Argentine Ant Populations Research Articles

Field evaluations of biodegradable boric acid hydrogel baits for the control of Argentine ants: Promising results in vineyards and citrus orchards

Argentine ants are a major pest in California. In this study, a biodegradable calcium alginate hydrogel with an aqueous boric acid bait was tested against Argentine ant populations in a citrus orchard and a vineyard. A new continuous method was developed to produce large quantities of hydrogel bait for the field test. Foraging activity levels of ants were compared between baited and untreated zones. For both study sites, four to five monthly bait applications throughout summer provided a greater than 80% reduction in ant activity. Based on spatial analyses by distance indices, the baited areas were characterized by gaps (areas with lower ant counts) and the untreated control zones were characterized by patches (areas with higher ant counts). This indicated area-wide suppression of Argentine ants. For the citrus orchard, post-baiting panel trap monitoring showed reductions of both ants and Asian citrus psyllid in the baited zone compared to the control. For the vineyard, mid-season soil analyses indicated that the impact of boric acid baiting on soil boron concentration was negligible. In sum, the calcium alginate hydrogel bait with boric acid as an active ingredient may provide a promising solution for Argentine ant baiting.

Variable viral loads and immune response in an invasive ant's native and introduced ranges

AbstractAimPathogens can play an important role in biological invasions. Introduced populations may be particularly vulnerable to pathogens due to factors such as low genetic diversity and high population density. However, introduced populations that escape their natural pathogens may reallocate resources away from immunity and towards growth and reproduction. Interestingly, introduced ants have been suggested to have increased tolerance to new pathogens, contributing to their success as introduced species. In this study, we aimed to investigate whether introduced Argentine ant populations harbour different viral loads compared to native populations and if these differences were related to immunity‐related gene expression.LocationThe study was conducted across the native range of Argentine ants in Argentina and four introduced regions in California, France, Australia and New Zealand.MethodsWe used RT‐qPCR to quantify viral loads and gene expression in the ants. We analysed 15 different potentially pathogenic viruses across the Argentine ant's native and introduced ranges.ResultsWe found that five viruses, LhuPcV1, LhuPiLV1, LhuCV1, Kashmir Bee virus and LHUV‐1, presented high loads in Argentine ants compared to the other viruses we screened. We found a significant effect of range on viral infections: high viral loads were commonly found in ants from introduced populations, which also exhibited increased immune gene expression. We found highly significant correlations between viral loads and expression of immune and metabolic genes. However, these associations were not fully consistent across the studied regions, indicating the complexity of eco‐immunological phenomena.Main ConclusionsOur results suggest that introduced Argentine ant populations host different viral communities compared to native populations and that these differences are correlated with changes in immunity‐related gene expression. The study highlights the complex role of pathogens in biological invasions and the importance of considering eco‐immunological factors when assessing the impact of introduced species.

Laboratory evaluations of biodegradable boric acid hydrogel baits for the control of Argentine ant (Hymenoptera: Formicidae).

Due to their mutualistic relationship with plant pests, the Argentine ant is considered a major pest in subtropical fruit orchards and vineyards. Besides insecticide sprays, liquid baiting has been demonstrated as an effective method to suppress the Argentine ant populations. To improve the economic feasibility of liquid baiting, hydrogel materials have been recently tested as a carrier for liquid baits containing various insecticidal active ingredients. Here, we tested boric acid as a toxicant in the aqueous sugar bait delivered in a biodegradable calcium alginate hydrogel. Laboratory tests demonstrated that boric acid (1%) liquid bait incorporated in the calcium alginate hydrogel effectively killed Argentine ant workers. Potassium sorbate (0.25%) added to the liquid bait as a preservative did not impact the efficacy of boric acid even though it significantly reduced the degree of swelling of the hydrogel beads in the bait solution. Testing with 2-month-old bait suggested that long-term storage might impact bait efficacy even with potassium sorbate preservative.

