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.     

Mating flight, metrosis, and semi-claustrality in the seed-harvester ant Pogonomyrmex salinus (Hymenoptera, Formicidae)

Here we examine dispersal, metrosis, and claustrality in the seed-harvester ant Pogonomyrmex salinus at an unusually large mating aggregation. We found that mode of queen dispersal from the mating aggregation is not a function of queen mass and that wing damage among queens was relatively rare. P. salinus is haplometrotic in the field and foundress queens placed together in forced associations eventually fight to the death. While queens of Pogonomyrmex salinus can survive claustrally, producing a single minim from their body reserves in the laboratory, fed queens produce up to four significantly larger minims along with concurrent larvae and pupae during the same period. Since queens forage in the field, we interpret claustrality as a secondary reserve strategy when foraging fails, and suggest that foraging is obligate for P. salinus queens in an overdispersed and temperate environment. Thus, nest founding strategies employed by P. salinus may be environmentally determined and represent a continuum between fully claustral and obligate foraging. We discuss our results with reference to theories of pleometrosis and claustral colony founding. Received 12 November 2004; revised 12 April 2004; accepted 29 July 2005.     

Optimal and central-place foraging theory applied to a desert harvester ant,Pogonomyrmex californicus

Summary Certain predictions of optimal- and central place-foraging theory were tested on the desert harvester ant, Pogonomyrmex californicus. Colonies were offered three different sizes of oat seed and found to maximize net energy intake (ei) over time (t _i ) by harvesting the seed sizes with the highest e _i /t _i rank. Two aspects of t _i were measured that were assumed constant in previous studies. The handling components of t _i (time required to manipulate the seed and travel time back to the colony with the food) were measured and found to be positively correlated with seed size. The manipulation success rate (the percentage of handled seeds successfully picked up) decreased with increased seed size. These results point out how important it is to measure all parameters of e _i /t _i rather than to assume constancy with both prey type and foraging distance. The relative abundance of less preferred food types was important in determining the proportion of preferred types in the diet. The food supply of eight colonies was manipulated experimentally over a 25-day period. Four deprived colonies were constrained within aluminum enclosures to prevented foraging. The remaining four satiated colonies were given food ad libitum. The niche breadths of the treated colonies were then compared to controls, but found not to differ significantly. Seed baits were offered at three distances from the colony to test whether selectivity increased with disance. Contrary to theoretical predictions, all colonies harcested about the same proportion of each seed size at each distance.     

Queen adoption in the polygynous and polydomous ant, Leptothorax curvispinosus

Alate female reproductives of the facultatively polygynous andpolydomous ant, Lepiothorax curuispinosus, were reared fromfield-collected nests, mated, and introduced into either theirparental nests or alien conspecific nests. The 41 queens introducedinto alien nests were usually attacked and rejected (97. 6%),but one queen was accepted after initial aggression. The 27queens introduced into their parental nests received a variableresponse. Some were accepted without any apparent aggression(59. 3%), but others were strongly attacked and rejected (40.7%). Sequential introductions of up to four queens into particularparental nests indicated that nests consistently either acceptor reject their mated offspring. The presence or absence ofresident queens in parental nests had no apparent influenceon the acceptance of offspring queens. Nests that accepted queenshad significantly fewer workers than those that did not, butthis slight difference is unlikely to explain these dichotomousresults and could be spurious. Dissections of the introducedqueens revealed that 79. 0% were inseminated and 98. 3% haddeveloping, yolked eggs in their ovarioles, but these variableshad no apparent effect on acceptability. Similarly, the sizeof the introduced queens and the time that elapsed between matingand introduction had no apparent effect. The consistent responseof parental nests in either accepting or rejecting their matedoffspring indicates a mechanism of queen number regulation inthis species that involves characteristics of the colony ornest rather than variability among offspring queens. This mechanismcould be responsible for maintaining relatively low numbersof queens and high genetic relatedness in colonies (or individualnests) while promoting flexibility in colony reproduction bycolony fission ("budding") and the dispersal of young queens.This mechanism could also involve an important conflict of interestbetween parental colonies and their mated offspring and mightcontribute to the evolution of socially parasitic colony foundationstrategies. Acceptance of mated offspring by their parentalcolonies might only occur during certain periods in colony development,depend on the level of genetic diversity within the colony (ornest), reflect the condition of the colony, nest, queen(s),brood, or local habitat, or result from a genetic polymorphism.     

