Origin of caulinary ant domatia and timing of their onset in plant ontogeny: evolution of a key trait in horizontally transmitted ant-plant symbioses

Many ant plants possess caulinary domatia, hollow and usually swollen stems. What are the evolutionary origins of this key trait of ant‐plant symbioses? In a single lineage, myrmecophytes often differ in the timing of the first appearance of domatia. What processes have led to diversification in the timing of expression of domatia in ontogeny? We suggest that an approach based on the analysis of leaf‐ stem size correlations, that appear general in trees, can supply answers to both these questions. Traits associated with increased primary diameter of twigs may have facilitated the evolution of domatia. Among lineages, differences in stem diameter may help to explain why domatia appeared in some, and not in others. Within species, because twig primary diameter increases over plant ontogeny, initially ants may have colonized only plants at later stages of development, whose twigs had reached a minimum size. We thus postulate that expression of domatia later in development is the primitive condition in lineages with domatia, and that increasing specialization of ants and plants enhanced both the probability of establishment and ant protection, favouring precocity of onset of domatia and other myrmccophytic characters. In the language of heterochrony, these characters are affected by prc‐displacement.     

The fitness consequences of bearing domatia and having the right ant partner: experiments with protective and non-protective ants in a semi-myrmecophyte

The fitness advantage provided by caulinary domatia to myrmecophytes has never been directly demonstrated because most myrmecophytic species do not present any individual variation in the presence of domatia and the removal of domatia from entire plants is a destructive process. The semi-myrmecophytic tree, Humboldtia brunonis (Fabaceae: Caesalpinioideae), is an ideal species to investigate the selective advantage conferred by domatia because within the same population, some plants are devoid of domatia while others bear them. Several ant species patrol the plant for extra-floral nectar. Fruit production was found to be enhanced in domatia-bearing trees compared to trees devoid of domatia independent of the ant associate. However, this domatium effect was most conspicuous for trees associated with the populous and nomadic ant, Technomyrmex albipes. This species is a frequent associate of H. brunonis, inhabiting its domatia or building carton nests on it. Ant exclusion experiments revealed that T. albipes was the only ant to provide efficient anti-herbivore protection to the leaves of its host tree. Measures of ant activity as well as experiments using caterpillars revealed that the higher efficiency of T. albipes was due to its greater patrolling density and consequent shorter lag time in attacking the larvae. T. albipes also provided efficient anti-herbivore protection to flowers since fruit initiation was greater on ant-patrolled inflorescences than on those from which ants were excluded. We therefore demonstrated that caulinary domatia provide a selective advantage to their host-plant and that biotic defence is potentially the main fitness benefit mediated by domatia. However, it is not the sole advantage. The general positive effect of domatia on fruit set in this ant–plant could reflect other benefits conferred by domatia-inhabitants, which are not restricted to ants in this myrmecophyte, but comprise a large diversity of other invertebrates. Our results indicate that mutualisms enhance the evolution of myrmecophytism.     

Geographical variation in an ant–plant interaction correlates with domatia occupancy,local ant diversity,and interlopers

Interactions between potentially mutualistic partners can vary over geographic areas. Myrmecophytes, which are plants harbouring ants, often do not exhibit sufficient intraspecific variability to permit comparative studies of myrmecophytic traits over space or time. Humboldtia brunonis (Fabaceae), a dominant, endemic myrmecophyte of the Indian Western Ghats, is unique in exhibiting considerable variability in myrmecophytic traits, e.g. domatia presence, as well as domatia occupancy and associated ant diversity throughout its geographic range. Although its caulinary domatia are occupied by at least 16 ant species throughout its distribution, young leaves and floral buds producing extrafloral nectar (EFN) are protected by ants from herbivory only in the southernmost region, where Technomyrmex albipes (Dolichoderinae) is the most abundant ant species. The extent of protection by ants was positively related to local species richness of ants and their occupancy of domatia. On the other hand, the highest abundance of interlopers in the domatia, including non‐protective ants, the arboreal earthworm Perionyx pullus, and other invertebrates, occurred in sites with the least protection from herbivory by ants. Whereas domatia morphometry did not vary between sites, domatia occupied by protective ants and invertebrate interlopers were longer and broader than empty ones at all sites. The lowest percentage of empty domatia was found at the southernmost site. There was a progressive decline in ant species richness from that found at the sites, to that feeding on H. brunonis EFN, to that occupying domatia, possibly indicating constraints in the interactions with the plants at various levels. Our study of this dominant myrmecophyte emphasizes the impact of local factors such as the availability of suitable ant partners, domatia occupancy, and the presence of interlopers on the emergence of a protection mutualism between ants and plants. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 538–551.     

