The Influence of Abiotic Factors and Temporal Variation on Local Invasion Patterns of the Argentine Ant (Linepithema humile)

The Argentine ant is a common pest of mediterranean-type ecosystems worldwide, causing widespread extirpation of many native ant species. This study examines spatial and temporal patterns of invasion at a local scale, investigates the effects of this invader on native ant communities and assesses causal mechanisms for these spatial, temporal and numerical patterns, particularly in relation to local climate. Argentine ant workers were more active in the drier months of summer and fall, and abundance was correlated with cumulative precipitation the previous winter. Argentine ant worker abundance greatly increased in conjunction with EI Niño events. No similar relationship existed for native ants. This invasive ant affected native ant abundances differently by species: some species were able to tolerate its presence temporarily. Overall, this study provides a framework of factors to consider when devising control strategies for the invasive Argentine ant.     

Argentine ant foraging activity and interspecific competition in complete vs. queenless and broodless colonies

Introduction experiments may prove useful in understanding the mechanisms underlying the successful establishment of invasive ant species into new areas. These manipulative introductions could be particularly helpful in exploring the interactions between invasive species and the local fauna and flora. However, the inherent risk of accidental establishment in such experiments poses unacceptable ethical concerns. Some of the worst invasive species are tramp ant species, which can adversely affect biodiversity and community structure after establishment. We conducted laboratory and field experiments investigating a safe methodology for carrying out introduction experiments using the sterile workers of the invasive Argentine ant, Linepithema humile, as a model. We found no difference in foraging rate between worker-only colonies of L. humile and complete colonies, containing queens, workers and brood. Worker-only L. humile colonies showed the same exploitative and interference ability as complete colonies in bait dominance trials with the odorous house ant, Tapinoma sessile, in both laboratory and field trials. We suggest that for those invasive ant species with sterile workers, worker-only colonies may be substituted for complete colonies in short-term field experiments in new areas. Received 18 January 2007; revised 19 June 2007; accepted 22 June 2007.     

Effect of Argentine ant invasions on ground-dwelling arthropods in northern California riparian woodlands

Although the Argentine ant (Linepithema humile) is a widespread invasive species that displaces native ants throughout its introduced range, the effects of these invasions on arthropods other than ants remain poorly known. This study documents the consequences of Argentine ant invasions on ants and other ground-dwelling arthropods in northern California riparian woodlands. Baits and unbaited pitfall traps were used to sample different components of the arthropod communities at five pairs of uninvaded and invaded sites. Sites occupied by Argentine ants supported almost no native epigeic ants except for the winter-active Prenolepis imparis. Sites with Argentine ants averaged four to ten times more ant workers than did sites with native ants, but ant worker biomass did not differ between uninvaded and invaded sites. Argentine ants recruited to baits in invaded areas, on average, in less than half the time of native ants in uninvaded areas. Despite the loss of epigeic native ants, higher Argentine ant worker abundance, and faster recruitment by Argentine ants at invaded sites, pitfall trap samples from uninvaded and invaded areas contained similar abundances and diversities of non-ant arthropods. These findings suggest that Argentine ants and the native ants they displace interact with the ground-dwelling arthropods of these habitats in a similar manner. Received: 24 February 1997 / Accepted: 9 November 1997     

江苏省外来种子植物的初步调查和分析

对江苏省外来种子植物的种类、来源地及分布状况进行了初步调查统计。调查结果显示,江苏省共有外来种子植物393种,大多数来源于欧洲和美洲,主要通过有意引种和无意引入2种方式引入。有意引种的栽培植物有276种,其中观赏植物132种、蔬菜46种、林木35种、工业原料植物17种、药用植物12种、牧草植物11种、粮食作物11种、果树7种以及油料作物5种。另外,外来种子植物中有逃逸种62种、归化种22种、入侵种33种;81．8％的入侵种在江苏全省均有分布;入侵种以菊科（Compositae）和禾本科（Poaceae）植物为主,分别有11种和7种。在此调查结果基础上,对江苏省外来种子植物的分类和危害进行了讨论,并对江苏省外来物种的利用和管理提出了一些建议。     

