Ants modulate stridulatory signals depending on the behavioural context.

Insect societies require an effective communication system to coordinate members' activities. Although eusocial species primarily use chemical communication to convey information to conspecifics, there is increasing evidence suggesting that vibroacoustic communication plays a significant role in the behavioural contexts of colony life. In this study, we sought to determine whether stridulation can convey information in ant societies. We tested three main hypotheses using the Mediterranean ant Crematogaster scutellaris: (i) stridulation informs about the emitter'caste; (ii) workers can modulate stridulation based on specific needs, such as communicating the profitability of a food resource, or (iii) behavioural contexts. We recorded the stridulations of individuals from the three castes, restrained on a substrate, and the signals emitted by foragers workers feeding on honey drops of various sizes. Signals emitted by workers and sexuates were quantitatively and qualitatively distinct as was stridulation emitted by workers on different honey drops. Comparing across the experimental setups, we demonstrated that signals emitted in different contexts (restraining vs feeding) differed in emission patterns as well as certain parameters (dominant frequency, amplitude, duration of chirp). Our findings suggest that vibrational signaling represents a flexible communication channel paralleling the well-known chemical communication system.

Speed and accuracy in nest-mate recognition: a hover wasp prioritizes face recognition over colony odour cues to minimize intrusion by outsiders.

Social insects have evolved sophisticated recognition systems enabling them to accept nest-mates but reject alien conspecifics. In the social wasp, Liostenogaster flavolineata (Stenogastrinae), individuals differ in their cuticular hydrocarbon profiles according to colony membership; each female also possesses a unique (visual) facial pattern. This species represents a unique model to understand how vision and olfaction are integrated and the extent to which wasps prioritize one channel over the other to discriminate aliens and nest-mates. Liostenogaster flavolineata females are able to discriminate between alien and nest-mate females using facial patterns or chemical cues in isolation. However, the two sensory modalities are not equally efficient in the discrimination of 'friend' from 'foe'. Visual cues induce an increased number of erroneous attacks on nest-mates (false alarms), but such attacks are quickly aborted and never result in serious injury. Odour cues, presented in isolation, result in an increased number of misses: erroneous acceptances of outsiders. Interestingly, wasps take the relative efficiencies of the two sensory modalities into account when making rapid decisions about colony membership of an individual: chemical profiles are entirely ignored when the visual and chemical stimuli are presented together. Thus, wasps adopt a strategy to 'err on the safe side' by memorizing individual faces to recognize colony members, and disregarding odour cues to minimize the risk of intrusion from colony outsiders.

High Varroa mite abundance influences chemical profiles of worker bees and mite-host preferences.

Honeybee disappearance is one of the major environmental and economic challenges this century has to face. The ecto-parasitic mite Varroa destructor represents one of the main causes of the worldwide beehive losses. Although halting mite transmission among beehives is of primary importance to save honeybee colonies from further decline, the natural route used by mites to abandon a collapsing colony has not been extensively investigated so far. Here, we explored whether, with increasing mite abundance within the colony, mites change their behaviour to maximize the chances of leaving a highly infested colony. We show that, at low mite abundance, mites remain within the colony and promote their reproduction by riding nurses that they distinguish from foragers by different chemical cuticular signatures. When mite abundance increases, the chemical profile of nurses and foragers tends to overlap, promoting mite departure from exploited colonies by riding pollen foragers.

Top-down sequencing of Apis dorsata apamin by MALDI-TOF MS and evidence of its inactivity against microorganisms.

Apis mellifera venom is one of the best characterized venoms among Hymenoptera, yet relatively little is known about venom belonging to other species in the genus Apis. Melittin, one of the most important bioactive peptides, has been isolated and characterized in A. mellifera, Apis cerana, Apis dorsata and Apis florea, while apamin has been only characterized in A. mellifera and A. cerana. At present, no information is available about the sequence of A. dorsata apamin. Moreover, while the antiseptic properties of melittin and MCD peptides are well documented, the antimicrobial activity of apamin has never been tested. In the present study, we isolated and characterized apamin from the venom of the giant honeybee A. dorsata. We tested the activity of apamin against bacteria and yeasts in a microbiological assay to gain a more complete understanding of the antimicrobial competence of the medium molecular weight venom fraction. We show that A. dorsata apamin toxin has the same primary sequence as apamin in A. mellifera and A. cerana, yet with a different C-terminal amidation. We did not find any antiseptic activity of apamin against any of the tested microorganisms. We discuss the evolutionary processes connected to the ecological context of venom use that drove the generation of Apis venom complexity.