Queen execution in a monogynous ant

Workers in many species of social insects are capable of laying unfertilized eggs, which can develop into haploid males. This causes a conflict about male parentage between queens and workers. In a few species, this may result in matricide, that is, workers kill the colony''s queen. Queen killing has so far been observed mainly in multi‐queen colonies or in annual species, when the queen''s fecundity declines at the end of the reproductive period. Here, we report queen expulsion and matricide in a monogynous, monandrous ant with perennial societies. Workers were seen to aggressively expel both related and unrelated queens from their nest shortly after the end of hibernation. Queen expulsion and matricide led to a significant decrease in the number of workers and brood, but eventually increased the direct fitness of workers through significant male production. Long‐term observations revealed a short lifespan of queens, while workers in orphaned colonies survived and produced male offspring over several years.     

Activity Patterns of Harvester Ants Pogonomyrmex pronotalis and Pogonomyrmex rastratus in the Central Monte Desert, Argentina

Seed-eating ants could have a significant effect on plant communities in deserts and semiarid zones. This effect is mediated through spatial and temporal foraging patterns, and seed selection within patches. Foraging patterns of harvester ants in South American deserts are almost unknown. The purpose of this work is to determine the temporal variations in the activity levels of Pogonomyrmex pronotalis and P. rastratus in the central Monte desert, and how these patterns may be related to abiotic factors, particularly to soil temperature. Activity levels and soil surface temperature were recorded at hourly intervals in five colonies for each species during the activity season (October, December, February, and April) in both 1999–2000 and 2000–2001 periods (except for October 1999). Surface ant activity starts in October, increases between December and February, and then ceases by April. Surface ant activity is diurnal throughout the season and usually has a unique peak during midday in October and April, and two peaks in the morning and the afternoon from December to February. The proportion of the activity budget devoted to nest's maintenance activity was similar for both species. Activity levels of foraging workers tended to be higher in P. pronotalis than in P. rastratus. P. pronotalis is active between 20 and 59°C, with higher levels of activity between 35 and 45°C, whereas P. rastratus shows activity between 18 and 58°C, with higher levels of activity between 30 and 40°C. Our results suggest that temporal changes in surface activity respond mainly to soil temperature fluctuations. However, at intermediate temperatures (those probably encompassing the thermal tolerance range of these ant species), temperature appears not to be a good predictor of daily and seasonal activity fluctuations.     

Polymorphism and size-pairing in the harvester antPogonomyrmex badius: a test of the ecological release hypothesis

Summary The role of caste polymorphism in the foraging strategy ofPogonomyrmex badius was studied in the field by measuring food items collected by foragers, and correlating food item size variables with forager size variables. The diet ofP. badius included seeds and insects. In two colonies examined, these food types comprised different proportions of the diet sample.Although some forager size variables showed close or significant correlations with food item size variables, we could identify no overall significant relationship between worker size and seed or prey size. Polymorphism inP. badius may be associated with omnivory. However, since minor workers serve as foragers and represent a portion of the total worker size variation, dietary expansion through caste proliferation appears to be only one aspect of the functional significance of polymorphism in this species.     

Life history traits of a European Messor harvester ant

We present life history traits of a Central European harvester ant, Messor cf. structor, determined by gyne and male dissections, behavioural assays, standardised brood photography and laboratory rearing of brood. Messor cf. structor is polygynous and builds up unicolonial populations. Sexuals develop from hibernated larvae in a univoltine cycle and become adult from late summer to late autumn. Neither intra- nor extranidal mating was observed in autumn, even though gynes and males were mature. In spring, after hibernation, intranidal mating without swarming flight took place, even though the flight muscles of alate sexuals were still fully developed. Received 3 March 2005; revised 4 May 2005; accepted 9 May 2005.     

