Independent evolution of geraniol-8-hydroxylase activity involved in iridoid formation in the Argentine ant (Linepithema humile).

Biological invasions pose significant threats to biodiversity, ecosystem functioning, and human well-being. Urban areas are particularly prone to biological invasions as high levels of human activity and connectivity enhance the arrival of alien species, and some characteristics of these areas (e.g. disturbance levels and reduced thermal variability) facilitate their establishment. Here, we study the establishment and distribution of the highly invasive Argentine ant, Linepithema humile (Mayr, 1868), in Bariloche, a Patagonian city ~1000km away from its native range, using a citizen science approach, complemented by outdoor ant sampling. Our findings confirm its establishment as an urban pest within a small downtown area and suggest it has not spread into surrounding urban or semi-natural habitats. However, ongoing urban development and climate change might facilitate its spread in the future. Through citizen science records, we also found other ants (including the invasive Monomorium pharaonis (Linnaeus, 1758), reported for the first time in the region) behaving as indoor pests in parts of the city where L. humile was not detected. This study highlights the value of citizen science in detecting invasive alien species and its added benefit of monitoring indoor pests that might otherwise go unnoticed using outdoor surveys and emphasize the need for coordinated management strategies from different stakeholders to address these invasions.

Ants (Hymenoptera: Formicidae) are a diverse and ecologically important group of social insects that play crucial roles in ecosystems worldwide. In this study, we aim to identify the community of ant species present in two important fruit crops of Central Chile. Ants were sampled from January to March in three cherry orchards and three vineyards, using standard pitfall traps to sample above-ground, and subterranean pitfall traps to sample below-ground. We found in total seven species of ants, three native and four introduced, with Strumigenys silvestrii Emery, 1906 being the first record of Attini species for central Chile. The introduced Hypoponera eduardi (Forel, 1894) was the most frequently sampled species, while the invasive Linepithema humile (Mayr, 1868) was found in only two sites. The ecological or functional roles of these species within the Chilean agroecosystems deserve further investigation.

Haplodiploid inheritance, in which females are diploid and males are haploid, is found in all species of Hymenoptera. Sex in haplodiploids is commonly determined by the alleles present at a complementary sex determination (CSD) locus, with heterozygosity triggering the female developmental pathway. The identity of this locus differs among taxa and is only known in a few species. Here, we map a single CSD locus to a 2 kbp region in the genome of the red mason bee Osmia bicornis. It overlaps the long noncoding RNA ANTSR, which has been identified as the sex-determining gene in the invasive ant Linepithema humile. This locus is homozygous in diploid males and exhibits extremely high levels of haplotype diversity, consistent with the action of frequency-dependent selection. The elevated levels of heterozygosity in the CSD locus enable us to fine-map potentially functional genetic variation within it. We also identify elevated levels of genetic diversity in the ortholog of the CSD locus in five other bee and ant genera, suggesting that it may govern sex determination widely in Hymenoptera. Our data are consistent with the hypothesis that ANTSR evolved a role in sex determination over 150 million years ago and is the ancestral sex-determination locus of bees and ants.

Haplodiploid inheritance, in which females are diploid and males are haploid, is found in all species of Hymenoptera. Sex in haplodiploids is commonly determined by the alleles present at a complementary sex determination (CSD) locus, with heterozygosity triggering the female developmental pathway. The identity of this locus differs among taxa and is only known in a few species. Here, we map a single CSD locus to a 2 kbp region in the genome of the red mason bee Osmia bicornis. It overlaps the long noncoding RNA ANTSR, which has been identified as the sex-determining gene in the invasive ant Linepithema humile. This locus is homozygous in diploid males and exhibits extremely high levels of haplotype diversity, consistent with the action of frequency-dependent selection. The elevated levels of heterozygosity in the CSD locus enable us to fine-map potentially functional genetic variation within it. We also identify elevated levels of genetic diversity in the ortholog of the CSD locus in five other bee and ant genera, suggesting that it may govern sex determination widely in Hymenoptera. Our data are consistent with the hypothesis that ANTSR evolved a role in sex determination over 150 million years ago and is the ancestral sex-determination locus of bees and ants.

