The ManhattAnt: Identi�cation, distribution, and colony structure of a new pest in New York City, Lasius emarginatus

A new ant species was discovered in the heart of New York City in 2011, which garnered national headlines and created the memorable nickname “ManhattAnt.” New York City is one of the oldest and largest cities in North America and has been the site of introduction for some of North America’s most damaging invasive pests. Nevertheless, there has been little follow up research on the ManhattAnt since its discovery, and it has yet to be formally identi�ed. Here we use genetic and morphological approaches to con�rm the identity of the ManhattAnt and characterize its introduced range and colony structure. Results from genetic analyses placed the ManhattAnt within the Lasius emarginatus species complex, and morphological comparisons ruled out closely related species to identify the ManhattAnt as the European ant L. emarginatus with 93–99% certainty depending on the nest sample. Since its initial discovery, the ManhattAnt has become one of the most common ants in New York City and has spread at a rate of 2km/yr into New Jersey and onto Long Island. Based on the climate it inhabits within its native range, L. emarginatus could expand to cover much of the eastern United states from Massachusetts to Georgia. Although many successful invasive ants display supercoloniality with little aggression between neighboring nests, we found no evidence that L. emarginatus colonies in New York City are supercolonial. Continued monitoring of L. emarginatus is warranted, as it has been increasingly reported as an indoor pest and is known to form mutualisms with honeydew producing pests of street trees.


Introduction
Cities are major introduction sites for invasive species due to their role as hubs for global trade and travel (Gotzek et  .By the time introduced species gain enough visibility to be discovered in cities, however, they have often already reached pest status.Even when species are noticed early, it takes time to make appropriate identi cation, and it is often too late to mitigate the impact of an invasive species or its spread.Despite our knowledge that cities are hotspots for species introductions, there are few, if any, policies in place to identify or monitor species before they reach pest status (Simberloff et al 2005).The lack of monitoring of newly introduced species in cities makes it di cult to catch problem species before they spread and become nearly impossible to control.
In 2011, a new species of ant was discovered in the heart of New York City, which made national headlines and produced the memorable nickname "ManhattAnt" (Karni 2012;Nuwer 2012).Despite initial media attention, there has been little to no follow up work on the ManhattAnt nor has it received an o cial identi cation.The species was noticeably absent during the rst survey of New York City ants in 2009 (Pećarević et al. 2010), and its populations were relatively small and con ned to forested parks when they were rst collected in 2011 as part of an extensive ant community survey (Savage et al. 2015).
Over the next ve years, however, their populations expanded to become among the most common ants in both parks and highly urban habitats in the city (Penick et al. 2015).New York City has been the site of introduction for some of North America's most damaging invasive pests-both chestnut blight and the Asian long-horned beetle were rst identi ed in New York City, which are together responsible for the deaths of an estimated 4 billion trees (Hepting 1974;Jacobs et al. 2012;Nowak et al. 2001).Therefore, it is essential that emerging pests, such as the ManhattAnt, are correctly identi ed and monitored after they are rst discovered.
Past research on invasive species has been hampered by uncertainty surrounding species identity and their source populations (Gotzek et al. 2012).It is estimated that the number of ant species introduced and established is much higher than those discovered, and even more so that those that have been accurately identi ed (Miravete et al. 2014).For ants and other insects, accurate identi cation can be di cult due to the diversity of insects globally and high resemblance among closely related species.This contributed to confusion around the identity of a recently discovered pest ant in Houston, TX, which was initially dubbed the "Rasberry Crazy Ant" after the exterminator who brought it to widespread attention, Tom Rasberry (Gotzek et al. 2012).Similar to the ManhattAnt, the Rasberry Crazy Ant's rapid expansion, potential for ecological harm, and initial lack of identity gained it media notoriety.After several years of confusion surrounding its identity, researchers eventually combined morphological and genetic approaches to identify it as Nylanderia fulva, a species that had been documented in Texas decades before and had been known previously called the "Tawny Crazy Ant" (Gotzek et al. 2012; Schär et al.