Responses of the trunk routes of a harvester ant to plant density

Messor barbarus is a Mediterranean harvester ant that constructs physically defined trunk routes on the ground to connect nest entrances with foraging areas. Some responses of these trunk routes to plant density (and therefore resource abundance) were analyzed by testing the preferential allocation of different parts (trunk route ends, segments and branching points) in a patchy environment. Maps of grass density in four categories and Messor barbarus trunk routes were compiled for a Mediterranean pasture in Central Spain over four consecutive years. The proportions of the density categories in each year were used to calculate random expected frequencies of the trunk route points and the predominance of higher or lower grass densities. Trunk route ends discriminate and selectively reach patches with a greater abundance of resources in all study years. Branching points are also allocated preferentially in areas with higher vegetation density, but only in years with a predominance of the higher categories of grass density. In these years, the colonies of Messor barbarus have a phalanx strategy at a colonial level, and branching is more profuse. Finally, trunk route segments do not indicate any preference for crossing determined vegetation densities, but rather connect successive branching points or trunk route ends by the shortest route. These results concur with a model of structural strategy change (guerilla — phalanx) (Hutchings 1988) at the level of trunk routes. They are probably constituted by transitory sections with few branches, that expand other more profusely branched sections which are more dedicated to foraging.     

Foraging energetics of the ant,Paraponera clavata

The energy currencies used by foraging animals are expected to relate to the energy costs and benefits of resource collection. However, actual costs of foraging are rarely measured. We measured the ratio of energetic benefit relative to cost (B/C) during foraging for the giant tropical ant, Paraponera clavata. The B/C ratio was 3.9 for nectar-foragers and 67 for insect prey foragers. In contrast, the B/C ratio during foraging for seed harvester ants (Pogonomyrmex occidentalis) is over 1000, demonstrating that the B/C ratio can vary widely among ants. The B/C ratio was 300 times lower for nectar-foraging Paraponera than for the seed-harvesting Pogonomyrmex because of: (1) a 5-fold lower energetic benefit per trip, (2) a 10-fold greater cost due to longer foraging distances, and (3) a 6-fold greater energy cost per meter due to larger body size. For Paraponera daily colonial energy intake rates are similar to expeditures and may limit colony growth and reproduction. In contrast, for Pogonomyrmex energy intake rates are an order of magnitude higher than estimated costs, suggesting that Pogonomyrmex colonies are unlikely to be limited by short-term energy intake. We suggest that variation in individual B/C ratios may explain why the foraging behavior of Paraponera but not Pogonomyrmex appears sensitive to foraging costs.     

Desiccation limits recruitment in the pleometrotic desert seed‐harvester ant Veromessor pergandei

The desert harvester ant Veromessor pergandei displays geographic variation in colony founding with queens initiating nests singly (haplometrosis) or in groups (pleometrosis). The transition from haplo‐ to pleometrotic founding is associated with lower rainfall. Numerous hypotheses have been proposed to explain the evolution of cooperative founding in this species, but the ultimate explanation remains unanswered. In laboratory experiments, water level was positively associated with survival, condition, and brood production by single queens. Queen survival also was positively influenced by water level and queen number in a two‐factor experiment. Water level also was a significant effect for three measures of queen condition, but queen number was not significant for any measure. Foundress queens excavated after two weeks of desiccating conditions were dehydrated compared to alate queens captured from their natal colony, indicating that desiccation can be a source of queen mortality. Long‐term monitoring in central Arizona, USA, documented that recruitment only occurred in four of 20 years. A discriminant analysis using rainfall as a predictor of recruitment correctly predicted recruitment in 17 of 20 years for total rainfall from January to June (the period for mating flights and establishment) and in 19 of 20 years for early plus late rainfall (January–March and April–June, respectively), often with a posterior probability > 0.90. Moreover, recruitment occurred only in years in which both early and late rainfall exceeded the long‐term mean. This result also was supported by the discriminant analysis predicting no recruitment when long‐term mean early and late rainfall were included as ungrouped periods. These data suggest that pleometrosis in V. pergandei evolved to enhance colony survival in areas with harsh abiotic (desiccating) conditions, facilitating colonization of habitats in which solitary queens could not establish even in wet years. This favorable‐year hypothesis supports enhanced worker production as the primary advantage of pleometrosis.     

