Les relations entre fourmis arboricoles et guêpes sociales sous les tropiques

Due to the brood that it contains, the nest of social wasps constitutes a major source of protein for eventual predators. In the tropics, ants are generally considered as the most efficacious among them. In the face of such predation, wasps have developed over the course of evolution a great number of strategies. In addition to guarding and defensive behaviours displayed by workers, social wasps have nests whose structure is a response to the predation pressure exerted by ants. Thus, the presence of a petiole attaching the nest to a natural support itself sometimes selected because of its limited accessibility constitutes a frequent example of architectural defence. The effectiveness of the petiole is often improved through the repeated application of repellent substances on its surface. In the same way, wasps that found their new societies through swarming construct nests protected by an envelope. In the American tropics, army ants which hunt on the ground and can rapidly take over a tree, regularly pillage wasp nests. Because no defensive strategy is efficacious against such predators, the choice of nest site becomes decisive. Paradoxically, it is by nesting in close proximity to an arboreal ant nest that certain wasp species have been able to find a way of responding to the threat exerted by other ants. These ants, which tolerate the presence of the wasp nest, are capable of efficaciously protecting the tree or the branch sheltering the nest.     

Founding Swarms in a Tropical Social Wasp: Adult Mortality, Emigration Distance, and Swarm Size

When colonies of swarm-founding wasps lose their nests to predation or accident, the entire adult population escapes, emigrates as an absconding swarm, and renests elsewhere. Such an event causes a reduction in the adult population due to losses during the emigration itself and to adult attrition without replacement during the subsequent preemergence growth period in the new nest. We addressed the first of these sources of mortality for 27 absconding swarms of Polybia occidentalis in Costa Rica. Adult mortality over the day that included swarm emigration averaged 0.044 ± 0.039 (SD) of the original population and was a weak positive function of distance moved, but not of swarm size. A larger data set showed that emigration distance increased with swarm size. This is the first study to measure mortality rates during emigration in a swarm-founding social insect.     

Behavior during artifically induced swarm emigration in an old world polistine wasp,Ropalidia romandi (Hymenoptera: Vespidae)

Absconding swarms of the Australian swarm-founding wasp,Ropalidia romandi, were artificially induced. Workers may initially search for new nesting sites and once a new nest site is decided, they recruit their nest mates including multiple queens there. During the swarm emigration, workers often land on prominent objects, such as tree leaves, and drag the metasoma; those that do not drag usually antennate the object surfaces. These findings suggest thatR. romandi workers use a scent trail during swarm emigration. Artificial transfer of colonies resulted in clustering of wasps from different colonies, indicating thatR. romandi may not act exclusively against non-nestmates. In the formation of tentative aggregation of wasps prior to commencing emigration, and also in recognizing a new nesting site by following swarm members that have been guided by a scent trail, a visual cue (aerial swarm formed around an aggregation of wasps) seems to play an important role.     

A mimetic nesting association between a timid social wasp and an aggressive arboreal ant

In French Guiana, the arboreal nests of the swarm-founding social wasp Protopolybia emortualis (Polistinae) are generally found near those of the arboreal dolichoderine ant Dolichoderus bidens. These wasp nests are typically protected by an envelope, which in turn is covered by an additional carton ‘shelter’ with structure resembling the D. bidens nests. A few wasps constantly guard their nest to keep D. bidens workers from approaching. When alarmed by a strong disturbance, the ants invade the host tree foliage whereas the wasps retreat into their nest. Notably, there is no chemical convergence in the cuticular profiles of the wasps and ants sharing a tree. The aggressiveness of D. bidens likely protects the wasps from army ant raids, but the ants do not benefit from the presence of the wasps; therefore, this relationship corresponds to a kind of commensalism.     