2022).
As was the case for the Rasberry Crazy Ant, accurate identi cation of new pest insects often requires a combination of genetic and morphological approaches.Genetic methods for identifying newly introduced species often rely on DNA barcoding of the COI gene (Gotzek et al. 2012;Rasool et al. 2020;Schär et al. 2022), but often COI barcoding is insu cient on its own to provide a positive identi cation.This is due, in part, to a lack of COI sequence data available for most of the world's insect species.In cases where DNA barcoding fails to provide a su cient match, morphological approaches can be useful to compare species to those in museum collections.One approach called numeric morphology-based alphataxonomy (NUMOBAT) has been used successfully to discriminate among closely related ant species that form cryptic species complexes (Centanni et al. 2022;Seifert 2009Seifert , 2019Seifert , 2020;;Seifert & Galkowski 2016;Seifert & Schultz 2009).NUMOBAT uses 15 unique morphological characters (linear or angular measurements and counts of elements) to compare specimens, and it can be used when COI data is unavailable or when DNA extraction is not possible.The bene t of using both approaches together is that, while both have their limitations, they can be used in combination to build con dence in species identi cations and reduce misidenti cations that can slow management decisions.
Beyond clarifying the taxonomy of introduced species, it is also important to identify key characteristics that may contribute to their successful establishment.Invasive ants often share a number of common traits that contribute to their success, especially their ability to form supercolonies that display little to no aggression between neighboring nests ( and the previously mentioned Rasberry Crazy ants (Nylanderia fulva) (Eyer et al 2018.This ability to exhibit supercoloniality allows introduced species to establish in high densities and dominate introduced habitats.Increased colony size and reduced interspeci c aggression within the population can increase a supercolony's ability to compete for local resources and displace native ant species (Holway & Case 2001).Therefore, identifying the colony structure and other traits that can facilitate invasion success (e.g., diet) is necessary to assess the potential of an introduced species to become an established pest.
Here we clarify the taxonomy of the ManhattAnt in New York City using a combination of genetic and morphological approaches.The ManhattAnt has been tentatively identi ed as the European ant Lasius emarginatus due to its characteristic red and black coloration.However, L. emarginatus is part of a species complex and shares morphological similarities with closely related species found across Europe and the Middle East (Seifert 2020).By clarifying its taxonomy, we identify the region from which the ManhattAnt was introduced as well as use information about its natural history to assess how this species could impact its newly invaded habitat.Because many successful invasive ants form supercolonies, we use aggression trials to test whether the ManhattAnt is supercolonial or whether they exhibit aggression between neighboring nests.Finally, we use published accounts and citizen science observations to build a distribution map of the ManhattAnt in New York City and surrounding areas, and we estimate its rate of spread.

Field collection
We collected workers of the newly introduced Lasius ant in Manhattan, New York City (NY, USA) during May 2022 from 4 locations (Table S1), which we stored in 70% ethanol for genetic and morphological analyses.During this time, we also made observations of nesting and foraging habits of Lasius colonies throughout the city, which included documenting nesting sites, feeding behavior, and interactions with cooccurring ant species.

