The raiding success of Pheidole megacephala on other ants in both its native and introduced ranges

We studied the behaviour of the invasive African myrmicine ant, Pheidole megacephala, when confronted with colonies of other common ant species in Cameroon, a part of its native range, and in Mexico, where it has been introduced. P. megacephala raided the nests of the other ants in both cases. Eleven species out of 12 put up a rather strong resistance to raiding P. megacephala workers in Cameroon compared to only three species out of 11 in Mexico, where only colonies of Solenopsis geminata, Dorymyrmex pyramicus and Dolichoderus bispinosus resisted these raids. We conclude that P. megacephala's heightened ability to successfully raid colonies of competing ants may help explain its success and the decline of native ants in areas where it has been introduced.     

The predatory behavior of Pheidole megacephala

We studied the foraging and predatory behaviors of the invasive African myrmicine ant, Pheidole megacephala (F.) in its native range. Workers can singly capture a wide range of insects, including relatively large prey items. For still larger prey, they recruit at short range those nestmates situated within reach of an alarm pheromone and together spread-eagle the insect. These behaviors are complimented by a long-range recruitment (of nestmates remaining in the nest) based on prey size. P. megacephala scouts also use long-range recruitment when they detect the landmarks of termites and competing ant species, thus permitting them to avoid confronting these termites and ants solitarily.     

The underdog invader: Breeding system and colony genetic structure of the dark rover ant (Brachymyrmex patagonicus Mayr)

Ants are among the most successful species at invading new environments. Their success undeniably comes from their various modes of reproduction and colony breeding structures, which influence their dispersal ability, reproductive potential, and foraging strategies. Almost all invasive ant species studied so far form supercolonies, a dense network of interconnected nests comprising numerous queens, without aggression toward non‐nestmates. This strategy results in invasive colonies that are able to grow extremely fast and large while avoiding intraspecific competition, allowing them to monopolize environmental resources and outcompete native species. Here, we developed and used 10 microsatellite markers to investigate the population structure and breeding system of the dark rover ant Brachymyrmex patagonicus Mayr in its introduced range. We determined whether this species exhibits a supercolonial structure by assessing whether different nests belonged to the same genetic colony. We inferred its dispersal ability by investigating isolation by distance and estimated the numbers of queens per colonies and mating per queen through parent‐offspring inferences. We found that most of the colonies of B. patagonicus were comprised of a single nest, headed by a single queen. Each nest was distinct from one another, without isolation by distance, which suggests strong dispersal ability through nuptial flights. These features are commonly observed in noninvasive and native ant species, but they are surprising for a successful invasive ant, as they strongly differ from other invasive ants. Overall, we discuss how this seemingly unfavorable strategy for an invasive ant might favor the invasive success of the dark rover ant in the United States.     

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.     

Genetics, behaviour and chemical recognition of the invading ant Pheidole megacephala

Introduced species often become ecologically dominant, displacing native species and posing a serious threat to ecosystem function and global biodiversity. Ants are among the most widespread and damaging alien species; introductions are often accompanied by population-level behavioural and genetic changes contributing to their success. We investigated the genetic structure, chemical profile and nestmate recognition in introduced populations of the invasive big-headed ant, Pheidole megacephala, in Australia. Behavioural analyses show that workers are not aggressive towards conspecifics from different nests, even at large geographical scales (up to 3000 km) and between populations encompassing a wide range of environmental conditions. By contrast, interactions with workers of other species invariably result in agonistic behaviours. Genetic analyses reveal that populations have low genetic diversity. No genetic differentiation occurs among nests of the same population; differentiation between populations, though significant, remains weak. Chemical analyses indicate that cuticular lipids are similar between colonies of a population, and that differentiation between populations is low. Altogether, these results indicate that the big-headed ant P. megacephala forms a large unicolonial population across northern/eastern Australia.     