Specialized predation on Wasmannia auropunctata by the army ant species Neivamyrmex compressinodis

Abstract:  We report here the first case of an efficient and specialized predator of the invasive ant species Wasmannia auropunctata : the army ant Neivamyrmex compressinodis . Our results are based on a study that we conducted in French Guiana, a part of the Wasmannia's native range. When N . compressinodis workers attacked W . auropunctata nests, the assaulted workers panicked and left the nests, some of them carrying brood. Nevertheless, during its raids on W . auropunctata , N . compressinodis was able to capture nearly all of the W . auropunctata brood and winged sexuals, whereas none of the attacks by N . compressinodis on other sympatric ant species were successful. Laboratory experiments revealed that the workers of eight compared sympatric species attacked the N . compressinodis individuals and that N . compressinodis workers accepted W . auropunctata brood as well as that of most of the tested species, showing that its specificity probably depends on the reaction of the W . auropunctata workers.     

Behavioral and Genetic Differentiation Between Native and Introduced Populations of the Argentine Ant

In this paper, we examine the hypothesis that reduced intraspecific aggression underlies the competitive prowess of Argentine ants in their introduced range. Specifically, we test three predictions of this hypothesis by comparing the genetic diversity, behavior, and ecology of Argentine ants in their native range to introduced populations. Differences between native and introduced populations of Argentine ants were consistent with our predictions. Introduced populations of the Argentine ant appear to have experienced a population bottleneck at the time of introduction, as evidenced by much reduced variation in polymorphic microsatellite DNA markers. Intraspecific aggression was rare in introduced populations but was common in native populations. Finally, in contrast to the Argentine ant's ecological dominance throughout its introduced range, it did not appear dominant in the native ant assemblages studied in Argentina. Together these results identify a possible mechanism for the widespread success of the Argentine ant in its introduced range.     

Aggressive interactions between the introduced Argentine ant, Linepithema humile and the native odorous house ant, Tapinoma sessile

The Argentine ant (Linepithema humile) is an invasive species that disrupts the balance of natural ecosystems by displacing indigenous ant species throughout its introduced range. The mechanisms by which Argentine ants effectively compete against native ant species have been previously addressed in field studies that centered on interference and exploitation competition at baits and mainly examined the colony-level performance of Argentine ants. Detailed behavioral observations explaining the basis for the strong competitive ability of L. humile are comparatively rare. To gain a better understanding of the mechanisms by which Argentine ants displace native ants we examined the aggressive interactions between the Argentine ants and the odorous house ant, Tapinoma sessile in four different aggression assays: (1) worker dyad interactions, (2) symmetrical group interactions, (3) intruder introductions into an established resident colony, and (4) a resource competition assay which focused on competition for food and nesting space. Our results demonstrate a clear disparity between worker-level and colony-level fighting ability of Argentine ants and provide behavioral evidence to explain the superior interference ability of Argentine ants in group assays. Argentine ants experienced mixed success in fighting against odorous house ants in dyad interactions, but gradually gained a numerical advantage in symmetrical group interactions by active cooperation among nestmates. Results of the resource competition assay indicate that Argentine ants recruit rapidly, numerically dominate food and nesting sites, and aggressively displace T. sessile from baits. Taken together, the results of these assays allow us to pinpoint the behavioral mechanisms responsible for the remarkable competitive ability of Argentine ants.     