A low-toxicity baiting program precipitates collapse of Argentine ant and ant-associated hemipteran pest populations in commercial citrus

The Argentine ant, Linepithema humile Mayr (Hymenoptera: Formicidae), disrupts biological control agents which induces population outbreaks of honeydew-producing hemipteran pests infesting agroecosystems. A multi-season liquid baiting program was evaluated for control of L. humile and ant-associated hemipteran pests in six commercial citrus orchards in southern California. Ant activity in treated trees fell by 96% within three months of treatment and was, on average, 95% lower in treated plots than untreated plots across the 18-month study period. Ant control resulted in the near elimination of hemipteran pest populations on trees in treated plots. Pre- and post-treatment estimates indicated that hemipteran pest infestation of twigs, flush, and fruit decreased by 97, 84, and 99% one year after treatment, respectively. Furthermore, cumulative infestation frequency was 10 times lower and cumulative colony size was 5 times lower on treated trees than control trees across the final year of study. Examining individual hemipteran pest species, the mean infestation rate by Diaphorina citri Kuwayama (Hemiptera: Liviidae) was 75% lower, Coccus hesperidum L. (Hemiptera: Coccidae) 95% lower, Planococcus citri Risso (Hemiptera: Pseudococcidae) 98% – 100% lower, and Aonidiella aurantii Maskell (Hemiptera: Diaspidae) 70 – 80% lower in treated trees relative to untreated trees. A surge in generalist predator activity and parasitism following ant bait deployment appear to be the driving forces behind the marked decrease in pest infestation observed in treated trees. Results underscore the importance of L. humile control for integrated pest management programs targeting ant-associated hemipteran pests in commercial citrus.

Native and introduced Argentine ant populations are characterised by distinct transcriptomic signatures associated with behaviour and immunity

Biological invasions can be influenced by trait variation in the invader, such as behavioural traits and ecological factors, such as variation in pathogen pressure. High-throughput nucleotide sequencing has increased our capacity to investigate the genomic basis of the functional changes associated with biological invasions. Here, we used RNA-sequencing in Argentina and California, Australia and New Zealand to investigate if native and introduced Argentine ant populations were characterised by distinct transcriptomic signatures. We focused our analysis on viral pressure and immunity, as well as genes associated with biogenic amines known to modulate key behaviour in social insects. Using a combination of differential expression analysis, gene co-expression network analysis and candidate gene approach, we show that native and introduced populations have distinct transcriptomic signatures. Genes associated with biogenic amines were overall up-regulated in the native range compared to introduced populations. Although we found no significant variation in overall viral loads amongst regions for viruses known to infect Argentine ants, viral diversity was lower in most of the introduced range which was interestingly associated with down-regulation of the RNAi immune pathway, primarily directed against viruses. Altogether, our data show that Argentine ant populations exhibit range-specific transcriptomic signatures, perhaps reflecting regional adaptations that may contribute to the ecological success of introduced populations.

Argentine Ant Management: Using Toxin-Laced Polyacrylamide Crystals to Target Ant Colonies in Vineyards

Summary Goals: Argentine ant (Linepithema humile) is an invasive species that is ecologically disruptive and agriculturally significant. In vineyards, Argentine ant activity compromises nonchemical approaches to mealybug management by interrupting biological control agents. Toxic baits have proven effective to control Argentine ant populations in urban and agricultural settings; however, it is resource intensive to use bait stations to deliver a toxin to ants, making it prohibitive to large-scale implementation by agricultural producers. We undertook this project to evaluate a simpler, more efficient delivery method, using toxin-laced polyacrylamide crystals. Key Findings: Polyacrylamide baits are quick and easy to prepare and apply in vineyards. Both boric acid (0.5%) and thiamethoxam (0.0006%) are compatible with the polyacrylamide crystal delivery method. In vineyards, two applications of thiamethoxam-laced bait eliminated ant foraging at sugar-soaked cotton balls for 5 to 6 wks and further suppressed activity for up to 6 mos. In vineyards, two applications of boric acid-laced bait eliminated ant foraging at sugar-soaked cotton balls for 4 to 6 wks and further suppressed activity for up to 5 mos. Overall temporal declines in ant populations in the trial blocks suggest that there are cumulative effects of the bait applications over multiple seasons. Impact and Significance: Toxic bait applied in commercial vineyards using a novel delivery method, polyacrylamide crystals, provided durable control of Argentine ant populations during the growing season. Following two bait applications, ant foraging at sugar-soaked cotton balls was eliminated for 5 to 6 wks and further suppressed for 3 to 6 mos; in several instances, this effect continued into the subsequent growing season. The broadcast toxin-laced crystals were spread across a large area of the vineyard, facilitating ant foraging at multiple point sources to target a greater number of nests within the infested areas. The sugary, aqueous bait mimicked their natural food source, making it attractive to foraging ants and therefore more likely to result in sustained population control. Using these baits to reduce Argentine ant populations in vineyard ecosystems can reduce their agricultural significance by enhancing biological control of phloem-feeding insects, particularly mealybugs. Other ecological benefits may also result from reducing populations of this invasive species in agricultural and natural systems.1,2,3

