Studies of a South East Asian ant-plant association: protection of Macaranga trees by Crematogaster borneensis

Summary In the humid tropics of SE Asia there are some 14 myrmecophytic species of the pioneer tree genus Macaranga (Euphorbiaceae). In Peninsular Malaysia a close association exists between the trees and the small, non-stinging myrmicine Crematogaster borneensis. These ants feed mainly on food bodies provided by the plants and have their colonies inside the hollow internodes. In a ten months field study we were able to demonstrate for four Macaranga species (M. triloba, M. hypoleuca, M. hosei, M. hulletti) that host plants also benefit considerably from ant-occupation. Ants do not contribute to the nutrient demands of their host plant, they do, however, protect it against herbivores and plant competition. Cleaning behaviour of the ants results in the removal of potential hervivores already in their earliest developmental stages. Strong aggressiveness and a mass recruiting system enable the ants to defend the host plant against many herbivorous insects. This results in a significant decrease in leaf damage due to herbivores on ant-occupied compared to ant-free myrmecophytes as well as compared to non-myrmecophytic Macaranga species. Most important is the ants' defense of the host plant against plant competitors, especially vines, which are abundant in the well-lit pioneer habitats where Macaranga grows. Ants bite off any foreign plant part coming into contact with their host plant. Both ant-free myrmecophytes and non-myrmecophytic Macaranga species had a significantly higher incidence of vine growth than specimens with active ant colonies. This may be a factor of considerable importance allowing Macaranga plants to grow at sites of strongest competition.     

Raiding and Foraging Behavior of the Blood-Red Ant,Formica sanguinea Latr. (Hymenoptera,Formicidae)

In this paper we report the results of a detailed study on the behavioral ecology of slave raiding and foraging activity in the European blood-red ant, Formica sanguinea Latr. The field study was conducted over an unbroken period of 78 days, during which the activity of two dulotic colonies of this facultative slave-maker was observed for 10 h each day. It was possible to observe 26 raids distributed over 23 days, among which 18 were followed by the sacking of nests belonging to the species F. cunicularia, F. fusca, and Lasius emarginatus, whereas 8 failed. Simple, continuous, and simultaneous raids occurred. We recorded the timing, frequency, distance, and direction of slave raids, including the number of participants and the type of booty. Particular attention was devoted to the scouting behavior and raiding organization. Moreover, every day, we observed foraging and predatory behavior, during which adult insects (mainly ants), seeds, and berries were retrieved to the dulotic colonies. On the basis of our observations F. sanguinea seems to be a very efficient slave-maker and predatory species of the Raptiformica subgenus. Moreover, its dulotic behavior may be regarded as a continuation and an expansion of its foraging and predatory behavior, as predicted by Darwin's hypothesis for the origin and evolution of slavery in ants.     

Host colony usurpation by the queen of the Japanese pirate ant, Polyergus samurai (hymenoptera: formicidae)

Colony usurpations by newly mated queens of Polyergus samurai were observed under artificial conditions. Newly mated queens of P. samurai were introduced into three kinds of Formica japonica host colonies: queenright, queenless (artificially orphaned), and workerless (only a queen remaining) colonies. In the queenright condition, the P. samurai queen intruded into the host nest and killed the host queen, and was subsequently adopted by the host workers. In all queenright and queenless host colonies, seven of 13 queens of P. samurai succeeded in colony usurpation, although the starting time of grooming, a nestmate behavior, by host workers in the queenright condition occurred earlier than in the queenless condition. In workerless conditions, four of five P. samurai queens ignored the F. japonica queen. The results suggest that while host-queen killing is not necessary, it is important to win acceptance by host workers.     

Host-plant selection,diet diversity,and optimal foraging in a tropical leafcutting ant

Summary A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.     

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.     