Hydrocarbon rank signatures correlate with differential oophagy and dominance behaviour in Polistes dominulus foundresses.

Social life offers animals increased fitness opportunities. However, the advantages are not evenly distributed and some individuals benefit more than others. The ultimate advantage of reaching the highest rank in a dominance hierarchy is the achievement of reproduction monopoly. In social insects, dominant individuals and queens keep their reproductive control through differential oophagy of unwanted eggs (egg policing). Egg recognition is the main proximate mechanism for maintaining reproductive dominance. In the social wasp Polistes dominulus, subordinate queens often lay eggs in the presence of the dominant individual. Combining gas chromatography-mass spectrometry (GC-MS) analysis and laboratory bioassays, we found that chemical differences between eggs of subordinate and dominant foundresses can explain the differential success in oophagy enjoyed by dominant individuals. We propose that dominance behaviour is an investigative behaviour as well as a ritualized agonistic behaviour. In fact, the frequency of dominance acts increases with the chemical similarity of the surfaces of dominant- and subordinate-laid eggs. Therefore, dominant individuals probably perform dominance behaviour to test the cuticular signatures of subordinates and so better assess the chemical profiles of subordinate eggs. Finally, we provide evidence that in particular social contexts, subordinate Polistes foundresses can develop ovaries as large as those of dominant individuals but nevertheless lay very few eggs. The subordinates probably lay a limited number of eggs to avoid unnecessary energy loss, as a result of efficient queen policing, but will start laying eggs as soon as the queen fails.

On status badges and quality signals in the paper wasp Polistes dominulus: body size, facial colour patterns and hierarchical rank.

To establish a dominance order, social animals often rely on indicators of fighting to avoid costly aggressive encounters. In some species, individuals use colour patterns to signal their social status. Recent studies claimed that facial markings in the eusocial paper wasp Polistes dominulus are status badges that allow co-foundresses to form a linear hierarchy based on individual quality. Here, we evaluated facial patterns in natural populations of P. dominulus, in its native range, to observe whether the marks reflect overall wasp quality in different contexts. We used the same measures of clypeus patterns used by earlier studies, but did not find that they functioned as status badges. Our analyses showed no evidence that visual markers are related to: (i) size, (ii) probability of surviving winter, (iii) social rank in spring associations, or (iv) health status (assessed by the presence of strepsipteran endoparasites). Size, however, is important. Larger wasps are more likely to survive the winter and to acquire the dominant position in spring associations. Larvae infected with endoparasites become smaller adult wasps. These findings suggest that body size is a reliable quality indicator on which wasps build their social networks, and that clypeus patterning is not involved.

Comparative analysis of epicuticular lipid profiles of sympatric and allopatric field populations of Anopheles gambiae s.s. molecular forms and An. arabiensis from Burkina Faso (West Africa).

We analysed by gas chromatography-mass spectrometry (GC-MS) and Gas Chromatography-Flame Ionization Detector (GC-FID) the epicuticular lipid profiles of field females of the major Afro-tropical malaria vector, Anopheles gambiae. The samples were collected in three villages in Burkina Faso (West Africa), where An. gambiae M and S molecular forms and An. arabiensis live sympatrically. The aim was to compare the cuticular hydrocarbon (CHC) composition of individual field specimens of these three taxa, to highlight possible differences among them. All the samples analysed by GC-MS (55 individuals and eight pools) were characterized by the same 48 CHCs and 10 oxygenated compounds. The 19 most abundant CHCs were quantified in 174 specimens by GC-FID: quantitative intra-taxon differences were found between allopatric populations of both An. arabiensis and S-form. Inter-taxa quantitative differences in the relative abundances of some hydrocarbons between pairs of sympatric taxa were also found, which appear to be mainly linked to local situations, with the possible exception of diMeC(35) between An. arabiensis and S-form. Moreover, MeC(29) shows some degree of differentiation between S- and M-form in all three villages. Possible causes of these differences are discussed.