Behavioural variation and plasticity along an invasive ant introduction pathway.

Once established in new areas, introduced species may exhibit changes in their biology due to phenotypic plasticity, novel selection pressures and genetic drift. Moreover, the introduction process itself has been hypothesised to act as a selective filter for traits that promote invasiveness. We tested the hypothesis that behaviours thought to promote invasiveness-such as increased foraging activity and aggression-are selected for during invasion by comparing traits among native and introduced populations of the widespread Argentine ant (Linepithema humile). We studied Argentine ant populations in the native range in Argentina and in three invaded regions along an introduction pathway: California, Australia and New Zealand. In each region, we set up 32 experimental colonies to measure foraging activity and interspecific aggression in a subset of the study regions. These colonies were subject to experimental manipulation of carbohydrate availability and octopamine, a biogenic amine known to modulate behaviour in insects, to measure variation in behavioural plasticity. We found variation in foraging activity among populations, but this variation was not consistent with selection on behaviour in relation to the invasion process. We found that colonies with limited access to carbohydrates exhibited unchanged exploratory behaviour, but higher exploitation activity and lower aggression. Colonies given octopamine consistently increased foraging behaviour (both exploration and exploitation), as well as aggression when also sugar-deprived. There was no difference in the degree of behavioural response to our experimental treatments along the introduction pathway. We did not find support for selection of behavioural traits associated with invasiveness along the Argentine ant's introduction pathway or clear evidence for an association between the introduction process and variation in behavioural plasticity. These results indicate that mechanisms promote behavioural variation in a similar fashion both in native and introduced ranges. Our results challenge the assumption that introduced populations always perform better in key behavioural traits hypothesised to be associated with invasion success.

Host-Tree Selection by the Invasive Argentine Ant (Hymenoptera: Formicidae) in Relation to Honeydew-Producing Insects.

The Argentine ant, Linepithema humile (Mayr; Hymenoptera: Formicidae), is one of the world's most hazardous invasive species, and thus its eradication from Japan is important. Physical and chemical controls can be expensive and cause strong adverse effects on local terrestrial ecosystems regardless of their high efficacy. Here, presence/absence of host-tree selection by Argentine ants was investigated to understand the ant-honeydew-producing insects interactions in order to develop new cultural controls compatible with biodiversity conservation. Abundance of Argentine ants and their tree utilization ratio was measured among dominant roadside trees (Cinnamomum camphora, Myrica rubra, Nerium indicum, Rhaphiolepis indica var. umbellata, Juniperus chinensis var. kaizuka) in two areas around Kobe, Japan. Almost all ants collected were Argentine ants suggesting that native ants would have been competitively excluded. Tree utilization of Argentine ants clearly differed among host trees. Abundance of both Argentine ants and honeydew-producing insects and tree utilization rate of the ants were significantly lower in especially C. camphora and J. chinensis. Few Argentine ants were observed trailing on C. camphora, J. Chinensis, and N. indicum, most probably due to low abundance of honeydew-producing insects on these trees with the toxic and repellent chemical components. On the other hand, high abundance of both Argentine ants and homopterans were found in M. rubra and especially R. indica. We suggest that reductions of R. indica and M. rubra would lead to a decrease in abundance of honeydew-producing insects, and thus effectively control populations of Argentine ants. At the same time, planting of C. camphora, J. Chinensis, and N. indicum may also play a role in restraint efficacy against invasion of the invasive ants.