The triple alliance: how a plant-ant, living in an ant-plant, acquires the third partner, a scale insect

Summary. Mutualistic associations between ants and plants often involve a third partner, scale insects (Hemiptera, Coccoidea). In southeast Asia, plant-ants of the genus Cladomyrma live together with coccoids in hollowed twigs of a wide range of ant-plants (myrmecophytes). Established colonies never lack sap-sucking scale insects and the ants appear to be dependent on the honeydew excretions of their trophobionts. Acquisition of scale insects thus seems to be an important step in the successful establishment of a new colony on a host plant. Coccoids may either be transported by a foundress ant on her nuptial flight (vertical transmission via co-dispersal) or ant and coccoid disperse on their own and associate anew during colony initiation (horizontal transmission). Here we test the assumption that founding gynes of Cladomyrma do not transport scale insects during their dispersal flight, and we determine when and how trophobionts are acquired. The results obtained in this study show that although coccoids are not carried by foundresses, their numbers within founding chambers of Cladomyrma increased with the founding stage. In all Cladomyrma species, colony-founding is claustral and the entrance hole into the nest chamber is first plugged with pith debris and later grows over by wound callus. However, access of trophobionts into hollow twigs is facilitated either by additional small openings (secondary slits) produced by the foundress ant after hollowing out a nest chamber or by the maintenance of an orifice at the entrance hole during the founding period. Immature pseudococcids experimentally placed onto twigs close to a Cladomyrma foundation readily entered these slits. Exclusion experiments suggest that foundresses of C. petalae are less successful in rearing their first offspring if entry of coccoids into founding chambers is prevented.Received 28 September 2003; revised 9 May and 11 October 2004; accepted 9 November 2004.     

Look before leaping: foraging selectivity of capuchin monkeys on acacia trees in Costa Rica

Acacia trees in Costa Rica have an obligate mutualism with three species of Pseudomyrmex ants, which vigorously defend their host tree from insect and mammalian herbivores. Depending on the size and species of ant colony, individual acacia trees may be differentially protected. For animals able to discern between weakly and highly aggressive ant colonies, costs of ant stings from less active colonies might be offset by nutritional value acquired from feeding on acacia fruit or ant larvae in swollen thorns. We examined foraging selectivity of capuchin monkeys on acacia trees in Santa Rosa National Park, Costa Rica. We measured four characteristics of the acacia trees from which capuchins fed and of acacias immediately adjacent to those in which the monkeys fed: diameter at breast height (DBH), accessibility, species of closest tree and ant species present. We found that capuchins prefer to forage in acacias that are large and accessible. We also made two measurements of ant colony activity on each tree, one before and one after disturbing the ant colony. We found that the three species of mutualistic ants differ in baseline activity levels and that mutualistic ants are more active than non-mutualistic ant species found in acacia trees. We also found that capuchins foraged more frequently in trees colonized by non-mutualistic ants, but the explanatory value (r ²) of this model was low. Furthermore, monkeys did not discriminate between acacias on the basis of baseline ant activity or the ant colony’s response to disturbance. We conclude that these monkeys select acacia trees in which to forage based on characteristics of the trees rather than the ants. In addition, our study suggests that white-faced capuchins act as predators on the acacia ants but they probably benefit the dispersal and reproductive success of acacia trees. Capuchins may in fact function as an additional mutualistic partner for acacia trees via seed dispersal, but they must overcome the ants’ defense of the trees to do so.     

Spartan defense in the Thermopylae pass: Strategic defense by aggregations of <Emphasis Type="Italic">Pseudomyrmex spinicola</Emphasis> (Hymenoptera,Formicidae) on the trunk of <Emphasis Type="Italic">Acacia collinsii</Emphasis> (Mimosaceae)