Costs and benefits of colony aggregation in the social wasp, Polistes annularis

Females of the social wasp, Polistes annularis, nest in largeaggregations onriverside cliffs in central Texas. Aggregationsoccur where there is an overhang under which wasps build theirnests. Colony aggregations appear to occur partly because individualfemales do notdisperse far from the natal site where most coloniesare successful. In most taxa greater reproductive success nearthe natal nest site is related to unique physical features orto advantages ofbeing in a group. In P. annularis, the physicalfactors and not the social factors appear to be most important.Advantages to remaining near the natal nest site include protectionprovided by the cliff overhang from rain and afternoon sun.Central colonies are as likely to suffer predation as are peripheralcolonies, which indicates an absence of selfish herd advantagesto being in the center of the group. A cost of being in a denseaggregation is often increased reproductive competition, andthis is the case for P. annularis. Colonies in the center ofthe aggregation suffer high usurpation rates by unrelated femalesand in 1 of 3 years produced fewer reproductives than edge colonies.     

Sociobiology of slave-making ants

Social parasitism is the coexistence in the same nest of two species of social insects, one of which is parasitically dependent on the other. Though parasitism in general is known to be of crucial importance in the evolution of host species, social parasites, though intriguing, are often considered as a phenomenon of marginal interest and are typically not taken into account in reviews on parasitism. Nevertheless, social parasites are rather common in social bees, wasps, and ants and therefore may offer unique model systems to study a number of fundamental problems in evolutionary biology. Here we review several aspects of the peculiar life history of slave-making ants, which is characterized by socially parasitic founding of colonies and the pillage of broods from neighboring host colonies during slave raids. In particular we focus on the evolution of slave-making habits (dulosis), communication mechanisms between slave makers and their hosts, sex-allocation ratios and reproductive conflict, and the effect of slave makers on host populations. Received: 2 February 2000 / Received in revised form: 21 December 2000 / Accepted: 8 January 2001     

Strike Fast,Strike Hard: The Red-Throated Caracara Exploits Absconding Behavior of Social Wasps during Nest Predation

Red-throated Caracaras Ibycter americanus (Falconidae) are specialist predators of social wasps in the Neotropics. It had been proposed that these caracaras possess chemical repellents that allow them to take the brood of wasp nests without being attacked by worker wasps. To determine how caracaras exploit nests of social wasps and whether chemical repellents facilitate predation, we: (1) video recorded the birds attacking wasp nests; (2) analyzed surface extracts of the birds'' faces, feet, and feathers for potential chemical repellents; and (3) inflicted mechanical damage on wasp nests to determine the defensive behavior of wasps in response to varying levels of disturbance. During caracara predation events, two species of large-bodied wasps mounted stinging attacks on caracaras, whereas three smaller-bodied wasp species did not. The “hit-and-run” predation tactic of caracaras when they attacked nests of large and aggressive wasps reduced the risk of getting stung. Our data reveal that the predation strategy of caracaras is based on mechanical disturbance of, and damage to, target wasp nests. Caracara attacks and severe experimental disturbance of nests invariably caused wasps to abscond (abandon their nests). Two compounds in caracara foot extracts [sulcatone and iridodial] elicited electrophysiological responses from wasp antennae, and were also present in defensive secretions of sympatric arboreal-nesting Azteca ants. These compounds appear not to be wasp repellents but to be acquired coincidentally by caracaras when they perch on trees inhabited with Azteca ants. We conclude that caracara predation success does not depend on wasp repellents but relies on the absconding response that is typical of swarm-founding polistine wasps. Our study highlights the potential importance of vertebrate predators in the ecology and evolution of social wasps.     

