Communication between the fungus garden and workers of the leaf-cutting ant, Atta sexdens rubropilosa, regarding choice of substrate for the fungus

Bait made from orange peel, containing the fungicide cycloheximide, was initially harvested by workers of Atta sexdens rubropilosa (Forel) and incorporated into the fungus garden as substrate for the fungus. The bait was subsequently rejected by the worker ants days later. Exposure of the fungus to cycloheximide, in laboratory sub-colonies, resulted in the fungus being ‘stressed’. By interchanging normal fungus garden with ‘stressed’ fungus garden, a change in the foraging behaviour of the workers was evident. –Two hypotheses to explain the behavioural changes were tested: a volatile semiochemical is produced by the fungus which affects the foragers directly, or contact between workers (and fungus garden) is necessary for information regarding fungal substrate to be transmitted through the worker force. When pairs of sub-colonies were connected (one colony of each pair exposed to cycloheximide in the bait) and workers were initially prevented from passing from one colony to the other, one colony continued to forage on orange bait while the other did not. When both colonies were allowed to make full contact then both colonies failed to accept orange bait. This discounted the first hypothesis, but supported the second, as a highly volatile chemical should be able to diffuse between colonies. When large foragers were prevented from making contact with the second colony, the information may be communicated by smaller workers.     

Nest architecture and colony size of the fungus-growing ant <Emphasis Type="Italic">Mycetophylax simplex</Emphasis> Emery, 1888 (Formicidae,Attini)

Detailed measurements on nest architecture and colony size of the fungus-growing ant Mycetophylax simplex Emery, 1888 (Formicidae, Attini) are reported for the first time, based on excavations of 55 nests from two sites in southern Brazil. All nests were subterranean, with a single entrance hole. Most nests consisted of two chambers, an upper and a lower chamber, but one and three-chamber nests were also found. The chambers were more cone-shaped than rounded, and located at a depth ranging from 4.0 cm to 32.5 cm below the nest entrance. The chamber dimensions generally increased as the depth of the chambers increased, and the lower chamber was mostly wider than the upper one. The fungus garden was always found resting on the chamber floor. The average colony size was 264.1 workers, ranging from 67 to 610 workers. Colonies produced most sexuals during the summer (from December to March) and a few during the winter (July). Direct observations showed that colonies were mostly monogynous, but more than one queen was recorded in two nests, suggesting that polygyny may also occur in this species. Received 30 November 2006; revised 20 April 2007; accepted 23 April 2007.     

Facultative sexual reproduction in the parthenogenetic ant Platythyrea punctata

Summary. In a few, scattered species of social Hymenoptera, unmated workers are capable of producing female offspring from unfertilized eggs through thelytokous parthenogenesis. Regular thelytoky has previously been demonstrated in a number of populations of the neotropical ant Platythyrea punctata. Nevertheless, the finding of males and inseminated queens and workers suggested the sporadic occurrence of sex. In this study we investigated the genetic structure of colonies from Puerto Rico and Costa Rica in order to detect traces of occasional sexual reproduction. Most Puerto Rican colonies had a clonal structure with all nestmates sharing the same multilocus genotype, indicating that thelytoky is the predominant mode of reproduction. Genetic variability was detected in six of 18 colonies and might have arisen from adoption of alien workers in one colony and from the adoption of alien workers, recombination during parthenogenesis, or sexual reproduction in the other colonies. The reproductive of one of these latter colonies was found to be an inseminated worker (gamergate), and the genotypes of its nestmates definitively suggested recombination and sexual reproduction. Three gamergates were found in a single colony collected in Costa Rica, and all produced offspring from fertilized eggs, while uninseminated workers were apparently incapable of reproducing by thelytoky.Received 10 August 2004; revised 20 October 2004; accepted 3 November 2004.     