Behaviour and chemical signature of pre-hibernating females of Polistes dominulus infected by the strepsipteran Xenos vesparum.

Polistes dominulus are social wasps which are the host of the strepsipteran endoparasite Xenos vesparum. In the hibernating phase, unparasitized and parasitized wasps leave natal nests and aggregate together in sheltered quarters. In aggregations, wasps are socially active, and some individuals perform helping behaviour. Here we investigated if castrated parasitized wasps perform worker tasks in mixed aggregations. Moreover, by gas chromatography and mass spectrometry, we examined the cuticular hydrocarbons of unparasitized and parasitized wasps to evaluate if the infection alters the composition of cuticular waxes that are recognition cues in social insects. In clusters, infected females do not perform helping behaviour and they are less active than unparasitized wasps. Cuticular hydrocarbons are slightly differentiated between unparasitized and parasitized wasps but, generally, unparasitized wasps are more similar to wasps infected by Xenos females compared to wasps infected by Xenos males. Wasps infected by Xenos males do not usually survive the winter. This chemical similarity is probably a consequence of the similar physiological condition of unparasitized and female-affected Polistes wasps. At this stage, it is difficult to affirm whether these modifications are a true parasite manipulation or a consequence of infection.

European Polistes venom allergy.

The American Polistes species venom mixture--that of P. annularis, P. fuscatus, P. metricus and P. exclamans--was the only commercially available mixture for diagnosis and therapy until 1996. However, these species of Polistes are not present in Europe, where P. dominulus and P. gallicus and to a lesser extent P. nimphus are widespread. The aim of this study was to assess the allergenic differences among the commercial American mix, P. dominulus and P. gallicus venom in European patients and therefore to verify if this mixture is suitable for diagnosis in these patients. We carried out skin tests, radioallergosorbent tests (RAST) and RAST inhibition in Italian patients with adverse reactions to Polistes stings. RAST inhibition results demonstrated that cross-reactivity between the American and European species is only partial and that P. dominulus and P. gallicus venoms have exclusive allergens. Skin tests and direct RAST confirmed these results and also showed that European Polistes venom is more suitable than the American mix in Italian patients. Moreover, we found a high rate of cross-reactivity between P. dominulus and P. gallicus. To conclude, P. dominulus and/or P. gallicus venoms are necessary for diagnosis and therefore in the therapy of European patients.

Social paper wasps as bioindicators: a preliminary research with Polistes dominulus (Hymenoptera Vespidae) as a trace metal accumulator.

The wasps of the genus Polistes (paper wasps), have a worldwide distribution and are widespread in human-built areas. Like other social wasps, they are at the top of food chains and are therefore exposed to the dangers of biomagnification, given that the larvae are fed predominantly with prey that consist of herbivorous insects. The larval faeces, larval fecal masses, in the form of a semi-solid ball, are made up of the residues of the diet of the larva, which are emitted and compressed on the floor of the cell during the larval metamorphosis. Larval fecal masses may accumulate lead (up to 36 times with respect to the adult body), therefore they were used as substrate for the analysis. From the analysis of sample nests of Polistes dominulus in various sites of the urban area of Florence, it emerges that the larval fecal masses are an analytical substrate with which it is possible to distinguish zones with differing degrees of lead pollution. The lead concentration measured in the larval fecal masses turns out to be directly correlated with vehicle traffic density, the main lead source in Florence when the survey was carried out. The notable increase in the lead concentration of larval fecal masses from the rural to the urban nest (11.15 times), in contrast with the much more limited level of pupae (4.39 times), seems to indicate the efficiency of the excretion and/or barrier mechanisms. These wasps seem to be a promising species for biomonitoring lead pollution in order to better understand its dynamics in anthropic ecosystems after the progressive diffusion of unleaded gasoline.