An alternative strategy for maintenance of eusociality after nest destruction: new nest construction in a primitively eusocial wasp

Although nests are central to colonial life in social insects, nests are sometimes damaged by predators or natural disasters. After nest destruction, individuals usually construct new nests. In this case, a sophisticated mechanism like the scent trail pheromone used in large insect colonies that recruit individuals to new nest sites would be important for the maintenance of eusociality. In independent-founding Polistes wasps, it is well known that queens enforce workers physiologically on the natal nests even if evidence of trail pheromone use has not been exhibited. We investigated the effect of the queen on an alternative strategy for the maintenance of eusociality by first females after nest destruction in the primitively eusocial wasp Polistes chinensis. We predicted that the first females in queen-absent colonies have various behavioral options after nest destruction. Even if the females construct new nests cooperatively with other individuals, the new nest construction should be conducted more smoothly in queen-present colonies because the queens regulate the behavior of wasps. We made wasps construct new nests by removing the entire brood from existing nests. The presence of the queen did not cause variation in the alternative strategy of the first females, as the first females (workers) usually constructed new nests cooperatively irrespective of the queen-presence. Thus, the workers in the queenpresent colonies affiliated to the new nest construction more smoothly and constructed new nests more efficiently than workers in the queen-absent colonies. Our results suggest that the presence of the queen is important for maintaining eusociality in primitively eusocial wasps after nest destruction. Received 8 February 2005; revised 5 October 2005; accepted 17 October 2005.     

Differentiated Anti-Predation Responses in a Superorganism

Insect societies are complex systems, displaying emergent properties much greater than the sum of their individual parts. As such, the concept of these societies as single ‘superorganisms’ is widely applied to describe their organisation and biology. Here, we test the applicability of this concept to the response of social insect colonies to predation during a vulnerable period of their life history. We used the model system of house-hunting behaviour in the ant Temnothorax albipennis. We show that removing individuals from directly within the nest causes an evacuation response, while removing ants at the periphery of scouting activity causes the colony to withdraw back into the nest. This suggests that colonies react differentially, but in a coordinated fashion, to these differing types of predation. Our findings lend support to the superorganism concept, as the whole society reacts much like a single organism would in response to attacks on different parts of its body. The implication of this is that a collective reaction to the location of worker loss within insect colonies is key to avoiding further harm, much in the same way that the nervous systems of individuals facilitate the avoidance of localised damage.     

The effects of colony characteristics on life span and foraging behavior of individual wasps (Polybia occidentalis,Hymenoptera: Vespidae)

Summary We studied the effects of intrinsic colony characteristics and an imposed contingency on the life span and behavior of foragers in the swarm-founding social waspPolybia occidentalis. Data were collected on marked, known-age workers introduced into four observation colonies.To test the hypothesis that colony demographic features affect worker life span, we examined the relationships of colony age and size with worker life span using survivorship analysis. Colony age and size had positive relationships with life span; marked workers from two larger, older colonies had longer life spans (¯X = 24.7 days) than those from two smaller, younger colonies (¯X = 20.1 days).We quantified the effects of experimentally imposed nest damage on forager behavior, to determine which of three predicted behavioral responses by foragers to this contingency (increased probability of foraging for building material, increased rate of foraging, or decrease in age of onset of foraging) would be employed. Increasing the colony level of need for materials used in nest construction (wood pulp and water) by damaging the nests of two colonies did not cause an increase in either the proportion of marked workers that gathered nest materials or in foraging rates of marked individuals, when compared with introduced workers in two simultaneously observed control colonies. Instead, nest damage caused a decrease in the age at which marked workers first foraged for pulp and water. The response to an increase in the need for building materials was an acceleration of behavioral development in some workers.     

Nest relocation and encounters between colonies of the seed-harvesting ant Messor andrei

Summary: Nest relocation in ants may be an attempt to escape areas of high competition. Encounters between colonies have been suggested to be the proximate cause of nest relocation. I examined the relation between nest relocation and encounters in a population of the seed-harvesting ant Messor andrei, a native of the western United States. Over 80% of colonies relocated their nest site during a year. Colonies moved up to ten times in a year, and new nest-sites were more distant from their nearest neighbour. However, encounters did not precipitate nest relocation. Relocation was not more likely to occur directly after an encounter, and colonies that moved did not experience more encounters than other colonies. Other possible cues for nest relocation, including predation, disease, microclimatic effects, and local resource depletion, are discussed.     