Several workers lay eggs in the same brood cell in queenless honey bee (Apis mellifera) colonies

Queenless honey bee (Apis mellifera) colonies are often characterized by the presence of multiple eggs in brood cells. This is surprising because only one egg can be reared to maturity per cell. Moreover, worker honey bees cannot produce many eggs per day. There are several reasons that could explain the presence of multiple eggs in single cells: a) workers cannot control how many eggs they release; in this case we would expect all eggs to be from the same mother; b) excess eggs could be provided as food for the first larva to hatch in the absence of adequate brood care and this would again result in all eggs in one cell sharing the same mother; c) the number of cells available for oviposition may be limiting, obliging workers to lay eggs in cells that already contain eggs, resulting in eggs of mixed maternity. Here we show that the majority of brood cells in queenless colonies contain eggs from multiple mothers. Therefore our results suggest that the presence of multiple eggs in brood cells arises from a limitation on the number of suitable cells available for oviposition. Received 29 September 2008; revised 21 November 2008; accepted 24 November 2008.     

Stay or drift? Queen acceptance in the ant <Emphasis Type="Italic">Formica paralugubris</Emphasis>

The acceptance of new queens in ant colonies has profound effects on colony kin structure and inclusive fitness of workers. Therefore, it is important to study the recognition and discrimination behaviour of workers towards reproductive individuals entering established colonies. We examined the acceptance rate of queens in populations of the highly polygynous ant F. paralugubris, where the genetic differentiation among nests and discrimination ability among workers suggest that workers might reject foreign queens. We experimentally introduced young queens in their natal nest and in foreign nests. Surprisingly, the survival rate of mated queens did not differ significantly when introduced in a foreign male-producing nest, a foreign female-producing nest, or the natal nest. Moreover, the survival of virgin queens in their natal nest was twice the one of mated queens, suggesting that mating status plays an important role for acceptance. The results indicate that other factors than queen discrimination by workers are implicated in the limited longdistance gene flow between nests in these populations. Received 8 April 2008; revised 16 June 2008; accepted 1 July 2008.     

Nestmate recognition by guards of the Asian hive bee <Emphasis Type="Italic">Apis cerana</Emphasis>

When a honey bee colony becomes queenless and broodless its only reproductive option is for some of its workers to produce sons before the colony perishes. However, for this to be possible the policing of worker-laid eggs must be curtailed and this provides the opportunity for queenless colonies to be reproductively parasitized by workers from other nests. Such reproductive parasitism is known to occur in Apis florea and A. cerana. Microsatellite analyses of worker samples have demonstrated that the proportion of non-natal workers present in an A. cerana colony declines after a colony is made queenless. This observation suggests that queenless A. cerana colonies may be more vigilant in repelling potentially parasitic non-natal workers than queenright colonies. We compared rates of nestmate and non-nestmate acceptance in both queenright and queenless A. cerana colonies using standard assays and showed that there is no statistical difference between the proportion of non-nestmate workers that are rejected in queenless and queenright colonies. We also show that, contrary to earlier reports, A. cerana guards are able to discriminate nestmate workers from non-nestmates, and that they reject significantly more non-nestmate workers than nestmate workers. Received 25 February 2008; revised 21 May 2008; accepted 25 June 2008.     

PARTIAL INCOMPATIBILITY BETWEEN ANTS AND SYMBIOTIC FUNGI IN TWO SYMPATRIC SPECIES OF ACROMYRMEX LEAF-CUTTING ANTS

Abstract We investigate the nature and duration of incompatibility between certain combinations of Acromyrmex leaf‐cutting ants and symbiotic fungi, taken from sympatric colonies of the same or a related species. Ant‐fungus incompatibility appeared to be largely independent of the ant species involved, but could be explained partly by genetic differences among the fungus cultivars. Following current theoretical considerations, we develop a hypothesis, originally proposed by S. A. Frank, that the observed incompatibilities are ultimately due to competitive interactions between genetically different fungal lineages, and we predict that the ants should have evolved mechanisms to prevent such competition between cultivars within a single garden. This requires that the ants are able to recognize unfamiliar fungi, and we show that this is indeed the case. Amplified fragment length polymorphism genotyping further shows that the two sympatric Acromyrmex species share each other's major lineages of cultivar, confirming that horizontal transfer does occasionally take place. We argue and provide some evidence that chemical substances produced by the fungus garden may mediate recognition of alien fungi by the ants. We show that incompatibility between ants and transplanted, genetically different cultivars is indeed due to active killing of the novel cultivar by the ants. This incompatibility disappears when ants are force‐fed the novel cultivar for about a week, a result that is consistent with our hypothesis of recognition induced by the resident fungus and eventual replacement of incompatibility compounds during force‐feeding.     

