Colony social structure in native and invasive populations of the social wasp Vespula pensylvanica

Invasion potential of hornets (Hymenoptera: Vespidae: Vespa spp.)

Hornets are large, predatory wasps that have the potential to alter biotic communities and harm honey bee colonies once established in non-native locations. Mated, diapausing females (gynes) can easily be transported to new habitats, where their behavioral flexibility allows them to found colonies using local food and nest materials. Of the 22 species in the genus Vespa, five species are now naturalized far from their endemic populations and another four have been detected either in nature or during inspections at borders of other countries. By far the most likely pathway of long-distance dispersal is the transport of gynes in transoceanic shipments of goods. Thereafter, natural dispersal of gynes in spring and accidental local transport by humans cause shorter-range expansions and contribute to the invasion process. Propagule pressure of hornets is unquantified, although it is likely low but unrelenting. The success of introduced populations is limited by low propagule size and the consequences of genetic founder effects, including the extinction vortex linked to single-locus, complementary sex determination of most hymenopterans. Invasion success is enhanced by climatic similarity between source locality and introduction site, as well as genetic diversity conferred by polyandry in some species. These and other factors that may have influenced the successful establishment of invasive populations of V. velutina, V. tropica, V. bicolor, V. orientalis, and V. crabro are discussed. The highly publicized detections of V. mandarinia in North America and research into its status provide a real-time example of an unfolding hornet invasion.

High Exploration Behavior of Termite Propagules Can Enhance Invasiveness

Social life is usually associated with enhanced propagule pressure, which increases the chance of introducing several individuals during a single introduction event. Social insects are therefore among the most successful invasive species, benefiting from rapid establishment and increased foundation success in new habitats. In termites, propagule pressure may also be increased by the development of reproductive individuals from a small group of foraging workers. This suggests that enhanced exploration activity may increase propagule pressure through an elevated chance of transporting isolated groups of foragers. Here, we analyzed the exploration behavior of three termite species of the Reticulitermes genus, comparing the invasive species Reticulitermes flavipes (testing both native and introduced populations) to the native species Reticulitermes grassei and Reticulitermes lucifugus. Different features representative of the exploration capacity were measured during 48 h, including: the number of tunnels, the length of tunnels, the number of foragers, and the interindividual distance of foragers in a straight line or through tunnels. Our results show that compared to the native Reticulitermes species, R. flavipes foragers from both populations dug more tunnels with a longer total length, and individuals were more spatially dispersed and covered a larger exploration zone. These findings suggest that the enhanced exploration ability of R. flavipes may have played a role in its invasion success, by increasing its propagule pressure through a higher chance of human-mediated transport. In addition, the absence of differences between the native and introduced populations of R. flavipes suggests that the exploration behaviors facilitating the worldwide invasion of this species originated in its native range.

Social structure of perennial Vespula squamosa wasp colonies

Many social species show variation in their social structure in response to different environmental conditions. For example, colonies of the yellowjacket wasp Vespula squamosa are typically headed by a single reproductive queen and survive for only a single season. However, in warmer climates, V. squamosa colonies sometimes persist for multiple years and can grow to extremely large size. We used genetic markers to understand patterns of reproduction and recruitment within these perennial colonies. We genotyped V. squamosa workers, pre-reproductive queens, and males from perennial colonies in the southeastern United States at 10 polymorphic microsatellite loci and one mitochondrial DNA locus. We found that V. squamosa from perennial nests were produced by multiple reproductives, in contrast to typical annual colonies. Relatedness of nestmates from perennial colonies was significantly lower than relatedness of nestmates from annual colonies. Our analyses of mitochondrial DNA indicated that most V. squamosa perennial colonies represented semiclosed systems whereby all individuals belonged to a single matriline despite the presence of multiple reproductive females. However, new queens recruited into perennial colonies apparently mated with non-nestmate males. Notably, perennial and annual colonies did not show significant genetic differences, supporting the hypothesis that perennial colony formation represents an instance of social plasticity. Overall, our results indicate that perennial V. squamosa colonies show substantial changes to their social biology compared to typical annual colonies and demonstrate variation in social behaviors in highly social species.