Differences in behavioural traits among native and introduced colonies of an invasive ant

Identifying the factors that promote the success of biological invasions is a key pursuit in ecology. To date, the link between animal personality and invasiveness has rarely been studied. Here, we examined in the laboratory how Argentine ant populations from the species’ native and introduced ranges differed in a suite of behaviours related to species interactions and the use of space. We found correlations among specific behavioural traits that defined an explorative-aggressive syndrome. The Main “European” supercolony (introduced range) more readily explored novel environments, displayed more aggression, detected food resources more quickly, and occupied more space than the Catalonian supercolony (introduced range) and two other Argentine supercolonies (native range). The two native supercolonies also differed in their personalities; one harbouring the less invasive personality, while the other is intermediate between the two introduced supercolonies. Therefore, instead of a binary pattern, Argentine ant supercolonies display a behavioural continuum that is independent on their geographic origin (native/introduced ranges). Our results also suggest that variability in personality traits is correlated to differences in the ecological success of Argentine ant colonies. Differences in group personalities may facilitate the persistence and invasion of animals under novel selective pressures by promoting adaptive behaviours. We stress that the concept of animal personality should be taken into account when elucidating the mechanisms of invasiveness.

Invasive ants carry novel viruses in their new range and form reservoirs for a honeybee pathogen.

When exotic animal species invade new environments they also bring an often unknown microbial diversity, including pathogens. We describe a novel and widely distributed virus in one of the most globally widespread, abundant and damaging invasive ants (Argentine ants, Linepithema humile). The Linepithema humile virus 1 is a dicistrovirus, a viral family including species known to cause widespread arthropod disease. It was detected in samples from Argentina, Australia and New Zealand. Argentine ants in New Zealand were also infected with a strain of Deformed wing virus common to local hymenopteran species, which is a major pathogen widely associated with honeybee mortality. Evidence for active replication of viral RNA was apparent for both viruses. Our results suggest co-introduction and exchange of pathogens within local hymenopteran communities. These viral species may contribute to the collapse of Argentine ant populations and offer new options for the control of a globally widespread invader.

Density-Dependent Effects of an Invasive Ant on a Ground-Dwelling Arthropod Community.

It is frequently assumed that an invasive species that is ecologically or economically damaging in one region, will typically be so in other environments. The Argentine ant Linepithema humile (Mayr) is listed among the world's worst invaders. It commonly displaces resident ant species where it occurs at high population densities, and may also reduce densities of other ground-dwelling arthropods. We investigated the effect of varying Argentine ant abundance on resident ant and nonant arthropod species richness and abundance in seven cities across its range in New Zealand. Pitfall traps were used to compare an invaded and uninvaded site in each city. Invaded sites were selected based on natural varying abundance of Argentine ant populations. Argentine ant density had a significant negative effect on epigaeic ant abundance and species richness, but hypogaeic ant abundance and species richness was unaffected. We observed a significant decrease in Diplopoda abundance with increasing Argentine ant abundance, while Coleoptera abundance increased. The effect on Amphipoda and Isopoda depended strongly on climate. The severity of the impact on negatively affected taxa was reduced in areas where Argentine ant densities were low. Surprisingly, Argentine ants had no effect on the abundance of the other arthropod taxa examined. Morphospecies richness for all nonant arthropod taxa was unaffected by Argentine ant abundance. Species that are established as invasive in one location therefore cannot be assumed to be invasive in other locations based on presence alone. Appropriate management decisions should reflect this knowledge.