Energetics of newly-mated queens and colony founding in the fungus-gardening ants Cyphomyrmex rimosus and Trachymyrmex septentrionalis (Hymenoptera: Formicidae)

Abstract.  The energetics of colony founding is investigated in the fungus gardening ants (Attini) Trachymyrmex septentrionalis and Cyphomyrmex rimosus . Similar to most ants, inseminated queens of these two species found nests independently unaccompanied by workers (haplometrosis). Whereas most ant founding queens seal themselves in a chamber and do not feed when producing a brood entirely from metabolic stores (claustral founding), the majority of fungus gardening ants must forage during the founding phase (semiclaustral founding). Laboratory-reared T. septentrionalis individuals comprise 84 dealate females collected after mating flights in June 2004. Twenty are immediately killed to obtain values for queen traits and another 20 after worker emergence for queen, fungus garden and worker traits. Cyphomyrmex rimosus comprise 22 dealate females collected in June 2005; ten of which are immediately killed and similarly prepared. Newly-mated T. septentrionalis queens have 25% of their dry weight as fat; whereas newly-mated C. rimosus queens contain 11% fat. These amounts are 50–75% less than most independently founding ant species. Trachymyrmex septentrionalis queens lose merely 5% of their energetic content during colony founding, whereas the total energetic content of their brood is more than three-fold the amount lost by the queen. Incipient T. septentrionalis colonies produce approximately half as much ant biomass per gram of fungus garden as do mature colonies. Similar to most ants, T. septentrionalis produces minim workers that are approximately 40% lighter than workers from mature colonies. Regardless of their size, T. septentrionalis workers contain much lower fat than do workers of claustral species. These data indicate that fungus gardening is adaptive because colonies can produce much cheaper offspring, making colony investment much lower.     

Larval isolation and brood care in Acromyrmex leaf-cutting ants

The larvae of leaf-cutting ants are maintained within the fungus gardens of their colonies and are fed pieces of fungus by the adult workers. However, little else is known about the nature of the worker-larva interaction in these ecologically important ants. To examine whether workers can gauge the needs of individual larvae, we isolated larvae without adult workers for different lengths of time. We then placed workers with the larvae and recorded the type and frequency of the subsequent behaviours of the workers. Workers scraped the mouthparts of larvae, ingested their faecal fluid, fed them with fungal hyphae, transported them around the fungus garden and, most frequently, licked their bodies. The workers were also observed to ‘plant’ fungal hyphae on the bodies of larvae. Workers interacted more frequently with larvae that had been isolated without workers than with those that had not, but there was no effect of the length of isolation. The results suggest that the interactions are complex, involving a number of behaviours that probably serve different functions, and that workers are to some extent able to assess the individual needs of larvae. Received 8 November 2004; revised 31 March 2005; accepted 22 April 2005.     

A new species of <Emphasis Type="Italic">Discostroma</Emphasis> and its anamorph <Emphasis Type="Italic">Seimatosporium</Emphasis> with two morphological types of conidia,isolated from the stems of <Emphasis Type="Italic">Paeonia suffruticosa</Emphasis>

Conidia of two morphologically different types, one with a basal appendage only and the other with appendage at both ends, were isolated from the stems of Paeonia suffruticosa. Single conidial isolates of both types of conidia yield identical colonies, which then produced both types of conidia on agar media depending on temperature, thus showing that both types of conidia belong to the same fungus. Seimatosporium botan is described based on its morphological characteristics. The teleomorph of the fungus was first found on sterilized P. suffruticosa stems placed on water agar, when grown at 5°C for 2 months in 12-h photoperiod. Discostroma botan is described for this fungus. The teleomorph is also found on the same host in the field.     