Arboreal thorn-dwelling ants coexisting on the savannah ant-plant, Vachellia erioloba, use domatia morphology to select nest sites

Nest site selection in arboreal, domatia-dwelling ants, particularly those coexisting on a single host plant, is little understood. To examine this phenomenon we studied the African savannah tree Vachellia erioloba, which hosts ants in swollen-thorn domatia. We found four ant species from different genera (Cataulacus intrudens, Tapinoma subtile, Tetraponera ambigua and an unidentified Crematogaster species). In contrast to other African ant plants, many V. erioloba trees (41 % in our survey) were simultaneously co-occupied by more than one ant species. Our study provides quantitative field data describing: (1) aspects of tree and domatia morphology relevant to supporting a community of mutualist ants, (2) how ant species occupancy varies with domatia morphology and (3) how ant colony size varies with domatia size and species. We found that Crematogaster sp. occupy the largest thorns, followed by C. intrudens, with T. subtile in the smallest thorns. Thorn age, as well as nest entrance hole size correlated closely with ant species occupant. These differing occupancy patterns may help to explain the unusual coexistence of three ant species on individual myrmecophytic trees. In all three common ant species, colony size, as measured by total number of ants, increased with domatia size. Additionally, domatia volume and species identity interact to predict ant numbers, suggesting differing responses between species to increased availability of nesting space. The proportion of total ants in nests that were immatures varied with thorn volume and species, highlighting the importance of domatia morphology in influencing colony structure.     

Size matters: nest colonization patterns for twig‐nesting ants

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A canopy dominant ant affects twig‐nesting ant assembly in coffee agroecosystems

One commonly studied driver of community assembly is the effect of dominant species on subordinate species. Dominant species may impact community assembly during competitive sorting, or recruitment. For ants, important and abundant species in the tropics, several factors may drive community assembly including competition, dispersal, priority effects, and environmental conditions. Although competition is a hallmark of ant ecology, few have examined the influence of patchily distributed dominant ants on other ant species and diversity, especially at the recruitment stage. Here, I consider the impacts of a canopy dominant ant species, Azteca instabilis, and changes in vegetation on twig‐nesting ant colony founding and ant community assembly in a coffee agroecosystem. I added artificial nests to coffee plants in areas with and without A. instabilis four times over a year, and then examined the occupation rate and identity of species colonizing nests. I also examined vegetation characteristics of sites where nests were added. The presence of A. instabilis on coffee plants drastically lowered colonization rates, but nest occupation increased with tree density, and with decreasing proportion of Inga spp. trees in the canopy. The presence of A. instabilis limited the number of nests occupied by six of the ten most common species; most rare species, however, were not affected by A. instabilis presence. Richness of colonizing ants in areas with A. instabilis was lower, but these effects did not significantly affect richness across broader scales. Despite large effects on individual species, species composition did not differ greatly in areas with and without A. instabilis, but some vegetation characteristics (basal area and tree richness) were predictive of ant composition. These results suggest that A. instabilis strongly affects founding events especially for common twig‐nesting species and that both vegetation and influences from this dominant species affect community assembly of twig‐nesting ants at the local scale.     

Strong influences of a dominant,ground‐nesting ant on recruitment,and establishment of ant colonies and communities

Many factors drive the organization of communities including environmental factors, dispersal abilities, and competition. In particular, ant communities have high levels of interspecific competition and dominance that may affect community assembly processes. We used a combination of surveys and nest supplementation experiments to examine effects of a dominant ground‐nesting ant (Pheidole synanthropica) on (1) arboreal twig‐nesting, (2) ground‐foraging, and (3) coffee‐foraging ant communities in coffee agroecosystems. We surveyed these communities in high‐ and low‐density areas of P. synanthropica over 2 years. To test for effects on twig ant recruitment, we placed artificial nesting resources on coffee plants in areas with and without P. synanthropica. The first sampling period revealed differences in ant species composition on the ground, in coffee plants, and artificial nests between high‐ and low‐density sites of P. synanthropica. High‐density sites also had significantly lower recruitment of twig ants and had species‐specific effects on twig ant species. Prior to the second survey period, abundance of P. synanthropica declined in the high‐density sites, such that P. synanthropica densities no longer differed. Subsequent sampling revealed no difference in total recruitment of twig ants to artificial nests between treatments. Likewise, surveys of ground and coffee ants no longer showed significant differences in community composition. The results from the first experimental period, followed by survey results after the decline in P. synanthropica densities suggest that dominant ants can drive community assembly via both recruitment and establishment of colonies within the community.     