Intraspecific competition influences the symmetry and intensity of aggression in the Argentine ant

Cooperative social groups rely on the ability to distinguishmembers from nonmembers. Accordingly, social insects have evolveda variety of systems that allow discrimination of nest matesfrom non–nest mates. In this study, we show that experiencecan modify patterns of intraspecific aggression in Argentineants (Linepithema humile). In laboratory experiments, we foundthat aggression between colonies was often asymmetrical, butin all five cases, this asymmetry shifted to symmetrical aggressionafter contact with a hostile colony. Moreover, in the field,aggression between workers collected from colony borders wassymmetrical, whereas polarized aggression occurred between workerscollected 500 m away from colony borders. Coinciding with thisshift in aggression symmetry, we also observed an increase inboth the overall level of aggression and the frequency of aggressionin both the field and laboratory bioassays. We found littleevidence for colony-level competitive asymmetries stemming frompolarities in aggression at the worker level, either in thelaboratory or in the field. These results illustrate that recognitionsystems in Argentine ants are surprisingly dynamic and provideexperimental evidence for how recognition can be adjusted inresponse to specific circumstances—in this case the presenceof intraspecific competitors.     

Habitat differences and occurrence of native and exotic ants on Okinawa Island

Ant fauna on Mallotus japonicus, a shrub with extrafloral nectaries, was investigated in two types of habitat (the villages and the forest‐edges) on subtropical Okinawa Island, Japan. Twenty and 16 ant species were found, including 11 and 6 tramp species in the villages and in the forest‐edges, respectively. Occurrence of tramp species was higher in the villages than in the forest edges, supporting the idea that tramp species tend to dominate in disturbed habitats. Nevertheless, the richness of native ants was almost the same across the two habitat types. Consequently, the ant species diversity on M. japonicus appeared higher in the villages. However, monitoring has to be continued to determine whether the above findings represent a stable phenomenon of the community or just a temporary state.     

An experimental study of the relative humidity preference and survival of the Argentine ant, <Emphasis Type="Italic">Linepithema humile</Emphasis> (Hymenoptera,Formicidae): comparisons with a native <Emphasis Type="Italic">Iridomyrmex</Emphasis> species in South Australia

Summary The Argentine ant, Linepithema humile (Mayr), is a widespread invasive ant species that commonly displaces native ants and other ground-dwelling invertebrates in its introduced range. Previous studies have documented a relationship between the spread of Argentine ants and abiotic conditions (particularly water availability) in invaded areas, suggesting that the invasion success of Argentine ants may be related to specific abiotic factors. This study describes the relative humidity preferences and survival of Argentine ants and a dominant native ant, Iridomyrmex rufoniger sp. group, in the laboratory. In a preference experiment, Argentine ant workers showed a significantly greater propensity than Iridomyrmex to locate themselves in containers with the highest relative humidity. A series of survival trials revealed that the survival of both species was related to the relative humidities within experimental containers. The survival of Argentine ant colonies was reduced in comparison with those of Iridomyrmex sp. after 4 h, however by 24 h both species displayed similar levels of survival in all relative humidity conditions. These findings confirm a relationship between the level of available moisture and the distribution and survival of Argentine ants, and may help to account for the current distribution and invasion success of Argentine ants in Australia.     

Genetic and behavioural evidence for a city-wide supercolony of the invasive Argentine ant Linepithema humile (Mayr) (Hymenoptera: Formicidae) in southeastern Australia

The success of invasive ants is frequently attributed to genetic and behavioural shifts in colony structure during or after introduction. The Argentine ant ( Linepithema humile ), a global invader, differs in colony genetic structure and behaviour between native populations in South America and introduced populations in Europe, Japan, New Zealand and North America. However, little is known about its colony structure in Australia. We investigated the genetic structure and behaviour of L. humile across Melbourne, Victoria by quantifying variation at four microsatellite loci and assaying intraspecific aggression at neighbourhood (30–200 m), fine (1–3.3 km) and regional (5–82 km) spatial scales. Hierarchical analyses across these scales revealed that most genetic variation occurred among workers within nests (∼98%). However, although low genetic differentiation occurred among workers between nests at the fine and regional scales (∼2%), negligible differentiation was detected among workers from neighbouring nests. Spatial genetic autocorrelation analysis confirmed that neighbouring nests were genetically more similar to each other. Lack of aggression within and across these scales supported the view that L. humile is unicolonial and forms a large supercolony across Melbourne. Comparisons of genetic structure of L. humile among single nests sampled from Adelaide, Brisbane, Hobart and Perth with Melbourne showed no greater levels of genetic differentiation or dissimilar spatial structure, suggesting an Australia-wide supercolony.     

