Polistes dominulus (Hymenoptera: Vespidae) larvae possess their own chemical signatures

Adverse effects of the fungal biopesticide Beauveria bassiana on a predatory social wasp

Biopesticides are considered eco-friendly alternatives to synthetic agrochemicals. However, their impact on non-target organisms is still poorly understood. Social wasps, in particular, are a largely neglected group when it comes to risk assessment of plant protection products, despite the relevant ecological and economic services provided by these insects. In the present study, we evaluated the impact of a common biopesticide, the entomopathogenic fungus Beauveria bassiana, on the paper wasp Polistes dominula. We adopted a holistic approach in ecotoxicology by focusing not only on the detrimental effects on isolated individuals, but also on the whole colony. Both adult wasps belonging to different castes and immature larvae were topically exposed to a field-realistic concentration of fungal spores from the commercial strain of B. bassiana ATCC 74040 to assess the impact of the biopesticide on their survival, behavior and physiology. Our results showed that the fungus causes a number of adverse effects on P. dominula, that include increased mortality, altered locomotion and feeding rate, selective ejection of exposed larvae from nests, reduced oviposition rate and ovary development in foundresses, and colony failure. Our findings provide new insights on the often-neglected sublethal effects of pollutants that can jeopardize not only individual beneficial insects, but also the delicate social balance of their colonies and their valuable ecosystem services, highlighting that the natural origin of plant-protection products does not always guarantee environmental safety.

Effect of temperature on the chemical profiles of nest materials of social wasps

Social insects depend on their nests for protection against predation and abiotic threats. Accordingly, the chemical compounds present in the material wasps use to build their nests can both facilitate communication and repel predators. It is herein hypothesized that different wasp species build their nests with different structure and substrate materials and that such materials consist of chemical compounds related to unique wasp behavior and outside temperature variation. To test this hypothesis, nests were collected from three species of social wasps, the samples of which were subjected to temperature variation under laboratory conditions. The compounds present in the substrate were analyzed by gas chromatography coupled to mass spectrometry. Chemical compounds identified in the nest material of the three species responded differently to temperature variation. Chemical compounds from Polybia nests were altered significantly when subjected to temperature variation, whereas the nests of Polistes versicolor did not significantly change in relation to the control. The differences found between Polistes and Polybia nests may be related to genetic factors, but also to the type of nest they construct. It is possible that divergent evolutionary strategies for maintaining colony temperature, as a function of the chemical composition of the nests, may have appeared between wasps that have open and closed nests. In relatively small colonies, nest substrate is more resistant to temperature variation because it is composed of a greater diversity of elements and thus capable of holding heavier, longer carbon chains. Our results suggest that chemical compounds in the nest material of the three wasp species analysed responded differently to fluctuating ambient temperatures and that such variation could result from the biochemical differences of unique wasp species or from thermoregulation strategies of colonies.

Social Complexity and Brain Evolution: Comparative Analysis of Modularity and Integration in Ant Brain Organization

The behavioral demands of living in social groups have been linked to the evolution of brain size and structure, but how social organization shapes investment and connectivity within and among functionally specialized brain regions remains unclear. To understand the influence of sociality on brain evolution in ants, a premier clade of eusocial insects, we statistically analyzed patterns of brain region size covariation as a proxy for brain region connectivity. We investigated brain structure covariance in young and old workers of two formicine ants, the Australasian weaver ant Oecophylla smaragdina, a pinnacle of social complexity in insects, and its socially basic sister clade Formica subsericea. As previously identified in other ant species, we predicted that our analysis would recognize in both species an olfaction-related brain module underpinning social information processing in the brain, and a second neuroanatomical cluster involved in nonolfactory sensorimotor processes, thus reflecting conservation of compartmental connectivity. Furthermore, we hypothesized that covariance patterns would reflect divergence in social organization and life histories either within this species pair or compared to other ant species. Contrary to our predictions, our covariance analyses revealed a weakly defined visual, rather than olfactory, sensory processing cluster in both species. This pattern may be linked to the reliance on vision for worker behavioral performance outside of the nest and the correlated expansion of the optic lobes to meet navigational demands in both species. Additionally, we found that colony size and social organization, key measures of social complexity, were only weakly correlated with brain modularity in these formicine ants. Worker age also contributed to variance in brain organization, though in different ways in each species. These findings suggest that brain organization may be shaped by the divergent life histories of the two study species. We compare our findings with patterns of brain organization of other eusocial insects.

The Role of Brood in Eusocial Hymenoptera

Study of social traits in offspring traditionally reflects on interactions in simple family groups, with famous examples including parent-offspring conflict and sibling rivalry in birds and mammals. In contrast, studies of complex social groups such as the societies of ants, bees, and wasps focus mainly on adults and, in particular, on traits and interests of queens and workers. The social role of developing individuals in complex societies remains poorly understood. We attempt to fill this gap by illustrating that development in social Hymenoptera constitutes a crucial life stage with important consequences for the individual as well as the colony. We begin by describing the complex social regulatory network that modulates development in Hymenoptera societies. By highlighting the inclusive fitness interests of developing individuals, we show that they may differ from those of other colony members. We then demonstrate that offspring have evolved specialized traits that allow them to play a functional, cooperative role within colonies and give them the potential power to act toward increasing their inclusive fitness. We conclude by providing testable predictions for investigating the role of brood in colony interactions and giving a general outlook on what can be learned from studying offspring traits in hymenopteran societies.