Hydrogel baits with low‐dose thiamethoxam for sustainable Argentine ant management in commercial orchards

AbstractArgentine ants, Linepithema humile (Mayr) (Hymenoptera: Formicidae), are a significant pest in various agricultural systems around the world, and are often associated with outbreaks of phloem‐feeding hemipteran insects. Previous research has evaluated a number of active ingredients and management approaches for controlling Argentine ant populations in agricultural systems, but various regulatory and economic issues have limited the development of effective management tools. Current chemical controls rely on residual sprays or toxic baits, each one posing unique disadvantages that limit their usefulness and efficacy. This study evaluated the potential of water‐storing crystals to effectively deliver liquid baits to Argentine ants. The efficacy of bait crystals containing 0.007% thiamethoxam was first evaluated in laboratory colonies. In addition, field studies were performed in a commercial plum orchard to determine the efficacy of the bait crystals. Protein marking was used within the orchard to examine the distribution of the bait in Argentine ant populations when delivered via water‐storing crystals. Results of laboratory tests showed that water‐storing crystals containing 0.007% thiamethoxam are highly attractive and effective against Argentine ants and require ca. 3–5 days to kill all castes and life stages. Results of the protein‐marking study demonstrated that the percentage of ants carrying protein‐labeled sugar water decreases sharply with increasing distance from the bait station. Bait movement was limited to within 17 m of the bait dispenser. Furthermore, bait efficacy tests in the field showed that Argentine ants can be effectively controlled using liquid thiamethoxam baits deployed via water‐storing crystals. The bait was highly effective and ant densities throughout the baited plots declined by 94 ± 2% within 14 days. The results of this study demonstrate that (1) thiamethoxam is highly effective for Argentine ant control in fruit orchards when used in low concentrations (0.007%), and (2) water‐storing crystals are an effective tool for delivering liquid baits to Argentine ants in agricultural settings.

Trail Pheromone of the Argentine Ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae)

The Argentine ant (Linepithema humile) is recognized as one of the world's most damaging invasive species. One reason for the ecological dominance of introduced Argentine ant populations is their ability to dominate food and habitat resources through the rapid mobilization and recruitment of thousands of workers. More than 30 years ago, studies showed that (Z)-9-hexadecenal strongly attracted Argentine ant workers in a multi-choice olfactometer, suggesting that (Z)-9-hexadecenal might be the trail pheromone, or a component of a trail pheromone mixture. Since then, numerous studies have considered (Z)-9-hexadecenal as the key component of the Argentine ant trails. Here, we report the first chemical analyses of the trails laid by living Argentine ants and find that (Z)-9-hexadecenal is not present in a detectible quantity. Instead, two iridoids, dolichodial and iridomyrmecin, appear to be the primary chemical constituents of the trails. Laboratory choice tests confirmed that Argentine ants were attracted to artificial trails comprised of these two chemicals significantly more often than control trails. Although (Z)-9-hexadecenal was not detected in natural trails, supplementation of artificial dolichodial+iridomyrmecin trails with an extremely low concentraion of (Z)-9-hexadecenal did increase the efficacy of the trail-following behavior. In stark contrast with previous dogma, our study suggests that dolichodial and iridomyrmecin are major components of the Argentine ant trail pheromone. (Z)-9-hexadecenal may act in an additive manner with these iridoids, but it does not occur in detectable quantities in Argentine ant recruitment trails.

Variation in the level of aggression, chemical and genetic distance among three supercolonies of the Argentine ant in Europe

In their invasive ranges, Argentine ant populations often form one geographically vast supercolony, genetically and chemically uniform within which there is no intraspecific aggression. Here we present regional patterns of intraspecific aggression, cuticular hydrocarbons (CHCs) and population genetics of 18 nesting sites across Corsica and the French mainland. Aggression tests confirm the presence of a third European supercolony, the Corsican supercolony, which exhibits moderate to high levels of aggression, depending on nesting sites, with the Main supercolony, and invariably high levels of aggression with the Catalonian supercolony. The chemical analyses corroborated the behavioural data, with workers of the Corsican supercolony showing moderate differences in CHCs compared to workers of the European Main supercolony and strong differences compared to workers of the Catalonian supercolony. Interestingly, there were also clear genetic differences between workers of the Catalonian supercolony and the two other supercolonies at both nuclear and mitochondrial markers, but only very weak genetic differentiation between nesting sites of the Corsican and Main supercolonies (F(ST) = 0.06). A detailed comparison of the genetic composition of supercolonies also revealed that, if one of the last two supercolonies derived from the other, it is the Main supercolony that derived from the Corsican supercolony rather than the reverse. Overall, these findings highlight the importance of conducting more qualitative and quantitative analyses of the level of aggression between supercolonies, which has to be correlated with genetic and chemical data.

