Tetramorium immigrans is famous for human-assisted dispersal and has already been reported in 39 states in the continental United States and three provinces in Canada. Because of the human-assisted spread and its dominance in urban environments, this species is commonly known as ‘immigrant pavement ant'.
A company in Ohio contacted the Entomology Department of The Ohio State University in late May 2024 inquiring about possible actions needed for one of their shipments that was contaminated with ants, either en route to Mexico or in Mexico.
Upon contacting Animal and Plant Health Inspection Services (APHIS), they advised the company to have the package delivered to the nearest customs and border protection location and have it treated. After the appropriate eradication method had been followed to ensure there were no ants alive, the shipment was delivered back to the company in Ohio.
The company then collected about two dozen dead ants and submitted them in a vial (dry form, not in alcohol) to the Plant Pest Diagnostic Clinic at The Ohio State University for identification.
Taxonomy and Identification
The ants were identified as Tetramorium immigrans or commonly known as immigrant pavement ants. These ants are dark brown to brownish-black. The average length is about 3 mm. Similar to other species of social ants, the colony has one or more queens, many workers, and can produce alates (nuptial flights occur in spring and summer). Workers have a small stinger that is broadened distally.
T. immigrans can be identified easily using the following combination of characters. The head and mesosoma have parallel rugae (series of parallel lines) running dorsally. The antennal insertions have a ridge that is formed by part of the clypeus and frons. The alitrunk has the same parallel rugae present dorsally. There is a single pair of sharp propodeal spines, and a small anteroventral tooth in the first node of the petiole.
Tetramorium is a genus of ants in the subfamily Myrmicinae. There are more than 750 species of Tetramorium in the world and nearly half of this number has been reported in the Afrotropical region. In the nearactic, it is believed that there are 6 species, but this number is still debated.
This ant species was first officially collected in Chile in 1922 and cataloged under the scientific name Tetramorium caespitum. In 1927, Santchi reassessed the specimens and renamed them as T. caespitum var. immigrans (Moss et. Al. 2022).
The earliest confirmed record of this species in North America was in 1927 from specimens collected in Maryland, New Jersey, New York, Pennsylvania, and Washington D.C., but unconfirmed records exist from as early as 1748. It is believed that the pavement ants were unintentionally shipped to the eastern United States from Europe in the early nineteenth century in soil ballast or cargo ships.
In 2017, Wagner et. al. identified 10 cryptic species from Europe using mitochondrial DNA, nuclear DNA, the morphology of workers and males, and their ecology. This included T. immigrans as one of the species in the T. caespitum species complex.
Zhang et. al. (2019) reassessed the identification of the Tetramorium species collected from 26 states across the continental United States using morphology and DNA sequencing (ddRADseq), and determined that all records were T. immigrans. No other cryptic species in the T. caespitum complex were found among these samples. Moreover, very little genetic difference (∼0.0%) was observed among T. immigrans found within the United States, suggesting that they may still be closely related to the T. immigrans in the native range. High genetic diversity would indicate that North American populations descended from multiple lineages of T. immigrans. A comparison of populations from the United States and Europe using a short fragment of the COI gene indicated that all haplotypes of T. immigrans found in the United States were identical to the most commonly collected haplotype from Europe.
The study also documented that the most common, widespread pavement ant in North America is unequivocally T. immigrans, as evidenced by COI and ddRADseq data from colonies sampled across 26 states in North America. No evidence of other T. caespitum complex species was found beyond the known presence of T. tsushimae (Steiner et al., 2006), which can be readily distinguished from T. immigrans based on morphology and geographic distribution.
Ecology, life cycle, and fascinating facts about this ant
The native range of this ant species is believed to be Anatolia and the Caucasus in Europe. The current distribution of the ant ranges from Mediterranean, Eastern, Western and Central Europe, Anatolia, Caucasus, and North and South America.
Similar to other ant species, T. immigrans forms large subterranean colonies. These ants are very common in cities where they readily consume food waste products for proteins, fats, and carbohydrates from plant to animal sources (Zhang et al. 2019). Colonies of T. immigrans can be found in human modified environments, especially near concrete paving, rock walls, railway tracks, inside buildings, under stones along roadsides, and in parks and gardens. They prefer warm, moist environments, but easily tolerate cold and dry conditions during the winter inactivity.
Vanek and Potter (2010) documented the ability of T. immigrans to protect Magnolia scale (Neolecanium cornuparvum, Hemiptera: Coccoidea) from its parasitoid by constructing earthen shelters. The shelters are like mud tubes that conceal the scales and prevent parasitism by Pelecinobaccha costata (syn= Ocyptamus costatus, Diptera: Syrphidae). This study was conducted in Lexington, KY in 2010 using several Magnolia spp., which had Magnolia scale infestations, the largest and most conspicuous native scale insect in the eastern United States. They observed ants cover immature scales as early as mid-May before the scale parasitoid fly was active. They also observed T. immigrans tending oak lecanium scale (Parthenolecanium quercifex (Fitch)) and calico scale (Eulecanium cerasorum (Cockerell)) at numerous locations. Even though ant-hemipteran mutualisms are well documented with other ant species, it is not fully understood why T. immigrans construct protective structures against natural enemies, especially when their workers are extremely aggressive towards intruders.