Genetic analysis
We ampli ed a portion of the mitochondrial cytochrome c oxidase subunit I gene (COI), which is the barcoding locus most often used to identify insects (Hebert et al. 2003).To extract DNA, we placed individual ants into separate plastic weigh boats and used a sterilized razor blade to break apart the tissue.We then extracted DNA using the QIAGEN DNeasy Blood and Tissue Kit (QIAGEN, Hilden, Germany) following manufacturer's instructions.To maximize compatibility with existing barcoding data, we used the primers LepF1 and LepR1 (Hebert et al 2004).We prepared reactions for two DNA extracts (ST009A and ST009B), and we tested two different annealing temperatures (45°C and 52°C) to optimize PCR conditions.We used Invitrogen Platinum SuperFi reagents (Thermo Fisher, Waltham, Massachusetts, USA), and each 25 µL reaction included: 5 µL 5x buffer, 0.5 µL dNTPs, 0.25 µL polymerase, 13.75 µL nuclease-free water, 1.25 µL 10 µM LepF1, 1.25 µL 10 µM LepR1, and 3 µL extracted DNA.We used the following thermal cycler conditions: 98°C for 30 seconds; then 40 cycles of 98°C for 10 seconds, 45°C or 52°C for 10 seconds, and 72°C for 20 seconds; then 72°C for 5 minutes.We then puri ed PCR products with a 2:1 SpeedBead:DNA product volume ratio (Rohland & Reich 2012), validated them on a 1% agarose gel, and quanti ed them using a Qubit 4 Fluorometer (Thermo Fisher, Waltham, Massachusetts, USA).We then pooled approximately 250 ng of puri ed PCR product from one reaction (ST009B; 52°C annealing temperature) with other amplicons from unrelated projects for library preparation.
To prepare amplicons for sequencing, we used a Ligation Sequencing Kit (SQK-LSK112) from Oxford Nanopore (Oxford, UK) and followed the manufacturer's protocol.We quanti ed the nal libraries and sequenced them on a MinION Flow Cell v. r10 (FLO-MIN112).We used the live, fast basecaller in Guppy v6.3.8 through MinKNOW v22.10.7 (Oxford Nanopore Technologies, Oxford, UK) and saved the rst 804,000 reads for downstream analysis.We then used medaka v1.7.3 (Oxford Nanopore Technologies, Oxford, UK) to align reads to a COI sequence from L. umbratus (median depth = > 8,000 reads) and to create and polish a consensus sequence from these aligned reads.Our raw sequencing data are available through the NCBI SRA (PRJNA977684), and our consensus sequence is available through NCBI GenBank (OR063821).
We used two approaches to identify the New York Lasius sp.First, we used an NCBI BLAST query (Altschul et al. 1990) for highly similar sequences using default parameters.Second, we used the "Animal Identi cation" tool to search the "Public Record Barcode Database" on the Barcode of Life Database (BOLD; Ratnasingham & Hebert 2007).For both approaches, we downloaded the 100 top matches from each search.Separately for each dataset, we added the consensus sequence from our New York Lasius sp. and a sequence from Lasius umbratus (GenBank accession = MZ610655) for use as an outgroup.We then aligned sequences using MUSCLE v3.8.31 (Edgar 2004) within AliView v1.28 (Larsson 2014).We then used RAxML-NG v1.1.0(Kozlov et al. 2019) to estimate maximum-likelihood phylogenies for each alignment.We used a GTR + GAMMA substitution model, 10 random + 10 parsimony-based starting trees, and used 1000 bootstrap replicates to assess con dence in relationships.

Morphometric analysis
We measured a total of 15 Numeric Morphology-Based Alpha-Taxonomy (NUMOBAT) characters (Table 1) (Seifert 2020) of three worker specimens from three nest samples collected in Manhattan in 2022 to compare with four closely related species: Lasius emarginatus (Olivier 1792) from Europe; L. illyricus Zimmermann 1935 from the Balkans, Asia Minor and Caucasus; Lasius tebessae Seifert 1992 from Morocco and Algeria; and Lasius maltaeus Seifert 2020 from Malta.To measure NUMOBAT characters of Lasius specimens collected in Manhattan, each ant was point-mounted and observed under a high-performance stereomicroscope (Leica M165C) tted with an a 2.0x planapochromatic objective (resolution 1050 lines/mm) at 120-360x magni cation.We then compared measurements to 292 Lasius specimens representing the four species described above.

Distribution mapping
To map the distribution of the ManhattAnt in New York City and surrounding areas, we combined previous data from ant surveys in New York City (Penick et al. 2015; Savage et al. 2015) with community reported data on iNaturalist (GBFI.org,2023; iNaturalist, n.d.).The introduced Lasius sp. is relatively easy to identify in photographs due to its size (3-5mm) and its characteristic black and red coloration, which is not found on other co-occurring ants.We individually veri ed research-grade photos reported to iNaturalist as L. emarginatus.In addition, we launched an iNaturalist citizen science project, Project ManhattAnt, to map the continued expansion of the ManhattAnt in the northeastern United States.

Aggression trials and colony structure
To determine colony structure and assess whether New York City Lasius sp. were exhibiting supercoloniality, we conducted aggression tests between workers collected from the same or different nests.We collected workers from 19 individual nests from sidewalks and street trees in New York City (Table S1) and stored them in 1.5ml vials until the start of aggression trials.An individual worker was randomly selected from two colonies (or two from a single colony for control trials) and placed into a plastic, 180 mL vial (5.08cm x 10.77cm) and observed for 15 minutes.We ran each trial within 24-hours of ant collection, and individual ants were used in only one trial.We conducted trials blind so that the observer did not know whether a trial was between members taken from the same or different nests.We quanti ed aggression as the performance of either biting or formic acid spraying.In addition, we quanti ed rapid antennation and mandible gaping as potentially non-aggressive behaviors for comparison as they are typically scored low in aggression trials (Giraud et

