Using genetic and morphological evidence, we have identified the “ManhattAnt” in New York City, NY (USA) as Lasius emarginatus, a species native to central and southern Europe. Genetic sequencing of the COI barcode gene placed the ManhattAnt within the genus Lasius and tentatively identified the species as L. emarginatus. Because COI data were lacking for other closely related Lasius species, we performed a morphological analysis using measurements from additional Lasius species, which confirmed the identity as L. emarginatus. While the ManhattAnt was first reported in New York City in 2011, this is the first confirmation of its identity, and it is the first documented population of L. emarginatus outside of its native range. Over the last decade, L. emarginatus populations have been expanding within and outside of New York City at a rate of 2 km per year, particularly within urban habitats where they have been reported as an emerging pest (Stewart 2022). Unlike other common invasive ants, we found no evidence that L. emarginatus exhibits supercoloniality, though their expanding range suggests they could have negative impacts on previously established ant species. In addition, they form mutualisms with honeydew-producing tree pests, which could increase stress on urban trees.
Successful management of emerging pests requires accurate and timely identification of newly introduced species, which can require both genetic and morphological approaches. We first used genetic sequencing of the COI barcode to place the ManhattAnt within the genus Lasius, and more specifically within a clade that included L. emarginatus. However, we acknowledge that this approach is limited by at least two issues: 1) mtDNA barcoding is unreliable as a primary method of species delimitation—a problem that is exacerbated by both biological (e.g., introgression) and methodological (e.g., misidentification) processes (Siefert 2018); and 2) the value of mtDNA barcoding depends upon the completeness of a reference database, and the barcoding data available for the L. emarginatus species complex are relatively sparse. In the future, as the amount of COI data available in public databases grows, this latter problem may be resolved. However, additional genomic resources (e.g., full-genome sequencing) will be necessary to fully explain evolutionary history of the L. emarginatus species complex, delimit species within it, and facilitate a more precise source of the introduced population in New York City. To account for the lack of barcoding data available for other members of the L. emarginatus species complex, we used morphological traits to distinguish among four closely related Lasius species found in Europe, the Middle East, and northern Africa and provide an identification of the ManhattAnt. Based on our analysis of 15 NUMOBAT characters, we were able to rule out other closely related species and identify the ManhattAnt as L. emarginatus with 93% certainty in the least clearly classified nest sample. In this case, genetic sequencing allowed us to quickly place the ManhattAnt within the L. emarginatus species complex, but detailed morphological measurements were needed to increase confidence in our identification.
Since its discovery in 2011, L. emarginatus has steadily expanded within and outside of New York City. L. emarginatus was absent from the first survey of New York City ants in 2009 but was collected during a second sampling of Manhattan in 2011, making it likely that it was introduced sometime within or shortly before this period. Initial collections of L. emarginatus occurred mainly in parks, with occurrences in 33% of park sites and 10% of traffic islands along Broadway (Savage et al. 2015). A subsequent study conducted in 2013 found an expansion of L. emarginatus in Manhattan, particularly in highly urban areas, with occurrences in 42% of park sites and 42% of traffic islands along Broadway (Penick et al. 2015). Compared to other ants, L. emargiantus is relatively easy to identify in low-resolution photographs, which has made it possible to track its spread using the sightings on the citizen science platform, iNaturalist. Research-grade observations from iNaturalist show that their populations have expanded across the Hudson River into New Jersey and onto Long Island at a rate of 2 km per year, which is consistent with natural expansion following mating flights.
The current distribution of L. emarginatus in Europe suggests their populations could find suitable habitat across a wide swath of the eastern United States. The range of L. emarginatus in Europe extends from 52.6°N in the north in southern England and the Netherlands to 37°N in the south across Iberia, Apennine, and the Balkans (Seifert 2018), which covers a span in mean annual temperature (MAT) from 9.2–18.8°C (NOAA 2023a). Translating this to the eastern United States, the introduced range of L. emarginatus could extend as far north as Portland, ME (9.2°C MAT) and as far south as Atlanta, GA (18.5°C MAT) (NOAA 2023a). However, we caution that we do not yet know what other biotic or abiotic factors may limit expansion. While populations of L. emarginatus in southeastern Europe are primarily found in closed-canopy broadleaf forests, populations in central Europe are more common in open forests as well as urban and suburban habitats. In parts of southeastern and northeastern Germany, in particular, L. emarginatus lives in close contact with humans and can commonly be found in homes and churches. There have also been observations of L. emarginatus workers in Europe foraging inside buildings and feeding on sugary human food sources, though research on ant community diets in New York City suggests that introduced populations of L. emarginatus populations do not heavily feed on human foods (Penick et al. 2015).
