Coupled adult-brood transport augments relocation in the Indian queenless ant Diacamma indicum

Colony relocation is an important aspect in the lives of social insects. In ants, the process of relocation is further complicated as brood, in addition to adults, have to be transported to the new nest. Here, we have investigated brood transport in the Indian ponerine ant Diacamma indicum, which uses tandem running—a primitive mode of recruitment—for the entire colony to relocate. We have found that there were no brood transport specialists and most of the brood was transported in the mandibles of followers that were being tandem run. Therefore, in a single tandem run, one adult and one brood item was effectively transported by tandem leaders augmenting the relocation process.     

An evaluation of the possible adaptive function of fungal brood covering by Attine ants

Fungus-growing ants (Myrmicinae: Attini) live in an obligate symbiotic relationship with a fungus that they rear for food, but they can also use the fungal mycelium to cover their brood. We surveyed colonies from 20 species of fungus-growing ants and show that brood-covering behavior occurs in most species, but to varying degrees, and appears to have evolved shortly after the origin of fungus farming, but was partly or entirely abandoned in some genera. To understand the evolution of the trait we used quantitative phylogenetic analyses to test whether brood-covering behavior covaries among attine ant clades and with two hygienic traits that reduce risk of disease: mycelial brood cover did not correlate with mutualistic bacteria that the ants culture on their cuticles for their antibiotics, but there was a negative relationship between metapleural gland grooming and mycelial cover. A broader comparative survey showed that the pupae of many ant species have protective cocoons but that those in the subfamily Myrmicinae do not. We therefore evaluated the previously proposed hypothesis that mycelial covering of attine ant brood evolved to provide cocoon-like protection for the brood.     

Anti-predator behaviour in the ant Pheidole desertorum: The importance of multiple nests

The relationship between emigrations and anti-predator behaviour in the ant Pheidole desertorum was investigated in the field. Frequent emigrations in P. desertorum result in the colony having multiple nests, of which only one is occupied at a time. A field procedure demonstrated that the ants quickly find and enter the unoccupied nests after a nest evacuation caused by the army ant Neivamyrmex nigrescens. When colonies were denied access to their unoccupied nests after an evacuation, survival of the brood and alates was significantly lower than in colonies allowed access to all their nests. These results suggest that emigrations in P. desertorum are part of a defence strategy against army ants.     

Foraging decisions of individual workers vary with colony size in the greenhead ant Rhytidoponera metallica (Formicidae, Ectatomminae)

Summary. The ability of worker ants to adapt their behaviour depending on the social environment of the colony is imperative for colony growth and survival. In this study we use the greenhead ant Rhytidoponera metallica to test for a relationship between colony size and foraging behaviour. We controlled for possible confounding ontogenetic and age effects by splitting large colonies into small and large colony fragments. Large and small colonies differed in worker number but not worker relatedness or worker/brood ratios. Differences in foraging activity were tested in the context of single foraging cycles with and without the opportunity to retrieve food. We found that workers from large colonies foraged for longer distances and spent more time outside the nest than foragers from small colonies. However, foragers from large and small colonies retrieved the first prey item they contacted, irrespective of prey size. Our results show that in R. metallica, foraging decisions made outside the nest by individual workers are related to the size of their colony.Received 23 March 2004; revised 3 June 2004; accepted 4 June 2004.     

Larger laboratory colonies consume proportionally less energy and have lower per capita brood production in Temnothorax ants

Colony size can affect individual- and colony-level behavioral and physiological traits in social insects. Changes in behavior and physiology in response to colony growth and development can affect productivity and fitness. Here, we used respirometry to study the relationship between colony size and colony energy consumption in Temnothorax rugatulus ants. In addition, we examined the relationship between colony size and worker productivity measured as per capita brood production. We found that colony metabolic rate scales with colony size to the 0.78 power and the number of brood scales with the number of workers to the 0.49 power. These regression analyses reveal that larger ant colonies use proportionally less energy and produce fewer brood per worker. Our findings provide new information on the relationships between colony size and energetic efficiency and productivity in a model ant genus. We discuss the potential mechanisms giving rise to allometric scaling of metabolic rate in ant colonies and the influence of colony size on energy consumption and productivity in general.     

