Donato A. Grasso

Colony founding in Polyergus rufescens: the role of the Dufour's gland.

Summary: In the European slave-making ant Polyergus rufescens, the occurrence of chemical strategies during the initial phase of dependent colony foundation or usurpation was investigated. To test this idea, we analysed the effect of the secretion of different glands (Dufours, poison, pygidial, rectal, and mandibular) on the behaviour of workers of its common host species, Formica cunicularia (subgenus Serviformica). Workers of another species, Formica rufibarbis (Serviformica), were daubed with these extracts, and introduced into colony fragments of F. cunicularia. The results of a set of laboratory aggression test showed that the secretion of the mandibular, pygidial, rectal, and poison glands do not alter the characteristic aggressive reactions generally performed by resident workers against alien ants. By contrast, the Dufours gland seems to play a crucial role in the appeasement of residents of the target host colony. In fact, its secretion drastically lowers the degree of overt attacks shown by F. cunicularia workers against the intruders. This chemical strategy probably allows an easier invasion and usurpation of host colonies by newly mated females of P. rufescens.

Thelytokous worker reproduction and lack of Wolbachia infection in the harvesting ant Messor capitatus

In hymenopteran societies, workers are not always sterile, and may produce parthenogenetically either males (arrhenotoky) or females (thelytoky). Thelytoky however is exceptional, and has been recorded in only four ant species. Here we provide evidence for worker thelytoky in an additional species, the harvesting ant Messor capitatus (Latreille) (Hymenoptera Formicidae Myrmicinae). Two orphaned colonies produced a large amount of worker offspring during 10 months in the laboratory. Dissections showed that reproduction was performed by workers and not by mated worker-like individuals (ergatoids). In some parasitoid wasps, parthenogenetic reproduction is caused by the maternally transmitted bacterium Wolbachia. Using a PCR-based assay we showed that Wolbachia can not be involved in parthenogenesis induction in this species. Finally, we point out reasons for the low Wolbachia susceptibility of parthenogenetic ant species.

Behavioural assays testing the appeasement allomone of Polyergus rufescens queens during host-colony usurpation.

The role of decyl butyrate (the main component of the secretion of Dufours gland in newly-mated queens) during host-colony usurpation was investigated in the European amazon ant Polyergus rufescens. The effect of this ester on the behaviour of workers of the common host species Formica cunicularia was analysed. We used a laboratory aggression test during which workers of the species Camponotus ligniperda, masked with synthetic decyl butyrate, were introduced into colony fragments of F. cunicularia. The results showed that this compound drastically lowers the degree of overt aggression against the introduced ants, and we suggest that it acts as an appeasement allomone. Moreover, pieces of filter-paper soaked with decyl butyrate were more attractive than pieces of paper treated with solvent. The results are discussed in relation to a chemical strategy that likely allows an easier invasion and usurpation of host colonies by queens of this obligatory slave-making ant.

The ant fauna of Castelporziano Presidential Reserve (Rome, Italy) as a model for the analysis of ant community structure in relation to environmental variation in Mediterranean ecosystems

Ants are often used as ecological indicators in environmental monitoring programs, but rarely in Europe, probably because of a lack of baseline knowledge on the relationship between ant community structure and environmental variation. We present this baseline knowledge for Italian ecosystems that are typical of the Mediterranean climate, in order to assess the value of ants as ecological indicators in this biogeographic zone. Sampling areas were selected inside the Castelporziano Natural Reserve (Rome, Italy). The elevated variety of habitats and diverse ecological conditions, as well as the different levels of human impact in the Reserve, make this area a suitable location to develop and test biological indicators of environmental quality. The ant community structure was compared among several Mediterranean ecosystems using pitfall trapping, multidimensional scaling, the indicator species analysis and the Functional Groups Approach. Key factors for differences in ant community structure seem to be the presence/absence of water, the level of light and microhabitat diversity. Our results suggest that Castelporziano is an informative area in order to develop a study model for the Italian ant fauna and the Functional Group Approach is a promising procedure to associate differences in ant community structures with differences in habitat stress and disturbance.

