Brown ants moving in mass along sidewalks



Hello, I am in Toronto and have located a colony of small reddish/brown ants living under the 6x6 wooden ties surrounding my lawn. My question is: are these ants beneficial to the eco-system and should therefore be simply left alone? I have a wooden porch: should I be concerned about 'an invasion'?
Secondly, I have occasionally seen the same type of ants moving in mass across sidewalks - thousands of them - so many that it looks like a brown stain on the sidewalk. Can you tell me what causes this phenomenon?
Thanks for your help,

Mary


Dear Mary,


Thanks for writing to the AntBlog! It was a pleasure to answer to your interesting questions.

Ants play a huge role in an ecosystem: they are diverse (we estimate 30,000 ant species living on Earth), and are in great numbers everywhere (all the ants weigh almost the same as the 7 billion human beings). Along their evolution, ants established ecological relationships with a large array of plants and animals. They are prey, predators, symbionts, parasites (there are even slave maker ant species!), seed dispersers, pollinators, and so on. Ants move more soil than earthworms. They impact and are impacted by almost everything surrounding them. More, they have a short lifespan, and that means their nest population is constantly being replace by new generations of ants. So, if something happens with an environment you will notice the effects faster and with more details if you look at the ants, and it will be much more effective than looking at birds or mammals, for example.

Just for curiosity, ants are important for other aspects of human societies. Their behavior is used as model to create smarter traffic lights, or to develop software that will evaluate the response of our bodies to the effect of new drugs (see here, here, and here). Anti-inflammatories, antibiotics, and even drugs to fight against cancer were/are being developed with substances ants secrete (here, here, here, here, and here). Finally, have you ever thought of including ants in your menu? Many human cultures around the world did! See here.


Salad of Oecophylla smaragdina queen brood mixed with some worker ants, mint leaves, spring onion, chili, and fish sauce. Popular in Thailand and Laos. Image by Joost Van Itterbeeck/AntWeb.org

Based on the behavior you described, I believe you found pavement ants (genus Tetramorium). Unlike carpenter ants (genus Camponotus), pavement ants don't cause any structural damage to your house (and just to take Camponotus out of the fire, those ants nest in decayed wood; so, if the wood in your house is in a good shape, carpenter ants will not be a problem).

Pavement ants get their name because they nest usually underneath or at the edge of sidewalks, and other hard surfaces. They are an introduced species from Europe; and in your garden they will: harvest seeds -- some of which will eventually grow around their nests; tend insects on plants, collecting sugary dropping they produce (A.K.A honeydew), and protecting them from predators; and predate other insects.

The pavement ant workers are dark reddish-black, about 2.5-4 mm long; the petiole, which connects the mesosoma (i.e., the modified thorax of ants) and gaster (modified abdomen), has two segments. The posterior part of the mesosoma has two spines that project upward, and they have a stinger in the last abdominal segment.


Lateral view of Tetramorium caespitum. Image by Will Ericson/AntWeb.org

When two pavement ant colonies overlap, worker ants leave the nest to establish their territory boundaries before ants from the other nest push them out of there. Then, ants coming from each nest collide in a massive battle. The combats are sometimes ritualized: they will just size each other strength, and produce very few casualties. In another occasions, they will ripe one another apart, and thousands of corpses will be left on the sidewalk afterwards.


Sidewalk ant war. Image by the fabulous Alex Wild (www.alexanderwild.com)


Cheers,

Flavia Esteves and the AntAsk Team

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