Nest-plugging: interference competition in desert ants (Novomessor cockerelli and Pogonomyrmex barbatus)

Summary Pogonomyrmex barbatus and Novomessor cockerelli, sympatric species of harvester ants in the Lower Sonoran desert, compete for seed resources. This study reports on a method of interference competition. Early in the morning, before P. barbatus' activity period, N. cockerelli fills the nest entrances of P. barbatus with sand. This delays the beginning of the P. barbatus activity period for 1–3 h. P. barbatus colonies near N. cockerelli nests were more likely to be plugged. Nest-plugging shifts the typical daily sequence of P. barbatus activities, including the onset of foraging, forward towards midday, when high temperatures force the colony back inside the nest. P. barbatus colonies do not compensate for late emergence or events impeding foraging by increasing foraging rate. Thus nest-plugging by N. cockerelli decreases the foraging capacity of P. barbatus colonies.     

Complex foraging ecology of the red harvester ant and its effect on the soil seed bank

Granivory is an important interaction in the arid and semi-arid zones of the world, since seeds form an abundant and nutritious resource in these areas. While species of the genus Pogonomyrmex have been studied in detail as seed predators, their impact on seed abundance in the soil has not yet been explored in sufficient depth. We studied the impact of the harvesting activities of the ant Pogonomyrmex barbatus on seed abundance in the soil of the Zapotitlán valley, Mexico. We found that P. barbatus activity significantly impacts the abundance of seeds in the soil, which is lower in the sites where P. barbatus forages than it is in sites with no recorded foraging. We also found that P. barbatus distributes intact seeds of three tree species, two of which are nurse plants, and could consequently be promoting the establishment of these species. Using tools derived from graph theory, we observed that the ant-seed interactions exhibit a nested pattern; where more depredated seed species seem to be the more spatially abundant in the environment. This study illustrates the complex foraging ecology of the harvester ant P. barbatus and elucidates its effect on the soil seed bank in a semi-arid environment.     

Harvester ant nests, soil biota and soil chemistry

Many ant species accumulate organic debris in the vicinity of their nests. These organic materials should provide a rich resource base for the soil biota. We examined the effect of harvester ant nests (Pogonomyrmex barbatus) on the soil community and soil chemistry. Ant nest soils supported 30-fold higher densities of microarthropods and 5-fold higher densities of protozoa than surrounding, control soils. The relative abundances of the major groups of protozoa differed as well: amoebae and ciliates were relatively overrepresented, and flagellates underrepresented, in ant nest versus control soils. Densities of bacteria and fungi were similar in the two soil types. Concentrations of nitrate, ammonium, phosphorus, and potassium were significantly higher in ant nest soils, while concentrations of magnesium, calcium, and water were similar in nest and control soils. Ant nest soils were marginally more acidic than controls. The results demonstrate that P. barbatus nests constitute a significant source of spatial heterogeneity in soil biota and soil chemistry in arid grasslands. Received: 17 March 1997 / Accepted: 10 June 1997     

Importance of large colony size for successful invasion by Argentine ants (Hymenoptera: Formicidae): Evidence for biotic resistance by native ants