Three queen morphs with alternative nest-founding behaviors in the ant, Temnothorax longispinosus

In many ant species, multiple modes of founding new colonies occur in the same population. These modes include dependent founding, independent founding by haplometrosis (single queen), and independent founding by pleometrosis (multiple queens). In several cases, a dimorphism in queen size has been found, such that each morph specializes in a particular nest-founding behavior. I investigated queen size in the ant Temnothorax longispinosus in several southern Wisconsin populations and found three distinct queen morphs: small queens with very low fat content and short wings, large queens with low fat and long wings, and large queens with high fat and very long wings. Several traits associated with founding behavior correlated with these queen sizes. Small queens were produced in lower numbers, were more common in polygynous nests, and returned to the nest in higher proportions than both large queen morphs. The size ranges and fat levels of each queen morph were similar to those of other species that specialize in either haplometrosis (very large, high fat), pleometrosis (large, low fat), or dependent founding (small and low fat). However, there was extensive overlap in several of the founding behaviors, suggesting that the morphs in these populations have some flexibility in founding behavior. The queen morphs in these populations of T. longispinosus may resemble early stages in the evolution of more specialized dispersal polymorphisms found in other ant species. Received 11 January 2006; revised 15 September 2006; accepted 18 September 2006.     

Queen transport during ant colony emigration: a group-level adaptive behavior

Ant colonies emigrate frequently from one nest site to another. Emigrations, however, are dangerous, particularly for colonieswith a single queen. The queen is a "vital organ" of the colony,and emigrations expose her to grave peril. The optimal strategyfor a monogynous ant colony, therefore, should be that thequeen moves during the middle of the emigration so that sheis transferred swiftly from the protection of half of the colony in the old nest to the protection of the other half colony inthe new nest. In the ant Leptothorax albipennis, the queenis carried during colony emigration. We tested the null hypothesisthat the queen has a random position in the sequence of transportevents during an emigration. The result of 32 emigrations demonstrated,for the first time, that the transport serial number of thequeen [calculated relative to the total number of all transportevents (i.e., of brood and adult ants together), brood transportevents, or adult ant transport events] is not random and furthermoreoccurs in the middle of the transport sequence. This resultrepresents a colony strategy because we found that the relativetransport serial number of the queen was related neither to emigration distance nor to colony size. Transporting queensin the middle of emigrations is a strategy probably favoredby selection and is an aspect of colonies behaving as group-leveladaptive units.     

Chaff piles of harvester ant (Messor spp.) nests in a desert ecosystem

Summary The plant species composition of the chaff piles of three species of harvester ant (Messor spp.) and the contribution of the chaff to the organic pool were studied from August 1985 to July 1987. There were distinct differences in the plant species composition of the chaff of the three species. We attribute this to the different diets of the three species, which reflect the relative sizes of their individuals and their foraging strategies. The amount of chaff accumulated varies greatly between the three species (Messor rugossus: 127–196 g · ha^–1 · y^–1;Messor ebeninus: 2823–4437 g · ha^–1 · y^–1;Messor arenarius: 2165–2535 g · ha^–1 · y^–1), although the number of nests per hectare is virtually the same. We found that the amount of chaff is related to the rate of activity and the size of the individuals of each of the three ant species. The total chaff accumulated during the study period was 19.2 kg · ha^–1, which is an important contribution to the organic matter in the soil in the Negev desert ecosystem.     

Intraspecific competition through food robbing in the harvester ant,Messor aciculatus (Fr. Smith), and its consequences on colony survival

