During a recent visit to a Glass City (a.k.a. Toledo) Metropark, my wife and I spotted swarms of ants blackening the edge of a sidewalk’s pavement. Fittingly, they were non-native Pavement Ants (Tetramorium immigrans, family Formicidae).
Pavement ants were introduced to the U.S. from Europe over 100 years ago. For many years, the pavement ants in North America were thought to be the non-native species, T. caespitum. Indeed, various web resources still cite this species.
However, researchers eventually discovered that T. immigrans is the species that was introduced into North America from Europe. Consequently, T. immigrans is now generally referred to as the “Immigrant Pavement Ant.” The immigrants have become established throughout the U.S., from the ice of Maine and Washington to the heat of Florida and southern California..
"Ant swarms" are most commonly associated with ants mating and establishing new colonies. However, immigrant pavement ants may also swarm for a more nefarious purpose: to conduct full-blown, no-holds-barred ant wars.
For Love and Food
The vast majority of the individuals found in ant colonies in Ohio are wingless sterile females, the "workers." Only queens can lay eggs, but once they start their colonies, they never leave.
Periodically, fertile winged ants, both males and females, are produced. These are called "alates," and the new winged queens will establish new colonies. The alates fly off to mate, and their departure is accompanied by swarms of wingless workers to see them off, presumably waving goodbye with their tarsi (I made that up).
Pavement ants generally produce mating swarms in late spring to early summer. However, if the colony size and environmental conditions induce dispersal, they can swarm at any time.
The immigrant pavement ants prefer to nest where there’s limited vegetation. Their habit of locating their underground colonies beside or beneath sidewalks gives rise to the “pavement” common name. Mounds of loose soil particles emerging from sidewalk cracks or expansion joints are often the work of pavement ants.
The ants scavenge for a wide variety of food, including tiny pieces of breadcrumbs and other detritus in our kitchens, live and dead insects, honeydew from aphids, grease, etc. The worker's ability to range widely to locate food and find their way unerringly back to their colonies while laying down a chemical trail for their sisters is phenomenal.
Pavement Ants Following Chemical Trail
As with other hymenopteran stinging insects (e.g., wasps and bees), immigrant pavement ants have stingers that are actually modified ovipositors (ovi = eggs). Their stingers are too small to penetrate our skin; reports of skin rashes are generally associated with their bites. However, the pavement ant's stingers do play an important role in laying down a chemical trail. Their stingers are broadened at the tip, which acts a bit like a spatula in depositing trailing pheromone.
Bare-Tarsal Brawls
Pavement ants are well known for their pugnacious behavior, with springtime being battle time. Instead of "make love, not war," they make love and war. However, they will also launch full-blown, no-holds-barred ant wars just about any time the mood strikes them.
Pavement ants are very protective of their feeding territory and intolerant of nearby colonies. They are well-known for their bare-tarsal brawls. Battles may occur as a massive, swirling clash of six-legged combatants or as a series of smaller, pitched skirmishes with constantly shifting battle lines.
A close examination of the melee will reveal ants locked mandible-to-mandible in ruthless combat. Battlefield injuries range from crushed abdomens to dismemberment. Ant wars provide questionable entertainment, and I've often wondered if the Romans got their gladiatorial ideas by watching pavement ants.
Some ant wars arise as territorial disputes resulting from scavenging workers based in adjacent colonies continually bumping into each other. Others occur as colonies try to expand their territories with two colonies "planting their ant flags" in each other's territories. These disputes are settled on a neutral battlefield between the two colonies, presumably with ant-drums and bugles blowing.
The most brutal battles happen when one colony decides to raid a nearby colony. These fights are bloody affairs with macerated bodies quickly piling up.
Colony raids occur right on top of the colony that's having a bad ant day. The defending colony quickly pours all available combatants into the fray. Even winged alates may be seen mixing it up with the opposing force. Although alates are much larger than their colony kin, they are built for love, not war. They do not fare well with their farewell marked by disassembly.
Pavement Ant War Alate Under Attack
Pavement Ant War Alate with Wings Ripped Off
Most ant wars are settled quickly. The engagements are over after a few hours, with nothing left on the battlefield to mark the epic confrontation. That's because the spoils of war for ants include the bodies of the defeated, which are trundled off to feed the victor's colony … a different twist on carry-out.
Selected References
Chalissery, J. M., Gries, R., Alamsetti, S. K., Ardiel, M. J., & Gries, G. (2022). Identification of the trail pheromone of the pavement ant Tetramorium immigrans (Hymenoptera: Formicidae). Journal of Chemical Ecology, 48(3), 302-311.
https://doi.org/10.1007/s10886-021-01317-3
Maxcer, M., Williams, J., & Lucky, A. (2023). Immigrant pavement ant Tetramorium immigrans Santschi (Insecta: Hymenoptera: Formicidae): EENY-600/IN1047, 3/2023. EDIS, 2023(2).
https://doi.org/10.32473/edis-in1047-2023
Moss, A. D., Swallow, J. G., & Greene, M. J. (2022). Always under foot: Tetramorium immigrans (Hymenoptera: Formicidae), a review. Myrmecological News, 32, 75
https://doi.org/10.25849/myrmecol.news_032:075
Wagner, H. C. (2017). Light at the end of the tunnel: Integrative taxonomy delimits cryptic species in the Tetramorium caespitum complex (Hymenoptera: Formicidae). Myrmecological News, 25, 95.
https://doi.org/10.25849/myrmecol.news_025:095