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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Mounds of mountain marvels: Allegheny mound ant, Formica exsectoides

 

With jaws engaged a drop of venom is squirted on the skin of the victim to leave a more lasting impression of the encounter with an Allegheny mound ant.

 

Formed some 480 million years ago, the ancient Allegheny Mountains support a rich community of insects. One of the most conspicuous of the residents is the Allegheny mound ant. On a recent visit to the hills near Mathias, West Virginia, I had the opportunity to get up close and personal with these earth movers of the eastern forest. Relatives of leaf cutter ants, carpenter ants, army ants, and fire ants we met in previous episodes, Alleghany mound ants form remarkable colonies that can number in the hundreds of thousands.

Even modest Allegheny mound ant nests can bring a smile to an entomologist’s face.

These busy ants create impressive mounds. Most mounds I observed ranged in size from about three to as much as nine feet in diameter but historical records describe colonies almost twice this large. Similarly impressive was the height of mounds that often bested two feet. Nests are all the more conspicuous by virtue of the ant’s tenacious vegetation management program that removes nearby plants from a wide zone surrounding the colony.  These very large domed shaped nests are believed to create ideal conditions of temperature and humidity in which to rear developing brood. Unlike other social insects, including honeybees and bumblebees, mound ant colonies contain multiple reproductive queens, a phenomenon known as polygyny, meaning “many women”. These multiple queens are capable of producing vast numbers of sterile workers tasked with the important jobs of tending the young, gathering food, and defending the colony. Ant larvae are can be found at several depths within the mound and their locations may be modified to adjust for changing conditions of temperature and humidity.

 

 

On the surface of a large Alleghany mound ant nest workers busily reposition grains of soil, while on a nearby foraging trail some workers return to the colony with food as others pass to pick up their loads.

Like other species of ants, Allegheny mound ants are omnivores consuming a wide variety of foods, including the honeydew of sucking insects like ants and scale insects. In addition, Alleghany mound ants have also been observed carrying plant parts and dismembered pieces of other insects and spiders to their nests. The following story of the ferocity of Alleghany mound ants was recounted by the naturalist Henry McCook in 1877 “ Into the little colony…was introduced a large female wolf spider (Lycosa leuta), one of the most powerful and ferocious species. The ants attacked her with demoniacal fury, and in a moment had torn off her limbs, and were hurrying the mutilated body into the galleries. The attack showed the courage that was quite characteristic, but the method and results I was wholly unprepared for….” 

Vents around the perimeter of the nest allow ants to enter and leave.

It’s always informative to test defensive capabilities of social insects like ants (see “In a warming world, here comes the red imported fire ant, Solenopsis invicata”), so I stood on a smallish Alleghany mound ant nest to witness their resolve. They passed the test with flying colors and rewarded me with numerous bites that were not very painful. However, these biting ants had one more trick. As I watched an ant on my finger I observed that once the ant’s mandibles were embedded in my flesh, she curled her abdomen beneath her body and delivered a healthy dose of venom to the skin near the bite wound.  Poison glands in the abdomen of many species of ants produce an array of irritants and toxicants with formic acid being quite common. Formic acid applied to skin broken by the bite creates much more discomfort than the bite itself. I quickly removed the remaining ants clinging to my body. Alleghany mound ants can be found from Nova Scotia to Georgia. Summer is a great time to head for the hills and enjoy these marvels of the mountains.       

 

A fierce attack by a fearless worker includes biting and delivering a dose of venom laced with formic acid.

References

“Mound-Making Ants of the Alleghenies, Their Architecture and Habits” by Henry C. McCook, and  “Nest structure and colony cycle of the Allegheny mound ant, Formica exsectoides Forel (Hymenoptera: Formicidae)” by C. M. Bristow, D. Cappaert , N. J. Campbell, and A. Heise were consulted for this episode. Bug of the Week thanks Nan, Dug, Barb, Bill, and Chris for the inspiration for this episode and the clever Dr. Shrewsbury for photographic assistance.