On The Trails: Ants

An Argentine ant, which has become an invasive species in Juneau. (Public domain photo by Penarc, republished under a CC BY-SA 4.0 license)

By Mary F. Willson

Ants?? Why think about ants when the outside temperatures at my house haven’t been over zero degrees (F) for days? Well, thoughts about ants are one kind of nice change from mentally feeling the cold (even in my warm house). And I was prompted by receiving two messages about some interesting ants.

Ants are highly social creatures, living in colonies presided over by a queen. She is the mother of all the larvae, most of which will be workers when they grow up. Meanwhile, the younger larvae need to be fed by their older siblings, which are now working as foragers. Workers make long foraging trips to bring back food for larvae, in some cases laying down long scent trails to good food sources, so other foragers know where to go. 

Larvae of most ant species are fed on a soup made up of chewed bits of prey and plant scraps, and maybe some honeydew produced by bugs that eat plant sap. This is transferred directly to the larval mouths.

Exceptions include the leaf-cutter ants of the tropics, famous for their long lines of workers, each one toting a cut-off piece of leaf back to the nest.  There, the workers use the leaf bits as a substrate to cultivate special fungi, which are fed to the colony’s larvae. 

Worker ants typically are non-reproductive females. They have become evolutionarily specialized for their foraging jobs by a process called kin selection in a way that benefits their kin (siblings and the queen).

Sometimes kin selection may go beyond such special job skills. When worker ants become sick, they typically go off to die alone. But very sick young ant pupae send out a special scent-signal from their cocoons to nearby workers to "Come and kill me." Workers respond, killing not only the pupae but also the responsible pathogens that made them sick. That prevents the pathogen from destroying the queen and remaining larvae. It thereby allows the shared genes of those self-sacrificing larvae to survive in their kin and, importantly, be passed on by the queen to the next generation. Queen pupae apparently don’t self-sacrifice; they probably have better immune systems. 

Here in Juneau, we do have ants, although I don’t see them very often. There are the usual little ground-foraging ants that crawl across pathways and gardens, carpenter ants that like wet, decomposing wood in the forest and houses, and reportedly some of the highly invasive Argentine ants. Argentine ants are native to southern South America, but they have been introduced, intentionally and unintentionally, to many places around the world. They are unusual in forming super-colonies, with numerous nests, and adjacent populations often meld together. A super-colony may have many queens and trillions of workers, sometimes extending for hundreds of miles. Super-colonies can be very territorial, enforcing their borders with pitched battles leading to multitudes of dead ants.

Argentine ants also interfere with native ants, attacking their nests. Depending on location, they may deplete the insect prey of other animals, displace ants that would disperse seeds, eat the larvae of pollinators, and foster the populations of insects (e.g. aphids) that attack important food plants. They like warm, moist environments and sometimes move from shallow ground nests to houses, where they nest in crevices and protected places. They are among the most invasive animals in the world.

There’s more on ants in a previous essay. 

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com.