Chemical and ethological studies of the trail pheromone of the ant Manica rubida (Hymenoptera: Formicidae)

ABSTRACT. The poison gland of Manica rubida contains nanogram quantities of four alkylpyrazines, methylpyrazine, 2,5-dimethylpyrazine, trimethylpyrazine and 3-ethyl-2,5-dimethylpyrazine, of which only the last induces pronounced trail-following behaviour. Myrmica rubra and M. rubida workers follow each other's trails equally well when allowance is made for the much greater size of M. rubida workers. The cross activity in trail-following between M. rubida, M. rubra and Tetramorium caespitum is understandable in terms of the amounts and proportions of the different pyrazines present in their glands and their responses to the synthetic substances and appropriate mixtures of them.     

Impact of Formica exsecta Nyl. on seed bank and vegetation patterns in a subalpine grassland ecosystem

The mound building ant Formica exsecta Nyl. is widely distributed in grassland ecosystems of the Central European Alps. We studied the impact of these ants on seed bank and vegetation patterns in a 11 ha subalpine grassland, where we counted over 700 active ant mounds. The mounds showed a distinct spatial distribution with most of them being located in tall‐grass, which was rarely visited by ungulates (red deer; Cervus elaphus L.). Heavily grazed short‐grass, in contrast, seemed to be completely avoided by ants as only few mounds were found in this vegetation type. The species composition of the ant mound and grassland seed banks was quite similar, i.e. from 15 common plant species 12 were found in both seed bank types. We found the same proportions of myrmecochorous seeds in ant mound and grassland soil samples. In contrast, the number of seeds was 15 times higher in mound compared with the grassland soil samples. Also, the vegetation growing on ant mounds significantly differed from the vegetation outside the mounds: graminoids dominated on ant mounds, herbaceous and myrmecochorous species in the grassland vegetation. We found significant continuous changes in vegetation composition on gradients from the ant mound centre to 1 m away from the mound edge. Overall, F. exsecta was found to have a considerable impact on seed bank and vegetation patterns in the grassland ecosystem studied. These insects not only altered grassland characteristics in the close surrounding of their mounds, but also seem to affect the entire ecosystem including, for example, the spatial use of the grassland by red deer.     

Macaranga ant-plants hide food from intruders: correlation of food presentation and presence of wax barriers analysed using phylogenetically independent contrasts

Many tropical ant-plants provide specialized ant partners with food, which may attract foreign ants parasitizing the mutualism. We present evidence for the ant-plant genus Macaranga , showing that ant competition has forced host plants to hide food resources and restrict access to the mutualists. In Macaranga myrmecophytes, the influence of ant competition strongly depends on the presence of slippery 'wax barriers'. Of all Macaranga ant-plant species, 50% have waxy stems that can be climbed only by the specific ant partners and not by other ant species. We compared the presentation of food (food bodies and extrafloral nectar) between waxy and non-waxy Macaranga host plants using traditional and phylogenetic comparative methods. Consistent with the hypothesized effect of ant competition, wax-free Macaranga host species had fewer extrafloral nectaries and more often produced food bodies under recurved or tubular stipules inaccessible to other ants; closed stipules were less persistent in waxy hosts. Several traits showed phylogenetic signal, but our finding of a more promiscuous food presentation in waxy Macaranga hosts was still supported by phylogenetic comparative analyses. We conclude that competition among ants is an important factor in the evolution of myrmecophytism, and that it has given rise to traits acting as protective filter mechanisms.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 177–193.     

The distribution of weaver ant pheromones on host trees

The visible anal spots deposited by Oecophylla smaragdina ants have been suggested to deter ant prey, affect interspecific competition and facilitate mutualists and parasites in tracking down Oecophylla ants. I measured the density of anal spots on host trees with and without ants and tested for correlations between spot density, ant activity and the likelihood of being detected by an ant. Spots were only found on trees with ants. On ant-trees, spots were distributed throughout the trees but with higher densities in areas with high ant activity and pheromone densities were higher on twigs compared to leaves. Also there was a positive correlation between spot density and the likelihood of being detected by ants. Anal spots may thus function as reliable cues to interacting species and be an important factor in shaping the community around Oecophylla colonies. Received 31 August 2006; revised 27 April 2007; accepted 2 May 2007.     