The social parasite wasp Polistes atrimandibularis does not form host races.

Parasites that exploit the parental behaviour of several host species may be selected to form distinct host-specific genetic lineages. This process is well documented in bird brood parasites, but not in insect social parasites. Polistes atrimandibularis is the only paper-wasp social parasite known to exploit four host species. It does not form genetically distinct host races according to analyses based on microsatellite loci. Also, there were no size-matching between parasites and host species. Instead, P. atrimandibularis queens seemed to be successful as parasites in this population only when they originated from nests of P. dominulus, the largest species. The other host species are a sink for P. atrimandibularis since adult females emerging from those nests appear too small to usurp colonies themselves. Traits that may help P. atrimandibularis infiltrate multiple species may include its nonaggressive usurpation tactics and its ability to acquire host cuticular hydrocarbon recognition labels.

Mating of Xenos vesparum (Rossi) (Strepsiptera, Insecta) revisited.

The controversial mating of the strepsipteran Xenos vesparum was studied to investigate the possible sperm routes for fertilization. The female, which is a neotenic permanent endoparasite of Polistes wasps, extrudes only its anterior region, the "cephalothorax," from the host abdomen. This region has an opening where both mating and larval escape occur. Observations with scanning and transmission electron microscopy revealed spermatozoa not only in the hemocoel, but also in the "ventral canal" (an extragenital duct peculiar to strepsipteran females) and in the "genital ducts" (ectodermal invaginations connecting the ventral canal to the hemocoel) of recently mated females. Xenos vesparum spermatozoa can reach the oocytes either through the hemocoel as a result of a hypodermic insemination, or by moving along the extragenital ducts, which are later used by first instar larvae to escape. The hypothesis of hypodermic insemination is reconsidered in the light of behavioral and ultrastructural evidence.

Expression of odorant-binding proteins and chemosensory proteins in some Hymenoptera.

The expression of chemosensory proteins (CSPs) and odorant-binding proteins (OBPs) in individuals of different castes and ages have been monitored in three species of social hymenopterans, Polistes dominulus (Hymenoptera, Vespidae), Vespa crabro (Hymenoptera, Vespidae) and Apis mellifera (Hymenoptera, Apidae), using PCR with specific primers and polyclonal antibodies. In the paper wasp P. dominulus, OBP is equally expressed in antennae, wings and legs of all castes and ages, while CSP is often specifically present in antennae and in some cases also in legs. In the vespine species V. crabro CSP is antennal specific, while OBP is also expressed in legs and wings. The three CSPs and the five OBPs of A. mellifera show a complex pattern of expression, where both classes of proteins include members specifically expressed in antennae and others present in other parts of the body. These data indicate that at least in some hymenopteran species CSPs are specifically expressed in antennae and could perform roles in chemosensory perception so far assigned only to OBPs.

Dynamics of chemical mimicry in the social parasite wasp Polistes semenowi (Hymenoptera: Vespidae).

Chemical cues are so important in the recognition mechanism of social insects that most social parasites (which rely on hosts to rear their brood) have been documented as overcoming the mechanism by which colony residents recognize non-nestmates, by mimicking the odour of the usurped colony. We simulated in the laboratory the process by which the obligate social parasite, Polistes semenowi, invades nests of the host species, Polistes dominulus, in the field and analysed the epicuticular lipid layer before and after host nest usurpation. The experiment documents that P. semenowi social parasites have an epicuticular hydrocarbon pattern which is very similar to that of their host but, after entering host colonies, parasites mimic the odour of the colonies they invade, to the point that they perfectly match the hydrocarbon profile peculiar to the colony they entered. However, both before and after host nest invasion, parasites show a tendency to possess diluted recognition cues with respect to their hosts.

Fine structure of the Nassonow's gland in the neotenic endoparasitic of female Xenos vesparum (Rossi) (Strepsiptera, Insecta).