Acacia ants (Pseudomyrmex spinicola) usually prune all of the vegetation growing in the vicinity of their host plant (Acacia collinsii). This lack of branches in contact with the host tree makes the main stem of the acacia the only point of access for ants invading the resident colony. The present study assessed whether the intrusion site affects the probability of the intruder being detected. It also investigated whether ants that remain inactive on the main trunk base (a) are involved in defensive duties and (b) are morphologically different from other colony members (e.g. foraging ants). In the field, intruder ants were placed on the trunk and leaves of acacias inhabited by P. spinicola; colony members on the trunk base and the leaves were marked and monitored; and head length, head width, and thorax length of ants that were either foraging on the leaves or immobile on the trunk were measured. P. spinicola ants found intruder ants entering their host acacia via the trunk more rapidly than those entering by means of the leaves. Ants on the base of the trunk do not exchange tasks with foragers: between 5 and 47% of the 1063 ants originally marked on the trunk were re-observed there, whereas only two of the 804 ants marked on the leaves were seen on the trunk. However, ants on the trunk were similar to foragers in morphology, suggesting that defensive ants may be part of a behavioral caste. Received 30 November 2007; revised 29 February 2008; accepted 17 March 2008.     

Effects of ants on the foraging of birds in spruce trees

I experimentally excluded ants from randomly selected spruce trees Picea abies near colonies of the wood ant Formica aquilonia. Foraging activity of birds in these trees was then compared to the foraging activity of birds in neighboring spruce trees, where ants were allowed to continue foraging. Birds which foraged in the foliage showed the effects of competition with ants: they visited the trees without ants more frequently, and for longer periods. In addition, the insects and spiders that they utilized as food were more abundant in the foliage of trees without ants. Cone-foraging birds, however, which fed on seeds in cones at the tops of the trees, did not show a preference for trees without ants. The differences of tree usage between foliage-gleaning and coneforaging birds can be explained by alteration of the birds' food supply by wood ants: ants did not feed on seeds in cones, and so did not compete with cone-foraging birds. However, foraging wood ants did feed on arthropods living in the foliage, thus reducing the amount of food available to birds there.     

Raiding Activity of an Obligate (Polyergus breviceps) and Two Facultative (Formica puberula and F. gynocrates) Slave-Making Ants

Slave-making ants are social parasites that exploit the labor of workers from their host species by keeping them captive in the slave-maker nest. Slave-makers vary in their degree of specialization, ranging from obligate slave-makers that cannot survive without captives, to facultative slave-makers, which are often found living independently. Our study system included one obligate slave-maker, Polyergus breviceps, two facultative slave-makers, Formica puberula and F. gynocrates, and two hosts, F. occulta and F. sp. cf. argentea. We observed all raids conducted during two raiding seasons by seven P. breviceps colonies, two F. puberula colonies, and two F. gynocrates colonies. We report on raiding frequency, average raid distances, and then compare the probability of being raided multiple times in a single raiding season for the two host species. We also report on the spatial distribution of slave raids, which suggests that slave-makers avoid raiding in areas used by other slave-maker colonies. This is the first report of raiding activity for P. breviceps in this location, and the first report of raiding activity of any kind for F. puberula and F. gynocrates.     

Leaf volatile compounds and the distribution of ant patrollingin an ant-plant protection mutualism: Preliminary results onLeonardoxa (Fabaceae: Caesalpinioideae) andPetalomyrmex(Formicidae: Formicinae)

While observations suggest that plant chemicals could be important in maintaining the specificity and permitting the functioning of ant-plant symbioses, they have been little studied. We report here the strongest evidence yet for chemical signalling between ants and plants in a specific ant-plant protection symbiosis. In the mutualism between Leonardoxa africana subsp. africana and Petalomyrmex phylax, ants continuously patrol young leaves, which are vulnerable to attacks by phytophagous insects. We provide experimental evidence for chemical mediation of ant attraction to young leaves in this system. By a comparative analysis of the related non-myrmecophytic tree L. africana subsp. gracilicaulis, we identify likely candidates for attractant molecules, and suggest they may function not only as signals but also as resources. We also propose hypotheses on the evolutionary origin of these plant volatiles, and of the responses to them by mutualistic ants.     