Invasive ants threaten biodiversity worldwide. They may benefit from being novel if native species fail to show appropriate responses to their cues. Cues include chemical footprints (or ‘home‐range markings' in ants) left by all walking insects, which resemble cuticular hydrocarbons (CHCs). Ants are known to rely heavily on such cues to forage and recognise other individuals. We investigated the response of Lasius niger ants in central Europe to cues of two invasive ant species ( Lasius neglectus and Linepithema humile ), studying whether they were shaped by experience and whether the ants discriminated between cues of different species. Groups of L. niger workers encountered invasive ant workers in a negative (aggressive), positive (food‐associated), or neutral (neither aggression nor food) context. Using Y‐mazes with one blank arm and one arm with footprints, we investigated how this affected their response to footprints compared to responses of naïve ants. Naïve L. niger avoided footprints of both species, walking on the cue‐bearing Y arm less than expected by chance (42.9 ± 0.6% SE versus 50% choosing the cue‐bearing arm). Avoidance became stronger after negative encounters (35.3 ± 1.9%). However, their response did not change significantly after neutral encounters, whereas they approached the footprints after positive experience (62 ± 1.3% choosing the cue‐bearing arm). Notably, there was no change in response to footprints of a species they had not encountered. Our results show that ants can link individual experience with living ants to their chemical footprints, thus transferring information between contexts. Also, they learn to respond specifically to cues of certain species, but do not generalise their responses to cues of other species. Ants hence might learn to interpret the wealth of chemical cues in their environment and use it to optimise foraging decisions. At least for other ants, invasive ants may not benefit from being novel.

BACKGROUNDInsects pose significant challenges in both pest management and ecological conservation. Often, the most effective strategy is employing toxicant‐laced baits, which also must be designed to specifically attract and be preferred by the targeted species for optimal species‐specific effectiveness. However, traditional methods for measuring bait preference are either noncomparative, meaning that most animals only ever taste one bait, or suffer from methodological or conceptual limitations. Here we demonstrate the value of direct comparison food preference assays using the invasive and pest ant Linepithema humile (Mayr, 1868) as a model.RESULTSWe compare the food preference sensitivity of noncomparative (one visit to a food source) and sequential comparative (visiting one type of food then another) assays at detecting low levels of aversive quinine in sucrose solution. We then introduce and test a novel dual‐choice feeder method for simultaneous comparative evaluation of bait preferences, testing its effectiveness in discerning between foods with varying quinine or sucrose levels. The nonsequential assay could not detect aversion to 1.25 mm quinine in 1 m sucrose, yet the sequential comparative approach detected aversion to quinine levels as low as 0.94 mm. The novel dual feeder method approach could detect aversion to quinine levels as low as 0.31 mm, and also preference for 1 m sucrose over 0.75 m sucrose.CONCLUSIONThe dual‐feeder method combines the sensitivity of comparative evaluation with high throughput, ease of use and avoidance of interpretational issues. This innovative approach offers a promising tool for rapid and sensitive testing of bait solutions, contributing to the development of targeted control strategies. The method also could be easily extended to other ant species. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

ABSTRACTAimThousands of non‐native species have established populations and spread in outdoor environments (i.e., Naturalised), yet some populations or species only occur indoors, potentially due to unsuitable climates. We assessed the hypothesis that non‐native ants are more often restricted to indoor environments when they invade regions with climates dissimilar from their native regions. Furthermore, we forecasted how climate change could influence the naturalisation of indoor‐restricted non‐native ants.LocationGlobal.MethodsUsing a global database of 323 non‐native ant taxa across 477 regions, we modelled how average climatic conditions in the native and invaded regions of each taxon determined whether they naturalised or were restricted indoors. We then modelled regional climatic suitability for the naturalisation of indoor‐restricted non‐native ants and projected future changes under climate change scenarios. We further assessed if climate change would facilitate the naturalisation of impactful non‐native ants using a global database describing their known impacts.ResultsNon‐native ants originating from warm regions were more likely restricted indoors when introduced to cold regions. Under 2°C and 4°C of warming, the number of indoor‐restricted non‐native ant species projected to find suitable regional climates for naturalisation increased by an average of 0.08 (maximum = 1.2) and 0.27 (maximum = 3.7) taxa per region, respectively. These anticipated naturalisations include high‐impact non‐native ants, such as the Argentine ant Linepithema humile and are expected to increase socioeconomic and environmental impacts under both warming scenarios, particularly in European regions.Main ConclusionsOur findings suggest that indoor environments serve as microclimatic beachheads for biological invasions, especially in cold regions in the Northern Hemisphere. Failure to limit climate warming and inadequate biosecurity management in indoor environments may facilitate the naturalisation of non‐native ants, with costly repercussions on nature and society.