Use of termites, Reticulitermes virginicus, as a springboard in the invasive success of a predatory ant, Pachycondyla (=Brachyponera) chinensis

Invasive ant species have general diet and nest requirements, which facilitate their establishment in novel habitats and their dominance over many native ants. The Asian needle ant, Pachycondyla chinensis, native throughout Australasia was introduced to the southeastern US where it has become established in woodland habitats, nests in close proximity to and consumes subterranean termites (Rhinotermitidae). P. chinensis do not occur in habitats lacking Rhinotermitidae. We suggest that subterranean termites are critical for P. chinensis success in new habitats. We demonstrate that P. chinensis is a general termite feeder, retrieving Reticulitermes virginicus five times more often than other potential prey near P. chinensis colonies. Odors produced by R. virginicus workers, as well as other potential prey, attract P. chinensis. Furthermore, P. chinensis occupy R. virginicus nests in the lab and field and display behaviors that facilitate capture of R. virginicus workers and soldiers. Termites are an abundant, high quality, renewable food supply, in many ways similar to the hemipteran honeydew exploited by most other invasive ant species. We conclude that the behavior of P. chinensis in the presence of termites increases their competitive abilities in natural areas where they have been introduced.     

Impacts of Argentine ants on avian nesting success

Biological invasions can have severe and widespread impacts on ecological communities. A few species of ants have become particularly damaging invaders but quantitative data of their impacts on many taxa is still lacking. We provide experimental evidence using artificial nests baited with quail eggs that the invasive Argentine ant (Linepithema humile) can be a significant avian nest predator – Argentine ants recruited to more nests and in higher abundance than the native ant species they displace. However, at a site invaded by Argentine ants, we monitored over 400 nests of a ground-nesting species, the Dark-eyed Junco (Junco hyemalis), and found that less than 2% of nests failed as a result of Argentine ant predation/infestation. A review of the literature also suggests that Argentine ants may not be a serious threat to bird nests relative to other predators or parasites. However, invasive ants with the capability of overwhelming prey though stinging (specifically the red-imported fire ant, Solenopsis invicta), may have a higher impact on avian nesting success. Received 14 January 2005; revised 28 April 2005; accepted 12 May 2005.     

Long-term impact of exotic ants on the native ants of Madeira

Abstract.  1. The earliest exotic records for two notorious invasive ants, the big-headed ant ( Pheidole megacephala ) and the Argentine ant ( Linepithema humile ), both come from the Atlantic islands of Madeira, where the two species underwent population explosions in the 1850s and 1890s respectively. Researchers have long assumed that these invaders spread across all of Madeira and exterminated most or all native ants, despite no research actually documenting such impact.
2. Re-examination of first-hand nineteenth century accounts suggest that P. megacephala and L. humile may never have spread beyond coastal lowland areas, representing < 10% of Madeira's land area. In 2002, native ants dominated most of Madeira; P. megacephala and L. humile were restricted to ≈ 0.3% and ≈ 6% of Madeira's land area respectively.
3. Of the 10 native ant species known from Madeira, only one ( Temnothorax wollastoni ) was not present in 1999–2002 surveys. Although exotic ants may have exterminated T. wollastoni , it seems likely that this species still survives.
4. Thus, even after 150 or more years of residence, P. megacephala and L. humile have come to occupy only a small part of Madeira, and appear to have had little impact.
5. Most of Madeira may be too cool for P. megacephala and perhaps too moist for L. humile to dominate. Also, Madeira's vast natural areas may generally lack weedy vegetation that can support high densities of plant-feeding Hemiptera critical for the ecological dominance of invasive ants. Finally, a dominant native ant, Lasius grandis , inhabiting ≈ 84% of Madeira, may actively exclude P. megacephala and L. humile .     