Abstract The Argentine ant, Linepithema humile (Mayr), is a widespread invasive ant species that has been associated with losses of native ant species and other invertebrates from its introduced range. To date, various abiotic conditions have been associated with limitations to the spread of Argentine ants, however, competitive interactions with native ant fauna may also affect the spread of Argentine ants. Here, we experimentally manipulated colony sizes of Argentine ants in the laboratory to assess whether Argentine ants were able to survive and compete for resources with a widespread, dominant native ant, Iridomyrmex‘rufoniger’. The results showed that over 24 h, the proportions of Argentine ants that were alive, at baits, and at sugar water decreased significantly in the presence of Iridomyrmex. In addition, Argentine ant mortality increased over time, however, the proportion of the colony that was dead decreased with the largest colony size. Argentine ants were only able to overcome Iridomyrmex when their colony sizes were 5–10 times greater than those of the native ants. We also conducted trials in which colonies of Argentine ants of varying sizes were introduced to artificial baits occupied by Iridomyrmex in the field. The results showed that larger Argentine ant colonies significantly affected the foraging success of Iridomyrmex after the initial introduction (5 min). However, over the first 20 min, when the Argentine ants were present at the baits, and over the entire 50 min experimental period, the numbers of Iridomyrmex at baits did not differ significantly with the size of the Argentine ant colony. This is the first experimental study to investigate the role of colony size in the invasion biology of Argentine ants in Australia, and the results suggest that Iridomyrmex may reduce the spread of Argentine ants, and that Argentine ants may need to attain large colony sizes in order to survive in the presence of Iridomyrmex. We address the implications of these findings for the invasion success of Argentine ants in Australia, and discuss the ability of Argentine ants to attain large colony sizes in introduced areas.     

The short-term regulation of foraging in harvester ants

In the seed-eating ant Pogonomyrmex barbatus, the return ofsuccessful foragers stimulates inactive foragers to leave thenest. The rate at which successful foragers return to the nestdepends on food availability; the more food available, the morequickly foragers will find it and bring it back. Field experimentsexamined how quickly a colony can adjust to a decline in therate of forager return, and thus to a decline in food availability,by slowing down foraging activity. In response to a brief, 3-to 5-min reduction in the forager return rate, foraging activityusually decreased within 2–3 min and then recovered within5 min. This indicates that whether an inactive forager leavesthe nest on its next trip depends on its very recent experienceof the rate of forager return. On some days, colonies respondedmore to a change in forager return rate. The rapid colony responseto fluctuations in forager return rate, enabling colonies toact as risk-averse foragers, may arise from the limited intervalover which an ant can track its encounters with returning foragers.     

β-eudesmol-induced aggression in the leaf-cutting ant Atta sexdens rubropilosa

Leaf‐cutting ants are highly polyphagous insects, but some plants escape their attack due to the presence of secondary metabolites that are toxic to the ant–fungus symbiosis. Previous studies have demonstrated that the terpenoid β‐eudesmol extracted from Eucalyptus species (Myrtaceae) is responsible for the deleterious behavior in colonies of leaf‐cutting ant species. The objective of this study was to evaluate the effect of β‐eudesmol on workers of the leaf‐cutting ant Atta sexdens rubropilosa Forel (Hymenoptera: Formicidae). This chemical caused behavioral modification in the colonies, leading to mutilation and death of workers. It is suggested that β‐eudesmol interferes with colony nestmate recognition. As a consequence, colony cohesion may be disrupted by β‐eudesmol what could be used as an additional control tactic against this important pest ant.     

Polygyny does not explain the superior competitive ability of dominant ant associates in the African ant‐plant,Acacia (Vachellia) drepanolobium

The Acacia drepanolobium (also known as Vachellia drepanolobium) ant‐plant symbiosis is considered a classic case of species coexistence, in which four species of tree‐defending ants compete for nesting space in a single host tree species. Coexistence in this system has been explained by trade‐offs in the ability of the ant associates to compete with each other for occupied trees versus the ability to colonize unoccupied trees. We seek to understand the proximal reasons for how and why the ant species vary in competitive or colonizing abilities, which are largely unknown. In this study, we use RADseq‐derived SNPs to identify relatedness of workers in colonies to test the hypothesis that competitively dominant ants reach large colony sizes due to polygyny, that is, the presence of multiple egg‐laying queens in a single colony. We find that variation in polygyny is not associated with competitive ability; in fact, the most dominant species, unexpectedly, showed little evidence of polygyny. We also use these markers to investigate variation in mating behavior among the ant species and find that different species vary in the number of males fathering the offspring of each colony. Finally, we show that the nature of polygyny varies between the two commonly polygynous species, Crematogaster mimosae and Tetraponera penzigi: in C. mimosae, queens in the same colony are often related, while this is not the case for T. penzigi. These results shed light on factors influencing the evolution of species coexistence in an ant‐plant mutualism, as well as demonstrating the effectiveness of RADseq‐derived SNPs for parentage analysis.     