The influence of hybridization on colony structure in the ant species Temnothorax nylanderi and T. crassispinus

The parapatric sibling ant species Temnothorax nylanderi and T. crassispinus hybridize in the contact zone in the Franconian Jura, Southern Germany. Aim of our study was to investigate the impact of hybridization on colony composition and fitness. We classified colonies as either ‘pure’ or containing hybrids by determining their allozyme pattern at GPI, an enzyme that is fixed for different alleles in the two parental species, and quantified their reproductive output. Most colonies with hybrid workers had a T. crassispinus queen. Colonies with heterozygous, hybrid workers produced more young workers than colonies of the parental species but similar numbers of male and female sexuals. Female sexuals from colonies with heterozygous workers had a significantly lower weight than female sexuals from pure colonies. Only a single reproductive queen was found to be heterozygous, suggesting reduced fitness of hybrid queens. As in the parental species, hybrid colonies appear to be frequently taken over by alien queens, which obscures the genetic colony structure. Received 6 April 2006; revised 10 June 2006; accepted 15 June 2006.     

Alate susceptibility in ants

Pathogens are predicted to pose a particular threat to eusocial insects because infections can spread rapidly in colonies with high densities of closely related individuals. In ants, there are two major castes: workers and reproductives. Sterile workers receive no direct benefit from investing in immunity, but can gain indirect fitness benefits if their immunity aids the survival of their fertile siblings. Virgin reproductives (alates), on the other hand, may be able to increase their investment in reproduction, rather than in immunity, because of the protection they receive from workers. Thus, we expect colonies to have highly immune workers, but relatively more susceptible alates. We examined the survival of workers, gynes, and males of nine ant species collected in Peru and Canada when exposed to the entomopathogenic fungus Beauveria bassiana. For the seven species in which treatment with B. bassiana increased ant mortality relative to controls, we found workers were significantly less susceptible compared with both alate sexes. Female and male alates did not differ significantly in their immunocompetence. Our results suggest that, as with other nonreproductive tasks in ant colonies like foraging and nest maintenance, workers have primary responsibility for colony immunity, allowing alates to specialize on reproduction. We highlight the importance of colony‐level selection on individual immunity in ants and other eusocial organisms.     

Fitness consequences of nest infiltration by the mutualist‐exploiter Megalomyrmex adamsae

1. Fungus‐growing ants are obligate mutualists. Their nutrient‐rich fungus garden provides a valuable food store that sustains the ant hosts, but can also attract social parasites. 2. The ‘guest ant' Megalomyrmex adamsae Longino parasitises the fungus‐growing Trachymyrmex zeteki Weber queen just after nest founding. The parasitic queen infiltrates the incipient nest, builds a cavity in the fungal garden, and lays eggs that develop into workers and reproductive males and females. 3. This study compared young parasitised and non‐parasitised laboratory colonies by measuring garden growth and biomass, and the number of host workers and reproductives. Host queen survival and parasite colony growth were also monitored. 4. Parasitised Trachymyrmex colonies had reduced host worker and alate numbers, as well as lower garden biomass, compared with non‐parasitised control colonies, confirming that M. adamsae is a xenobiotic social parasite. Host queen survival was not significantly different between parasitised and control colonies. 5. This is the first study that experimentally infects host colonies with a xenobiotic social parasite to measure fitness cost to the host. The natural history of M. adamsae and the fungus‐growing ant mutualism are evaluated in the context of three general predictions of (Bronstein, Ecology Letters, 4 , 277–287, 2001a) regarding the cost of mutualism exploiters.     