Seasonal polydomy in a polygynous supercolony of the odorous house ant,Tapinoma sessile

Abstract 1. The odorous house ant, Tapinoma sessile, is a native ant species common throughout North America. In its natural habitat, T. sessile is a low‐key species that consists of small colonies. In invaded urban areas, T. sessile exhibits extreme polygyny and polydomy and becomes a dominant invasive pest. 2. The current study examined: (i) the density, persistence, and the spatial distribution of nests in a large supercolony of T. sessile, (ii) trail abundance and overall colony connectivity as facilitated by the network of trails, (iii) the abundance and the spatial distribution of competing ant species, and (iv) the effect of environmental factors on the number and distribution of T. sessile nests. 3. A distinct pattern of seasonal polydomy was observed, whereby the colony undergoes an annual fission‐fusion cycle. The colony occupies one or a few nests during the winter, experiences rapid exponential growth in the spring to colonize available nesting sites, reaches maximum nest density in the summer, and again coalesces in the winter, returning to the same winter location year after year. The trails show spatio‐temporal variation as well, depending on the location of nesting and foraging sites. Furthermore, nest movements may be driven by soil microclimate and proximity to man‐made structures. 4. In total, 119 ant nests were discovered in a 3.15 ha plot, 90 (76%) of which belonged to T. sessile. Tapinoma sessile exhibited strong colony connectivity as 78/90 (87%) of nests were connected to at least one other nest by a trail. Mean persistence time for T. sessile nests was 133 ± 5 days. 5. Results indicate that T. sessile is a highly adaptable native ant species that exhibits a high degree of flexibility in its colony social structure. A high degree of polygyny and polydomy may contribute to its ecological dominance and pest status in urban environments.     

Predatory behavior of a seed-eating ant:Brachyponera senaarensis

The great flexibility of the feeding strategies exhibited by the ponerine ant Brachyponera senaarensis (Mayr) allows it to exploit either seeds or animal prey items as food resources. Predation is generally limited to small prey and is very similar to scavenging behavior. In laboratory conditions, the predatory behavior of B. senaarensis is not different in structure from that known in other carnivorous ants species. The workers forage individually and return to the nest using a series of cues involving light, a chemical graduated marking system near the nest entrance, and memory. During nest-moving, recruitment by tandem running was observed. However, in colonies where the food supply is regular, workers that discover food do not recruit nestmates, but make repeated trips between the nest and the food source. On the contrary, in starved colonies, the introduction of prey may produce a massive exit of foragers, corresponding to a primitive form of mass recruitment similar to that observed in some other ant species.     

Effects of experimental small-scale grassland fragmentation on spatial distribution, density, and persistence of ant nests

Abstract.  1. Grassland fragmentation is expected to influence the abundance of different invertebrate species to a different extent. Fragmentation-related effects are of particular importance in species that interact with many other species.
2. The density and spatial distribution of nests of 15 ant species in experimentally fragmented calcareous grasslands at three sites in the Northern Swiss Jura mountains were examined. Fragments of different size (0.25 m², 2.25 m², and 20.25 m²) were isolated by a 5-m wide strip of frequently mown vegetation. Control plots of corresponding size were situated in adjacent undisturbed grassland.
3. Three years after initiation of the experiment, ant nest density did not differ between fragments and control plots. Six years after initiation of the experiment, however, ant nest density and forager abundance were higher in large fragments than in large control plots. Ant nests tended to occur more frequently along the edge of fragments than in the core area. Persistence time of nests of the most abundant species, Lasius paralienus , tended to be shorter in fragments than in control plots. Furthermore, persistence time was longer in nests situated close to the fragment edge than in nests in the core area.
4. Effects on nest density, edge effects on the spatial distribution of nests, and the relationships between nest density and environmental factors were more pronounced when only nests of L. paralienus were considered. The implications of these findings for plant and other invertebrate species are discussed.     

Driver Ants Invading a Termite Nest: Why Do the Most Catholic Predators of All Seldom Take This Abundant Prey?

Driver ants ( i.e. , epigaeic species in the army ant genus Dorylus , subgenus Anomma ) are among the most extreme polyphagous predators, but termites appear to be conspicuously absent from their prey spectrum and attacks by driver ants on termite nests have not yet been described. Here, we report a Dorylus ( Anomma ) rubellus attack on a colony of the fungus-growing termite Macrotermes subhyalinus that was observed during the dry season in a savannah habitat in Nigeria's Gashaka National Park. It was estimated that several hundred thousand termites (probably more than 2.4 kg dry mass) were retrieved. The apparent rarity of driver ant predation on Macrotermes nests may be explained by different habitat requirements, by the fact that these ants mostly forage aboveground, by efficient termite defense behavior and nest architecture that make entry into the nest difficult, and finally by driver ant worker morphology, which differs remarkably from that of subterranean Dorylus species that regularly invade and destroy termite colonies.     