Lethal and sublethal effects of thiacloprid on non-target carpenter ant,Camponotus japonicus Mayr (Hymenoptera: Formicidae)

To control population of Monochamus beetles that transmit pine wood nematode, Bursaphelenchus xylophilus, a variety of insecticides have been applied to forest ecosystems in Korea. Non-target predatory insects can be directly or indirectly exposed to insecticides. We evaluated potential lethal and sublethal effects of thiacloprid on survival and behavior of carpenter ants, Camponotus japonicus Mayr. Field-collected ants were directly exposed to several food items such as thiacloprid-exposed Monochamus beetles, 10% sugar water with thiacloprid concentrations, and 10% sugar water at group and individual levels. In experiment for groups of individuals, dead beetle bodies generally had possible adverse effects on ants through dietary exposure, because two forager ants were dead or paralysis after they were exposed to thiacloprid-exposed Monochamus beetles. At individual level, dietary exposure to thiacloprid at concentrations of 10 and 50?mg/L was lethal to ants, causing paralysis and impaired walking. Mortality of ant workers was higher in direct or indirect exposure treatments than that in the control. Some ants exposed to thiacloprid showed abnormal behavior within a few days, especially at thiacloprid concentrations of 10 and 50?mg/L. However, some of them sometimes recovered from the abnormal behavior in a day. In consequence, application of thiacloprid in pine forests may disrupt species interaction and foraging behavior of ants, but the effect of thiacloprid through trophallaxis behavior should be further assessed using more extensive colonies composed of queen, workers, brood, and so on.     

Division of labor inPonera pennsylvannica (Formicidae: Ponerinae)

Summary We examined division of labor and colony demography in the antPonera pennsylvannica. Observation of three colonies with individually marked workers revealed a high degree of interindividual behavioral variation and a rough but consistent division of labor between brood tenders and foragers. This division was present both in colonies consisting entirely of workers produced in the previous summer and in colonies containing freshly eclosed ants. Two colonies showed typical age-based polyethism, with young ants focusing on brood care and overwintered ants on foraging. No such age basis was detected in the third colony. This difference may relate to variability in brood production schedules. Colonies showing temporal polyethism had two peaks of brood production and thus had relatively large brood populations when the first young workers eclosed, while the third colony had only one peak and little brood for the young workers to tend. Even if young ants have a lower threshold for brood care, it may have been concealed in the latter situation. Demographic data indicate that natural colonies produce one brood per year and that workers typically eclose into colonies with relatively low brood care demands. This suggests that overwintered workers do most of a colony's work and that the division of labor among overwintered ants is the more important one under natural conditions. The basis of this division is as yet unknown. These results also suggest that small colony size, univoltine brood schedule and a close association between foraging and brood care do not preclude division of labor among specialized castes, as has been suggested for another ponerine species (Traniello 1978).     

Behavioural and chemical studies of discrimination processes in the leaf-cutting ant Acromyrmex laticeps nigrosetosus (Forel, 1908).

Leaf-cutting ants live in symbiosis with a basidiomycete fungus that is exploited as a source of nutrients for ant larvae. Tests of brood transport revealed that Acromyrmex laticeps nigrosetosus workers did not discriminate a concolonial brood from an alien brood. The same result was observed with tests of fungus transport. Adult workers showed no aggressive behaviour to workers from other alien colonies (non-nestmates). There was no qualitative variation in the chemical profiles of larvae, pupae and adult workers from the different colonies. However, quantitative differences were observed between the different colonies. Hypotheses about the lack of intraspecific aggression in this subspecies of ants are discussed.     