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

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

Natural variation in colony inbreeding does not influence susceptibility to a fungal pathogen in a termite

Reduced genetic diversity through inbreeding can negatively affect pathogen resistance. This relationship becomes more complicated in social species, such as social insects, since the chance of disease transmission increases with the frequency of interactions among individuals. However, social insects may benefit from social immunity, whereby individual physiological defenses may be bolstered by collective‐level immune responses, such as grooming or sharing of antimicrobial substance through trophallaxis. We set out to determine whether differences in genetic diversity between colonies of the subterranean termite, Reticulitermes flavipes, accounts for colony survival against pathogens. We sampled colonies throughout the United States (Texas, North Carolina, Maryland, and Massachusetts) and determined the level of inbreeding of each colony. To assess whether genetically diverse colonies were better able to survive exposure to diverse pathogens, we challenged groups of termite workers with two strains of a pathogenic fungus, one local strain present in the soil surrounding sampled colonies and another naïve strain, collected outside the range of this species. We found natural variation in the level of inbreeding between colonies, but this variation did not explain differences in susceptibility to either pathogen. Although the naïve strain was found to be more hazardous than the local strain, colony resistance was correlated between two strains, meaning that colonies had either relatively high or low susceptibility to both strains regardless of their inbreeding coefficient. Overall, our findings may reflect differential virulence between the strains, immune priming of the colonies via prior exposure to the local strain, or a coevolved resistance toward this strain. They also suggest that colony survival may rely more upon additional factors, such as different behavioral response thresholds or the influence of a specific genetic background, rather than the overall genetic diversity of the colony.     

Ecology of spiders inhabiting abandoned mammal burrows in South African savanna

Summary We marked the sites chosen by 338 foundress queens of two desert ant species (Veromessor pergandei and Myrmecocystus flaviceps) and monitored changes in the spacing of both species and the foraging activity at V. pergandei young nests. Although the long established colonies of both species tend intraspecifically toward regular dispersion, queens of both species were intraspecifically clumped. After 3 months, when the first workers emerged, the young colonies (reduced to a total of 42 colonies) were randomly spaced intraspecifically. We also followed the spatial patter of queens with respect to established colonies of both species. Queens founded nests away from the nests of all established colonies on the site. After three months, the young colonies were dispersed away from conspecifics only. During June through August 1986, we censused the number of foragers at the surviving V. pergandei nests. Young colonies that were more active also tended to be far from established conspecifics in July and August. There was no correlation of forating activity with distance to heterospecific established colonies in any month. These results indicate that established conspecifics may reduce the survivorship of young ant colonies.     

Genetic Diversity of Fungi Occurring in Nests of Three <Emphasis Type="Italic">Acromyrmex</Emphasis> Leaf-cutting Ant Species from Córdoba,Argentina

It is assumed in current literature that the fungus garden cultivated by leaf-cutting ants consists of a single fungus species, the putative mutualistic fungus. However, most studies report a very high rate of fungi contamination (fungi isolated from fungus gardens that are considered not to be the mutualistic fungus). In this article, we report a genetic similarity analysis of all fungi (regardless of their mutualistic condition) isolated from 16 fungus gardens of three Acromyrmex species in Córdoba, Argentina, using intersimple sequence repeat (ISSR) as genetic markers. We isolated 60 clones, of which the three primers employed yielded 53 loci. The patterns revealed a high interclone polymorphism, with a few bands shared by the clones. Of all possible pairwise comparisons, 99% showed a genetic similarity (S) lower than 0.5, the threshold level assumed for fungus Operational Taxonomy Unit (OTU). We found more than one fungus OTU in all studied nests (range 2–11). Cumulative number of OTUs increased linearly with the number of nests sampled. The number of fungus OTUs common to both ant species and sites was very small. We did not find a core group composed of few very common OTUs, as expected if a set of truly mutualistic OTU was present. A simple explanation for the high number of OTUs found is that they are regular components of the fungus garden, which may be used as food source by the ants.     