Arboreal Ant Abundance and Leaf Miner Damage in Coffee Agroecosystems in Mexico

Shaded coffee agroecosystems traditionally have few pest problems potentially due to higher abundance and diversity of predators of herbivores. However, with coffee intensification (e.g., shade tree removal or pruning), some pest problems increase. For example, coffee leaf miner outbreaks have been linked to more intensive management and increased use of agrochemicals. Parasitic wasps control the coffee leaf miner, but few studies have examined the role of predators, such as ants, that are abundant and diverse in coffee plantations. Here, we examine linkages between arboreal ant communities and coffee leaf miner incidence in a coffee plantation in Mexico. We examined relationships between incidence and severity of leaf miner attack and: (1) variation in canopy cover, tree density, tree diversity, and relative abundance of Inga spp. shade trees; (2) presence of Azteca instabilis, an arboreal canopy dominant ant; and (3) the number of arboreal twig‐nesting ant species and nests in coffee plants. Differences in vegetation characteristics in study plots did not correlate with leaf miner damage perhaps because environmental factors act on pest populations at a larger spatial scale. Further, presence of A. instabilis did not influence presence or severity of leaf miner damage. The proportion of leaves with leaf miner damage was significantly lower where abundance of twig‐nesting ants was higher but not where twig‐nesting ant richness was higher. These results indicate that abundance of twig‐nesting ants in shaded coffee plantations may contribute to maintenance of low leaf miner populations and that ants provide important ecosystem services in coffee agroecosystems.     

Association between the African lycaenid,Anthene usamba,and an obligate acacia ant,Crematogaster mimosae

The African lycaenid butterfly, Anthene usamba, is an obligate myrmecophile of the acacia ant, Crematogaster mimosae. Female butterflies use the presence of C. mimosae as an oviposition cue. The eggs are laid on the foliage and young branches of the host plant, Acacia drepanolobium. Larvae shelter in the swollen thorns (domatia) of the host tree, where they live in close association with the acacia ants, and each larva occupies a domatium singly. Anthene usamba are tended by ants that feed from the dorsal nectary organ at regular intervals. Larvae also possess tentacle organs flanking the dorsal nectary organ and appear to signal to ants by everting these structures. Larvae were observed to spend most of their time within the domatia. Stable isotope analysis of matched host plant–ant–butterfly samples revealed that Anthene usamba are δ¹⁵N enriched relative to the ants with which they associate. These data, based on the increase in δ¹⁵N through trophic levels, indicate that the caterpillars of these butterflies are aphytophagous and either exploit the ant brood of C. mimosae within the domatia, or are fed mouth to mouth by adult workers via trophallaxis. This is the first documented case of aphytophagy in African Anthene. Pupation occurs inside the domatium and the imago emerges and departs via the hole chewed by the larva. The adult females remain closely associated with their natal patch of trees, whereas males disperse more widely across the acacia savannah. Females prefer to oviposit on trees with the specific host ant, C. mimosae, an aggressive obligate mutualist, and avoid neighbouring trees with other ant species. Adult butterflies are active during most months of the year, and there are at least two to three generations each year. Observations made over a 5‐year period indicate that a number of different lycaenid species utilize ant‐acacias in East Africa, and these observations are summarized, together with comparisons from the literature. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013 , 109 , 302–312.     

Induced biotic responses to herbivory and associated cues in the Amazonian ant-plant Maieta poeppigii

Ants inhabiting ant‐plants can respond to cues of herbivory, such as the presence of herbivores, leaf damage, and plant sap, but experimental attempts to quantify the dynamic nature of biotic defenses have been restricted to a few associations between plants and ants. We studied the relationship between certain features of the ant‐shrub Maieta poeppigii Cogn. (Melastomataceae) and the presence or absence of ant patrolling on the leaf surface in plants occupied by the ant Pheidole minutula Mayr (Hymenoptera: Formicidae). We also carried out field experiments to examine ant behavior following plant damage, and the potential cues that induce ant recruitment. These experiments included clipping of the leaf apex, as well as the presentation of a potential herbivore (live termite worker) and a foliar extract from Maieta on treatment leaves. The presence of ants patrolling the leaves of M. poeppigii is influenced by the number of domatia on the plant. Ant patrolling on the leaves of M. poeppigii was constant throughout a 24 h cycle, but the mean number of patrolling ants decreased from young to mature leaves, and from leaves with domatia to those without domatia. There was an overall increase in the number of ants on experimental leaves following all treatments, compared to control leaves. Visual and chemical cues associated with herbivory are involved in the induction of ant recruitment in the Maieta–Pheidole system. The continuous patrolling behavior of ants, associated with their ability to respond rapidly to foliar damage, may result in the detection and repellence/capture of most insect herbivores before they can inflict significant damage to the leaves.     