Sex-ratio regulation: the economics of fratricide in ants

In many insect societies, workers can manipulate the reproductive output of their colony by killing kin of lesser value to them. For instance, workers of the mound-building Formica exsecta eliminate male brood in colonies headed by a single-mated queen. By combining an inclusive fitness model and empirical data, we investigated the selective causes underlying these fratricides. Our model examines until which threshold stage in male brood development do the workers benefit from eliminating males to rear extra females instead. We then determined the minimal developmental stage reached by male larvae before elimination in F. exsecta field colonies. Surprisingly, many male larvae were kept until they were close to pupation, and only then eliminated. According to our model, part of the eliminated males were so large that workers would not benefit from replacing them with new females. Moreover, males were eliminated late in the season, so that new females could no longer be initiated, because matings take place synchronously during a short period. Together, these results indicate that workers did not replace male brood with new females, but rather reduced total brood size during late larval development. Male destruction was probably triggered by resource limitation, and the timing of brood elimination suggests that males may have been fed to females when these start to grow exponentially during the final larval stage. Hence, the evolution of fratricides in ants is best explained by a combination of ecological, demographic and genetic parameters.     

The economics of brood raiding and nest consolidation during ant colony founding

Summary The most dangerous time for an ant colony is during the founding stage when the small colony is vulnerable to predation and competition. Colonies can grow more rapidly when multiple queens cooperate in raising the first worker brood (pleometrosis) or by raiding other incipient colonies for their brood. This brood raiding has been proposed to be the primary force selecting for pleometrosis, i.e. multiple-queen colonies may have a considerable advantage in destroying neighbours by aggressively stealing their brood. An alternative hypothesis is that incipient nests are part of a larger, interconnected population structure and that brood raiding reflects cooperative pleometrosis with subdivided colonies. A simple mathematical model supports the second hypothesis: workers of incipient colonies are especially favoured to peaceably abandon their nest and join with other colonies if the queens are related or queens from raided colonies can infiltrate the raiding colony. The latter condition is often met in ant species that brood raid and particularly exemplified in fire ants (Solenopsis invicta), where brood raiding involves little mortal combat and combines with pleometrosis to rapidly increase colony size. It is proposed that the term nest consolidation should replace brood raiding to more accurately reflect the relatively non-aggressive and potentially apparently cooperative nature of interactions between incipient ant colonies.     

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.     

INTRASPECIFIC VARIATION IN ANT SEX RATIOS AND THE TRIVERS-HARE HYPOTHESIS

We consider worker-controlled sex investments in eusocial Hymenoptera (ants in particular) and assume that relatedness asymmetry is variable among colonies and that workers are able to assess the relatedness asymmetry in their own colony. We predict that such “assessing” workers should maximize their inclusive fitness by specializing in the production of the sex to which they are relatively most related, i.e., colonies whose workers have a relatedness asymmetry below the population average should specialize in males, whereas colonies whose workers have a higher than average relatedness asymmetry should specialize in making females. Our argument yields the expectation that colony sex ratios will be bimodally distributed in ant populations where relatedness asymmetry is variable owing to multiple mating, worker reproduction, and/or polygyny. No such bimodality is expected, however, in ant species where relatedness asymmetry is known to be constant, or in cases where relatedness asymmetry is supposed to be irrelevant due to allospecific brood rearing under queen control, as in the slave-making ants. Comparative data on colony sex ratios in ants are reviewed to test the predictions. The data partly support our contentions, but are as yet insufficient to be considered as decisive evidence.     

The Role of Habitat in the Persistence of Fire Ant Populations

The association of the exotic fire ant, Solenopsis invicta with man-modified habitats has been amply demonstrated, but the fate of such populations if ecological succession proceeds has rarely been investigated. Resurvey of a fire ant population in a longleaf pine plantation after 25 years showed that the recovery of the site from habitat disturbance was associated with a large fire ant population decline. Most of the persisting colonies were associated with the disturbance caused by vehicle tracks. In a second study, mature monogyne fire ant colonies that had been planted in experimental plots in native groundcover of the north Florida longleaf pine forest had mostly vanished six years later. These observations and experiments show that S. invicta colonies rarely persist in the native habitat of these pine forests, probably because they are not replaced when they die. A single site harbored a modest population of polygyne fire ants whose persistence was probably facilitated by reproduction through colony fission.     