Effects of Vegetation Cover, Presence of a Native Ant Species, and Human Disturbance on Colonization by Argentine Ants

The spread of non-native invasive species is affected by human activity, vegetation cover, weather, and interaction with native species. We analyzed data from a 17-year study of the distribution of the non-native Argentine ant (Linepithema humile) and the native winter ant (Prenolepis imparis) in a preserve in northern California (U.S.A.). We conducted logistic regressions and used model selection to determine whether the following variables were associated with changes in the distribution of each species: presence of conspecifics at neighboring sites, distance to development (e.g., roads, buildings, and landscaped areas), proportion of vegetation cover taller than 0.75 m, elevation, distance to water, presence of both species at a site, temperature, and rainfall. Argentine ants colonized unoccupied sites from neighboring sites, but the probability of appearance and persistence decreased as distance to development, vegetation cover, and elevation increased. Winter ants appeared and persisted in sites with relatively high vegetation cover (i.e., highly shaded sites). Presence of the 2 species was negatively associated in sites with high vegetation cover (more winter ants) and sites near development (more Argentine ants). Probability of colonization of Argentine ants decreased where winter ants were most persistent. At sites near development within the preserve, abundant Argentine ant populations may be excluding winter ants. The high abundance of Argentine ants at these sites may be due to immigration from suburban areas outside the preserve, which are high-quality habitat for Argentine ants. In the interior of the preserve, distance from development, low-quality habitat, and interaction with winter ants may in combination exclude Argentine ants. Interactions among the variables we examined were associated with low probabilities of Argentine ant colonization in the preserve.

Site occupancy and detection probability of Argentine ant populations

AbstractEffective sampling and detection plays an important role in the surveillance and management of invasive species. Invasive ants are prime candidates for imperfect detection because of their small size, cryptic nature and also because of their strong association with human transportation around the globe. We examined site occupancy, colonization‐extirpation dynamics and detection probability of Argentine ant populations in the Auckland region of New Zealand. Comparison of 175 sites from 2002 to 2007 shows the dynamic nature of Argentine ants, with an extirpation rate of 33% and a colonization rate of 8%. Baited vials gave relatively poor detection for Argentine ants, despite repeated sampling. If Argentine ants were present at a location, their probability of detection using baited vials was, on average, only P = 0.55 (±SE = 0.04). However, this probability depended on the duration that baited vials were left out. Detectability was highest for vials set out for three (P = 0.52–0.75) or 6 h (P = 0.53–0.82). Detection probability for visual searching was 0.895. We use these data to suggest improvements in the design of surveys for Argentine ants. Research on detection theory and its applications for surveillance and eradication is a major gap for invasive ants.

Ecological and evolutionary perspectives on "supercolonies": a commentary on Moffett