The Argentine ant (Linepithema humile), listed among the world's 100 worst invasive alien species, is notoriously difficult to control due mainly to its formation of large, expansive supercolonies. Despite the drawbacks of chemical control, biological alternatives have not been previously explored for this species. In this study, we evaluated six native entomopathogenic fungal strains against Argentine ants from four behaviorally distinct supercolonies, identified through aggression assays and collected from both urban and natural sites within the species' native range. Ants were inoculated with 1 × 108 conidia/mL using three methods: topical application, spray, and immersion. Mortality was recorded over 14 days, and the cause of death was confirmed by fungal outgrowth from cadavers. Among all strains, Beauveria bassiana Li053 consistently induced high mortality across all supercolonies and inoculation methods, with LT50 values between 2 and 5 days and final mortality rates exceeding 80%. Fungal infection was confirmed in 87-92% of cadavers. Dose-response assays revealed that higher conidial concentrations accelerated and increased mortality, with an LC50 estimated at 1 × 106 conidia/mL. These results demonstrate that B. bassiana Li053 is a promising candidate for the biological control of L. humile and merits further evaluation under field conditions.

The big-headed ant, Pheidole megacephala, is an ecologically disruptive invader of tropical and subtropical environments worldwide. In April 2014 an established infestation of P. megacephala was discovered in a residential neighborhood in Costa Mesa, Orange County, California, and in 2019 a second infestation was found in a residential neighborhood (Talmadge / City Heights) in San Diego, San Diego County, California. Although big-headed ants are regularly detected in commerce in California, the records from Costa Mesa and Talmadge / City Heights represent the first established infestations documented from the state. In 2024 and 2025, four additional infestations were discovered or confirmed in other residential neighborhoods in San Diego. To assess whether or not P. megacephala will expand its range in this region, we delineated infestations in Costa Mesa and Talmadge / City Heights in 2023 and 2024 and compared this species to another widespread invader, the Argentine ant (Linepithema humile), with respect to desiccation tolerance and δ15N. The delineated P. megacephala infestations extend over multiple hectares of suburban and urban development, with the Talmadge / City Heights infestation exceeding 100 ha and the Costa Mesa infestation exceeding 10 ha. Between 2023 and 2024 the size of the Talmadge / City Heights infestation increased by 12 ha. Comparisons of the two focal species revealed overlapping δ15N values and estimates of desiccation tolerance. Our findings indicate that established populations of P. megacephala will continue to spread in urban environments in coastal southern California and potentially cause impacts comparable to those resulting from invasion by the Argentine ant.

Repellent Effects of Bay, Neem, and Scent Leaf Charcoal Briquettes on Argentine Ants

Foraging ant colonies often face the challenge that food items can appear unpredictably across their territory. This is analogous to traveling salesman/salesperson problems (TSP), wherein solutions to visiting multiple possibly-rewarding sites can vary in cost, travel distance, or site revisits. However, TSP solutions for ants are likely also constrained by cognitive limitations. Rather than envisioning entire routes, ants probably determine their paths by individual-level responses to immediate stimuli, such as nestmate presence or avoiding revisiting an already explored site. Thus, complex group-level search and food retrieval patterns may self-organize from simple individual-level movement rules. Here we derive solutions through simulations that optimize net foraging gains across groups of ant-like agents. Agent search strategies evolve in three spatial networks that differ in travel distances to nests, connectivity, and modularity. We compare patterns from simulations to observed foraging of Argentine ants (Linepithema humile) in identical spatial networks. The simulations and ant experiments find foraging patterns are sensitive to both network characteristics and predictability of food appearance. Simulations are consistent in multiple ways with observed ant behavior, particularly in how network arrangements affect search effort, food encounters, and forager distributions (e.g., clustering in the more connected cells). In some distributions, however, ants find food more successfully than simulations predict. This may reflect a greater premium on encountering food in ants versus increasing find exploitation rates for agents. Overall, the results are encouraging that evolutionary optimization models incorporating relevant ant biology can successfully predict expression of complex group-level behavior.