Divide and conquer: Multicolonial structure,nestmate recognition,and antagonistic behaviors in dense populations of the invasive ant Brachymyrmex patagonicus

The ecological success of ants has made them abundant in most environments, yet inter‐ and intraspecific competition usually limit nest density for a given population. Most invasive ant populations circumvent this limitation through a supercolonial structure, eliminating intraspecific competition through a loss of nestmate recognition and lack of aggression toward non‐nestmates. Native to South America, Brachymyrmex patagonicus has recently invaded many locations worldwide, with invasive populations described as extremely large and dense. Yet, in contrast with most invasive ants, this species exhibits a multicolonial structure, whereby each colony occupies a single nest. Here, we investigated the interplay between genetic diversity, chemical recognition, and aggressive behaviors in an invasive population of B. patagonicus. We found that, in its invasive range, this species reaches a high nest density with individual colonies located every 2.5 m and that colony boundaries are maintained through aggression toward non‐nestmates. This recognition and antagonism toward non‐nestmates is mediated by chemical differentiation between colonies, as different colonies exhibit distinct chemical profiles. We highlighted that the level of aggression between colonies is correlated with their degree of genetic difference, but not their overall chemical differentiation. This may suggest that only a few chemical compounds influence nestmate recognition in this species or that weak chemical differences are sufficient to elicit aggression. Overall, this study demonstrates that invasive ant populations can reach high densities despite a multicolonial structure with strong aggression between colonies, raising questions about the factors underlying their ecological success and mitigating negative consequences of competitive interactions.     

Social structure and nestmate discrimination in two species of Brachyponera ants distributed in Japan

The ponerine ant Brachyponera chinensis was introduced to the USA, where it has become invasive. Although various ecological data have been collected for B. chinensis populations in the USA, most aspects concerning the biology and ecology of native populations in Japan, a presumed origin, remain unknown. Here we investigated the social structure and nestmate discrimination in native populations of B. chinensis and a closely related species, B. nakasujii. Both species showed functional polygyny over seasons. Only in B. nakasujii was there a seasonal change in the numbers of queens and workers per nest. In arena tests, workers of neither species showed aggressive behaviors to conspecific non‐nestmates from the same population, and the mean aggression score did not increase with the distance between nests. However, some differences in non‐aggressive responses were detected between nestmate and non‐nestmate pairs in both species. In an experiment to introduce a single worker into a nest, B. chinensis accepted non‐nestmates with a high probability just like nestmates, whereas in B. nakasujii non‐nestmates were less accepted than nestmates. These findings suggest that native populations of B. chinensis already possess some of the key characteristics shared by many invasive exotic ants in introduced ranges, such as stable polygyny, weak internest aggression and acceptance of non‐nestmates. These tendencies are remarkable in comparison to the closely related B. nakasujii.     

红火蚁工蚁受侵扰的行为反应强度与蚁巢大小和侵扰强度之间的关系研究

红火蚁Solenopsis invicta Buren的防御行为是其被认定为社会性昆虫的关键特征之一。红火蚁蚁巢受到侵扰时,工蚁会快速涌出搜寻侵扰物并展开攻击。然而,蚁巢受侵扰后工蚁的行为反应与蚁巢大小和侵扰强度之间的关系有待进一步明确。为此,本研究通过野外视频记录并结合室内统计分析的方法来明确不同大小的红火蚁蚁巢受侵扰后工蚁反侵扰行为的动态过程。结果表明,蚁巢大小和受侵扰强度对蚁巢受侵扰后工蚁的反应速度具有显著影响。同等侵扰强度下,与中、大型蚁巢相比,小型蚁巢的工蚁往往表现出更慢的行为反应。尤其在低侵扰强度时,小蚁巢工蚁的反应时间平均0.8 s,而中蚁巢和大蚁巢均仅为0.34 s。当侵扰强度相同时,蚁巢越大,受侵扰后召集工蚁的数量越多;当蚁巢大小相同时,侵扰强度越高,召集工蚁的数量越多。但无论是蚁巢大小还是侵扰强度的不同,红火蚁蚁巢受侵扰后,召集工蚁的数量到达峰值的时间均为30 s左右。研究结果为进一步了解红火蚁的防御行为,科学预防红火蚁的攻击,以及有效开展红火蚁的防治提供了重要的实验依据。     