Early queen joining and long-term queen associations in polygyne colonies of an invasive wasp revealed by longitudinal genetic analysis

Invasive social insects rank among the most damaging of terrestrial species. They are responsible for extensive damage and severely threaten the biodiversity of environments where they are introduced. Variation in colony social form commonly occurs in introduced populations of yellowjacket wasps (genus Vespula). In particular, invasive colonies may contain multiple queens (i.e., polygyne) and persist several years, while in the native range, the colonies are usually annual and harbor a single queen (i.e., monogyne). In this study, we used genome-wide loci obtained by double digest restriction site-associated DNA sequencing (RADseq) to investigate the genetic structure and queen turnover in colonies of the western yellowjacket, Vespula pensylvanica, in their introduced range in Hawaii. Of the 27 colonies monitored over four months (October-January), 19 were polygyne and already contained multiple queens on the first day of sampling. Contrary to previous speculation, this finding suggests that polygyny often arises early in the annual colony cycle, before the production of new queens in the fall. Furthermore, polygyne colonies exhibited a prolonged average lifespan relative to those headed by a single queen. As a result, there is no clear window during which colony eradication efforts would be more effective than upon first discovery. The relatedness among nestmate queens was slightly above zero, indicating that these colonies are generally composed of nonrelatives. The queen turnover within each colony was low, and we detected some full-sibling workers sampled up to four months apart. Finally, we did not detect any population structure among colonies, suggesting that queens disperse up to several kilometers. Taken together, our results provide the first insights into the requeening dynamics in this invasive and incipiently polygyne population and illuminate the early establishment of multiple long-lasting queens in these damaging colonies.

Distinct colony boundaries and larval discrimination in polygyne red imported fire ants (Solenopsis invicta)

Evaluating the factors that promote invasive ant abundance is critical to assess their ecological impact and inform their management. Many invasive ant species show reduced nestmate recognition and an absence of boundaries between unrelated nests, which allow populations to achieve greater densities due to reduced intraspecific competition. We examined nestmate discrimination and colony boundaries in introduced populations of the red imported fire ant (Solenopsis invicta; hereafter, fire ant). Fire ants occur in two social forms: monogyne (colonies with a single egg-laying queen) and polygyne (colonies with multiple egg-laying queens). In contrast with monogyne nests, polygyne nests are thought to be interconnected due to the reduced antagonism between non-nestmate polygyne workers, perhaps because polygyne workers habituate the colony to an odour unique to Gp-9^b -carrying adults. However, colony boundaries and nestmate discrimination are poorly documented, particularly for worker-brood interactions. To delimit boundaries between field colonies, we correlated the exchange of a ¹⁵ N-glycine tracer dissolved in a sucrose solution with social form. We also evaluated nestmate discrimination between polygyne workers and larvae in the laboratory. Counter to our expectations, polygyne colonies behaved identically to monogyne colonies, suggesting both social forms maintain strict colony boundaries. Polygyne workers also preferentially fed larval nestmates and may have selectively cannibalized non-nestmates. The levels of relatedness among workers in polygyne colonies was higher than those previously reported in North America (mean ± standard error: 0.269 ± 0.037). Our study highlights the importance of combining genetic analyses with direct quantification of resource exchange to better understand the factors influencing ant invasions.

Breeding structure and invasiveness in social insects

Plasticity in life history traits is commonly used to explain the invasion success of social insects. While intraspecific plasticity is often recognized, interspecific variability is easily overlooked, whereby different species exhibit different strategies. The presence of many queens per colony and the collapse of colony boundaries have favored invasiveness for many ant species. However, these strategies are absent from other successful social invaders. Here, we report that various life-history traits may differentially enhance the invasion success in social insects. We suggest that other aspects of their breeding system, like asexual reproduction, intranidal mating and pre-adaptation to inbreeding may enhance their invasion success. Thorough comparative studies between native and introduced populations or studies of closely related species will help identify additional traits favoring the invasion success of social insects, and ultimately provide a more comprehensive picture of the evolutionary factors enhancing invasiveness across this phylogenetically and ecologically diverse group.

Emerging patterns in social wasp invasions

Invasive species are a main driver of biodiversity loss and ecological change globally. Consequently, there is a need to understand how invaders damage ecosystems and to develop effective management strategies. Social wasps (Hymenoptera: Vespidae) include some of the world's most ecologically damaging invasive insects. In recent decades, the invasive social wasp literature has grown rapidly. This may be due in part to increased rate of introduction as well as greater public awareness of invasive wasps and their potential negative impacts on bees. Here, we investigate trends in invasive social wasp research, identifying the emergence of Vespa invasions, the mechanism-based inquiry into Vespula invasions, and the increased application of molecular methods to track invasive species through the invasion process.

How does climate change affect social insects?

Climate change poses a major threat to global biodiversity, already causing sharp declines of populations and species. In some social insect species we already see advanced phenologies, changes in distribution ranges, and changes in abundance Rafferty (2017) and Diamond et al. (2017). Physiologically, social insects are no different from solitary insects, but they possess a number of characteristics that distinguish their response to climate change. Here, we examine these traits, which might enable them to cope better with climate change than solitary insects, but only in the short term. In addition, we discuss how climate change will alter biotic interactions and ecosystem functions, and how it will affect invasive social insects.

Founder effects on sex determination systems in invasive social insects

Invasive populations are often established from a small number of individuals, and thus have low genetic diversity relative to native-range populations. Social ants, bees and wasps (social Hymenoptera) should be vulnerable to such founder effects on genetic diversity because sex in these species is determined genetically via Complementary Sex Determination (CSD). Under CSD, individuals homozygous at one or more critical sex loci are inviable or develop as infertile diploid males. Low diversity at sex loci leads to increased homozygosity and diploid male production, increasing the chance of colony death. In this review, we identify behavioral, social and reproductive traits that preserve allele richness at sex loci, allow colonies to cope with diploid male production, and eventually restore sex allele diversity in invasive populations of social Hymenoptera that experience founding bottlenecks.

Microbiome of the wasp Vespula pensylvanica in native and invasive populations, and associations with Moku virus

Invasive species present a worldwide concern as competition and pathogen reservoirs for native species. Specifically, the invasive social wasp, Vespula pensylvanica, is native to western North America and has become naturalized in Hawaii, where it exerts pressures on native arthropod communities as a competitor and predator. As invasive species may alter the microbial and disease ecology of their introduced ranges, there is a need to understand the microbiomes and virology of social wasps. We used 16S rRNA gene sequencing to characterize the microbiome of V. pensylvanica samples pooled by colony across two geographically distinct ranges and found that wasps generally associate with taxa within the bacterial genera Fructobacillus, Fructilactobacillus, Lactococcus, Leuconostoc, and Zymobacter, and likely associate with environmentally-acquired bacteria. Furthermore, V. pensylvanica harbors-and in some cases were dominated by-many endosymbionts including Wolbachia, Sodalis, Arsenophonus, and Rickettsia, and were found to contain bee-associated taxa, likely due to scavenging on or predation upon honey bees. Next, we used reverse-transcriptase quantitative PCR to assay colony-level infection intensity for Moku virus (family: Iflaviridae), a recently-described disease that is known to infect multiple Hymenopteran species. While Moku virus was prevalent and in high titer, it did not associate with microbial diversity, indicating that the microbiome may not directly interact with Moku virus in V. pensylvanica in meaningful ways. Collectively, our results suggest that the invasive social wasp V. pensylvanica associates with a simple microbiome, may be infected with putative endosymbionts, likely acquires bacterial taxa from the environment and diet, and is often infected with Moku virus. Our results suggest that V. pensylvanica, like other invasive social insects, has the potential to act as a reservoir for bacteria pathogenic to other pollinators, though this requires experimental demonstration.

Viral load, not food availability or temperature, predicts colony longevity in an invasive eusocial wasp with plastic life history

Social insect colonies exhibit a variety of life history strategies, from the annual, semelparous colonies of temperate bees and wasps to the long-lived colonies of many ants and honeybees. Species introduced to novel habitats may exhibit plasticity in life history strategies as a result of the introduction, but the factors governing these changes often remain obscure. Vespula pensylvanica, a yellowjacket wasp, exhibits such plasticity in colony longevity. Multi-year (perennial) colonies are relatively common in introduced populations in Hawaii, while source populations in the western United States are typically on an annual cycle. Here, we use experiments and observational data to examine how diet, disease, nest thermal environment, and nest location influence colony longevity in a population with both annual and perennial colonies. Counter to our predictions, experimental feeding and warming did not increase colony survival in the winter in the introduced range. However, Moku Virus load and wasp colony density predicted colony survival in one year, suggesting a potential role for disease in modulating colony phenology. We also found that local V. pensylvanica colony density was positively correlated with Moku Virus loads, and that Arsenophonus sp. bacterial loads in V. pensylvanica colonies were positively associated with proximity to feral honeybee (Apis mellifera) hives, suggesting potential transmission routes for these poorly understood symbionts. The factors influencing colony longevity in this population are likely multiple and interactive. More important than food availability, we propose winter precipitation as a critical factor that may explain temporal and spatial variation in colony longevity in these invasive wasps.