Moffett provides a very thorough review of the ‘‘supercolony’’ concept, focusing on the biology of the Argentine ant (Linepithema humile). The term supercolony is being used more and more frequently in the social insect literature, however, not always to describe the same colony structure. Despite invoking an almost science fiction-like quality, we agree with Moffett that the concept of the supercolony has been valuable, providing considerable insight from both an ecological and an evolutionary perspective. Ecologically, the term supercolony is often used to describe a polydomous colony that occupies such a large number of nests that it is impossible for all members of the colony to interact in their lifetime (Pedersen et al. 2006). That is, the society occupies many nest sites over such a large area that queens and workers who live on one side of the supercolony will never meet individuals from the other side. As Moffett points out, this is likely true for many social insects, even those that occupy relatively few nest sites over smaller areas. Moffett suggests that we restrict the definition of supercolony to societies with at least 1 million workers. However, this seems a bit arbitrary; a number of ant species are known to have more than 1 million workers yet occupy a single nest site. Are these supercolonies in the same sense that a spatially extensive polydomous Argentine ant colony is? Species that form supercolonies are also often ecologically dominant where they occur, both outcompeting other species for resources and overwhelming species through numerical superiority (Holway et al. 2002). However, as pointed out by Holldobler and Wilson (1977), research is still needed to determine if ants become ecologically dominant because they form supercolonies or if ecologically dominant ants monopolize resources and subsequently form supercolonies as a result. From an evolutionary perspective, a supercolony can be defined as the aggregation of ants across many nests that share a common genotype (usually inferred using microsatellites or mitochondrial DNA) and phenotype (e.g., cuticular hydrocarbon profile). This is likely due to having a shared common ancestry. Subsequently, this approach has been used to examine the source and introduction history of invasive ants. Recent holistic work combining behavior, chemistry, and genetics has provided incredible insight into the worldwide spread of Argentine ants (Wilgenburg et al. 2010), and supercolony identity has been used to infer recent exchange of ants between different introduced populations (Tsutsui et al. 2001; Corin et al. 2007; Sunamura et al. 2009; Vogel et al. 2010; Suhr et al. 2011). As Moffett points out, genetic work on both native and introduced populations of Argentine ants suggests little to know gene flow among supercolonies. However, we disagree that each supercolony should be considered its own species (much as each individual is not considered its own species in clonal organisms). More work is needed to determine the spatial extent of gene flow in species that form supercolonies. We agree with Moffett that we need to move past semantic arguments when discussing supercolonies. However, we believe that both ecological and evolutionary examinations of supercolonies need to be conducted from the perspective of the population. A population can be defined as a group of colonies that have the potential to exchange genes or compete for resources. A population can therefore have many colonies, even many supercolonies. However, the term unicolonial, which is often considered synonymous with supercolony (Pedersen et al. 2006), should be restricted to describe situations in which the entire population consists of only one supercolony (a ‘‘unicolonial population’’ Holldobler and Wilson 1977; Suarez et al. 2008). Research is still clearly needed on the biology of species that form supercolonies as many unanswered questions remain. For example, how do supercolonies arise and how long do they last? How much variation in social parameters exists in native populations of species that can form supercolonies (e.g., colony size, queen number, intravs. intercolony genetic diversity, gene flow among colonies)? Is there conflict over reproduction within supercolonies or are they as harmonious as Moffett implies? Over what spatial scales are food and information exchanged (Heller et al. 2008)? These last 2 questions in particular address whether or not supercolonies are effectively made up of aggregations of smaller colonies. Despite being one of the best-studied social insects, we still know surprisingly little about the biology of Argentine ants, particularly in their native range. From an evolutionary perspective, we often know even less about species closely related to those that form supercolonies. For example, what characteristics are shared among sibling species in the humile complex or are unique to the widespread L. humile?