Summary Intraspecific interference competition in the harvester ant,Messor aciculatus, was studied. Colonies of this species were found not to have territories. Some nests were located very close to each other, and the foraging areas of the neighbors usually overlapped. Even though the frequency with which alien and resident ants met was very high in the vicinity of the nest entrances, aggressive interactions between them rarely occurred. However, when hostile workers encountered each other, they exhibited a kind of ritualized combat and the winner ejected, but did not injure the loser. If any aliens entered the nest, some of them were pulled out, mainly by the residents.Aliens roaming near a neighbor's nest entrance ferociously attacked the residents carrying seeds in their mandibles and robbed them. On other occasions, aliens entered the nest and stole the collected seed. Although seed robbing and stealing occurred among neighboring colonies, there were remarkable differences in the frequency of their occurrence. The results of field observations and experiments suggest the existence of a dominance order among the neighbors. In one instance, extermination of an inferior colony by its neighbor was observed. The raider colony transferred the stored seeds from the nest of the inferior colony to its own and deposited the larvae and workers some distance away from the nest.The influence of ritualized combat and food robbing on colony activities, and the ecological significance of this interference behavior in terms of spatial distribution and temporal persistence of the nest sites, is discussed.     

Queen number in a supercolony of the invasive garden ant, <Emphasis Type="Italic">Lasius neglectus</Emphasis>

Summary We have analysed the distribution of queens under stones at the core and at the periphery of a supercolony of Lasius neglectus that occupies 14 ha at Seva (NE Spain). Queens were not found alone, but rather within worker groups. Density at the center (mean ± s.d.: 1.38 ± 2.87 queens/stone; n = 100 stones; range 0–14) was not different from density at the periphery (1.18 ± 2.38; range 0–12). The estimate of the number of queens found under stones for the whole colony is about 35500. Egg-laying rates for queens from these two zones were obtained in the laboratory, at three different temperatures, and there were no differences detected. The presence of brood stages, from eggs to cocoons, was also similar in both zones. The homogeneous distribution of colony components may indicate that the area occupied by L. neglectus has already reached saturation. With a different technique – soil core extraction – we could estimate the density of workers in the soil: 800 workers per m². Soil cores had 6.28 ± 20.0 workers/core (range: 0–173), giving a rough estimate of 1.12 × 10⁸ workers in the soil, for the entire colony. Though few, some queens were also recovered from soil cores. Queen numbers for the supercolony, based on queens found in the soil, reaches the astounding level of 360000. Numbers are consistent with previous predictions.Received 10 September 2003; revised 1 December 2003; accepted 9 December 2003.     

Effect of mating stage on water balance, cuticular hydrocarbons and metabolism in the desert harvester ant, Pogonomyrmex barbatus

Water-loss rates increase after mating in queens of the harvester ant Pogonomyrmex barbatus (Formicidae: Myrmicinae), then increase again after the mated queens excavate an incipient nest. We determined the mechanistic basis for these increased water-loss rates by examining cuticular permeability, respiratory water loss, metabolic rates, and cuticular hydrocarbons for queens at three stages in the mating sequence: unmated alate queens, newly mated dealate queens, and mated queens excavated from their incipient nest. Both total water loss and cuticular transpiration increased significantly following mating, with cuticular transpiration accounting for 97% of the increased water loss. In contrast, metabolic rate and respiratory water loss were unaffected by mating stage. The total quantity of cuticular hydrocarbons did not vary by mating stage. However, relative amounts of four of the most abundant cuticular hydrocarbons did vary by mating stage, as did quantities of n-alkanes and methylalkanes. The general pattern was that percent composition of n-alkanes decreased through the mating sequence, while percent composition of methylalkanes increased over the same sequence. We discuss three mechanisms that might cause these post-mating increases in cuticular permeability. Our data support the hypothesis that part of this increase results from soil particles abrading the cuticle during the process of nest excavation.     

Ant reproductive strategies

In contrast to what is generally believed, the reproductive strategies of ants are remarkably diverse and include such different phenomena, as wingless female and male sexuals, reproduction by mated workers, thelytokous parthenogenesis, and complete workerlessness. We review the various reproductive life histories and investigate them in the light of recent models on the evolution of dispersal strategies and multiple-queening. It appears that most deviations from “normal” colony propagation can be explained by a decreased success of dispersal and solitary founding by solitary queens in certain types of habitats. Consequently, alternative reproductive strategies are found especially in those species, in which environmental conditions or a highly specialized way of life are thought to make solitary founding costly. Among the key factors, which determine the success of reproductive strategies, appear to be spatial and temporal distribution of habitats and the availability of nest sites.