Many of the most destructive recent invasive ant species display supercoloniality with little aggression between neighboring colonies (Holoway 2002), but we did not find evidence of supercoloniality in L. emarginatus. We observed aggression between ants collected from neighboring colonies in over 80% of trials, which suggests that colonies are independent. This differs from another invasive Lasius species found in Europe, L. neglectus, which forms massive supercolonies in urban areas (Seifert 2018)— one L. neglectus supercolony in Budapest was estimated to have more than 1010 workers and over 30 million queens, and it expanded in size at a rate of 89m/yr over 17 years (Tartally 2006). Like L. emarginatus, L. neglectus has largely invaded urban habitats and is absent from surrounding natural habitats (Seifert 2018). Though supercoloniality is a common trait among successful invasives, there are successful invasive species that remain territorial, such as monogynous fire ants, Solenopsis invicta, that are abundant throughout the southeastern United States, and the pavement ant, Tetramorium immigrans, which is abundant in temperate urban habitats, including New York City (Jelley & Moreau 2023).
While L. emarginatus does not appear to be supercolonial, there are a number of characteristics that have likely led to their success in urban habitats. Based on studies in their native range, L. emarginatus tends to perform best in relatively open habitats with smooth surfaces with low spatial resistance, such as a mixture of concrete and stony surfaces that are common in cities (Seifert 2018). Foragers of L. emarginatus have the fastest walking speed and the best visual system of any Lasius species occurring in western Palaearctic cities, which may facilitate their success in urban habitats with smooth stone or concrete surfaces. Most of their foraging takes place at ground level, but occasionally high up into apartments of the 8th floor (pers. observation of BS in the city of Leipzig), and they typically forage long distances (of up to 40m) with faster recruitment rates compared to co-occurring Lasius (Seifert 2018). They also have a relatively broad diet, which may contribute to their success in urban habitats where generalists tend to perform better than those with more specialized diets (Penick et al. 2015). L. emarginatus exploits a wide range of resources, which includes hunting or scavenging dead insects as well as consuming plant sap, nectar, and elaiosomes found on seeds (Seifert 2018). In particular, L. emarginatus forms mutualisms with honeydew producing insects, such as aphids and scale insects, which are abundant in urban street trees (Meineke et al 2013; Seifert 2018).
The impact of L. emarginatus on local ecosystems is not known, but they display a number of characteristics that could make them serious pests. First, they have reached high abundances in New York City and have been increasingly reported as pests inside human dwellings. In 2022, local hardware stores in New York City reported selling out of ant baits when New Yorkers found L. emarginatus colonies infesting their apartments (Stewart 2022). Second, their tendency to form mutualisms with tree pests, such as scale insects and aphids, could have negative impacts on urban trees. New York City, alone, is home to 7 million trees that are estimated to remove 51,000 tons of carbon per year, 1,100 tons of air pollution, and to reduce annual residential energy costs by $17.1 million USD (Nowak 2018). The total cost of replacing New York City trees is estimated at $5.2 billion USD, with insect pests identified as a primary concern (Nowak 2018). We observed that L. emarginatus workers foraged in trees throughout the entire day and night, as well as during rain events where the bark was dry. These foraging behaviors are similar to foraging behavior observed in another introduced species, the Argentine ant (Linepithema humile) (Burford et al., 2018). Third, L. emarginatus could have a negative impact on local insect diversity, particularly other ant species. We observed L. emarginatus workers surrounding and dismembering pavement ant workers (T. immigrans) in sidewalk habitats where both species are abundant. In parks, which have a higher diversity of native ant species, we observed L. emarginatus less frequently, so their negative impacts on these communities may be reduced.
Additional research is needed to assess potential long-term impacts of L. emarginatus in urban habitats, particularly as they are expected to spread throughout the northeastern region. We have established a community science project through iNaturalist to monitor and document L. emarginatus expansion. Because L. emarginatus was detected early after its introduction, it may be possible to assess their impacts on native insect populations by resampling diversity before and after their expansion into previously sampled habitats. In addition, further research on their interactions with honeydew-producing insects and subsequent impacts on urban trees could also assess any potential negative impacts on tree health. Finally, it remains unclear what traits of L. emarginatus have facilitated their success as urban pests. We showed that they do not form supercolonies in New York City, but it is likely that they possess other traits that have contributed to their success. Continued monitoring of their introduced population and studies of their interactions with co-occurring species will be necessary to assess their potential impacts on urban ecosystems as they continue to expand their introduced range.