Statistical analyses
For our morphometric analysis, we used a linear discriminant analysis (LDA) to test the probability that the Lasius specimens collected from New York City were members of a known species using the software package SPSS 15.0.We ran the New York samples as "wild-cards" (i.e.without imposing a species hypothesis) against the background of Lasius emarginatus, L. illyricus Zimmermann, L. tebessae Seifert 1992 and L. maltaeus.The LDA was run with the morphological characters corrected for allometric variance according to the algorithms given in Seifert (2020).For aggression trials, we used Fisher's exact test using R version 4.3.2(R Core Team 2023) to determine whether aggression was signi cantly more common in between colony trials compared to within colony control trials.If colonies displayed supercoloniality, we predicted that we would observe little to no aggression among trials and that we would not observe a signi cant difference in aggression for between-colony or within-colony trials.If colonies were not supercolonial, then we predicted that we would observe signi cantly higher aggression in between-colony trials compared to within colony-trials.

Species identi cation
Our genetic and morphological analyses supported identi cation of the New York ManhattAnt as Lasius emarginatus, a European species.We ampli ed a portion of the mitochondrial cytochrome c oxidase subunit I gene (COI), the barcoding locus used most often to identify insects.In the phylogeny that included samples from GenBank, we found strong support (bootstrap support = 88%) for the placement of the New York specimens in a clade that included only L. emarginatus and two specimens identi ed only as L. sp.In the phylogeny that included samples from the Barcode of Life Data System (BOLD) we found similarly strong support (bootstrap support = 90%) for the placement of New York Lasius specimens in a clade that included only L. emarginatus, one sample identi ed only as L. sp., and one sample identi ed as L. brunneus (Fig. 2).Genetic data were not available for the other three members of the L. emarginatus species complex.
Due to a lack of COI data from other closely related Lasius species, we could not resolve the species identity using genetic data alone.Therefore, we conducted a morphological analysis using 15 allometrically corrected NUMOBAT characters that we compared with morphological measurements taken from L. emarginatus and three other closely related species (L.illyricus, L. maltaeus, and L. tebessae).We ran a linear discriminant analysis (LDA) comparing three New York specimens against the four members of the L. emarginatus species complex with the New York specimens run as "wild-cards'' (i.e., without imposing a species hypothesis).There was over a 93% posterior probability that the New York specimens were L. emarginatus compared to less than 7% match with the other three species in the L. emarginatus complex (Table 2).We then ran a cluster analysis with L. malteus excluded due to a zero percent probability match, which showed that the New York specimens clustered within the European L. emarginatus and showed that they could not be allocated to the northwest African L. tebessae nor to the L. illyricus from the southern Balkans, Asia Minor, or the Middle East (Fig. 3).
Distribution and spread L. emarginatus was initially absent from a survey of ant diversity conducted in New York City in 2006 (Pećarević et al. 2010) but appeared in 33% of park sites and 10% of tra c island sites during a sample in 2011 (Savage et al. 2015).The percent of sites in which L. emarginatus were collected in studies of New York City ants increased to 42% in park sites and 42% in tra c island sites by 2013 (Penick et al. 2015).In addition to formal sampling efforts, there have been over 570 sightings reported to iNaturalist from 2014-2023 (GBFI.org2023; iNaturalist n.d.).From the initial discovery of L. emarginatus in Midtown and upper Manhattan, they have expanded to lower Manhattan and across the Hudson River into New Jersey (Fig. 4).When comparing the maximum range of L. emarginatus each year from 2011-2023, we found that their populations are expanding at a rate of 2 km per year, which is consistent with expansion through typical mating ights, though we cannot rule out anthropogenic dispersal.

Colony structure
Many introduced ants exhibit supercoloniality and display little aggression among neighboring nests, but we found no evidence of supercoloniality in L. emarginatus in New York City.In dyadic trials between workers collected from the same or neighboring nests, we found a signi cant difference in aggression, quanti ed as performance of biting or formic acid spraying (Fisher's exact test, N same =10, N neighboring =10 p = 0.023, Fig. 5).Workers from neighboring nests displayed aggression in 80% of trials, while workers collected from the same nests displayed aggression in only 20% of trials.In addition to clearly aggressive behaviors, we also quanti ed incidences of mandible gaping and rapid antennation, which were performed in over 80% of trials regardless of treatment and did not signi cantly differ between workers paired from the same or neighboring nests (Fisher's exact test, N same =10, N neighboring =10 p = 0.47).