Rubbing behavior and morphology of van der Vecht's gland inBelonogaster petiolata (Hymenoptera: Vespidae)

Females of the social wasp, Belonogaster petiolata,rub the secretion of van der Vecht's gland, located on their terminal gastral sternite, onto the nest pedicel. In bioassays, the secretion was repellent to two species of ants, while shortchain acids were effective releasers of rubbing behavior. Rubbing was associated with pedicel enlargement and departure from the nest in preemergence colonies. Its frequency was high where wasps were often exposed to ants and low where ants were rare or absent. Rubbing also decreased significantly from the pre-to the postemergence stage of the colony cycle. In both stages, subordinate foundresses rubbed more often than queens or workers. These observations support the hypothesis that rubbing behavior and the secretion of van der Vecht's gland function in chemical defense of the nest against ant predation. The general morphology of the gland in B. petiolataresembles that of the four other independent-founding polistine wasp genera.     

A test of adaptive hypotheses for rapid nest construction in a swarm-founding wasp

Most social insect species enlarge their nests gradually and in close correlation with the growing need for space for brood and/or stored food. In contrast, some species of swarm-founding eusocial wasps construct the nest rapidly to a final size in the first two to three weeks of the founding stage. We considered four hypotheses on the functions of rapid nest construction in the wasp Polybia occidentalis and directly tested two of them. The first hypothesis is that rapid construction maximizes output of the worker force when there are few other work demands; it predicts that construction rate remains high until the first eggs begin to hatch, following which it declines as increasing amounts of worker effort are allocated to the feeding of larvae. The second says that rapid nest construction minimizes the time the adults in the swarm are exposed to predation and the elements; it predicts that nest-construction rate should drop steeply after the nest is large enough to house all the adults in the swarm. We measured pulp-foraging rates for the first 12 days of the founding stage in control colonies and in colonies whose nests we manipulated to prevent housing of the swarm. The treatment and control groups did not differ in construction rate for several days following the housing event, contradicting the adult-protection hypothesis. Late in nest construction, treatment colonies were building at significantly higher rates than were control colonies. If demand for brood care were a major factor in determining construction rate, both groups would have responded to the eclosion of larvae in the same way and shown a parallel decline in construction rate, but this did not happen. Instead, the patterns of nest construction rate we observed provided indirect support for the two remaining hypotheses. The first of these is that rapid construction minimizes exposure of the brood to natural enemies and desiccation. The second is that rapid construction promotes competition among queens by providing empty cells for oviposition, thereby facilitating the selecting out of the less fecund of the multiple reproductive females. Also consistent with this hypothesis is the apparent absence of explosive nest construction in monogynous, eusocial bees. Received 13 October 2007; revised 31 March 2008; accepted 6 April 2008.     

Social paper wasp (Agelaia pallipes) predates songbird nestling

The social paper wasp Agelaia pallipes is known to eat carrion and scavenge on vertebrates. There are few records of wasps predating vertebrates, including an attack on an adult hummingbird and the predation of bird nestlings. During a project monitoring reproductive behaviour of a neotropical songbird, the Lined Seedeater Sporophila lineola in south-eastern Brazil, we recorded the predation of a four-day-old nestling by a social paper wasp. In the video, the adult female bird attempted to visit the nest prior to the predation. The male could be seen with its crest feathers erect after a wasp left the nest, when the nestling was presumably already dead. When we arrived at the nest to remove the camera, we found the nestling dead, and did not observe the parents in the vicinity. We also registered two other dead nestlings in a different nest with similar wounds. However, the conclusive cause of death of those nestlings is unknown. Nest predation is a major selective pressure in birds, and insects are rarely assumed to play a notable role in this process. Further research is needed to better understand the nature of the relationship between wasps and birds.     