Nassonow's gland consists of a number of cells with ducts that open on to the ventral surface of the brood canal in the cephalothoracic region of a neotenic female strepsipteran. The structural organization of the gland is reminiscent of the class 3 of the epidermal gland cells as defined by Noirot and Quennedey [Ann. Rev. Entomol. 19 (1974) 61], which consists of secretory and duct forming cells. The ultrastructure of the Nassonow's gland is described in female Xenos vesparum (Rossi) parasitic in the social wasp Polistes dominulus Christ. The large secretory cells are clustered in groups of three to four, rich in smooth endoplasmic reticulum and produce a secretion made up of lipids. In young females, just before mating, the ultrastructure of the cells and their inclusions indicate that they are active. In old-mated females the Nassonow's gland degenerates. Microvilli line an extracellular cavity and there are pores present in the irregularly thick cuticle of the efferent duct. The small duct forming cells, intermingle with epidermal cells, overlap secretory cells and produce a long efferent duct, the cuticle of which becomes thick close to its opening in the brood canal. Nassonow's gland could be the source of a sex pheromone, which might be capable of attracting the free-living male to a permanently endoparasitic female.

Variation in cuticular hydrocarbon signatures, hormonal correlates and establishment of reproductive dominance in a polistine wasp.

In many social insects the relationship between reproductive dominance and physiological correlates is poorly understood. Recent evidence now strongly suggests that cuticular hydrocarbons are important in reproductive differentiation in these societies where they are used as signals of ovarian activity in reproductive females. In this study we investigated the relationship between reproductive dominance, size of the corpora allata (CA, producer of Juvenile Hormone, JH) and the proportions of cuticular hydrocarbons present on the cuticle in overwintering foundresses and both associative (polygynous) and solitary (monogynous) pre-emergence colonies of the social wasp Polistes dominulus. Size of the CA was positively correlated with ovarian development in polygynous colonies. In contrast, solitary foundresses possessed significantly smaller CAs than dominant foundresses from polygynous nests, yet ovarian activity was similar for both female types. CA size variation was associated with variation in cuticular hydrocarbon proportions. Overwintering, solitary, dominant and subordinate (from associative nests) females all possessed distinctive cuticular chemical profiles revealed by multivariate discriminant analyses. Our data indicate that the social environment strongly affects reproductive physiology in this wasp, and we discuss the role of cuticular hydrocarbons in reproductive signaling in P. dominulus and other social insects.

Rank integration in dominance hierarchies of host colonies by the paper wasp social parasite Polistes sulcifer (Hymenoptera, Vespidae).

In multiple-foundress nests of the wasp Polistes dominulus, dominance hierarchies are established among foundresses, and only the dominant (=alpha) individual lays eggs. The alpha female can be distinguished from subordinate females and workers on the basis of the proportions of some hydrocarbons present on the cuticle, suggesting that chemical signaling of her reproductive status could occur. P. dominulus is also the host species of the obligate social parasite Polistes sulcifer. After aggressively usurping host colonies and behaviorally replacing the host alpha female, parasites are characterized by a change in the proportions of their cuticular hydrocarbons to match that of the host cuticular profile at both species and colony levels. In the current study, we demonstrate that P. sulcifer queens also modify their cuticular hydrocarbon proportions after usurpation to match that of the host alpha female. Parasite females, therefore, acquire the dominant rank in host colonies both reproductively and chemically by mimicking the typical alpha profile of the host. Parasite females were not able to fully inhibit ovary development in host foundresses, and 10 days after usurpation, parasites, alpha and beta foundresses show similar chemical profiles and ovarian development.

Worker interests and male production in Polistes gallicus, a Mediterranean social wasp.

The resolution of social conflict in colonies may accord with the interests of the most numerous party. In social insect colonies with single once-mated queens, workers are more closely related to the workers' sons than they are to the queens' sons. Therefore, they should prefer workers to produce males, against the queen's interests. Workers are capable of producing males as they arise from unfertilized eggs. We found Polistes gallicus to have colonies of single, once-mated queens, as determined by microsatellite genotyping of the workers, so worker interests predict worker male production. In colonies lacking queens, workers produced the males, but not in colonies with original queens. Thus worker interests were expressed only when the queen was gone. The high fraction of missing queens and early end to the colony cycle relative to climate so early in the season is surprising and may indicate a forceful elimination of the queen.