The Ohio State University's Department of Entomology received information about an infestation of field ants from a homeowner in Holmes County, Ohio in August 2024. The homeowner took an image from his County office and sent it to the OSU Forest Entomologist, Kayla Perry highlighting the extent of the colonies, and requesting information about the ants. The infested area was about 3 acres of forested land. A recent aerial image clearly showed a large number of mounds scattered in proximity to each other. The homeowner counted and flagged 41 ant mounds on the property.
The homeowner collected a few ants and brought them to the OSU, Department of Entomology We identified these ants as Allegheny mound ants, Formica exsectoides.
These ants are native to North America. They are reported from the eastern and central United States and eastern Canada (Ontario, Quebec, and Nova Scotia). They are one of the largest mound-building ants in North America. In the Great Lakes region, these ants are reported to be abundant in jack pine (Pinus banksiana) forests (Bristow et al 1992).
F. exsectoides are about 4.5 to 7 mm in size. Their head and alitrunk are reddish brown while the gaster is black. The species can be identified using the following characteristics: 12 segmented antennae, metapleuron with distinct oval orifice, triangular mandibles with 8 or more teeth, propodium rounded but distinctly angulate, top margin of the head is strongly concave (when viewed from the front), the presence of erect hairs on the posterior half of the gaster, and the lack of erect hairs on the pronotum.
Colony location and construction
F. exsecoides has been observed to construct nests mainly in openings of oak and pine groves, as well as in open meadows and prairies. I observed all 41 colonies scattered in about 3 acres of land on the homeowner's property in Holmes County. These nests were abundant in the wooded areas and are expanding towards the nearby open grassland.
The image shows 6 mounds closer to each other, possibly with underground connections. All the mounds were constructed in well-lit areas. No mounds were found in the shade. These ants prefer warm sunlit areas to build their nests. So, they select open areas and kill the vegetation around them by injecting the formic acid they produce. In this picture, you can see one of the trees (arrow) closer to a mound is dying. This strategy helps ants to clear the vegetation around the area so they can expand the colonies.
These ants make complex colonies that are interconnected with underground tunnels. One colony may have more than one queen. The colony can be up to 4 feet underground, and the ants can construct sub-conical, above-ground soil mounds up to 4 feet as well. The largest mound we encountered on the property was 3 feet high and 4 feet in diameter. Some of the fresh ant mounds are covered with a thick layer (2-3 inches) of finely chopped twigs. These were placed on top of the mound to act as a roof for protection from extreme weather events like rain. The above image shows such a mound ‘thatched’ with a ‘roof’.
Ant mounds are indeed fascinating feats of engineering. The ants excavate soil from underground tunnels and use it to build up the mound aboveground. The structure of the mound is an intricate combination of tunnels and chambers, each layer built as the ants dig deeper and expand their underground network. This dual construction method, excavating soil from below while simultaneously building up the structure above, demonstrates the impressive coordination and organization within an ant colony. Each layer of the mound can help regulate temperature and humidity within the nest, showing how these small creatures create complex and functional habitats.
These ants usually kill the surrounding vegetation as they build the nest, but I have seen they often incorporate live plants into their nests for various reasons. It is not clear why they leave these grasses and branches alive. As I tried to see the cross section of one of the mounds, I could see the live grass blades inside the galleries. One possibility is that the live plants may help stabilize the structure of the nest. Live vegetation could provide additional support and insulation, enhancing the nest’s durability and protecting the ants from environmental extremes.
F. exsectoides are aggressive and will bite when they are threatened. When the ant nest is disturbed, it is not possible to go near the mound without getting attacked. They often communicate with each other through pheromones. It is amazing how they prepare to attack quickly and in large numbers within a short period of time.
A primary function of the mound is microclimatic regulation. The studies conducted using F. exsectoides found that the mounds are built in reference to the sunlight and all parts of the mound are warmer than the surrounding environment. Most interestingly, different faces of the mound have different temperatures, suggesting that these chambers are constructed to control the temperature inside the mound (Andrews, 1927, Holldobler and Wilson, 1990)
The main food source of F. exsecoides is sweetened liquids like honeydew secretions by aphids, mealy bugs, and scale insects. Also, they extract nectar from nectar-producing flowers and trees. I observed spicebush (Lindera benzoin) under a poplar tree (Populus sp.) completely covered with black sooty mold fungus, suggesting honeydew-secreting insects are living in the nearby poplar tree. A trail of ants was observed going up and down the poplar, as workers harvested honeydew from the insects living in the canopy.
Sooty mold-infested spicebush plant below the poplar tree. An ant trail was seen climbing the poplar tree to harvest honeydew from the insects. Sooty mold is a good indicator for the presence of honeydew-secreting insects in the canopy above.
In addition to collecting honeydew, the ants will prey upon small arthropods and various phytophagous insects, which has been beneficial for pest suppression in some cropping systems such as wild blueberries. While beneficial effects have been documented, this ant species can cause damage to trees and vegetation around their nests. The homeowner in Holmes County had an extremely large population of F. exsectoides that would be difficult to control. In this situation, the adverse effects of the ants likely outweigh the benefits they may have.