Influence of temperature on survival and cuticular chemical profile of social wasps

The cuticle of social insects is a barrier against desiccation and a channel for chemical communication, two characteristics fundamental to the success of this group. The compounds present in the cuticle interact dynamically in order to achieve a balance between these two functions. Thus, viscosity correlates with waterproofing, whereas fluidity correlates with effective communication. Temperature variation can cause the cuticular hydrocarbon (CHC) layer of the cuticle to change in order to maintain body homeostasis. Thus, in this study, we tested the hypothesis that wasps with different body sizes and nest types will differ in their tolerance to temperature variation and ability to respond by changing cuticular chemical composition. To test this hypothesis, workers of three species of social wasps with different body sizes and nests, both with or without envelope, were subjected to different temperatures under controlled conditions. Cuticular compounds were analyzed by gas chromatography coupled to mass spectrometry (GC/MS). Results show that tolerance to temperature variation is not directly related to either wasp size or nesting type. An increase in the percentage of linear alkanes and a decrease in the percentage of branched alkanes were correlated with increased temperature. Thus, instead of either body size or nest type, tolerance to temperature variation seems to be mediated by the changing chemical composition of the cuticle.

Novel insights into the ontogeny of nestmate recognition in Polistes social wasps

The importance of early experience in animals' life is unquestionable, and imprinting-like phenomena may shape important aspects of behaviour. Early learning typically occurs during a sensitive period, which restricts crucial processes of information storage to a specific developmental phase. The characteristics of the sensitive period have been largely investigated in vertebrates, because of their complexity and plasticity, both in behaviour and neurophysiology, but early learning occurs also in invertebrates. In social insects, early learning appears to influence important social behaviours such as nestmate recognition. Yet, the mechanisms underlying recognition systems are not fully understood. It is currently believed that Polistes social wasps are able to discriminate nestmates from non-nestmates following the perception of olfactory cues present on the paper of their nest, which are learned during a strict sensitive period, immediately after emergence. Here, through differential odour experience experiments, we show that workers of Polistes dominula develop correct nestmate recognition abilities soon after emergence even in absence of what have been so far considered the necessary cues (the chemicals spread on nest paper). P. dominula workers were exposed for the first four days of adult life to paper fragments from their nest, or from a foreign conspecific nest or to a neutral condition. Wasps were then transferred to their original nests where recognition abilities were tested. Our results show that wasps do not alter their recognition ability if exposed only to nest material, or in absence of nest material, during the early phase of adult life. It thus appears that the nest paper is not used as a source of recognition cues to be learned in a specific time window, although we discuss possible alternative explanations. Our study provides a novel perspective for the study of the ontogeny of nestmate recognition in Polistes wasps and in other social insects.

The chemical basis of host nest detection and chemical integration in a cuckoo paper wasp

Insect social life is governed by chemicals. A great number of studies have demonstrated that the blend of hydrocarbons present on the cuticle (CHCs) plays a pivotal role in intra- and inter-specific communication. It is not surprising, therefore, that social parasites, specialized in exploiting the costly parental care provided by host workers, exploit the host chemical communication system too. Throughout their life cycle, social parasites intercept and break this CHC-based code. Recently, however, several polar compounds (mainly peptides) have been found in addition to CHCs both on the cuticle and on the comb surface of social insects, and their semiochemical role has been demonstrated in some circumstances. In the present study, we used the paper wasp social parasite-host system Polistes sulcifer (Zimmerman)-Polistes dominulus (Christ) to evaluate the relative importance of the CHCs and polar compounds in two different steps of the host exploitation process: host nest detection by the pre-usurping parasite and parasite chemical integration into the host colony. After separating the polar and apolar fractions of the host nest as well as those of pre- and post-usurpation parasites, we carried out laboratory assays based on the binary choice model. Our results show that nest polar compounds neither are used by the parasite to detect the host's nest nor play a role in parasite chemical integration into the host colony. In contrast, we demonstrate that CHCs are fundamental in both steps, thus confirming their primary role in social insect life and consequently in social parasite-host interactions.

Polistes dominulus (Hymenoptera, Vespidae) larvae show different cuticular patterns according to their sex: workers seem not use this chemical information

During reproductive phase, larvae of male and female are intermingled in nest of social wasps. Workers care for and feed larvae that gives them an opportunity to bias investment with respect to sex, or even to kill some larvae, if they can distinguish between immature males and females. Cuticular hydrocarbon (CHC) mixtures are the most studied cues for species, nestmate, and caste recognition in social Hymenoptera. In this study, we investigate the paper wasp Polistes dominulus to see if male and female larvae show different patterns of CHCs and if workers are able to discriminate between male and female larvae on this basis. We performed gas chromatography-mass spectrometry analysis on cuticular extracts of larvae, and then we genotyped them to assign sex. We found sex-based variation in CHC-profiles sufficient for discrimination. However, our behavioral assays do not support the view that adults discriminate between male and female larvae within nests.