Changes in Species Richness,Abundance, and Composition of Arboreal Twig‐nesting Ants Along an Elevational Gradient in Coffee Landscapes

The distribution, diversity, and assembly of tropical insects have long intrigued ecologists, and for tropical ants, can be affected by competitive interactions, microhabitat requirements, dispersal, and availability and diversity of nesting sites. Arboreal twig‐nesting ants are limited by the number of hollow twigs available, especially in intensive agricultural systems. Ant diversity and abundance may shift along elevation gradients, but no studies have examined if the proportion of occupied twigs or richness of arboreal twig‐nesting ants vary with elevation. In coffee agroecosystems, there are over 40 species of arboreal twig‐nesting ants. We examined communities of twig‐nesting ants in coffee plants along an elevational gradient to answer the following questions: (1) Do species richness and colony abundance decline with elevation or show a mid‐elevation peak? (2) Does community composition change with elevation? (3) Is elevation an important predictor of change in ant abundance, richness, and relative abundance of common species? We surveyed 42 10 × 10 m plots in 2013 from 450 to1550 m elevation across a coffee landscape in Chiapas, Mexico. We sampled a total of 2211 hollow coffee twigs, 77.1 percent of which were occupied by one of 28 species of ants. Pseudomyrmex simplex was more abundant in lower elevations, whereas Pseudomyrmex ejectus dominated in high elevations. Species richness and the percent of occupied hollow twigs both peaked at mid‐elevations (800–1050 m). In sum, we found that species richness, abundance, and composition of arboreal twig‐nesting ants shift with elevation. These findings may provide important insights for understanding ant communities in coffee agroecosystems.     

Reduced ant defenses in <Emphasis Type="Italic">Macaranga</Emphasis> myrmecophytes (Euphorbiaceae) infested with a winged phasmid <Emphasis Type="Italic">Orthomeria cuprinus</Emphasis>

Macaranga is a tree genus that includes many species of myrmecophytes, which are plants that harbor ant colonies within hollow structures known as domatia. The symbiotic ants (plant–ants) protect their host plants against herbivores; this defense mechanism is called ‘ant defense’. A Bornean phasmid species Orthomeria cuprinus feeds on two myrmecophytic Macaranga species, Macaranga beccariana and Macaranga hypoleuca, which are obligately associated with Crematogaster ant species. The phasmids elude the ant defense using specialized behavior. However, the mechanisms used by the phasmid to overcome ant defenses have been insufficiently elucidated. We hypothesized that O. cuprinus only feeds on individual plants with weakened ant defenses. To test the hypothesis, we compared the ant defense intensity in phasmid-infested and non-infested M. beccariana trees. The number of plant–ants on the plant surface, the ratio of plant–ant biomass to tree biomass, and the aggressiveness of plant–ants towards experimentally introduced herbivores were significantly lower on the phasmid-infested trees than on the non-infested trees. The phasmid nymphs experimentally introduced into non-infested trees, compared with those experimentally introduced into phasmid-infested trees, were more active on the plant surface, avoiding the plant–ants. These results support the hypothesis and suggest that ant defenses on non-infested trees effectively prevent the phasmids from remaining on the plants. Thus, we suggest that O. cuprinus feeds only on the individual M. beccariana trees having decreased ant defenses, although the factors that reduce the intensity of the ant defenses remain unclear.     