Evidence of intracolony transmission of Thelohania solenopsae (Microsporidia: Thelohaniidae) in red imported fire ants (Hymenoptera: Formicidae) and the first report of spores from pupae.

Red imported fire ant, Solenopsis invicta, colonies were infected horizontally by introducing live brood (mainly larvae and pupae) infected with Thelohania solenopsae. Live, infected brood introduced into uninfected colonies were adopted and raised to adulthood instead of being executed by the recipient colony. Introductions of infected larvae with uninfected pupae, which eclose into adult worker caste fire ants, resulted in an 80% infection rate of the inoculated colonies. Infections from introductions of infected pupae with uninfected larvae resulted in a 37.5% infection of inoculated colonies. Infections were also detected in 11.6 and 3.7% of the adult worker caste ants that eclosed from uninfected large larvae and pupae, respectively, that were held with infected adult workers. Microscopic examination of infected brood revealed sporoblasts and large numbers of spores of T. solenopsae in S. invicta pupae.     

Intraspecific usurpation of incipient fire ant colonies

Brood raiding, or reciprocal stealing of brood, is common amongincipient colonies of the fire ant Solenopsis invicta. Paradoxically,workers from a colony that loses its brood during a raid oftenabandon their nest and join the winning colony. Queens abandonedby their workers may then migrate from their original nest siteand attempt to forcefully usurp another incipient colony bydisplacing that colony's queen or queens. This study examinedfactors that influence the success of usurpation attempts. Queensattempting to usurp a nearby colony after laboratory brood raidswere successful in less than 30% of trials. Usurpation attemptswere more successful if workers familiar to the migrating queenwere present in the nest, as would happen if a queen were tofind the colony to which her workers had moved. Cross-fosteringexperiments showed that this effect was due to familiarity ratherthan relatedness. Usurpation attempts were less successful ifthey were delayed by 16 h. The probability of usurpation successwas not reduced by doubling the number of defending workersin the invaded colony. However, colonies founded by three queenswere almost always able to resist usurpation attempts. Theseresults support the hypothesis that workers abandon their natalcolony after losing a brood raid to increase the likelihoodthat their queen can usurp the colony to which they migrate.These results also provide the first evidence that coloniesof ants founded by several queens are better able to resistusurpation attempts than colonies founded by a single queen     

Brood stimulation controls the phasic reproductive cycle of the parthenogenetic ant Cerapachys biroi

Several groups of ants display a reproductive cycle in which two phases of adult activity alternate in synchrony with the brood instars. The brood stimulation hypothesis (Schneirla, 1957) was developed for ecitonine army ants to explain the proximate control of such biphasic cycles. According to it, onsets of cyclic activities are triggered by social stimulations arising from the developing brood, rather than by innate pace-makers inbuilt in adult ants. While it seemed to provide an acceptable explanation, this hypothesis failed to be experimentally demonstrated, in spite of numerous field observations. We used colonies of thelytokous populations of the phasic ant Cerapachys biroi as a model in order to test the brood stimulation theory. Brood removal and substitution experiments allowed us to confirm, first, that the periodicity of the cycle is not controlled by an endogenous rhythm in adults. Moreover, we could also characterise the influence of each brood instar on the activity of adult ants. Although we confirmed the existence of a brood stimulation involved in the control of the cycle, experiments revealed that it was not performed accordingly to Schneirla’s hypothesis. In effect, our study suggests a primacy of larval influence: the foraging phase was triggered and sustained by larvae- induced excitement rather than by stimulation from the newly-emerged callows. Received 21 March 2005; revised 1 June 2005; accepted 6 june 2005.     