ABSTRACTLateralization, or the presence of left–right asymmetry, is a widespread phenomenon in vertebrates and has been shown to confer various adaptive advantages, as lateralized individuals tend to outperform non‐lateralized ones in specific tasks. In contrast, much less is known about lateralization in invertebrates. Further investigation into lateralization in understudied invertebrate groups is crucial for deepening our understanding of its evolutionary origins. In this study, we evaluated asymmetries during food probing behaviors in two ant species, Lasius niger and Linepithema humile. Overall, both species exhibited asymmetries, favoring either a particular leg or antenna when investigating a sugar drop. Interestingly, L. niger favored the right side, while L. humile favored their left. These results imply the absence of a strong driver for a bias on a specific side preference for food probing in ants, but a potential benefit of lateralization in food probing. Supporting this, individuals fully lateralized on the opposite side of the majority were observed in both species. The collective bias found in both species supports the theory that population‐level lateralization may have evolved in species that need to coordinate their behaviors. This study provides novel insights into the lateralization of ant behaviors and highlights the need for further research into its evolutionary drivers.

Social insects such as ants possess a battery of behavioural mechanisms protecting their colonies against pathogens and toxins. Recently, active abandonment of poisoned food was described in the invasive ant Linepithema humile. During this abandonment, foraging declines by 80% within 6–8 h after baits become toxic—a reduction not due to satiety, diminished motivation, or mortality. Here we explore the mechanisms behind this behaviour, testing two hypotheses: (1) the presence of ‘no entry’ pheromones near toxic food, and (2) the formation of aversive memories linked to the toxic food site. In field trials, we placed bridges leading to sucrose, nothing, or poisoned sucrose on an active trail. Within hours, 80% of ants abandoned poisoned bait bridges. By swapping bridges strategically, we confirmed that aversive memories formed at toxic bait sites, while no evidence of a ‘no entry’ pheromone was found. Then, in the laboratory, we asked how ants may be sensing the toxicity of the bait, hypothesising poison-induced malaise. Motility, used as a proxy for malaise, was 29% lower in toxicant-exposed ants after 3 h, linking malaise to abandonment. Developing toxicants with delayed malaise, not just delayed mortality, may improve toxic bait control protocols.

Invasive ant species like Linepithema humile cause significant ecological and economic harm, making effective control strategies essential. Insecticide baits are currently the most effective approach for controlling ants. Therefore, quantifying how palatable or unpalatable baits, bait additives, or toxicants are, is critical for developing effective control methods. Recent research shows that when animals can compare a test food containing a bitterant with another option, they are much better at detecting the bitterant and thus rejecting the test food. Here, we deploy a newly developed comparative evaluation methodology to examine the palatability to L. humile workers of three toxicants commonly used in invasive ant control: fipronil, spinosad, and imidacloprid. Additionally, we tested egg-white protein in sucrose solutions to assess its impact on bait acceptance. Ants showed no significant preference between pure sucrose and sucrose-toxicant solutions, indicating that they either cannot detect the toxicants or do not find them distasteful. Survival tests confirmed that the toxicant concentrations used, fipronil at 0.0001% and 0.001%, spinosad at 0.015% and 0.15%, and imidacloprid at 0.005%, were lethal, with a survival rate of 50% or below after 72 h. However, ants found egg protein additive unpalatable, significantly preferring pure sucrose to a sucrose egg-white protein mix. These findings confirm that three major toxicants are suitable for use in baits, and that reported abandonment or avoidance of toxic baits is not due to the unpalatability of these toxicants. However, the addition of egg protein alone to sucrose baits, even at ratios which optimise colony growth, is likely counterproductive. Future research should investigate the relative preference of invasive ants for various bait matrixes over naturally available food, ensuring more effective pest management strategies.

The Argentine ant ( Linepithema humile ) is an invasive ant species found across California. Many invasive ant species, including the Argentine ant, can use chemical defense compounds to ward off predators or compete with native ant species, which aids in invasive spread. Previously, Stanford undergraduate researchers found variation in the potency of Argentine ant chemical profiles (collected in varying locations on campus) in repelling Caenorhabditis elegans during chemotaxis assays. Here, we asked if variation in Argentine ant chemical extracts was related to collection location on the Stanford campus. We collected Argentine ants from five different locations and analyzed differences in their chemical profiles. Using gas chromatography coupled to mass spectrometry, we found variation in ant chemical profiles based on location collected, where many metabolites contributed to these differences. Five of these compounds were successfully annotated, including pyrazines that are known to have a repulsive function in insects. This work highlights the importance of sampling location impacting naturally-derived ant compounds and the importance of this variable in future chemotaxis assays in the classroom.

Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), is a pest in southern California citrus orchards because it protects honeydew-producing hemipteran pests from natural enemies. A major impediment to controlling L. humile is estimating ant densities in orchards. Ants use irrigation lines to travel across orchard floors to reach trees infested with hemipterans. However, for making ant control decisions, it is the number of ants in trees, not on pipes that is critical. Work completed here demonstrates that the number of ants counted on pipes is highly correlated with the number of ants counted on trunks. Densities of ants counted on trunks are correlated with trunk diameter, citrus variety, and time of year and time of day counts. Six regression models, linear regression, zero-inflated Poisson regression, and zero-inflated negative binomial regression models, and each of their mixed model extensions, indicated a strong positive relationship between ant counts on irrigation pipes and ant counts on tree trunks. Mean squared prediction error and 5-fold cross-validation analyses indicated that the best performing of these 6 models was the zero-inflated Poisson mixed regression model. A binary classification model developed using support vector machine learning for ant infestation severity levels, categorized as low (<100 ants counted in 1 min) or high (≥100 ants counted in minutes), predicted ant densities on trunks with 85% accuracy. These models can be used to estimate the number of ants on the trunks of citrus trees by using counts of ants made on irrigation pipes.

Intertidal environments receive energy from marine ecosystems in the form of marine wrack, which makes up the base of a food web that includes both intertidal and terrestrial consumers. Consumption of wrack by terrestrial consumers can elevate their abundance and alter how they interact with organisms in adjacent terrestrial environments. Although rarely documented, terrestrial invaders may exploit marine wrack subsides and potentially disrupt intertidal and terrestrial food webs. Here, we examine consumption of marine wrack resources by the introduced Argentine ant (Linepithema humile), which occurs commonly on beaches in southern California. In controlled trials the Argentine ant readily scavenged arthropod detritivores (amphipods and flies) abundant in wrack. In spite of obvious risks (e.g., exposure to tides, desiccation, thermal stress) associated with intertidal foraging, Argentine ant activity on beaches was comparable to that in spatially-paired, scrub environments. Foraging on beaches allowed ants to access higher densities of arthropod prey and carrion compared to those found in scrub environments. Stable isotope analyses provide evidence for extensive assimilation of marine-derived resources. Values of δ15N and δ13C for the Argentine ant were higher at beach sites than at scrub sites, and Argentine ant δ15N values broadly overlapped those of intertidal consumers at beach sites. Although ants are known to forage in intertidal environments, this study provides a novel example of an introduced ant species exploiting a cross-boundary subsidy.

The cover image is based on the article Fipronil‐infused sodium polyacrylate gels provide effective management of Argentine ants in conservation areas by Grzegorz Buczkowski and Theresa Wossler https://doi.org/10.1002/ps.8409. image

Abstract Invasive species are major drivers of global ecosystem change. They often displace native species through competitive exclusion, creating novel species interactions. These interactions can lead to ecological cascades, where the presence of one invasive species enhances the fitness of another. Despite this, the mechanisms that underpin these mutualistic interactions and promote secondary invasions remain poorly understood. We assessed how the introduction of the invasive Argentine ant (Linepithema humile) alters mutualistic interactions within the recipient community, enhancing the dispersal and fitness of species of a secondary invasive plant genus (Acacia spp.). Our study examines these interactions at the community, individual species, and trait levels for a range of native and non‐native ant and plant species. Specifically, we aimed to investigate the direct and indirect effects of L. humile on seed dispersal mutualisms. We found that in L. humile invaded locations, seeds of the invasive Acacia plant species were approximately three times more likely to be dispersed than seeds of native endemic plant species. This preference is driven by indirect changes in the community structure of native seed‐dispersing ants, rather than direct interactions between the invasive ant and plant species. Moreover, we found that native ant species that co‐exist with L. humile, such as Tetramorium sericeiventre, respond to a narrower range of seed traits—traits which the invasive Acacia seeds possess. Our findings suggest that the invasion of L. humile has cascading effects on ant‐plant mutualisms, potentially increasing community permeability to secondary invasions. These results highlight that the impact of invaders like the Argentine ant may be greater than initially perceived, driving losses in both biological and functional plant diversity. Read the free Plain Language Summary for this article on the Journal blog.