Observations onApoica pallens,a nocturnal neotropical social wasp (Hymenoptera: Vespidae,Polistinae, Epiponini)

Summary Colonies and nests ofApoica pallens in the llanos region of Venezuela range from small foundress nests to large mature colonies. Nests are sited on small diameter, near-horizontal branches in a variety of shrub and tree species. During the day, adult wasps cluster on the face of the nest in an array that seems to be determined by orientation to gravity; defense of the colony against parasitoids and ants by the resting wasps may be more a passive than an active behavior. Wasps fan their wings to cool the colony during the day, but no foraging for water accompanies the fanning behavior. Nightly foraging activity begins with the explosive departure from the nest of hundreds of wasps, most of which rapidly return. Moderate foraging levels early at night give way to very low foraging levels in pre-dawn hours. The period of moderate foraging may be extended for longer hours during increased moonlight. Foraging wasps collect arthropod provisions for larvae. Larvae produce a trophallactic saliva; adults engage in inter-adult trophallaxis; brood are cannibalized. During cluster formation prior to swarm emigration, adult wasps do not appear to scent-mark substrates such as leaves. Instead,A. pallens exhibits a calling behavior, unique among polistine wasps studied to date, in which the gaster is held rigidly away from the thorax and metasomal sternal glands are exposed. Swarms can emigrate during the day.A. pallens may incorporate absconding and colony relocation as features of its colony cycle in the highly seasonal llanos.     

Thorn‐dwelling ants provide antiherbivore defence for camelthorn trees,Vachellia erioloba,in Namibia

Ants are widely employed by plants as an antiherbivore defence. A single host plant can associate with multiple, symbiotic ant species, although usually only a single ant species at a time. Different plant‐ant species may vary in the degree to which they defend their host plant. In Kenya, ant–acacia interactions are well studied, but less is known about systems elsewhere in Africa. A southern African species, Vachellia erioloba, is occupied by thorn‐dwelling ants from three different genera. Unusually, multiple colonies of all these ants simultaneously and stably inhabit trees. We investigated if the ants on V. erioloba (i) deter insect herbivores; (ii) differ in their effectiveness depending on the identity of the herbivore; and (iii) protect the tree against an important herbivore, the larvae of the lepidopteran Gonometa postica. We show that experimental exclusion of ants leads to greater levels of herbivory on trees. The ants inhabiting V. erioloba are an effective deterrent against hemipteran and coleopteran, but not lepidopteran herbivores. Defensive services do not vary among ant species, but only Crematogaster ants exhibit aggression towards G. postica. This highlights the potential of the V. erioloba–ant mutualism for studying ant–plant interactions that involve multiple, simultaneously resident thorn‐dwelling ant species.     

Ant- and plant-mediated indirect effects induced by aphid colonization on herbivorous insects on tall goldenrod

We studied the indirect effects of an aphid Uroleucon nigrotuberculatum on density and performance of herbivorous insects through tending ants and modification of plant traits on a tall goldenrod Solidago altissima in Japan. To examine ant-mediated indirect effects of the aphid on the leafhopper and geometrid moth caterpillars, we conducted an experiment in which we manipulated aphid densities. The aphid decreased the density of these herbivorous insects through ant-mediated indirect effects, because honeydew scattered by the aphid-attracted ants that then removed them. To examine plant-mediated indirect effects of the aphid on two temporally separated insects, a scale insect and a grasshopper, we compared the density and performance of these herbivorous insects on aphid-inoculated plants and aphid-free plants. Aphid-induced plant modifications had different effects on the scale insect and grasshopper. The aphid indirectly decreased the density and survivorship of the scale insect. On the other hand, the number of grasshoppers increased as a result of the increased number of leaves and the increased nitrogen content induced by prior aphid feeding. However, aphid infestation did not affect the survival of the grasshopper. Thus, the aphid has large indirect effects on co-occurring herbivorous insects through the removal behavior of tending ants and on temporally separated herbivorous insects through changes in quality and quantity of the tall goldenrod.     