Global-scale analyses of chemical ecology and population genetics in the invasive Argentine ant

Ants are some of the most abundant and ecologically successful terrestrial organisms, and invasive ants rank among the most damaging invasive species. The Argentine ant is a particularly well-studied invader, in part because of the extreme social structure of introduced populations, known as unicoloniality. Unicolonial ants form geographically vast supercolonies, within which territorial behaviour and intraspecific aggression are absent. Because the extreme social structure of introduced populations arises from the widespread acceptance of conspecifics, understanding how this colonymate recognition occurs is key to explaining their success as invaders. Here, we present analyses of Argentine ant recognition cues (cuticular hydrocarbons) and population genetic characteristics from 25 sites across four continents and the Hawaiian Islands. By examining both hydrocarbon profiles and microsatellite genotypes in the same individual ants, we show that native and introduced populations differ in several respects. Both individual workers and groups of nestmates in the introduced range possess less diverse chemical profiles than ants in the native range. As previous studies have reported, we also find that introduced populations possess much lower levels of genetic diversity than populations in the native range. Interestingly, the largest supercolonies on several continents are strikingly similar to each other, suggesting that they arose from a shared introduction pathway. This high similarity suggests that these geographically far-flung ants may still recognize and accept each other as colonymates, thus representing distant nodes of a single, widely distributed supercolony. These findings shed light on the behaviour and sociality of these unicolonial invaders, and pose new questions about the history and origins of introduced populations.     

The Argentine ant, Linepithema humile, in Japan: Assessment of impact on species diversity of ant communities in urban environments

The Argentine ant Linepithema humile (Mayr) invaded the Hiroshima Prefecture in south‐west Japan some time before 1990. In this report, we describe the distribution of this exotic ant species and assess its impact on indigenous ant communities in urban areas. L. humile is now widely distributed mainly in urban areas and surrounding secondary vegetation of the cities Hatsukaichi and Hiroshima. The impact assessment suggested that L. humile reduced species diversity of local, indigenous ant communities. There was differential sensitivity of indigenous ant species to the invasion of L. humile. Some ant species disappeared in parks infested with L. humile; for example, Pheidole noda, Pheidole indica and Lasius japonicus. L. humile seemed to be superior to these ant species in certain traits and habits, such as mobility, recruitment ability, aggressiveness and omnivory. In contrast, Paratrechina sakurae and Camponotus vitiosus were less affected by L. humile infestation. The mechanisms allowing such coexistence seemed to be small body size (P. sakurae) and arboreal nesting habits (C. vitiosus).     

Targeted control of widespread exotic species for biodiversity conservation: The Red Fox (Vulpes vulpes) in New South Wales, Australia

Summary   Widespread exotic species provide one of the greatest challenges to biodiversity conservation because they often have devastating impacts on native biota but are near impossible to eradicate. The Red Fox ( Vulpes vulpes ) is established across most of mainland Australia, where it has been linked to severe declines and extinctions of a broad suite of native fauna. A targeted approach to reducing the impacts of Foxes on biodiversity was instigated in New South Wales in 2001 under the New South Wales Fox Threat Abatement Plan. It is based on three simple steps. First, explicit priorities for Fox control are established by identifying which native species are at greatest risk from Fox predation and at which sites Fox control for these species is most critical. Second, high-frequency broad-area Fox-control programmes are established across all land tenures at these priority sites. Third, monitoring programmes are established to measure the response of targeted native fauna and Foxes to Fox control. Monitoring helps refine the priorities for control and the methods used over time. This approach provides a model for the strategic control of widespread exotic pests which threaten biodiversity.     