Extrafloral nectar as a driver of arboreal ant communities at the site‐scale in Brazilian savanna

Despite years of study, it remains unclear if and to what extent the effects of extra‐floral nectaries (EFNs) on arboreal ants observed on individual trees scale up to larger spatial scales. Here, we address this issue in Brazilian savanna and tested three predictions: (i) Trees with EFN have higher richness of arboreal ant species than trees without; (ii) Arboreal ant species richness increases with the proportion of total EFN‐bearing trees at the site scale, due to a higher occurrence of non‐core ant species; (iii) Ant species composition changes with the proportion of EFN‐bearing trees at the site scale. We sampled arboreal ants in 32 plots with EFN‐bearing trees ranging from 0% to 60% of all trees. We sampled 72 ant species, from which 17 (mostly belonging to Camponotus, Cephalotes and Crematogaster) were identified as core species in at least one of the ant‐EFN networks in the 32 plots. Ant species richness was significantly higher on EFN‐bearing trees. We identified 11 ant species that preferentially occurred on EFN‐bearing trees, all of which were core partners in networks. Species richness at the site scales increased with the proportion of EFN‐bearing trees, regardless of tree density and richness; this pattern was due to a higher occurrence of non‐core ant species. Finally, species composition also varied with the proportion of EFN‐bearing trees. Therefore, we found that the presence of EFNs not only influences arboreal ants on individual trees but also has a substantial effect on the ant‐EFN network on a broader community scale. The increase in non‐core species site scale reveals that this interaction is unlikely to result in substantially enhanced protection services for EFN‐bearing plants.     

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.     

Mutualism between Maieta guianensis Aubl., a myrmecophytic melastome,and one of its ant inhabitants: ant protection against insect herbivores

Summary The hypothesis that ants (Pheidole minutula) associated with the myrmecophytic melastome Maieta guianensis defend their host-plant against herbivores was investigated in a site near Manaus, Amazonas, Brazil. M. guianensis is a small shrub that produces leaf pouches as ant domatia. Plants whose ants were experimentally removed suffered a significant increase in leaf damage compared with control plants (ants maintained). Ants patrol the young and mature leaves of Maieta with the same intensity, presumably since leaves of both ages are equally susceptible to herbivore attack. The elimination of the associated ant colony, and consequent increase in herbivory, resulted in reduced plant fitness. Fruit production was 45 times greater in plants with ants than in plants without ants 1 year after ant removal.     

Host‐plant dissections reveal contrasting distributions of Crematogaster ants and their symbionts in two myrmecophytic Macaranga species

1. Ant–plant mutualisms are among the most widespread and ecologically important insect–plant interactions in the tropics. The multitrophic mutualism involving Macaranga plants (Euphorbiaceae) and Crematogaster ants (Formicidae) is the most diverse in Southeast Asia. This interaction also includes trophobiotic scale insects (Coccidae) and nematodes inhabiting ant refuse piles. 2. Here two myrmecophytic systems were compared, Macaranga trachyphylla with Crematogaster captiosa (Mt + Cc) and Macaranga beccariana with Crematogaster decamera (Mb + Cd), using a fine‐scale dissection of the stems. For the two plant species, for each internode, both contents (ants, coccids, refuse piles) and structure (internode height, numbers of open and occluded ant holes) were recorded. 3. There were significant patterns in the vertical distribution of ant colonies and their symbionts in the plant stems. Most coccids were kept in the highest sections of both systems, although Mb + Cd hosted a broader range of coccid species than Mt + Cc. Three nematode species were recorded, but with a rather low specificity to plant or ant species. Furthermore, the fine‐scale distribution showed aggregation of closed holes with ant brood and separation of nematode‐infested refuse piles from eggs. 4. The results of this study indicate that ants manipulate spatial colony structure via distribution of brood, holes and the symbionts. It is suggested that ants optimise the location of refuse piles and occluded holes via spatial heterogeneity in their distribution among internodes. This paper discusses the protective role of occluded holes and demonstrates some general interactions with other symbiotic fauna.     

Plant Ontogeny as a Conditionality Factor in the Protective Effect of Ants on a Neotropical Tree

The outcome of any interspecific interaction is often determined by the ecological context in which the interacting species are embedded. Plant ontogeny may represent an important source of variation in the outcome of ant–plant mutualisms, as the level of investment in ant rewards, in alternative (non‐biotic) defenses, or both, may be modulated by the plant's developmental stage. In addition, the abundance and identities of the ants involved in the interaction may change during ontogeny of the host‐plant. Here, we evaluated if plant ontogeny affects the interaction between ants and a savanna tree species (Caryocar brasiliense) that produces extrafloral nectar. We found fewer ants per branch and fewer species of ants per tree in juvenile than in reproductive trees of medium and large size. In addition, large‐sized reproductive trees were more likely to host more aggressive ants than were medium‐sized reproductive or juvenile trees. Such differences strongly affected the outcome of the interaction between ants and their host‐plants, as the magnitude of the effect of ants on herbivory was much stronger for large trees than for juvenile ones. The fact that we did not find significant ontogenetic variation in the concentration of leaf tannins suggests that the observed differences in herbivory did not result from a differential investment in chemical defenses among different‐sized plants. Overall, the results of our study indicate that the developmental stage of the host plant is an important factor of conditionality in the interaction between C. brasiliense and arboreal foraging ants.     