Bait distribution among multiple colonies of Pharaoh ants (hymenoptera: Formicidae)

Pharaoh ant, Monomorium pharaonis (L.), infestations often consist of several colonies located at different nest sites. To achieve control, it is desirable to suppress or eliminate the populations of a majority of these colonies. We compared the trophallactic distribution and efficacy of two ant baits, with different modes of action, among groups of four colonies of Pharaoh ants. Baits contained either the metabolic-inhibiting active ingredient hydramethylnon or the insect growth regulator (IGR) pyriproxyfen. Within 3 wk, the hydramethylnon bait reduced worker and brood populations by at least 80%, and queen reductions ranged between 73 and 100%, when nests were in proximity (within 132 cm) to the bait source. However, these nest sites were reoccupied by ants from other colonies located further from the bait source. The pyriproxyfen bait was distributed more thoroughly to all nest locations with worker populations gradually declining by 73% at all nest sites after 8 wk. Average queen reductions ranged from 31 to 49% for all nest sites throughout the study. Even though some queens survived, brood reductions were rapid in the pyriproxyfen treatment, with reductions of 95% at all locations by week 3. Unlike the metabolic inhibitor, the IGR did not kill adult worker ants quickly, thus, more surviving worker ants were available to distribute the bait to all colonies located at different nest sites. Thus, from a single bait source, the slow-acting bait toxicant provided gradual, but long-term control, whereas the fast-acting bait toxicant provided rapid, localized control for a shorter duration.     

Thievery,home ranges,and nestmate recognition inEctatomma ruidum

Summary Thievery of food items among colonies of a ponerine ant,Ectatomma ruidum was common; nonnestmates in colonies or near the colony entrances receive incoming food items and carry them to their own colony. In our study area 7 of 10 colonies were victimized by thief ants. Colonies have discrete home ranges and home range size is correlated with the number of workers in the colony. Worker ants discriminate nestmates from non-nestmates when non-nestmates are presented at colony entrances, but individuals from different colonies were not observed to engage in agonistic interactions away from nest entrances. Non-nestmates gain entrance to colonies when the entrance is unguarded. Many instances of non-nestmates being removed from colonies by residents were observed. The costs and benefits of theft under these circumstances are considered.     

Colony sex ratios vary with breeding system but not relatedness asymmetry in the facultatively polygynous ant Pheidole pallidula

Abstract.— We investigated sex allocation in a Mediterranean population of the facultatively polygynous (multiple queen per colony) ant Pheidole pallidula . This species shows a strong split sex ratio, with most colonies producing almost exclusively a single-sex brood. Our genetic (microsatellite) analyses reveal that P. pallidula has an unusual breeding system, with colonies being headed by a single or a few unrelated queens. As expected in such a breeding system, our results show no variation in relatedness asymmetry between monogynous (single queen per colony) and polygynous colonies. Nevertheless, sex allocation was tightly associated with the breeding structure, with monogynous colonies producing a male-biased brood and polygynous colonies almost only females. In addition, sex allocation was closely correlated with colony total sexual productivity. Overall, our data show that when colonies become more productive (and presumably larger) they shift from monogyny to polygyny and from male production to female production, a pattern that has never been reported in social insects.     

The role of competition in task switching during colony emigration in the ant Leptothorax albipennis

The decision-making process that determines when an animal should switch between tasks is a fundamental issue in the study of animal behaviour. We investigated, for the first time, such task switching in terms of the dynamics of worker populations in ant colonies. During colony emigration in the ant Leptothorax albipennis, the colony has to carry out the following three tasks: (1) transport of brood and nestmates to the new nest; (2) sorting of the brood into its characteristic pattern; and (3) building the nest wall. At the beginning of the emigration, the stimuli for all three tasks increase simultaneously but the tasks are performed sequentially by populations of workers in the colony. The issue here is how decision making at the colony level is based on the behaviour of individual workers. We used a mathematical model to explore the hypothesis that such colony level task switching is based on tasks competing for workers. The essential feature of this model is that the sequence of tasks performed by an individual worker need not match the sequence of tasks on which the colony concentrates. We base the parameterization of our model on our detailed experimental study of eight emigrations, one for each of eight L. albipennis colonies. We compared our results with earlier work that emphasizes the role of response thresholds in task-related decisions. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.