Sunday, March 20, 2016

Amazing Maryland Bugs: Allegheny Mound Ant (Formica exsectoides)

"Amazing Maryland Bugs" is a WMD101 series showcasing a variety of Maryland's fascinating terrestrial invertebrates, including insects, centipedes and millipedes, isopods, and others. Butterflies and moths, and spiders are featured in their own series: "Maryland Butterflies and Moths" and "Maryland Spiders."

Ants are well-known for exhibiting some of the most complex social structures in the animal world.  Allegheny mound ants (Formica exsectoides) are a native Maryland ant species whose fascinating adaptations and behavior is more than worthy of a closer look.

  • large size
  • red-orange head and thorax, black gaster (abdomen)
  • head is distinctly concave between the eyes
  • creates large thatched mounds in sunny areas
  • during the warm months, the mounds are crawling with busy workers (during the cold months, the overwintering queen and workers rest deep below the mound)
A hard-working Allegheny mound ant - notice the strongly concave head shape.
Despite their common name suggesting a montane habitat range, Allegheny mound ants are actually very widespread and are among the most common ants along the eastern coast of North America.  They are a member of the Formica genus of ants, and like most members of this group, they are large in size, highly predatory, active during the day, and known for secreting formic acid.

A colony of Allegheny mound ants starts like many ant colonies - with a new queen looking for a place to raise her young.  However, an Allegheny mound ant queen is not capable of raising a brood herself - she needs workers to help care for her eggs and larva.  To accomplish this, she infiltrates a colony of a closely related Formica ant, typically the silky field ant (Formica subsericea).  This infiltration is risky business - many invading queens are likely killed by workers before they have the chance to reach the colony's resident queen.  But those that do, will subsequently kill the resident queen and take over the colony.  The dead queen's workers feed the impostor queen and rear her brood until they die of old age and are completely replaced by the new queen's daughters and the colony takeover is complete.  This style of colony establishment is known as temporary social parasitism

An entrance into the mound.
Once an Allegheny mound ant colony is established, it may have more than one queen operating within a supercolony of many mounds, chambers, and networks of tunnels.  A particularly large colony may fragment into several individual colonies, similar to honeybees. Evidence of mound ant colonies are distinctive - open, sunny spaces marked with several mounds of thatched grasses, dirt and wood fragments.  These mounds are seething with workers as they maintain the mound, remove detritus, and return to the colony with food.  

The mounds are solar incubators for the colony's brood, warming the eggs and pupae as they develop.  Shade is the Achilles' heel of the mound ant colony - without the warming sun, the colony will fail as the next generation struggles to reach maturity. Nature, however, abhors a vacuum, and sunny habitats often succeed into woodlands and eventually shady forest.  Allegheny mound ants have a unique method of insuring their sunny paradises are not overshadowed.  When tall vegetation sprouts up around their mounds, workers bite the stems of the plants, then inject formic acid into the wound, gradually killing the plant.  

A mound made by Allegheny mound ants. 
Growers of plants are generally split in their opinion of mound ants as pests or beneficial insects - while their activity will kill plants immediately around the mound, their voracious predatory nature acts as highly efficient pest control over a wide area around the colony. These ants are ecologically significant, maintaining sunny clearings in areas that would have otherwise succeeded to woodland.  Some birds, butterflies, and other animals who prefer open habitats may be able to thrive in areas cleared by mound ants in the midst of less suitable habitat.  It has even been suggested that these ants could be used to help clear areas of invasive plants.

While the dietary needs of an ant colony are complex, the most important food sources fall into two broad categories: proteins and sugar.  Allegheny mound ants obtain most of their protein by predating a wide variety of other insects and spiders, with their large size and numbers allowing them to overpower even very formidable prey.  They are adept at killing caterpillars, a major pest of gardens and crops.  It is fascinating to watch a mound ant colony as workers parade through with bits and bodies of an assortment of local insects.  However, many members of the returning foraging party do not carry prey. Rather, their abdomens are swollen with fluid.  These ants are carrying the second most important food source - sugars - in the form of nectar and honeydew.  Mound ants can often be seen tending and protecting aphids and leafhoppers in return for the honeydew they secrete. 

A habitat maintained by Allegheny mound ants - notice the two mounds and lack of tall vegetation in their vicinity.
The activity of mound ants change throughout the year. In early spring, workers begin to care for the brood at the surface of the colony structure, but not in the mound itself, which is still too cold and exposed to be used as an incubator.  By the summer, brood production is in full swing, with the queen producing many eggs, and the brood is cared for in the mound, which is now warmed by the summer sun.  Workers move the brood up and down the mound to ideal development temperatures and humidity. Winged males and new queens are produced mid-summer, when they mate outside the colony and the males die shortly after. Not every nest produces reproductives each year, and each nest that does seems to invest in either males or reproductive females. Early season queens may return to their original hive to reproduce or parasitize a related species.  Late-season queens may return to the hive to await dispersal the next spring. By fall, egg production stops, and workers prepare themselves and the queens to overwinter.  Through the winter, the colony is torpid, lying in wait for the coming spring. 

Because Allegheny mound ants are common and very conspicuous, they are typically not hard to find and observe.  Colonies can be observed in Soldiers Delight's serpentine barren savanna, but mounds can also be easily spotted and identified from a moving vehicle on the edges of highways!


Bristow, C. M.; Cappaert, D.; Campbell, N. J.; Heise, A.; (1992). "Nest structure and colony cycle of the Allegheny mound ant, Formica exsectoides Forel (Hymenoptera: Formicidae)." Insectes Sociaux 39(4): 385-402. 

Michigan State University Extension: Mound Ants

University of Maine Cooperative Extension: Insects - 195-Beneficial Insect Series 1: Allegheny Mound Ant

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