Parasitic ants
Among the ants, fewer than 2% of species are known to be parasites. It is believed that the ant species Tetramorium atratulum (Syn - Anergates atratulus), a workerless inquiline social parasitic species of T. immigrans, was also reported in North America along with T. immigrans. A parasitic T. atratulum queen will invade the host’s nest but usually does not kill the host queen. While the host queen continues to produce her own workers, the parasitic queen only produces males and new queens while she is continuously fed by the host workers. T. atratulum is widespread in the eastern United States and commonly collected along with T. immigrans. Link In its native range, T. atratulum parasitizes several species of Tetramorium (Helms et. Al., 2019).
Tetramorium inquilinum is another inquiline parasite of T. immigrans, but this species is only reported in some parts of Europe and Turkmenistan. Currently, T. inquilinum has not been reported in the United States.
Management and control
T. immigrans is widespread in urban environments in Ohio (Ivanov, 2019). The absence of a strong population structure suggests that these ants spread with human-assisted dispersal (Zheng 2019), thus making it quite difficult to control them.
The nesting locations of this ant are diverse. Nests can be found under rocks, in lawns, under wood piles, under patios and concrete pavements, under mulch, in leaf litter, in foundation cracks, in spaces between pavers, etc. When their colony grows, they often gather large numbers and fight with nearby colonies for resources. Sometimes they will come indoors in search of food and may create nests indoors where there is high moisture, including during winter months.
When controlling these ants, it is important to use multiple control methods. Any control methods should include prevention of the pest first, inspection of their habits and habitat, and then the use of appropriate measures to manage and control them. Ants are no exception, prevention is the best way to stop this nuisance pest indoors.
In outdoor settings, it is important to remove any potential nesting sites such as rocks to discourage colony establishment. Pavement ants are small and can easily exploit cracks and other spaces in the concrete or paved surfaces to make their nests. Caulking these spaces can help discourage colony establishment in these sites. Because these ants will move their nest in response to a threat or disturbance, simply filling these cracks with water does not eliminate the colony.
Bait stations
These ants are active and will encroach indoors starting in the spring and summer. When you find a few scouting ants indoors, that’s the first sign of a possible infestation. All food items need to be properly stored in containers with tight lids, and at the same time, maintain sanitation in cupboards, in corners, and on surfaces by cleaning any food crumbs, flour, etc. This will help to discourage the ants from venturing indoors and establishing a colony. When ants are searching indoors for resources, they leave pheromone trails behind so other ants in the colony can follow them to locate the food. These small ‘invasions’ can be managed by sweet liquid baits that include boric acid. These baits are readily available in most supermarkets and garden stores. Workers carry the bait back to the colony and share the food with the young and queen/s. These baits are slow-acting. A continuous supply of the bait is important until there are fewer numbers of ants visiting the bait.
Baits come in two forms, liquid and granular. Granular baits are for outdoor use to sprinkle over the colony. Liquid baits are not effective outdoors, and thus mainly for indoor use. Make sure to keep these baits out of reach for children and pets as they are toxic to humans and other animals. Under warm conditions, liquid baits will evaporate fast leaving thick sugary material behind which ants won’t be able to consume. Always check the condition of the bait and replace it as necessary.
Insecticides
It may not be possible to eliminate all nesting sites at once, but observing their habits and their movements is a better approach for the long-term management of these ants. Treating the colony with chemicals followed by observations of the active workers will give you an indication of the effectiveness of the treatment. Combining several methods to achieve control is the most effective approach. There are soil drenches to directly treat the nesting sites. However, pavement ant nests can be located deep inside the soil, making insecticide drenches less effective.
Perimeter barrier application is often a good idea but these need to be used wisely. If you have pavement ant problems both indoors and outdoors, you can use bait stations as a control option indoors, and then use non-repellant insecticides to control the ants outdoors. While repellent insecticides are good for direct nest drenches or quickly stopping ants from entering a structure or house, they will isolate the colony and the bait stations will become less effective. Non-repellent, non-residual insecticides act slowly and do not interrupt the foraging behaviors and pheromone trails. Instead, when ants come in contact with the non-repellant, non-residual insecticides, they will take the chemical with them to the colony along with any bait chemicals and share them with the nest mates.
Insecticides have to be used as a last resort. Follow all the label instructions and wear appropriate clothing before using them as they are highly toxic to humans and other animals. Read the label instructions carefully, and find out if the chemical is non-repellent, or non-residual depending on your need. Usually, these applications need to be handled by professionals as they have better knowledge of how the baits and other chemicals work depending on the situation.
Glossary of words
Afrotropical - The Afrotropical realm is one of the Earth's eight biogeographic realms. It includes Sub-Saharan Africa, the southern Arabian Peninsula, the island of Madagascar, and the islands of the western Indian Ocean. It was formerly known as the Ethiopian Zone or Ethiopian Region.
COI - Cytochrome c oxidase subunit I (CO1 or MT-CO1) is one of three mitochondrial DNA (mtDNA) encoded subunits (MT-CO1, MT-CO2, MT-CO3) of respiratory complex IV.
ddRADseq - Double digest restriction-site associated DNA sequencing (ddRADseq) technology combines genome-reduced representation by digestion with two restriction enzymes and next-generation sequencing (NGS) to obtain thousands of markers (SNP, SSR, and InDels) and genotype tens to hundreds of samples simultaneously.
Haplotype - a group of genes within an organism that was inherited together from a single-parent
Inquiline - ‘Lodger’ or ‘tenant’ from Latin, refers to the dwelling of an animal of another species.
Nearactic - Geographical region comprising North America as far south as northern Mexico, together with Greenland.
Rugae – series of asymmetrical or parallel ridges running along a surface.