Recruitment Behavior During Foraging in the Neotropical Ant Gnamptogenys moelleri (Formicidae: Ponerinae): Does the Type of Food Matter?

Gnamptogenys moelleri nests in bromeliads and feeds on an array of food items, including dead and live animals, and nectar. Field data in Brazilian forests indicate that G. moelleri hunts solitarily, while retrieving is performed both by solitary workers for small items, or by a group of recruited workers for large items. This flexible foraging strategy was investigated in the laboratory through a series of experiments to assess the context in which recruitment is elicited. Three types of food were used: 50% honey solution, large insect prey, and cluster of small insects. For all food types the first encounter by a scout resulted in increased numbers of ants leaving the nest and finding the food in the arena. After finding liquid food or large prey, the forager returns to the nest and transmits information to nestmates about food location on the substrate. The successful scout repeatedly taps the sting on the ground, and recruited ants collectively retrieve the large insect to the nest. On the other hand, there is no transmission of information to nestmates about the location of small clumped prey, although the returning scout induces nestmates to leave the nest and hunt. Because foraging in G. moelleri is restricted mostly to the nest bromeliad, and small worker size (0.5 cm) precludes capturing large prey solitarily, recruitment behavior widens the spectrum of food items consumed by this ant species. Although recruitment behavior in ponerines has already been reported to vary with the type and size of a food source, this study also shows that the transmission of information about food location depends on the type of food found (large prey or liquid food versus cluster of small prey).     

Plasticity in queen number and social structure in the invasive Argentine ant (Linepithema humile)

In many polygynous social insect societies, ecological factors such as habitat saturation promote high queen numbers by increasing the cost of solitary breeding. If polygyny is associated with constrained environments, queen number in colonies of invasive social insects should increase as saturation of their new habitat increases. Here I describe the variation in queen number, nestmate relatedness, and nest size along a gradient of time since colonization in an invading population of Argentine ants (Linepithema humile) in Haleakala, Hawaii. Nest densities in this population increase with distance from the leading edge of the invasion, reaching a stable density plateau approximately 80 m from the edge (> 2 years after colonization). Although the number of queens per nest in Haleakala is generally lower than previously reported for Argentine ants, there is significant variation in queen number across this population. Both the observed and effective queen numbers increase across the density gradient, and nests in the center of the population contain queen numbers three to nine times higher than those on the edge of the invasion. The number of workers per nest is correlated with queen number, and nests in the center are six times larger than nests at the edge. Microsatellite analysis of relatedness among nestmates reveals that all nests in the Haleakala population are characterized by low relatedness and have evidence of multiple reproducing queens. Relatedness values are significantly lower in nests in the center of the population, indicating that the number of reproducing queens is greater in areas of high nest density. The variation in queen number and nestmate relatedness in this study is consistent with expectations based on changes in ecological constraints during the invasion of a new habitat, suggesting that the social structure of Argentine ant populations is strongly influenced by ecological factors. Flexibility in social structure may facilitate persistence in variable environments and may also confer significant advantages to a species when introduced into new areas.     