Does exogenic food benefit both partners in an ant-plant mutualism? The case of Cecropia obtusa and its guest Azteca plant-ants

In the mutualisms involving the myrmecophyte Cecropia obtusa and Azteca ovaticeps or A. alfari, both predatory, the ants defend their host trees from enemies and provide them with nutrients (myrmecotrophy). A. ovaticeps provisioned with prey and then ¹⁵N-enriched food produced more individuals than did control colonies (not artificially provisioned). This was not true for A. alfari colonies, possibly due to differences in the degree of maturity of the colonies for the chosen range of host tree sizes (less than 3 m in height). Myrmecotrophy was demonstrated for both Azteca species as provisioning the ants with ¹⁵N-enriched food translated into higher δ¹⁵N values in host plant tissues, indicating that nitrogen passed from the food to the plant. Thus, the predatory activity of their guest ants benefits the Cecropia trees not only because the ants protect them from defoliators since most prey are phytophagous insects but also because the plant absorbs nutrients.     

Raiding behavior of the dulotic antChalepoxenus muellerianus (Finzi) in the field (Hymenoptera: Formicidae,Myrmicinae)

Summary Little information is available regarding the raiding behavior in nature of dulotic ants belonging to the tribeLeptothoracini. Between July 17 and 24, 1991, several raids ofChalepoxenus muellerianus (Finzi) were observed in nature near Tignale/Lago di Garda, Italy. Apparently, this species raids frequently during its summer raiding season (5 raids were observed during 7 days spent observing 8 colonies). A singleChalepoxenus colony sometimes raids more than one host colony more or less simultaneously. Observations during which oneChalepoxenus colony raided another and captured slavemaker brood indicate that intraspecific raids can occur either accidentally, or as a result of competition or territoriality when there is a sufficient dense slavemaker population.     

Host plant influence on chemical defense in conifer sawflies (Hymenoptera: Diprionidae)

Host diet affects the defensive efficacy of Neodiprion sawflies. In laboratory assays with wood ants (Formica obscuripes), secretions from larvae reared on Pinus banksiana were the most repellent, while those from P. resinosa feeders were the least so. This was explained diterpene resin acid content, but not total monoterpene content. The terpene content of regurgitant generally reflected dietary concentrations. Compounds were sequestered nonselectively by larvae. Host-based differences in defense persisted at the behavioral level. P. banksiana feeders regurgitated greater volumes of fluid and were less likely to be disabled or killed by ants in one-on-one interactions than were larvae fed on P. resinosa. The defensive advantages of host diet conflicted with developmental requirements. N. sertifer reared on P. banksiana (the best diet for defense in all cases) had lower cocoon weights (a correlate of fecundity) than those reared on other diets, and had prolonged larval development compared to insects fed P. sylvestris. No such tradeoff was detected in N. lecontei. Larvae of both species strongly preferred P. banksiana over P. resinosa in feeding choice assays.     

Behavior of ergatoid males in the ant,Cardiocondyla nuda

Summary Ergatoid males of the ant,Cardiocondyla nuda, attack and frequently kill young males during or shortly after eclosion. Smaller colonies therefore contain typically only one adult male, which may inseminate all alate queens which are reared in the colony over a few weeks. In larger colonies, several males may be present, however, fighting among adult males was not observed. We discuss the significance of male fighting behavior in ants.     

Coordinated group ambush: A new predatory behavior inAzteca ants (Dolichoderinae)

Summary This article describes a new technique used by ants (Azteca cf. lanuginosa) in the Brazilian Cerrado to capture large mobile insects. Large numbers of ants position themselves along a leaf margin in sit-and-wait groups and ambush prey by simultaneously attacking insects that land on the upper surface of the leaf. The workers are able to seize insects such as beetles, bees, and butterflies, most of them being two times longer than the ants.