Effects of food rewards offered by ant–plant Macaranga on the colony size of ants

Myrmecophytes (ant–plants) have special hollow structures (domatia) in which obligate ant partners nest. As the ants live only on the plants and feed exclusively on plant food bodies, sap-sucking homopterans in the domatia, and/or the homopterans honeydew, they are suitable for the study of colony size regulation by food. We examined factors regulating ant colony size in four myrmecophytic Macaranga species, which have strictly species-specific association with Crematogaster symbiont ants. Intra- and interspecific comparison of the plants showed that the ant biomass per unit food biomass was constant irrespective of plant developmental stage and plant species, suggesting that the ant colony size is limited by food supply. The primary food offered by the plants to the ants was different among Macaranga species. Ants in Macaranga beccariana and Macaranga bancana relied on homopterans rather than food bodies, and appeared to regulate the homopteran biomass and, as a consequence, regulate the ants own biomass. In contrast, ants in Macaranga winkleri and Macaranga trachyphylla relied primarily on food bodies rather than homopterans, and the plants appeared to manipulate the ant colony size. Per capita plant investment in ants (ant dry weight plant dry weight^–1) was different among the four Macaranga species. The homoptera-dependent M. beccariana and M. bancana harbored lower biomass of ants than the food-body dependent M. winkleri, suggesting that energy loss is involved in the homoptera-interposing symbiotic system which has one additional trophic level. The plants investment ratio to the ants generally decreased as plants grew. The evolution of the plant reward-offering system in ant–plant–homopteran symbioses is discussed with an emphasis on the role of homopterans.     

Observations on the Ecology of Weaver Ants (Oecophylla smaragdina Fabricius) in a Thai Mangrove Ecosystem and Their Effect on Herbivory of Rhizophora mucronata Lam.

Ants of the genus Oecophylla are predators of other insects and are able to protect a variety of terrestrial plants against pest insects; however, observations on the ecology of these ants in mangrove forests are lacking. General observations on the ecology of Oecophylla smaragdina were carried out in a Thai mangrove forest to determine if these ants can protect their host plants in less favorable mangrove habitats. Leaf herbivory and the density of O. smaragdina ants were measured on Rhizophora mucronata trees at two sites. The results showed a negative correlation between ant density and herbivory. At both sites, the mean percent damaged leaf area was more than four times higher on trees without ants compared to “ant‐trees.” A significant negative correlation was found between tree mean percent leaf damage and the density of ants on the tree. Furthermore, on trees with ants, there was less herbivory on leaves close to ant nests compared to other leaves on the tree. Most damage was caused by chrysomelid beetles (62%) and sesarmid crabs (25%) and both types of herbivory were significantly reduced on ant‐trees.     

Plant‐Inhabiting Ant Utilizes Chemical Cues for Host Discrimination

The Neotropical ant Pseudomyrmex triplarinus is involved in an obligate and complex symbiotic association with Triplaris americana trees. The ants inhabit trunk and branch domatia and respond aggressively to foreign invaders. Their degree of host specificity and basis for recognition of host trees has not been studied. We determined that, in contrast to T. americana seedlings, heterospecific seedlings set around the host trees suffered continuous pruning. Ants also removed 80–100 percent of heterospecific leaves attached to the trunk in contrast to only 10–30 percent of conspecific leaves. True species specificity was demonstrated by the selective removal of leaves from Triplaris poeppigiana pinned to host trees. This selectivity was also observed in a matrix‐independent bioassay using leaf cuticular extracts on glass microfiber strips. Strips treated with leaf wax extracts from host trees and pinned to the trunk of host trees received only 42 percent of the number of ant visits recorded on solvent‐treated controls by the end of the experiment. Strips treated with extracts of a related species, T. poeppigiana, received 64 percent of the number of ant visits compared with solvent‐treated controls. These experiments also suggest that P. triplarinus recognizes surface chemicals of their host tree, independent of the texture or architecture of the carrier material; although these factors may still play some role in recognition. This is the first study that we are aware of to investigate the mechanism of host discrimination related to pruning behavior. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .