Small-bodied males invest in larger testes when highly ornamented

Sperm competition and male mating rate are two non-mutually exclusive key evolutionary pressures selecting for larger testes within and across animal taxa. A few studies have tried to test the role of mating rate in the absence of sperm competition. Under the mating rate hypothesis, particular phenotypes of a given population that are expected to gain more mates (e.g., more ornamented males) are expected to make higher investments in testes size (a proxy for sperm production). We test this prediction in Polistes simillimus, a neotropical paper wasp in which females are single mated (no sperm competition) and males can mate with multiple partners. Testes size was predicted by body size (positive association), sexual ornamentation (negative association), and their interaction (among small males, testes size was positively related to ornamentation, but the opposite pattern was observed among large males). We propose that small-bodied well-ornamented males may face the highest risk of sperm depletion. Small-bodied males make relatively higher investment in testes size when highly ornamented. This strategy might be less profitable to large males, as they have overall larger testes. Our results provide strong evidence for the mating rate hypothesis.

A mixture of mesotrione and atrazine harms adults and larvae of the predatory wasp Polistes satan

Herbicides have been intensively used for weed control, raising concerns about their potentially adverse effects on non-target organisms. Research on the effects of these common agrochemicals on beneficial insects and the ecosystem services they provide (e.g., predation and pollination) is scarce. Therefore, we tested whether a commercial formulation comprising a mixture of mesotrione and atrazine was detrimental to adult females and larvae of the Neotropical predatory social wasp Polistes satan, which is an effective natural enemy of crop pests. Wasps were individually fed syrups contaminated with different concentrations of the herbicide above and below the maximum label rate (MLR = 12 mL/L). Survival was assessed. The locomotor activity, immune response, and midgut morphology of adults as well as the immune response of the larvae were also studied. Herbicide concentrations far above the MLR (12, 40, and 100 times) caused adult mortality, whereas lower concentrations (0.5, 1, and 6 times) did not. Herbicide exposure at 0.5 to 12 times the MLR increased adult activity. Adult exposure at 0.1 or 0.5 times the MLR did not affect melanotic encapsulation of foreign bodies but led to changes in the morphology of the midgut epithelium and peritrophic matrix. In larvae, the ingestion of herbicide at 0.1 or 0.2 times the MLR (corresponding to 9.6 and 19.2 ng of herbicide per individual) did not cause mortality but decreased their melanization-encapsulation response. Increased locomotor activity in herbicide-exposed adults can affect their foraging activity. The altered midgut morphology of adults coupled with the decreased immune response in larvae caused by herbicide exposure at realistic concentrations can increase the susceptibility of wasps to infections. Therefore, herbicides are toxic to predatory wasps.

Immune response in paper wasp workers: Task matters more than age

Workers of social hymenopterans (ants, bees and wasps) display specific tasks depending on whether they are younger or older. The relative importance of behavior and age in modulating immune function has seldom been addressed. We compared the strength of encapsulation-melanization immune response (hereafter melanotic encapsulation) in paper wasps displaying age polyethism or experimentally prevented from behavioral specialization. Foragers of Polybia paulista had higher melanotic encapsulation than guards, regardless of their age. Nevertheless, melanotic encapsulation decreased with age when wasps were prevented from behavioral specialization. Thus, in this species, worker melanotic encapsulation seems more sensitive to task than age. Foraging is considered one of the riskier behaviors in terms of pathogen exposure, so upregulating melanotic encapsulation in foragers can possibly improve both individual and colony-level resistance against infections.

A predatory social wasp does not avoid nestmates contaminated with a fungal biopesticide

Fungus-based biopesticides have been used worldwide for crop pest control as a safer alternative to chemical pesticides such as neonicotinoids. Both agrochemicals can be lethal and may also trigger side effects on the behavioral traits of non-target social insects, which play a crucial role in providing essential biological pest control services in agroecosystems. Here, we evaluated whether a commercial formulation of the entomopathogenic fungus Beauveria bassiana or the neonicotinoid imidacloprid causes mortality in foragers of Mischocyttarus metathoracicus. These social wasps are natural enemies of caterpillars and other herbivorous insects and inhabit both urban and agricultural environments in Brazil. We also tested whether wasps discriminate between biopesticide-exposed and unexposed conspecifics. Through a combination of laboratory (survival assay) and field experiments (lure presentation), along with chemical analyses (cuticular hydrocarbon profiles), we showed that topic exposure to the label rate of each pesticide causes a lethal effect, with the biopesticide exhibiting a slower effect. Moreover, wasps do not discriminate biopesticide-exposed from unexposed conspecifics, likely because of the similarity of their cuticular chemical profiles 24 h after exposure. Overall, the delayed lethal time at the individual level, combined with the indistinctive chemical cues of exposure and the lack of discrimination by conspecifics suggests that the fungal biopesticide may ultimately pose a threat to the colony survival of this predatory wasp.

Sex recognition does not modulate aggression toward nest intruders in a paper wasp

During social interactions, the behavior of an individual often depends on the sex of its social partner. Many animal societies have males and females that play very different behavioral roles, although they coexist and interact non-sexually. At specific phases of the colony cycle, social wasp females and males are contemporaries within a nest, they often interact, although mating occurs mostly off the nest, therefore providing an opportunity to test sex discrimination in contexts other than classical sexual ones. We performed a lure presentation experiment to test if Mischocyttarus metathoracicus discriminate between conspecifics of the 2 sexes during on-nest social interactions. Female wasps discriminated conspecific sex during experimentally simulated nest intrusions. Visual and chemical cues may account for this sex discrimination. Despite sex discrimination (evidenced by differential inspective behavior from the nest females toward the female and the male lures), female wasps were as aggressive toward lures of both sexes. In the female-dominated hymenopteran societies, males are often subordinate and not aggressive on nest, resulting in females directing less aggression to them compared to other females. Instead, M. metathoracicus males and females are both aggressive toward nestmates, so they might be perceived as similar threat during on-nest social interactions.

Recognition of conspecific immunological status in a neotropical paper wasp

Host immune activation is common under a pathogen invasion. This physiological response can promote changes in the body surface compounds, thus providing chemical cues related to health that might be useful to conspecifics. By recognizing the current immunological status of social partners, individuals can modulate their behavior to minimize the risk of infection. Tegument wounding, an immune elicitor, is a required step for many parasites to become established in a host. By using the neotropical eusocial paper wasp Mischocyttarus metathoracicus as a model organism, we first performed a lure presentation experiment in the field to test if wasps discriminate conspecific immunological status (experimentally manipulated by wounding) during on nest social interactions. Then, we performed gas chromatography–mass spectrometry analyses to test if immunostimulation by wounding alters the wasps’ cuticular hydrocarbon profile. We found that wasps reduce the duration of aggressive physical contact when interacting with wounded lures, despite displaying a similar frequency of inspective and aggressive behaviors toward both the wounded and the control lures. Besides, we found a subtle increase in a single cuticular hydrocarbon in the wounded wasps. Thus, wasps recognize conspecific immunological status, likely by chemical cues, and modulate their behavior in order to defend the colony against intruders while minimizing the personal risk of infection.

Ingestion of polystyrene microparticles impairs survival and defecation in larvae of Polistes satan (Hymenoptera: Vespidae)

Microplastics (MPs) are widespread pollutants of emerging concern, and the risks associated with their ingestion have been reported in many organisms. Terrestrial environments can be contaminated with MPs, and terrestrial organisms, including arthropods, are predisposed to the risk of ingesting MPs. In the current study, the larvae of the paper wasp Polistes satan were fed two different doses (6 mg or 16 mg at once) of polystyrene MPs (1.43 mm maximum length), and the effects of these treatments on immature development and survival till adult emergence were studied. Ingestion of the two doses resulted in mortality due to impaired defecation prior to pupation. The survival of larvae that ingested 16 mg of MPs was significantly lower than that of the control. The ingestion of 16 mg of MPs also reduced the adult emergence (11.4%) in comparison to the control (44.4%). MPs were not transferred from the larvae to the adults that survived. These findings demonstrate that MP ingestion can be detrimental to P. satan, e.g. larval mortality can decrease colony productivity and thus the worker force, and that MPs can potentially affect natural enemies that occur in crops, such as predatory social wasps.

Sexually dimorphic traits and male fertility in a paper wasp

Females of many social Hymenoptera commit their reproductive potential to a single male; they mate once and never re-mate. Sexual selection theory predicts that under this regimen female precopulatory mate choice is crucial. The targets of female choice, male secondary sexual traits, should convey information about the functional fertility of the male. Here, we evaluated semen quality and its correlates in the paper wasp Polistes dominula, a monandric species with a lek-based mating system. The main goal of the study was to test whether sperm viability and the total number of sperm covaried with sexually dimorphic abdominal spots, body size and parasitic infection by Xenos vesparum. Surprisingly, superior male fertility was not predicted by attractive sexual traits, such as little round spots and large body size. We also found no effect of the parasite on male fertility, sexual ornamentation or body size. We found only that sperm viability was positively associated with the number of sperm. Moreover, there was evidence of a counterintuitive relationship between (sexually unattractive) large irregular spots and large body size of dominant successful males. Overall, our results suggest a condition-dependent trade-off between body size and costly sexual signals.

WASPnest: a worldwide assessment of social Polistine nesting behavior

Cooperative breeding decreases the direct reproductive output of subordinate individuals, but cooperation can be evolutionarily favored when there are challenges or constraints to breeding independently. Environmental factors, including temperature, precipitation, latitude, high seasonality, and environmental harshness have been hypothesized to correlate with the presence of cooperative breeding. However, to test the relationship between cooperation and ecological constraints requires comparative data on the frequency and variation of cooperative breeding across differing environments, ideally replicated across multiple species. Paper wasps are primitively social species, forming colonies composed of reproductively active dominants and foraging subordinates. Adult female wasps, referred to as foundresses, initiate new colonies. Nests can be formed by a single solitary foundress (noncooperative) or by multiple foundress associations (cooperative). Cooperative behavior varies within and among species, making paper wasps species well suited to disentangling ecological correlates of variation in cooperative behavior. This data set reports the frequency and extent of cooperative nest founding for 87 paper wasp species. Data were assembled from more than 170 published sources, previously unpublished field observations, and photographs contributed by citizen scientists to online natural history repositories. The data set includes 25,872 nest observations and reports the cooperative behavioral decisions for 45,297 foundresses. Species names were updated to reflect modern taxonomic revisions. The type of substrate on which the nest was built is also included, when available. A smaller population-level version of this data set found that the presence or absence of cooperative nesting in paper wasps was correlated with temperature stability and environmental harshness, but these variables did not predict the extent of cooperation within species. This expanded data set contains details about individual nests and further increases the power to address the relationship between the environment and the presence and extent of cooperative breeding. Beyond the ecological drivers of cooperation, these high-resolution data will be useful for future studies examining the evolutionary consequences of variation in social behavior. This data set may be used for research or educational purposes provided that this data paper is cited.

Sexual ornaments reveal the strength of melanization immune response and longevity of male paper wasps

It has been recently suggested that female mate choice, based on sexually selected ornaments, is an important component of social wasps' reproductive biology. The correlates of male ornaments that could be of a female's interest, however, remain to be investigated. Males of the Neotropical paper wasp Polistes simillimus have sexually dimorphic melanin-based black spots on their faces. In this species, male spots work like sexual ornaments, as it has been experimentally demonstrated that females prefer sexual partners with a higher proportion of black pigment on their faces. We have shown that, under laboratory conditions, male sexual ornamentation positively predicts the strength of the melanization immune response and longevity. Therefore, in P. simillimus, melanin-based facial patterns (ornaments) seem to be honest indicators of male quality.

Ontogenic Caste Differences in the Van der Vecht Organ of Primitively Eusocial Neotropical Paper Wasps

Recent studies have reported incipient morphological caste dimorphism in the Van der Vecht organ size of some temperate Polistes paper wasps. Whether species other than the temperate ones show a similar pattern remains elusive. Here, we have studied some Neotropical Polistes species. By comparing females collected through the year, we showed caste related differences in the size of the Van der Vecht organ in P. ferreri (body size corrected Van der Vech organ size of queens = 0.45 ± 0.06, workers = 0.38 ± 0.07 mm², p = 0.0021), P. versicolor (body size corrected Van der Vech organ size of queens = 0.54 ± 0.11, workers = 0.46 ± 0.09 mm², p = 0.010), but not P. simillimus (body size corrected Van der Vech organ size of queens = 0.52 ± 0.05, workers = 0.49 ± 0.06 mm², p = 0.238). Therefore, it seems that queens and workers of some Neotropical Polistes have diverged in their ontogenic trajectory of the Van der Vecht organ size, providing clear evidence for incipient morphological caste dimorphism. As Polistes are distributed mostly in the tropics, we propose that physical caste differences may be widespread in the genus. Also, we highlight that morphological divergence in the queen–worker phenotypes may have started through differential selection of body structures, like the Van der Vecht organ.

Sexy faces in a male paper wasp

Sexually selected signals are common in many animals, though little reported in social insects. We investigated the occurrence of male visual signals mediating the dominance relationships among males and female choice of sexual partner in the paper wasp Polistes simillimus. Males have three conspicuous, variable and sexually dimorphic traits: black pigmentation on the head, a pair of yellow abdominal spots and body size differences. By conducting behavioral assays, we found that none of the three visual traits are associated with male-male dominance relationship. However, males with higher proportion of black facial pigmentation and bigger yellow abdominal spots are more likely chosen as sexual partners. Also, after experimentally manipulating the proportion of black pigment on males' face, we found that females may evaluate male facial coloration during the choice of a sexual partner. Thus, the black pigmentation on P. simillimus male's head appears to play a role as a sexually selected visual signal. We suggest that sexual selection is a common force in Polistes and we highlight the importance of this group as a model for the study of visual communication in insects.