Rita Cervo

Unrelated helpers in a social insect

High-resolution genetic markers have revolutionized our understanding of vertebrate mating systems, but have so far yielded few comparable surprises about kinship in social insects. Here we use microsatellite markers to reveal an unexpected and unique social system in what is probably the best-studied social wasp, Polistes dominulus. Social insect colonies are nearly always composed of close relatives; therefore, non-reproductive helping behaviour can be favoured by kin selection, because the helpers aid reproductives who share their genes. In P. dominulus, however, 35% of foundress nestmates are unrelated and gain no such advantage. The P. dominulus system is unlike all other cases of unrelated social insects, because one individual has nearly complete reproductive dominance over subordinates who could have chosen other reproductive options. The only significant advantage that subordinates obtain is a chance at later reproduction, particularly if the queen dies. Thus, P. dominulus societies are functionally unlike other social insects, but similar to certain vertebrate societies, in which the unrelated helpers gain through inheritance of a territory or a mate.

Polistes dominulus (Hymenoptera, Vespidae) invading North America: some hypotheses for its rapid spread

Summary:Polistes dominulus, a common Polistes species with Old World distribution, is now invading the United States. We discuss those characteristics of P. dominulus that may explain its successful establishment in its new American environment. A versatile diet, the ability to colonize new environments and a short development time of the immature brood might have played an important role in the rapid spread in P. dominulus.

PHYLOGENETIC RELATIONSHIPS AMONG PAPER WASP SOCIAL PARASITES AND THEIR HOSTS (HYMENOPTERA: VESPIDAE; POLISTINAE)

Abstract— Cladistic analyses of data from allozyme polymorphisms in paper wasp social parasites and their hosts do not support the hypothesis that social parasites are most closely related to their hosts. Electrophoretic data are adduced for nine species of Polistes, including all three known species of social parasites (Sulcopolistes) and their hosts. Three different coding methods are investigated; in no case do the social parasites cluster most closely with their hosts. Rather, there is limited evidence that they form a monophyletic group. However, formal taxonomic recognition of Sulcopolistes is not justified, as it renders Polistes sensu stricto paraphyletic. Although the social parasites are not most closely related to their hosts, hosts and parasites belong in the same subgenus and share many characteristics that may have facilitated the exploitation and deception practised by the parasites on the hosts.

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.

Social hackers: integration in the host chemical recognition system by a paper wasp social parasite.

Abstract Obligate social parasites in the social insects have lost the worker caste and the ability to establish nests. As a result, parasites must usurp a host nest, overcome the host recognition system, and depend on the host workers to rear their offspring. We analysed cuticular hydrocarbon profiles of live parasite females of the paper wasp social parasite Polistes sulcifer before and after usurpation of host nests, using the non-destructive technique of solid-phase micro-extraction. Our results reveal that hydrocarbon profiles of parasites change after usurpation of host nests to match the cuticular profile of the host species. Chemical evidence further shows that the parasite queen changes the odour of the nest by the addition of a parasite-specific hydrocarbon. We discuss the possible role of this in the recognition and acceptance of the parasite and its offspring in the host colony.

Social parasites in polistine wasps are monophyletic: implications for sympatric speciation

A social parasite uses workers of another social insect species to rear its own progeny. They are often so closely related to their hosts that it has been suggested that they could have evolved sympatrically from them. To address the question of whether social parasites evolved from their hosts we present a partial sequence of the mitochondrial 16S rRNA gene for nine species of Polistes, comprising all known species of social parasites, their hosts and two outgroups. Parsimony and maximum-likelihood analyses of the data support monophyly for these social parasites. The trees supporting monophyly are significantly shorter than the trees supporting sympatric speciation of parasites from their hosts. These data support the hypothesis that speciation occurred allopatrically and independently of the evolution of social parasitism. Where the social parasite parasitizes more than one species, the two species used are most closely related to each other. Although social parasites are monophyletic and did not evolve sympatrically from their hosts, it is clear that relatedness among species is important in the host–parasite relation.

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.

A quantitative threshold for nest-mate recognition in a paper social wasp

Nest-mate recognition is fundamental for protecting social insect colonies from intrusion threats such as predators or social parasites. The aggression of resident females towards intruders is mediated by their cuticular semiochemicals. A positive relation between the amount of cues and responses has been widely assumed and often taken for granted, even though direct tests have not been carried out. This hypothesis has important consequences, since it is the basis for the chemical insignificance strategy, the most common explanation for the reduction in the amount of semiochemicals occurring in many social parasites. Here we used the social wasp Polistes dominulus, a model species in animal communication studies and host of three social parasites, to test this hypothesis. We discovered that different amounts of cuticular hydrocarbons (CHC) of a foreign female evoke quantitatively different behavioural reactions in the resident foundress. The relation between CHC quantity and the elicited response supports the idea that a threshold exists in the chemical recognition system of this species. The chemical insignificance hypothesis thus holds in a host–parasite system of Polistes wasps, even though other explanations should not be discarded.

The Cost of Queen Loss in the Social Wasp Polistes dominulus (Hymenoptera: Vespidae)

Abstract Loss of the queen is a crisis for a social insect colony. The process of queen succession could cause increased aggression and work inefficiencies, and the new queen, if the colony can produce one, could be slow to develop mature eggs. We evaluated the cost of queen replacement in Polistes dominulus by removing the queen from a set of single-foundress colonies while leaving a control set with their queens. At 2 and 11 days after queen removal, we found that the queenless colonies had increased levels of some dominance behaviors, chewing and climbing, but not of the far more common lunging and biting. However, foraging behavior did not decrease on nests without queens as compared to nests with their original queens. Nest growth diminished as compared to control nests, as would be expected if new queens were not as competent at egg laying or if dominance behavior interfered with nurturing activities. Furthermore, replacement queens did not mate in the first 12 days after queen removal and few had mature eggs in their ovaries, though after a month most had mated and had developed ovaries. The degree of ovarian dominance of the top egg-layer over the others was also diminished at 12 days, but by a month the new queen was as dominant as control queens. The high cost of replacing the queen may indicate that workers are kept reproductively suppressed enough not to be a threat to the existing queen.

Effects of the Diet on the Microbiota of the Red Palm Weevil (Coleoptera: Dryophthoridae)

Rhynchophorus ferrugineus, also known as the red palm weevil, is regarded as the major pest of palm trees. Although studies of the microbiota associated with this species have been performed in recent years, little attention has been dedicated to the influence of the diet in shaping the host bacterial community. Here, we investigated the influence of food sources (i.e. palm tissues vs apple based substrate) on the microbial diversity associated with RPW, which was compared with the microbiota associated with wild individuals of the sister species Rhynchophorus vulneratus. The bacterial characterization was performed using a culture independent approach, i.e. the 16S rRNA pyrotag, and a culture dependent approach for a subset of the samples, in order to obtain bacterial isolates from RPW tissues. The bacterial community appeared significantly influenced by diet. Proteobacteria resulted to be the most abundant clade and was present in all the specimens of the three examined weevil groups. Within Proteobacteria, Enterobacteriaceae were identified in all the organs analysed, including hemolymph and reproductive organs. The apple-fed RPWs and the wild R. vulneratus showed a second dominant taxon within Firmicutes that was scarcely present in the microbiota associated with palm-fed RPWs. A comparative analysis on the bacteria associated with the palm tissues highlighted that 12 bacterial genera out of the 13 identified in the plant tissues were also present in weevils, thus indicating that palm tissues may present a source for bacterial acquisition.