Divide and conquer: Multicolonial structure, nestmate recognition, and antagonistic behaviors in dense populations of the invasive ant Brachymyrmex patagonicus

The ecological success of ants has made them abundant in most environments, yet inter- and intraspecific competition usually limit nest density for a given population. Most invasive ant populations circumvent this limitation through a supercolonial structure, eliminating intraspecific competition through a loss of nestmate recognition and lack of aggression toward non-nestmates. Native to South America, Brachymyrmex patagonicus has recently invaded many locations worldwide, with invasive populations described as extremely large and dense. Yet, in contrast with most invasive ants, this species exhibits a multicolonial structure, whereby each colony occupies a single nest. Here, we investigated the interplay between genetic diversity, chemical recognition, and aggressive behaviors in an invasive population of B. patagonicus. We found that, in its invasive range, this species reaches a high nest density with individual colonies located every 2.5 m and that colony boundaries are maintained through aggression toward non-nestmates. This recognition and antagonism toward non-nestmates is mediated by chemical differentiation between colonies, as different colonies exhibit distinct chemical profiles. We highlighted that the level of aggression between colonies is correlated with their degree of genetic difference, but not their overall chemical differentiation. This may suggest that only a few chemical compounds influence nestmate recognition in this species or that weak chemical differences are sufficient to elicit aggression. Overall, this study demonstrates that invasive ant populations can reach high densities despite a multicolonial structure with strong aggression between colonies, raising questions about the factors underlying their ecological success and mitigating negative consequences of competitive interactions.

Polyandry and paternity affect disease resistance in eusocial wasps

Polyandry (multiple mating by females) is a central challenge for understanding the evolution of eusociality. Several hypotheses have been proposed to explain its observed benefits in eusocial Hymenoptera, one of which, the parasite–pathogen hypothesis (PPH), posits that high genotypic variance among workers for disease resistance prevents catastrophic colony collapse. We tested the PPH in the polyandrous wasp Vespula shidai. We infected isolated workers with the entomopathogenic fungus Beauveria bassiana and quantified their survival in the laboratory. Additionally, we conducted a paternity analysis of the workers using nine microsatellite loci to investigate the relationship between survival and the matriline and patriline membership of the workers. As predicted by the PPH, nestmate workers of different patrilines showed differential resistance to B. bassiana. We also demonstrated variation in virulence among strains of B. bassiana. Our results are the first to directly support the PPH in eusocial wasps and suggest that similar evolutionary pressures drove the convergent origin and maintenance of polyandry in ants, bees, and wasps.

Extreme polyandry aids the establishment of invasive populations of a social insect

Although monandry is believed to have facilitated the evolution of eusociality, many highly eusocial insects have since evolved extreme polyandry. The transition to extreme polyandry was likely driven by the benefits of within-colony genetic variance to task specialization and/or disease resistance, but the extent to which it confers secondary benefits, once evolved, is unclear. Here we investigate the consequences of extreme polyandry on the invasive potential of the Asian honey bee, Apis cerana. In honey bees and other Hymenoptera, small newly founded invasive populations must overcome the genetic constraint of their sex determination system that requires heterozygosity at a sex-determining locus to produce viable females. We find A. cerana queens in an invasive population mate with an average of 27 males (range 16-42) that would result in the founding queen/s carrying 75% of their source population's sex alleles in stored sperm. This mating frequency is similar to native-range Chinese A. cerana (mean 29 males, range 19-46). Simulations reveal that extreme polyandry reduces the risk, relative to monandry or moderate polyandry, that colonies produce a high incidence of inviable brood in populations that have experienced a founder event, that is, when sex allele diversity is low and/or allele frequencies are unequal. Thus, extreme polyandry aids the invasiveness of A. cerana in two ways: (1) by increasing the sex locus allelic richness carried to new populations with each founder, thereby increasing sex locus heterozygosity; and (2) by reducing the population variance in colony fitness following a founder event.