Group nesting and individual variation in behavior and physiology in the orchid bee <Emphasis Type="Italic">Euglossa nigropilosa</Emphasis> Moure (Hymenoptera,Apidae)

The Euglossini are a key group for studying the traits that promote or hinder highly social behavior in bees because it is the only tribe in the Apine clade without large colonies or females with distinct life histories, e.g. queens and workers. There have been few studies on behavior of orchid bee females in nests because these nests are not found easily. Taking advantage of the relatively high abundance of Eg. nigropilosa nests at Reserva Natural La Planada, Colombia, we examined social behavior of Eg. nigropilosa individuals in five nests (3 original and 2 reused) for nine months. We report this species to have the largest colonies known for Euglossa, with nests reaching up to 22 individuals, and all nests containing more than one female bee from the same generation. These nests presented many traits that correspond to communal insect colonies. No generational overlap and no cooperative brood care were detected. We examined natural enemies and resource limitation as important factors for group nesting. We examined parasitoid attacks to cells in a nest with females and one without females. We also searched for nesting locations and examined nest re-use as indicators of nest site limitation. Lastly, we examined behavioral and physiological differences among females in the same nest. Such differences could be the bases for evolution of alternative life histories among group living females. We examined extent of ovary development and oviposition rates in similarly aged females in the same nest. We found large variation in reproductive effort of young females. We also examined differences in resin foraging and cell usurpation behaviors. Behavioral specialization was observed, with some individuals bringing only resin to the nest. Inside the nests, bees had territories in which they constructed and defended cells. This territoriality may be a defense against usurpation of provisioned cells by nest mates. Received 10 December 2007; revised 2 May 2008; accepted 7 May 2008.     

Trail‐sharing among tropical ants: interspecific use of trail pheromones?

1. Trail‐sharing between different ant species is rare and restricted to a small number of species pairs. Its underlying mechanisms are largely unknown. For trail‐sharing to occur, two factors are required: (i) one or both species must recognise the other species or its pheromone trails and (ii) both species must tolerate each other to a certain extent to allow joint use of the trail. A species that follows another's trails can efficiently exploit the other's information on food sources contained in the pheromone trails. Hence, food competition and thus aggressive interactions between a species following another's trail and the species being followed, seem likely. 2. In the present study, we investigated interspecific trail following and interspecific aggression in trail sharing associations (i) among Polyrhachis ypsilon, Camponotus saundersi, and Dolichoderus cuspidatus, and (ii) among Camponotus rufifemur and Crematogaster modiglianii. We tested whether trail‐sharing species follow each other's pheromone trails, and whether the ants tolerated or attacked their trail‐sharing partners. In both associations, we confronted workers with pheromone trails of their associated species, and, for the former association, measured interspecific aggression among the trail‐sharing species. 3. In our assays, D. cuspidatus and C. rufifemur regularly followed heterospecific pheromone trails of P. ypsilon and C. modiglianii, respectively. However, only few workers of the remaining species followed heterospecific pheromone trails. Thus, shared trails of P. ypsilon and C. saundersi cannot be explained by interspecific trail‐following. 4. Interspecific aggression among P. ypsilon, C. saundersi, and D. cuspidatus was strongly asymmetric, C. saundersi being submissive to the other two. All three species differentiated between heterospecific workers from the same or another site, suggesting habituation to the respective trail‐sharing partners. We therefore hypothesise that differential tolerance by dominant ant species may be mediated by selective habituation towards submissive species and this way determines the assembly of trail‐sharing associations.     

Sex allocation in fungus‐growing ants: worker or queen control without symbiont‐induced female bias

The fungal cultivars of fungus‐growing ants are vertically transmitted by queens but not males. Selection would therefore favor cultivars that bias the ants’ sex ratio towards gynes, beyond the gyne bias that is optimal for workers and queens. We measured sex allocation in 190 colonies of six sympatric fungus‐growing ant species. As predicted from relatedness, female bias was greater in four singly mated Sericomyrmex and Trachymyrmex species than in two multiply mated Acromyrmex species. Colonies tended to raise mainly a single sex, which could be partly explained by variation in queen number, colony fecundity, and fungal garden volume for Acromyrmex and Sericomyrmex, but not for Trachymyrmex. Year of collection, worker number and mating frequency of Acromyrmex queens did not affect the colony sex ratios. We used a novel sensitivity analysis to compare the population sex allocation ratios with the theoretical queen and worker optima for a range of values of k, the correction factor for sex differences in metabolic rate and fat content. The results were consistent with either worker or queen control, but never with fungal control for any realistic value of k. We conclude that the fungal symbiont does not distort the ants’ sex ratio in these species.