Colony characteristics influence the risk of nest predation of a polydomous ant by a monotreme

Although predation of individual social insect workers has little effect on colony fitness, nest predation may be a significant selective agent because it can result in substantial loss of reproductive success. Surprisingly, the consequences of predation on social insect nests are poorly understood. In the present study, we investigate the factors that correlate with the probability of predation by echidnas, Tachyglossus aculeatus , on nests and colonies of the facultatively polydomous meat ant, Iridomyrmex purpureus. In particular, we investigate whether colony fragmentation provides a mechanism for reducing the costs of echidna predation. Over 2 years, 138 of the 140 colonies on our study site were depredated. Nest predation was most common in woodlands but with no obvious seasonal patterns. The probability of nest predation was positively correlated with the size of the nest, and negatively correlated with the density of surrounding nests. Although polydomous colonies are at a similar risk of predation by echidnas, the proportion of depredated nests is negatively correlated with the number of nests; thus, the probability that one or more nests avoid predation is increased with increasing nest numbers. Surprisingly, we found no influence of the level of echidna predation on colony growth, measured by either changes in the number of nests or the number of nest entrance holes.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 1–8.     

A latitudinal gradient in intensity of applying ant-repellent substance to the nest petiole in paper wasps (Hymenoptera: Vespidae)

Summary Observations on pre-emergence, single-foundress colonies of Japanese paper wasps (Polistes) revealed that there was a latitudinal gradient in intensity of application of an ant-repellent substance (secreted by the metasomal sternum VI glands) to the nest petiole. Thus the lower the latitude, the more frequently a foundress rubbed ant repellent onto the nest petiole. Estimation of potential ant predation on wasp brood using bait traps showed that there was a positive correlation between the frequency of rubbing and potential predation pressure from ants which are guided predominantly by substrate cues for foraging. There was also a latitudinal gradient in the degree of temporal association of rubbing behavior with foraging: the lower the latitude, the more closely foundress departure from the nest was associated with rubbing. Enlargement of the nest petiole by applying oral secretion potentially obliterated previous coats of ant repellent; however, this behavior was not always followed by rubbing behavior. The ant-repellent chemical barrier around the nest petiole may have evolved in tropical regions of the world as a defense against ant predation on wasp brood. I argue that as ant predation pressure diminishes towards the cooler regions, so does selection maintaining the behavioral sequence where foundress departure from the nest is preceded by rubbing behavior.     

Adult mortality rates in young colonies of a swarm-founding social wasp (Polybia occidentalis)

The swarm founders are unusual among the social wasps in having socialized the dispersal stage of the life cycle. Colonies are initiated by groups of workers accompanied by smaller numbers of queens. Thus, swarm founders avoid the colony size bottleneck faced by the independent founders, whose colonies start with one or a few queens. Among the advantages of swarm founding is a reduction of the risk of colony failure due to attrition of the founding adults. Stochasticity in adult mortality is less likely to lead to outright extinction of a large founding group before new workers are produced (pre-emergence period). However, it is not known how important pre-emergence mortality is as a selective force on founding and dispersal behaviour in swarm founders, since colony-wide mortality rates have never been reported for a large-colony social wasp. Sixty-eight swarms of Polybia occidentalis were censused just before colony initiation and again 24 days later. Mortality over this period averaged 0.41±0.12 of the founding swarm population. Including mortality on the day of emigration and extrapolating to day 30, when the first adult offspring eclose, the original absconding swarm would be reduced by 0.52 of its initial size. Rates of loss during the first week, while the colonies engaged in nest construction, did not differ from rates over the full 24 days. Thus, colony founding in P. occidentalis is both costly and highly variable in terms of mortality of the founding adults.     

