible, then domesticated the last few sheep and became pastoralists. 

Holway and his co-authors tracked a Linepithema invasion in Rice Canyon in southern California, documenting the near-extirpation of native ants as the newcomers moved in; native diversity fell from 23 species to two. They used a technique called stable isotope analysis to determine what the Argentine ants had been eating, and identify when their diet changed. Comparing the ratio of heavy to light nitrogen isotopes allowed the scientists to distinguish carnivores from herbivores (or, in the case of the Argentines, carbovores.) 

So it appears that Argentine ants are flexible enough in their behavior to avoid the consequences of ecological overkill. As long as they have their scales or aphids, they’re in fine shape. 

And things are only going to get better for them. Species with limited ranges and narrow habitat and food requirements may be pushed to extinction by global climate change, but not the Argentine ant. Another recent study, headed by Nuria Roura-Pascual at the University of Girona in Catalonia, suggests that unoccupied areas in East Asia, northeastern North America, and elsewhere will become more suitable for invasion in a warmer world. Linepithema marches on. 



Joe Eaton’s “Wild Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Ron Sullivan’s “Green Neighbors” column on East Bay trees.