Seeing the forest and the trees

Moffett (2012) misrepresents the position of Gordon, Heller, and others on supercolonies. We recognize the existence of Argentine ant supercolonies, populations of colonies all descended from common ancestors. A substantial body of work shows the common origins of certain Argentine ant populations of colonies, or supercolonies (e.g., Vogel et al. 2010). We consider that a colony, unlike a supercolony, involves functional ecological interactions among the ants. A group of ants functions as a colony when it shares resources and reproduction. Heller et al. (2008) showed that nests share resources only at scales of about 100 m. Ingram and Gordon (2003) showed that there is isolation by distance, demonstrating limited genetic mixing, also at scales of 100 m. These results indicate that there are colonies on the scale of 100 m at the study site in northern California. Ants that belong to the same supercolony have a shared ancestry, but if the supercolony spans many kilometers, or regions that ants cannot cross, then it includes ants that cannot possibly meet and thus cannot possibly share resources or interact ecologically. Many studies of Argentine ants show that ants from different locations are unlikely to fight (e.g., van Wilgenburg, Clemencet, et al. 2010). For ants, the absence of fighting is not equivalent to belonging to the same colony. It is not the case, as Moffett says, that ‘‘in a healthy society, [ants] invariably avoid or kill alien ants with different cues.’’ In fact, ants of different colonies do not always fight. Many studies of nestmate recognition show variation, in trials with ants of the same colony or in repeated trials with the same individual, in the extent of fighting or avoidance. Species differ in propensity to fight, and within a species, whether ants fight depends on context. For example, Roulston et al. (2003) showed that whether Argentine ants fight in bioassays depends on the number of ants. As the recent work of Newey (2011) and others shows, the extent of fighting is not necessarily a measure of genetic relatedness or of similarity in hydrocarbon profile. Instead, the extent of fighting, genetic relatedness, and similarity in hydrocarbon profile are related in complex ways (Sturgis and Gordon 2011) and in Argentine ants depend on many factors, including food supply (Liang and Silverman 2000), social context (Buczkowski and Silverman 2005), and familiarity (van Wilgenburg, Torres, et al. 2010). Moffett’s article proceeds by means of journalistic hyperbole rather than specific statements; as he says, many of his arguments are ‘‘semantic’’ arguments, not based on facts. He extols the unity of the Argentine ant supercolony without clearly distinguishing evidence from opinion. We hope that continued discussion of the intriguing biology of Argentine ants will be based on scientific investigation. What is needed is not further discussion of definitions but further data on the organization of tasks and resource sharing among nests within colonies, on the interactions among colonies within supercolonies, and on how these produce the spatial and genetic structure of populations of colonies.

The widespread collapse of an invasive species: Argentine ants ( Linepithema humile ) in New Zealand

Synergies between invasive species and climate change are widely considered to be a major biodiversity threat. However, invasive species are also hypothesized to be susceptible to population collapse, as we demonstrate for a globally important invasive species in New Zealand. We observed Argentine ant populations to have collapsed in 40 per cent of surveyed sites. Populations had a mean survival time of 14.1 years (95% CI = 12.9-15.3 years). Resident ant communities had recovered or partly recovered after their collapse. Our models suggest that climate change will delay colony collapse, as increasing temperature and decreasing rainfall significantly increased their longevity, but only by a few years. Economic and environmental costs of invasive species may be small if populations collapse on their own accord.

Behavioural and chemical evidence for multiple colonisation of the Argentine ant, Linepithema humile, in the Western Cape, South Africa

BackgroundThe Argentine ant, Linepithema humile, is a widespread invasive ant species that has successfully established in nearly all continents across the globe. Argentine ants are characterised by a social structure known as unicoloniality, where territorial boundaries between nests are absent and intraspecific aggression is rare. This is particularly pronounced in introduced populations and results in the formation of large and spatially expansive supercolonies. Although it is amongst the most well studied of invasive ants, very little work has been done on this ant in South Africa. In this first study, we investigate the population structure of Argentine ants in South Africa. We use behavioural (aggression tests) and chemical (CHC) approaches to investigate the population structure of Argentine ants within the Western Cape, identify the number of supercolonies and infer number of introductions.ResultsBoth the aggression assays and chemical data revealed that the Western Cape Argentine ant population can be divided into two behaviourally and chemically distinct supercolonies. Intraspecific aggression was evident between the two supercolonies of Argentine ants with ants able to discriminate among conspecific non-nestmates. This discrimination is linked to the divergence in cuticular hydrocarbon profiles of ants originating from the two supercolonies.ConclusionsThe presence of these two distinct supercolonies is suggestive of at least two independent introductions of this ant within the Western Cape. Moreover, the pattern of colonisation observed in this study, with the two colonies interspersed, is in agreement with global patterns of Argentine ant invasions. Our findings are of interest because recent studies show that Argentine ants from South Africa are different from those identified in other introduced ranges and therefore provide an opportunity to further understand factors that determine the distributional and spread patterns of Argentine ant supercolonies.