Colony Membership, Division of Labor, and Genetic Relatedness Among Females of Colonies of Eustenogaster fraterna (Hymenoptera, Vespidae, Stenogastrinae)

To compare the social structure of primitively social wasps with that of communal breeding vertebrates, we used a new technique based on micro videocameras applied to the nest envelope to study the organization of behavior in Malaysian colonies of the stenogastrine wasp Eustenogaster fraterna. The reproductive division of labor in this species appears to be different from that reported so far in other species of Stenogastrinae: it is at a very primitive stage because the helpers work less hard and take fewer risks than the egg-layers. Nevertheless, the very low-risk tasks performed by the helpers (nest guarding and supply of abdominal secretion to the young larvae) are important for the colonies. Behavioral characters and ovarian development of the helpers suggest that females are only temporary helpers and that they wait to start their own production of offspring in the hope of inheriting the nest. Females of the same colony can be highly related, but in some colonies we found low-related individuals. The social organization of these wasps resembles that of vertebrate groups with helpers at the den, i.e., in the small number of individuals, the division of labor with limited reproductive inhibition, and the prolonged external work of the reproductive individuals. However, the main drive to sociality in these insects appears to be different, being based mainly on problems connected with short adult life and long immature brood development.     

Productivity in a social wasp: per capita output increases with swarm size

We measured the productivity of newly-founded colonies of Polybiaocddentalis, a Neotropical swarm-founding social wasp, overtheir first 25 days. By both of the measures we used, numberof nest cells built by the swarm and dry weight of brood produced,colony-level productivity was a significant positive quadraticfunction of the number of adults in the swarm, indicating thatper capita output increased with swarm size. Subdividing adultsinto queens and workers did not improve significantly on thesemodels, but the proportion of queens was a significant factorexplaining brood production in one of two sampling years. Earlierwork on P. ocddentalis suggests that the mechanism behind thepattern is that workers transferring materials to one anotherexperience increasing queuing delays as group size decreases.The largest colony in each of the two years produced unusuallylow outputs of brood. One interpretation is that the curve ofgroup-size related brood productivity peaks at intermediategroup size and that these colonies are on the downward partof the curve. That these same two colonies also had the lowestproportions of queens suggests a second interpretation: thesecolonies were constrained to low brood production by a low colony-leveloviposition rate. A third possibility is diat these were maturecolonies, and mature colonies may allocate a smaller fractionof resources to brood rearing than do younger colonies. Ourresult contradicts earlier findings for a variety of socialand subsocial Hymenoptera that per capita productivity declinesas group size increases. We suspect that Michener's result forswarm-founding wasps is an artifact of his having to lump coloniesof different species and different stages of development toobtain adequate sample sizes to plot. If our result for P. ocddentaliscan be generalized to other swarm-founders, then these waspshave evolved a mode of colony organization fundamentally differentfrom that of other wasps. Thus, our result places new significanceon the role of group dynamics as a factor affecting group sizein different taxa.     

An ecological paradox: More woodland predators and less artificial nest predation in landscapes colonized by noisy miners

Many passerine bird populations, particularly those that have open‐cup nests, are in decline in agricultural landscapes. Current theory suggests that an increase in habitat generalist predators in response to landscape change is partially responsible for these declines. However, empirical tests have failed to reach a consensus on how and through what mechanisms landscape change affects nest predation. We tested one hypothesis, the Additive Predation Model, with an artificial nest experiment in fragmented landscapes in southern Queensland, Australia. We employed structural equation modelling of the influence of the relative density of woodland and habitat generalist predators and landscape features at the nest, site, patch and landscape scales on the probability of nest predation. We found little support for the Additive Predation Model, with no significant influence of the density of woodland predators on the probability of nest predation, although landscape features at different spatial scales were important. Within woodlands fragmented by agriculture in eastern Australia, the presence of noisy miner colonies appears to influence ecological processes important for nest predation such that the Additive Predation Model does not hold. In the absence of colonies of the aggressive native bird, the noisy miner, the influence of woodland predators on the risk of artificial nest predation was low compared with that of habitat generalist predators. Outside noisy miner colonies, we found significant edge effects with greater predation rates for artificial nests within woodland patches located closer to the agricultural matrix. Furthermore, the density of habitat generalist predators increased with the extent of irrigated land‐use, suggesting that in the absence of noisy miner colonies, nest predation increases with land‐use intensity at the landscape scale.