Genetic structure of native ant supercolonies varies in space and time

Ant supercolonies are the largest cooperative units known in nature. They consist of networks of interconnected nests with hundreds of reproductive queens, where individuals move freely between nests, cooperate across nest boundaries and show little aggression towards non‐nestmates. The combination of high queen numbers and free mixing of workers, queens and brood between nests results in extremely low nestmate relatedness. In such low‐relatedness societies, cooperative worker behaviour appears maladaptive because it may aid random individuals instead of relatives. Here, we provide a comprehensive picture of genetic substructure in supercolonies of the native wood ant Formica aquilonia using traditional population genetic as well as network analysis methods. Specifically, we test for spatial and temporal variation in genetic structure of different classes of individuals within supercolonies and analyse the role of worker movement in determining supercolony genetic networks. We find that relatedness within supercolonies is low but positive when viewed on a population level, which may be due to limited dispersal of individuals and/or ecological factors such as nest site limitation and competition against conspecifics. Genetic structure of supercolonies varied with both sample class and sampling time point, which indicates that mobility of individuals varies according to both caste and season and suggests that generalizing has to be carried out with caution in studies of supercolonial species. Overall, our analysis provides novel evidence that native wood ant supercolonies exhibit fine‐scale genetic substructure, which may explain the maintenance of cooperation in these low‐relatedness societies.     

Low relatedness among cooperatively breeding workers of the greenhead ant Rhytidoponera metallica

The greenhead ant Rhytidoponera metallica has long been recognized as posing a potential challenge to kin selection theory, because it has large queenless colonies where apparently many of the morphological workers are mated and reproducing. However, this species has never been studied genetically and important elements of its breeding system and kin structure remain uncertain. We used microsatellite markers to measure the relatedness among nestmates, unravel the fine‐scale population genetic structure, and infer the breeding system of R. metallica. The genetic relatedness among worker nestmates is very low but significantly greater than zero (r=0.082 ± 0.015), which demonstrates that nests contain many distantly related breeders. The inbreeding coefficient is very close to and not significantly different from zero, indicating random mating and lack of microgeographic genetic differentiation. On average, closely located nests are not more similar genetically than distant nests, which is surprising, as new colonies form by budding and female dispersal is restricted. Lack of inbreeding and absence of population viscosity indicates high gene flow mediated by males. Overall, the genetic pattern detected in R. metallica suggests that a high number of moderately related workers mate with unrelated males from distant nests. This breeding system results in the lowest relatedness among nestmates reported for social insect species where breeders and helpers are not morphologically differentiated.     

Nest raiding by the invasive Argentine ant on colonies of the harvester ant, Pogonomyrmex subnitidus

Invasive ants often displace native ants, and published studies that focus on these interactions usually emphasize interspecific competition for food resources as a key mechanism responsible for the demise of native ants. Although less well documented, nest raiding by invasive ants may also contribute to the extirpation of native ants. In coastal southern California, for example, invasive Argentine ants (Linepithema humile) commonly raid colonies of the harvester ant, Pogonomyrmex subnitidus. On a seasonal basis the frequency and intensity of raids vary, but raids occur only when abiotic conditions are suitable for both species. In the short term these organized attacks cause harvester ants to cease foraging and to plug their nest entrances. In unstaged, one-on-one interactions between P. subnitidus and L. humile workers, Argentine ants behaved aggressively in over two thirds of all pair-wise interactions, despite the much larger size of P. subnitidus. The short-term introduction of experimental Argentine ant colonies outside of P. subnitidus nest entrances stimulated behaviors similar to those observed in raids: P. subnitidus decreased its foraging activity and increased the number of nest entrance workers (many of which labored to plug their nest entrances). Raids are not likely to be the result of competition for food. As expected, P. subnitidus foraged primarily on plant material (85% of food items obtained from returning foragers), but also collected some dead insects (7% of food items). In buffet-style choice tests in which we offered Argentine ants food items obtained from P. subnitidus, L. humile only showed interest in dead insects. In other feeding trials L. humile consistently moved harvester ant brood into their nests (where they were presumably consumed) but showed little interest in freshly dead workers. The raiding behavior described here obscures the distinction between interspecific competition and predation, and may well play an important role in the displacement of native ants, especially those that are ecologically dissimilar to L. humile with respect to diet. Received 15 July 2005; revised 19 October 2005; accepted 26 October 2005.