Population genetic structure of the predatory, social wasp Vespula pensylvanica in its native and invasive range

Invasive species cause extensive damage to their introduced ranges. Ocean archipelagos are particularly vulnerable to invasive taxa. In this study, we used polymorphic microsatellite markers to investigate the genetic structure of the social wasp Vespula pensylvanica in its native range of North America and its introduced range in the archipelago of Hawaii. Our goal was to gain a better understanding of the invasion dynamics of social species and the processes affecting biological invasions. We found that V. pensylvanica showed no significant genetic isolation by distance and little genetic structure over a span of 2000 km in its native range. This result suggests that V. pensylvanica can successfully disperse across large distances either through natural- or human-mediated mechanisms. In contrast to the genetic patterns observed in the native range, we found substantial genetic structure in the invasive V. pensylvanica range in Hawaii. The strong patterns of genetic differentiation within and between the Hawaiian Islands may reflect the effects of geographic barriers and invasion history on gene flow. We also found some evidence for gene flow between the different islands of Hawaii which was likely mediated through human activity. Overall, this study provides insight on how geographic barriers, invasion history, and human activity can shape population genetic structure of invasive species.

Relationship between invasion success and colony breeding structure in a subterranean termite

Factors promoting the establishment and colonization success of introduced populations in new environments constitute an important issue in biological invasions. In this context, the respective role of pre-adaptation and evolutionary changes during the invasion process is a key question that requires particular attention. This study compared the colony breeding structure (i.e. number and relatedness among reproductives within colonies) in native and introduced populations of the subterranean pest termite, Reticulitermes flavipes. We generated and analysed a data set of both microsatellite and mtDNA loci on termite samples collected in three introduced populations, one in France and two in Chile, and in the putative source population of French and Chilean infestations that has recently been identified in New Orleans, LA. We also provided a synthesis combining our results with those of previous studies to obtain a global picture of the variation in breeding structure in this species. Whereas most native US populations are mainly composed of colonies headed by monogamous pairs of primary reproductives, all introduced populations exhibit a particular colony breeding structure that is characterized by hundreds of inbreeding reproductives (neotenics) and by a propensity of colonies to fuse, a pattern shared uniquely with the population of New Orleans. These characteristics are comparable to those of many invasive ants and are discussed to play an important role during the invasion process. Our finding that the New Orleans population exhibits the same breeding structure as its related introduced populations suggests that this native population is pre-adapted to invade new ranges.

Colony size is linked to paternity frequency and paternity skew in yellowjacket wasps and hornets

BACKGROUND

The puzzle of the selective benefits of multiple mating and multiple paternity in social insects has been a major focus of research in evolutionary biology. We examine paternity in a clade of social insects, the vespine wasps (the yellowjackets and hornets), which contains species with high multiple paternity as well as species with single paternity. This group is particularly useful for comparative analyses given the wide interspecific variation in paternity traits despite similar sociobiology and ecology of the species in the genera Vespula, Dolichovespula and Vespa. We describe the paternity of 5 species of yellowjackets (Vespula spp.) and we perform a phylogenetically controlled comparative analysis of relatedness, paternity frequency, paternity skew, colony size, and nest site across 22 vespine taxa.

RESULTS

We found moderate multiple paternity in four small-colony Vespula rufa-group species (effective paternity 1.5 - 2.1), and higher multiple paternity in the large-colony Vespula flavopilosa (effective paternity ~3.1). Our comparative analysis shows that colony size, but not nest site, predicts average intracolony relatedness. Underlying this pattern, we found that greater colony size is associated with both higher paternity frequency and reduced paternity skew.

CONCLUSIONS

Our results support hypotheses focusing on the enhancement of genetic diversity in species with large colonies, and run counter to the hypothesis that multiple paternity is adaptively maintained due to sperm limitation associated with large colonies. We confirm the patterns observed in taxonomically widespread analyses by comparing closely related species of wasps with similar ecology, behavior and social organization. The vespine wasps may be a useful group for experimental investigation of the benefits of multiple paternity in the future.

Competitive impacts of an invasive nectar thief on plant-pollinator mutualisms

Plant-pollinator mutualisms are disrupted by a variety of competitive interactions between introduced and native floral visitors. The invasive western yellowjacket wasp, Vespula pensylvanica, is an aggressive nectar thief of the dominant endemic Hawaiian tree species, Metrosideros polymorpha. We conducted a large-scale, multiyear manipulative experiment to investigate the impacts of V. pensylvanica on the structure and behavior of the M. polymorpha pollinator community, including competitive mechanisms related to resource availability. Our results demonstrate that V. pensylvanica, through both superior exploitative and interference competition, influences resource partitioning and displaces native and nonnative M. polymorpha pollinators. Furthermore, the restructuring of the pollinator community due to V. pensylvanica competition and predation results in a significant decrease in the overall pollinator effectiveness and fruit set of M. polymorpha. This research highlights both the competitive mechanisms and contrasting effects of social insect invaders on plant-pollinator mutualisms and the role of competition in pollinator community structure.