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.     

Habitat segregation based on soil texture and body size in the seed‐harvester ants Pogonomyrmex rugosus and P. barbatus

1. The seed‐harvester ants Pogonomyrmex rugosus and P. barbatus are ecologically equivalent sister species that have broadly overlapping distributions in the south‐western U.S.A.; however the two species are only sympatric in localised contact zones. 2. Soil regimes at 25–50 cm below the surface were quantified across contact zones to assess abiotic habitat factors related to distribution pattern. Physiological parameters related to foundress survival were also measured in order to test for a correlation between these parameters and distribution pattern. 3. The two species segregated among microhabitats based on soil texture; P. barbatus occurred alone in soils with a higher clay content and/or higher moisture retention. In areas of sympatry, soil texture was similar for both species but was intermediate to that in areas where the two species occurred allopatrically. The pattern of microhabitat segregation was similar across three sites that encompassed a broad range of soil regimes. 4. The only measure of foundress survival correlated with microhabitat differences was an ≈ 8% greater dry mass for alate females of P. rugosus. This resulted in their surviving significantly longer than did alate females of P. barbatus under desiccating conditions. 5. This microdistribution pattern may be caused indirectly by soil texture affecting plant species distribution and hence the seeds available to ants. A companion laboratory experiment demonstrated, however, that soils could also cause this distribution pattern of both ant species directly via effects on foundress wet mass.     

First Record of Polydomy in a Monogynous Ponerine Ant: A Means to Allow Emigration Between Pachycondyla goeldii Nests

The ponerine ant Pachycondyla goeldii is a monogynous (i.e. one queen per colony) arboreal species that colonizes pioneer areas. Founding queens and first generation workers initiate their own ant garden by building a cardboard-like structure into which epiphyte seeds are integrated. Following the growth of the epiphyte, the colony establishes its nest within the root system. This particular nest-building behavior is crucial in an environment where suitable nest sites are rare. Nevertheless, the slow growing process of ant gardens does not allow this species to readily evacuate and find another refuge in the advent of an attack by a predator or worsening climatic conditions. Previous field studies of P. goeldii were performed after forest destruction and subsequent colonization by P. goeldii. As a result, the colonies studied where relatively young and monodomous (i.e. one nest per colony). Our study of nest composition, worker exchanges between ant gardens in the field, and dyadic encounters shows that mature colonies of P. goeldii are polydomous (i.e. multiple nests per colony). In ants, the association of polydomy with monogyny has infrequently been reported. To our knowledge, P. goeldii represents the first record of a Ponerinae exhibiting both these particular characteristics. Our field and laboratory experiments suggest that polydomy is adaptively advantageous in coping with the microclimatic instability of pioneer areas by providing colonies with easily accessible nests.     

Does the host plant affect the benefits from mutualisms? The invasive mealybug and ghost ant association

1. The benefits to trophobionte hemipterans are affected by the ant tending level, which is a widely accepted statement. The ant tending level is closely related to multiple factors. It is clear that the ant tending level can be affected by the temporal factor, age‐specific, the density of the hemipterans, and quantity and quality of honeydew produced by hemipterans. 2. Few studies of ant–hemipteran mutualisms have reported the patterns of host plants‐dependent effects, and whether host plants influence the ant tending level that is also unclear. As such, laboratory experiments were conducted to test whether the colony growth rate of an invasive mealybug Phenacoccus solenopsis Tinsley, parasitism of Aenasius bambawalei Hayat, an dominant parasitoid of P. solenopsis, are affected by tending by ghost ants (Tapinoma melanocephalum(Fabricius)], host plants (tomato and cotton), and interactions between the two factors. The difference in the ant tending level between the host plants was also determined. 3. The results showed that mealybug colony growth and parasitism were significantly affected by ant tending and host plant separately. There were significant interactions between the independent factors on the mealybug colony growth rate and percentage parasitism. These results suggest that benefits to mealybugs are host plant‐dependent.     