Life cycle
F. exsectoides has a fascinating strategy for colony establishment due to their temporarily parasitic nature. Newly mated queens of F. exsectoides seek out and infiltrate the nests of their host F. subsericea, another Formica species reported in North America. Upon entering a host nest, the parasitic queen kills the resident host queen. Initially, the host workers, who are still present, care for the brood (eggs, larvae, and pupae) of the invading F. exsectoides queen. The host ants unwittingly raise the parasitic ants' young. Over time, the host workers die off, leaving behind only the workers of F. exsectoides. This gradual takeover ensures that the host colony is eventually replaced by the parasitic ants. Once all the host workers are gone, the F. exsectoides workers remain and continue to maintain and expand the colony. The newly established colony is now composed entirely of the parasitic ants, with the host’s original colony structure and workers completely replaced.
At the homeowner’s property in Holmes County, Formica exsectoides appears to be the dominant Formica species present. I did not observe any other Formica species on site or in the samples collected. It appears that F. exsectoides has successfully established itself there.
F. exsectoides can complete its life cycle in 8-12 weeks. In the eastern United States, queens and workers normally hibernate from October to May of the following year. As the weather warms in the spring, the queen starts laying eggs in June. After a few weeks, the eggs will hatch, and existing workers will tend the newly emerged larvae with small arthropods until honeydew-producing insects become active. After about 5 weeks, the larvae will pupate, and adult workers will emerge another 2 weeks later.
Brostow et al. (1992) found that only workers and queens were found inside the ant colonies during March and April in central Michigan. No brood or pupae were found during this period. During early June, they discovered the early stages of the brood, and in late July and early August, the colony had late brood, pupae, and winged males. Only pupae were present in the ant colonies in mid-September. They also discovered that the spatial distribution of the brood varied within the nest depending on the time. For example, early in the season, most broods were located in the upper half of the nest above the soil surface. Midseason excavations found most of the pupae deeper in the mound at a depth of 150 cm. Knowing this information may be vital to control these ants. Late in the season, only a few pupae were found inside the colony but all were present in the upper portion of the mound. These findings suggest that the ants are moving around the brood depending on the time/season of the year. While Brostow et al. (1992) did not observe any flight activity or mating, alate queens and winged males were observed on low foliage during late July and early August.
Interesting facts about F. exsectoides
Haris and Savage (2020) observed three snake species, Diadophis punctatus (Ring-necked Snake), Storeria occipitomaculata (Red-bellied Snake) and Thamnophis sirtalis (Eastern Gartersnake) inhabiting active F. exsectoides nests in the fall in northeastern Pennsylvania. It is well documented that snakes seek shelter deep inside the soil to escape cold temperatures. Often these ant mounds are warmer than the surrounding environment during the winter (Haris & Savage, 2020). While this observation is interesting, it is unclear why an active ant colony would allow the snakes to enter and reside in their nest.
Allegheny mound ants, along with termites, worms, and many other mammals contribute to bioturbation. Bioturbation is a process by which individual behaviors alter or displace soil or sediment. This process plays a very important role in shaping the environment via changes to soil structure and composition. Heather et al. (2021) found that a mound built by F. exsectoides contained more than a ton of earth.
Management of F. exsectoides
Since these ants construct very large mounds and part of the nest is located underground, it is not an easy task to completely control them in one season. This is especially true for well-established colonies like those found on the property in Holmes County. The homeowner said there were more than 10 ant colonies when he purchased the land more than two decades ago, and since then, the mounds have expanded into the nearby meadow, totaling 41 colonies. Most of the time these colonies are interconnected, which makes control more difficult.
The best scenario in this situation is to use slow-acting granular bait without disturbing the colonies. There are baits one can buy online and in garden stores that are labeled for field ants. These baits will work for Allegheny mound ants since they are honeydew/nectar harvesting ants and will take the sweet baits to their colony to feed the young workers and queens. Timing is important to do this task. These ants are active during spring and summer. This is the best time to apply these granules.
Some residual insecticides require the complete removal of the top portion of the mound to reveal the ant nest before application. The disadvantage of this method is that ants are very sensitive to disturbances. As soon as the nest is disturbed, they will communicate through pheromones, and alert the other ants in the colony. This will lead to the relocation of the eggs, their young, and the queen/s into a safer place ensuring their survival.
Regardless of the control method, it is highly unlikely that large populations of these ants can be eradicated within a single season. The homeowner in Holmes County will have to monitor for ant activity and re-apply the control measures again next year. It is not uncommon for reapplication to be required for several years to achieve control.
These ants are foraging for honeydew/nectar in trees and shrubs. Managing nectar-producing bushes and honeydew-secreting insects like aphids, mealy bugs, and scales will be helpful in controlling these ants.
It's true that there are many unofficial methods and anecdotal reports regarding ant control, including using a brush hog to destroy ant mounds and success stories by doing so. A brush hog can break apart the mounds and disturb the colony, potentially killing ants in the immediate area and disrupting their nest. This approach might provide temporary relief from an ant infestation by physically removing the colony and its mounds. Ants are highly resilient and can often rebuild their colonies or establish new ones nearby, so this method might not offer a long-term solution.
As always, consider insecticides as a last resort. Some insecticides are not available to purchase or ship to Ohio, so check the label and restrictions before you buy. Also, these insecticides are toxic to the environment, humans, and other animals when exposed. Use proper precautions when you apply them and always follow the label instructions.