Queen execution in the Argentine ant, Iridomyrmex humilis

ABSTRACT. Field censuses and laboratory experiments show that in the Argentine ant, Iridomyrmex humilis (Mayr), c. 90% of the queens are executed by workers in May, at the beginning of the reproductive season. The reduction in the number of queens probably decreases the inhibition exerted by queens on the differentiation of sexuals and thus allows the production of new queens and males shortly thereafter. In the laboratory, there was no correlation between the percentage of queens executed and their weight or fecundity. At the time of execution of queens, nearly all queens were of the same age; less than 1 year. Therefore it is not likely that the age of queens plays any role in the choice that workers make in the queens they executed. Execution of these queens results in a heavy energetic cost for the colony which amounts c. 8% of the total biomass. This behaviour of workers executing nestmate queens is discussed with regard to possible evolutionary significance at the queen and worker level.     

Establishment of chasmogamous and cleistogamous seedlings of an ant-dispersed UNDERSTORY HERB,CALATHEA MICANS (Marantaceae)

This study compares the establishment success of cleistogamous (CL, obligately selfed) and chasmogamous (CH, potentially outcrossed) seeds of the neotropical understory herb, Calathea micans (Marantaceae). In this species, CH and CL seeds are morphologically identical and are both dispersed by ants. I measured dispersal distances and analyzed seedling recruitment and seedling size for ant-planted CH and CL seeds, a protocol made possible by the fact that the seed coat of C. micans can be indelibly marked and remains attached to the base of the seedling for a few months. Seeds were taken by ants from experimental depots placed in natural conditions. In a second experiment, I planted CH and CL seeds in gap and understory sites to determine the effects of seed type and light on germination percentage, date of emergence, and seed and seedling survivorship. Ant-planted CH and CL seedlings did not differ in dispersal distance. CH and CL seeds did not differ significantly in recruitment probability and seedling size in either experiment. However, for the ant-planted seedlings, CH recruitment was higher in the understory than in the gap, while CL recruitment was uniform across light environments. In conclusion, I found a difference in the establishment success of CH and CL seeds in this understory herb, only after dispersal by ants in field conditions.     

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.     

Interaction between ants, extrafloral nectaries and insect herbivores in Neotropical coastal sand dunes: herbivore deterrence by visiting ants increases fruit set in Opuntia stricta (Cactaceae)

1. This study examines the anti-herbivore effect of ants visiting the extrafloral nectaries (EFNs) of Opuntia stricta (Cactaceae) and its possible influence on the plant's reproductive output in Mexican coastal sand dunes. Opuntia 's EFNs are located in the areoles of the developing tissue of emerging cladodes and flower buds.
2. Ants visited the EFNs of O . stricta on a round-the-clock basis. The associated ant assemblage was formed by nine species distributed in four subfamilies, and the species composition of the principal ant visitors changed markedly from day to night period.
3. Cladodes of control (ants present) and treatment (ants excluded) plants of Opuntia were equally infested by sucking bugs and mining dipterans. Damage to buds by a pyralid moth, however, was significantly higher on treatment than on control plants. Ant visitation to Opuntia 's EFNs translated into a 50% increase in the plant's reproductive output, as expressed by the number of fruits produced by experimental control and treatment branches. Moreover, fruit production by ant-visited branches was positively and significantly associated with the mean monthly rate of ant visitation to EFNs.
4. This is the first demonstration of ant protection leading to increased fruit set in the Cactaceae under natural conditions. Although the consequences of damage by sucking and mining insects remain unclear for Opuntia , the results show how the association of EFNs with vulnerable reproductive plant organs can result in a direct ant-derived benefit to plant fitness.