Extreme life history plasticity and the evolution of invasive characteristics in a native ant

Disturbance resulting from urbanization is a leading cause of biotic homogenization worldwide. Native species are replaced with widespread non-native species and ants are among the world’s most notorious invaders. To date, all documented cases of ant invasions involve exotic introduced species that are spread around the world by human-mediated dispersal. I investigated the effect of urbanization on the evolution of invasive characteristics in a native ant species, the odorous house ant, Tapinoma sessile (Say). Colony social structure, life history traits, and the spatial pattern of nest distribution were compared by sampling T. sessile across a gradient of three distinct habitats: natural, semi-natural, and urban. Results demonstrate a remarkable transition in colony social and spatial structure and life history traits between natural and urban environments. In natural habitats, T. sessile colonies are comprised of small, monogyne (single queen), and monodomous (single nest) colonies. In urban areas, T. sessile often exhibit extreme polygyny and polydomy, form large supercolonies, and become a dominant pest. Results also suggest that urban T. sessile colonies may have a negative impact on native ant abundance and diversity. In the natural environment T. sessile coexisted with a wide array of other ant species, while very few ant species were present in the urban environment invaded by T. sessile. Habitat degradation and urbanization can lead to extreme changes in social and spatial colony structure and life history traits in a native ant species and can promote the evolution of invasive characteristics such as polygyny, polydomy, and supercolonial colony structure.     

A native ant armed to limit the spread of the Argentine ant

Argentine ants (Linepithema humile) usually actively displace native ants through a combination of rapid recruitment, high numerical dominance and intense aggressive fights. However, in some cases, native ants can offer a strong resistance. In Corsica, a French Mediterranean island, local resistance by the dominant Tapinoma nigerrimum has been proposed as a factor limiting Argentine ant invasion. With the aim of evaluating the abilities of T. nigerrimum in interference and exploitative competition, this study tested in the laboratory the aggressive interactions between this native dominant ant and the invasive Argentine ant. We used four different assays between L. humile and T. nigerrimum: (1) worker dyadic interactions, (2) symmetrical group interactions, (3) intruder introductions into an established resident colony, and (4) a competition for space and food. This study confirms the ability of Argentine ants to compete with native species, by initiating more fights, using cooperation and simultaneously deploying physical and chemical defenses. However, despite Argentine ant fighting capabilities, T. nigerrimum was more efficient in both interference and exploitative competition. Its superiority was obvious in the space and food competition assays, where T. nigerrimum dominated food in 100% of the replicates after 1 h and invaded Argentine ant nests while the reverse was never observed. The death feigning behavior exhibited by Argentine ant workers also suggests the native ant’s superiority. Our study thus demonstrates that T. nigerrimum can offer strong competition and so may be able to limit the spread of Argentine ants in Corsica. This confirms that interspecific competition from ecologically dominant native species can affect the invasion success of invaders, notably by decreasing the likelihood of incipient colony establishment and survival.     

Effects of food rewards offered by ant–plant Macaranga on the colony size of ants

Myrmecophytes (ant–plants) have special hollow structures (domatia) in which obligate ant partners nest. As the ants live only on the plants and feed exclusively on plant food bodies, sap-sucking homopterans in the domatia, and/or the homopterans honeydew, they are suitable for the study of colony size regulation by food. We examined factors regulating ant colony size in four myrmecophytic Macaranga species, which have strictly species-specific association with Crematogaster symbiont ants. Intra- and interspecific comparison of the plants showed that the ant biomass per unit food biomass was constant irrespective of plant developmental stage and plant species, suggesting that the ant colony size is limited by food supply. The primary food offered by the plants to the ants was different among Macaranga species. Ants in Macaranga beccariana and Macaranga bancana relied on homopterans rather than food bodies, and appeared to regulate the homopteran biomass and, as a consequence, regulate the ants own biomass. In contrast, ants in Macaranga winkleri and Macaranga trachyphylla relied primarily on food bodies rather than homopterans, and the plants appeared to manipulate the ant colony size. Per capita plant investment in ants (ant dry weight plant dry weight^–1) was different among the four Macaranga species. The homoptera-dependent M. beccariana and M. bancana harbored lower biomass of ants than the food-body dependent M. winkleri, suggesting that energy loss is involved in the homoptera-interposing symbiotic system which has one additional trophic level. The plants investment ratio to the ants generally decreased as plants grew. The evolution of the plant reward-offering system in ant–plant–homopteran symbioses is discussed with an emphasis on the role of homopterans.     

The spatial scale of seed collection by harvester ants

Colonies of the seed-eating ant, Pogonomyrmex barbatus, compete with neighboring colonies for foraging areas. In a conflict over foraging area, what is at stake? This depends on how resources are distributed in time and space: if certain regions consistently provide particularly nutritious seed species, or especially abundant seeds, such regions will be of greater value to a colony. During the summer, seeds were taken from returning foragers in colonies located in 4 different vegetation types. There was no relation between the vegetation currently growing in the foraging area, and the species of seeds collected by ants. During the summer, ants collect mostly seeds produced in previous seasons and dispersed by wind and flooding. In 1991, colonies in all vegetation types collected mostly Bouteloua aristidoides; in 1992, Eriastrum diffusum and Plantago patagonica. There was no relation between colony density and numbers of seeds collected. Seed species collected by ants were compared in different colonies, and on different foraging trails within a colony. The results show that seed patches are distributed on the scale of distances between nests, not the smaller scale of different foraging trails of one colony. It appears that colonies are competing for any space in which to search for seeds, not competing for certain regions of consistently high value.     

Genetic relationship of myrmecophyte (Anthorrhiza caerulea) individuals within and among territories of the arboreal ant (Dolichoderus sp.) detected using random amplified polymorphic DNA markers

The polymorphic bands of random amplified polymorphic DNA (RAPD) markers of the population of epiphytic myrmecophyte, Anthorrhiza caerulea Huxley & Jebb (Rubiaceae: Hydnophytinae), and those of its obligatory occupant ant, Dolichoderus sp. (Formicidae: Dolichoderinae), were analyzed in this study in a Papua New Guinean secondary forest. All the reproducible polymorphic bands were used as RAPD markers. Euclidean distances were calculated as the similarity indices of the band patterns, and similarities of the band patterns were analyzed using cluster analysis (group average method). It was frequently observed that the occupant ants gathered the seeds of A. caerulea and buried them into their carton trails on the bark of host trees. The colony boundaries of Dolichoderus sp. were assessed using RAPD analysis, and were clearly detected by this method. From the analyses of genetic relationships in the population of A. caerulea, it was revealed that the myrmecophytes within a certain ant territory consisted of close relatives. Therefore, it was inferred that the descendants of A. caerulea were dispersed only within the territory of a certain ant colony.     

The structured diversity of specialized gut symbionts of the New World army ants

Symbiotic bacteria play important roles in the biology of their arthropod hosts. Yet the microbiota of many diverse and influential groups remain understudied, resulting in a paucity of information on the fidelities and histories of these associations. Motivated by prior findings from a smaller scale, 16S rRNA‐based study, we conducted a broad phylogenetic and geographic survey of microbial communities in the ecologically dominant New World army ants (Formicidae: Dorylinae). Amplicon sequencing of the 16S rRNA gene across 28 species spanning the five New World genera showed that the microbial communities of army ants consist of very few common and abundant bacterial species. The two most abundant microbes, referred to as Unclassified Firmicutes and Unclassified Entomoplasmatales, appear to be specialized army ant associates that dominate microbial communities in the gut lumen of three host genera, Eciton, Labidus and Nomamyrmex. Both are present in other army ant genera, including those from the Old World, suggesting that army ant symbioses date back to the Cretaceous. Extensive sequencing of bacterial protein‐coding genes revealed multiple strains of these symbionts coexisting within colonies, but seldom within the same individual ant. Bacterial strains formed multiple host species‐specific lineages on phylogenies, which often grouped strains from distant geographic locations. These patterns deviate from those seen in other social insects and raise intriguing questions about the influence of army ant colony swarm‐founding and within‐colony genetic diversity on strain coexistence, and the effects of hosting a diverse suite of symbiont strains on colony ecology.     

Fungus gardens of the leafcutter ant Atta colombica function as egg nurseries for the snake Leptodeira annulata

Attine ants are well known for their mutualistic symbiosis with fungus gardens, but many other symbionts and commensals have been described. Here, we report the discovery of two clusters of large snake eggs in neighboring fungus gardens of a mature Atta colombica colony. The eggs were completely embedded within the fungus garden and were ignored by the host ants, even when we placed them into another, freshly excavated fungus garden of the same colony. All five eggs contained embryos and two snakes eventually hatched, which we identified as being banded cat eyed snakes Leptodeira annulata L. Ant fungus gardens are likely to provide ideal climatic conditions for developing snake eggs and almost complete protection from egg predation. Our observations therefore indicate that mature banded cat eyed snakes are able to enter and oviposit in large and well defended Atta colonies without being attacked by ant soldiers and that also newly hatched snakes manage to avoid ant attacks when they leaving their host colony. We speculate that L. annulata might use Atta and Acromyrmex leafcutter ant colonies as egg nurseries by some form of chemical insignificance, but more work is needed to understand the details of this interaction. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Biological notes on a fungus-growing ant, Trachymyrmex cf. zeteki (Hymenoptera, Formicidae, Attini) attacked by a diverse community of parasitoid wasps (Hymenoptera, Diapriidae)

A number of wasps in the family Diapriidae, subfamily Diapriinae (Proctotrupoidea), are parasitoids that specialize on ant larvae. These wasps are abundant and diverse in the Neotropics, but little is known about their biology. We studied parasitism rates by an array of diapriine wasps that attack the larvae of fungus-growing ants, Trachymyrmex cf. zeteki, in a single population (near Gamboa, Panamá). Relatively little is known about the biology and natural history of these ants, so we also present data on colony size and nest architecture. We excavated 136 colonies in central Panamá from June to September 2006, and 20 nests from July 2009. We reared six wasp morphotypes; two of them in the genus Mimopriella Masner and Garcia, one Oxypria Kieffer, two Szelenyiopria Fabritius and one Acanthopria Ashmead. The mean intensity of larval parasitism per ant colony was 33.9% (2006), and its prevalence across all ant populations was 27.2% (2006 and 2009). Parasitism rates were not positively correlated with host colony size. A single case of super-parasitism was documented in which two Oxypria males were reared from the same host larva.     

Sensu stricto individual conditioning,and imitation,in the ant Myrmica sabuleti (Hymenoptera: Formicidae)

Summary

Conditioning of isolated ants was attempted in the ant Myrmica sabuleti. Isolated ants did not die and could be visually as well as olfactorily conditioned. They acquired and kept ? or partly lost ? their visual ? or their olfactory ? conditioned responses as do ants living in a colony. Each individual of an ant colony is thus able to learn and memorize by itself. Any collective conditioning, learning and memory actually reflect the individuals’ performance. Naive workers paired with isolated previously conditioned ants apparently acquired olfactory conditioning in a shorter time than isolated ants, reaching a conditioning score identical to that of isolated ants. But they lost their apparent conditioning as soon as the olfactory cue was removed. Thus, imitation of nestmates exists in ants but does not lead, by itself, to learning sensu stricto since nothing of what has been imitated is retained.