Natural history observations
During eld work in New York City, we made observations of L. emarginatus nesting sites, feeding behavior, and interactions with other ants.We observed workers walking into and out of nesting holes in the ground, which is consistent with observations in Europe that indicate L. emarginatus is a groundnesting species (Seifert 2018).We also observed what appeared to be satellite nests at the base of trees and in holes in trees more than 1m off the ground in park and sidewalk habitats.More recently, we observed L. emarginatus workers on the balcony of a fth-oor apartment, and there have been growing reports of L. emarginatus workers foraging inside buildings at ground level and above (Stewart 2022).One colony of L. emarginatus was even found nesting in a small ower planter in the middle of Times Square far away from street trees or large ornamental plants that would typically serve as nesting habitat.Often, the only visual non-humans in Times Square are humans dressed as Disney animals and yet, hidden in this ower pot was a whole society of animal beings.
In our observations of foraging, we saw L. emargaintus workers interacting with aphids on vegetation in city parks, and we observed them carrying small herbivorous insects, including scale insects, while foraging on street trees.We observed several occasions of L. emarginatus workers surrounding and dismembering workers of the second most common urban ant in New York City, Tetramorium immigrans (Penick 2021), another introduced species.We also observed L. emarginatus workers carrying some of their sister workers up and down trees.We found that L. emarginatus colonies foraged on street trees all hours of the night, even during and after rain events.They continued to forage on dry portions of smooth barked London plane (Platanus sp.) trees and seemed less deterred by the rain on coarser barked locust trees (Robinia sp.).
For New York City populations, L. emarginatus mating ights were observed ying in the evening 24 July 2020 through the morning of 25 July 2020 (Ivan Lacroix, personal communication).Other Lasius species were also observed ying that night, including L. murphyi.According to NOAA weather station data (NOAA 2023b) for July 2020, there was rain on the 22nd (3.6 cm, evening), 23rd (trace amount, sporadic), and 24th (0.5 cm, morning), with relatively dry conditions for the week preceding these dates and the week following.L. emarginatus queens were observed and reported to iNaturalist on 21 June 2020, 23 July 2022, 27 July 2022, and 4 August 2022.No rain events occurred on or immediately before 21 June 2020 or 4 August 2022, but there were trace amounts of rain on 22 July 2022 and 27 July 2022 (2.5 cm, afternoon, trace amount in the evening).

Discussion
Using genetic and morphological evidence, we have identi ed 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 identi ed 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 con rmed the identity as L. emarginatus.While the ManhattAnt was rst reported in New York City in 2011, this is the rst con rmation of its identity, and it is the rst 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 identi cation of newly introduced species, which can require both genetic and morphological approaches.We rst used genetic sequencing of the COI barcode to place the ManhattAnt within the genus Lasius, and more speci cally 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., misidenti cation) 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 identi cation 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 classi ed 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 con dence in our identi cation.
Since its discovery in 2011, L. emarginatus has steadily expanded within and outside of New York City.L. emarginatus was absent from the rst 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 tra c 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 tra c 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 ights.
The current distribution of L. emarginatus in Europe suggests their populations could nd 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°CMAT) (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 closedcanopy 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 nd 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 10 10 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 re 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 oor (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 identi ed 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.

Table 1
Description of 15 Numeric morphology-based alpha-taxonomy (NUMOBAT) characters measured for each Lasius specimen : mean of the large (EL) and small diameter (EW) of the elliptic compound eye GuHL Maximum length of setae on the underside of head nGen* Number of setae on head sides in full face view (frontal of anterior eye margin) nGu* Number of setae on underside of head in full pro le nHT* Number of setae on extensor pro le of hind tibia excluding the most apical setae nOcc* Number of setae projecting from hind margin of vertex frontal to caudal end of eye nSc* Number of setae on dorsal plane of scape excluding of the most apical setae nSt* Number of setae on lateral and caudolateral surface of metapleuron excluding protective setae fringing the ori ce of the metapleural gland PnHL Length of the longest hair on pronotum PoOc Postocular distance (caudal measuring point: median occipital margin; frontal measuring point: median head at the level of the posterior eye margin) SL Maximum scape length excluding the articular condyle sqPDCL Square root of pubescence distance on clypeus (PDCL; the number of pubescence hairs crossing or just touching a census line from caudomedian clypeus to lateral clypeal depression.Hairs crossing/touching the census line are counted as 1/0.5) *Bilateral sum is halved

Table 2
Posterior probabilities for allocation of New York City Lasius sp. to four closely related species based on 15 NUMOBAT characters using a linear discriminant analysis (LDA) with New York specimens run as wild-cards