Experimental evidence that the introduced fire ant, Solenopsis invicta, does not competitively suppress co-occurring ants in a disturbed habitat

1. The fire ant, Solenopsis invicta, is a globally distributed invasive ant that is largely restricted to disturbed habitats in its introduced range. For more than half a century, biologists have believed its success results from superior competitive abilities relative to native ant species, as well as an escape from their natural enemies. 2. We used large volumes of hot water to kill fire ant colonies, and only fire ant colonies, on experimental plots in pastures, and found that populations and diversity of co-occurring ants did not subsequently increase. 3. These results are contrary to classical predictions and indicate that S. invicta is not a superior competitor that suppresses native ants, and that the low diversity and abundance of native ants in degraded ecosystems does not result from interaction with fire ants. Instead, other factors such as prior disturbance and recruitment limitation may be the primary limiting factors for native species in these habitats.     

Effects of gaster extract trail concentration on the trail following behaviour of the Argentine ant, Iridomyrmex humilis (Mayr)

In the Argentine ant, optimum trail following to gaster extracts was displayed to 0.1 and 1.0 equivalents/50 cm of circular trail. Trail following to airborne components was demonstrated when ants exhibited normal trail following behaviour while walking 3 or 6 mm below a 0.1 ant equivalent trail. However, at 8 or 12 mm separation, following ceased, indicating that the height of the active space was ca. 6–8 mm. The average horizontal distance from the centre of the trail at which ants exhibited following behaviour increased with concentration to 3–4 mm beyond the applied trail boundaries, indicating both an ability to follow airborne chemicals, and possibly a non-tolerance of excessively high concentration. Activity of 0.1 ant equivalent trails on filter paper declined to about half the original level by four hours; after eight hours, responses were significantly different from, but almost as low as, solvent controls.     

Macroevolutionary patterns in the origin of mutualisms involving ants

Ants are a diverse and abundant insect group that form mutualistic associations with a number of different organisms from fungi to insects and plants. Here, we use a phylogenetic approach to identify ecological factors that explain macroevolutionary trends in the mutualism between ants and honeydew-producing Homoptera. We also consider association between ant-Homoptera, ant-fungi and ant-plant mutualisms. Homoptera-tending ants are more likely to be forest dwelling, polygynous, ecologically dominant and arboreal nesting with large colonies of 10(4)-10(5) individuals. Mutualistic ants (including those that garden fungi and inhabit ant-plants) are found in under half of the formicid subfamilies. At the genus level, however, we find a negative association between ant-Homoptera and ant-fungi mutualisms, whereas there is a positive association between ant-Homoptera and ant-plant mutualisms. We suggest that species can only specialize in multiple mutualisms simultaneously when there is no trade-off in requirements from the different partners and no redundancy of rewards.     

Genetic relationships among populations of the Argentine ant Linepithema humile introduced into Japan

Since some colonies of Argentine ant Linepithema humile were discovered in Japan in 1993, populations of this invasive alien ant species have been expanding their distribution. To resolve the number of invasions and the genetic structure in the early stages of introduction, we inferred the genetic structure and relationships among colonies from eight localities, from analyses of eight nuclear microsatellite DNA markers. F _ST analysis, principal component analysis and assignment test showed that at least three highly genetically differentiated groups of Argentine ants are present in Japan. Populations from Hiroshima Bay were grouped together (Hiroshima, Hatsukaichi, Otake, Iwakuni Central and Iwakuni Kuroiso), while those from Kobe and Yanai were both genetically distant from each other and from the Hiroshima Bay group. Hatsukaichi and Kobe are international seaports, suggesting that the by-ship invasion occurred at least twice. The invasion route of the Yanai population is unknown at this moment. The Aichi population was genetically distant from that of the Hiroshima Bay group by the difference in allele frequencies, and it was plausible that the Aichi population was introduced from the Hiroshima Bay group by human-mediated jump dispersal.