Mandible associated glands in queens of the slave-making ant Polyergus rufescens (Hymenoptera, Formicidae)

SummaryA first description of the structural organization of two exocrine glands associated with the mandibles of queens of the obligatory slave-making ant Polyergus rufescens is reported. The mandibular gland consists of clustered bicellular secretory units connected by means of cuticular ducts to a big reservoir. The reservoir continues in a duct that opens proximally on the mandible cuticle. Intramandibular glands are isolated bicellular secretory units connected to the external pores through a cuticular duct. No reservoir has been observed. In both cases the secretory cells belong to the 3rd class. Functional aspects of the investigated glands are discussed taking also into account for the parasitic habit of this slave-making species.

Climate mediates the effects of disturbance on ant assemblage structure

Many studies have focused on the impacts of climate change on biological assemblages, yet little is known about how climate interacts with other major anthropogenic influences on biodiversity, such as habitat disturbance. Using a unique global database of 1128 local ant assemblages, we examined whether climate mediates the effects of habitat disturbance on assemblage structure at a global scale. Species richness and evenness were associated positively with temperature, and negatively with disturbance. However, the interaction among temperature, precipitation and disturbance shaped species richness and evenness. The effect was manifested through a failure of species richness to increase substantially with temperature in transformed habitats at low precipitation. At low precipitation levels, evenness increased with temperature in undisturbed sites, peaked at medium temperatures in disturbed sites and remained low in transformed sites. In warmer climates with lower rainfall, the effects of increasing disturbance on species richness and evenness were akin to decreases in temperature of up to 9°C. Anthropogenic disturbance and ongoing climate change may interact in complicated ways to shape the structure of assemblages, with hot, arid environments likely to be at greatest risk.

Dufour’s gland contents of queens of the slave-making ant Polyergus rufescens and its host species Formica cunicularia

Dufour’s gland contents of the queens of the slave-making ant species Polyergus rufescens and its host, Formica cunicularia, were analysed by gas chromatography-mass spectrometry. In P. rufescens, the gland contains a mixture of several esters, one of which, decyl isobutyrate, was consistently present in large quantities (constituting more than 80% of glandular contents). Glandular contents in the host species, were found to be different, containing, in addition to two substances also found in P. rufescens, a mixture of acetates and several hydrocarbons. Decyl isobutyrate was not present. We discuss the possible role of these substances in the life history of P. rufescens.

Extrafloral-nectar-based partner manipulation in plant–ant relationships

Many plant-derived chemicals may have an impact on the functioning of the animal brain. The mechanisms by which the psychoactive components of these various products have their effects have been widely described, but the question of why they have these effects has been almost totally ignored. Recent evidence suggests that plants may produce chemicals to manipulate their partner ants and to make reciprocation more beneficial. In the present review we propose that these plant-derived chemicals could have evolved in plants to attract and manipulate ant behaviour; this would place the plant–animal interaction in a different ecological context and open new ecological and neurobiological perspectives for drug seeking and use.

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 Darwins hypothesis for the origin and evolution of slavery in ants.

Glandular sources of recruitment, trail, and propaganda semiochemicals in the slave-making ant Polyergus rufescens

The chemical sources involved in raiding activities of the slave-making ant Polyergus rufescens were investigated in the laboratory. In particular, we identified the sources of substances the raiding workers employ to convey information to nestmates and to disrupt reactions by defending workers during host nest invasion. Different secretions were offered to the dulotic ants on the tip of a blotting paper strip inserted a few centimetres into the nest entrance. The Dufours gland secretion induced the characteristic mass exit from the nest and is likely the source of a recruitment signal. Moreover, laboratory tests with secretions from different anatomical structures revealed that the trail signals for homing raiding workers are conveyed by their hindgut contents released during the outbound trip. Finally, to ascertain if P. rufescens raiding workers use “propaganda” pheromones, the behavioural response of the Formica (Serviformica) cunicularia host workers to different secretions of the slave-makers was investigated. Only mandibular secretions caused the typical panic reactions shown by residents at the moment of nest invasion. This probably facilitates both nest invasion and brood sacking by raiding workers.