Impacts of inbreeding on bumblebee colony fitness under field conditions

Background  

Inbreeding and the loss of genetic diversity are known to be significant threats to small, isolated populations. Hymenoptera represent a special case regarding the impact of inbreeding. Haplodiploidy may permit purging of deleterious recessive alleles in haploid males, meaning inbreeding depression is reduced relative to diploid species. In contrast, the impact of inbreeding may be exacerbated in Hymenopteran species that have a single-locus complementary sex determination system, due to the production of sterile or inviable diploid males. We investigated the costs of brother-sister mating in the bumblebee Bombus terrestris. We compared inbred colonies that produced diploid males and inbred colonies that did not produce diploid males with outbred colonies. Mating, hibernation and colony founding took place in the laboratory. Once colonies had produced 15 offspring they were placed in the field and left to forage under natural conditions.     

Rearing techniques for hornets with emphasis on Vespa velutina (Hymenoptera: Vespidae): A review

Of the 34 vespid species recorded worldwide as invasive aliens, particular attention is currently being given to the yellow-legged hornet Vespa velutina that has invaded Europe, Japan, and Korea. The hornet is a voracious predator of bees and a serious threat to bee colonies and bee pollination. Control measures are needed, but their development has been challenging as biological and ecological studies are limited by the short field season and the cryptic nesting behavior of these hornets. A Vespa rearing process can generate the large numbers of workers, gynes, and males that are essential for studying chemical ecology and life history and for experimental testing of various hypotheses. We present a synthesis of suitable methods and techniques for year-round Vespa hornet rearing. Particular reference is given to Chinese know-how and experience with Vespa rearing for medicinal and culinary motivations. We also draw on observations with reared insects that have a similar life history as Vespa hornets, namely Bombus bumblebees and Vespula yellowjackets. Key challenges to optimizing Vespa hornet rearing are identified and discussed.     

Cover Caption

Vespa velutina is the first Vespidae predator of honeybees to be accidentally introduced into Europe from Asia. This invasive hornet species typically hunts domestic honeybees by hovering at the hive entrance. Our capture‐markrecapture study shows that within the apiary, the distribution of V. velutina is heterogeneous. This is not related to the honeybee colony size but could reflect the variability in defensiveness between colonies (see pages 765–774). Photoed by Karine Monceau.     

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

Social insects rank among the most invasive of terrestrial species. The success of invasive social insects stems, in part, from the flexibility derived from their social behaviors. We used genetic markers to investigate if the social system of the invasive wasp, Vespula pensylvanica, differed in its introduced and native habitats in order to better understand variation in social phenotype in invasive social species. We found that (1) nestmate workers showed lower levels of relatedness in introduced populations than native populations, (2) introduced colonies contained workers produced by multiple queens whereas native colonies contained workers produced by only a single queen, (3) queen mate number did not differ significantly between introduced and native colonies, and (4) workers from introduced colonies were frequently produced by queens that originated from foreign nests. Thus, overall, native and introduced colonies differed substantially in social phenotype because introduced colonies more frequently contained workers produced by multiple, foreign queens. In addition, the similarity in levels of genetic variation in introduced and native habitats, as well as observed variation in colony social phenotype in native populations, suggest that colony structure in invasive populations may be partially associated with social plasticity. Overall, the differences in social structure observed in invasive V. pensylvanica parallel those in other, distantly related invasive social insects, suggesting that insect societies often develop similar social phenotypes upon introduction into new habitats.     

Do hornets have zombie workers?

Colonies of the European hornet, Vespa crabro, are typically founded by a single queen mated to a single male. From the resulting colony relatedness pattern we predicted strong worker-queen conflict over male production where both the workers and the queen attempt to produce the colony's males. To test for this conflict, male production was studied in 15 hornet nests using a combination of DNA microsatellite analysis (282 males), worker ovary dissections (500 workers from eight nests) and 50 h of observation (four nests). In contrast to our prediction, the data show that hornet males are queens' sons, that workers never attempt to lay eggs, rarely have activated ovaries, and that there is no direct aggression between the queen and the workers. This contrasts with other data for vespine wasps, which support relatedness predictions. Dolichovespula arenaria has the same kin structure as V. crabro and workers produce males in many colonies. The similarity between these two species makes it difficult to explain why workers do not reproduce in V. crabro. Self-restraint is expected if worker reproduction significantly reduces colony productivity but there is no obvious reason why this should be important to V. crabro but not to D. arenaria. Alternatively, queen control may be important. The absence of expressed queen-worker conflict rules out physical control. Indirect pheromonal control is a possibility and is supported by the occurrence of royal courts and queen pheromone in Vespa but not Dolichovespula. Pheromonal queen control is considered evolutionarily unstable, but could result from a queen-worker arms race over reproductive control in which the queen is ahead. The genetic data also revealed diploid males in one colony, the first example in the vespine wasps, and two colonies with double matrilines, suggesting that occasional usurpation by spring queens occurs.     

Diploid males and their triploid offspring in the paper wasp Polistes dominulus

Although the hymenopteran sex-determining mechanism generally results in haploid males and diploid females, diploid males can be produced via homozygosity at the sex-determining locus. Diploid males have low fitness because they are effectively sterile or produce presumably sterile triploid offspring. Previously, triploid females were observed in three species of North American Polistes paper wasps, and this was interpreted as indirect evidence of diploid males. Here we report what is, to our knowledge, the first direct evidence: four of five early male-producing Polistes dominulus nests from three populations contained diploid males. Because haploid males were also found, however, the adaptive value of early males cannot be ignored. Using genetic and morphological data from triploid females, we also present evidence that both diploid males and triploid females remain undetected throughout the colony cycle. Consequently, diploid male production may result in a delayed fitness cost for two generations. This phenomenon is particularly relevant for introduced populations with few alleles at the sex-determining locus, but cannot be ignored in native populations without supporting genetic data. Future research using paper wasp populations to test theories of social evolution should explicitly consider the potential impacts of diploid males.     

Relationship between the age of Vespa velutina workers and their defensive behaviour established from colonies maintained in the laboratory

As the structural bases of insect societies are essential to colony survival, nests must be protected from predation. Nest defence behaviours are among the most important roles assigned to worker members. However, in hornet societies, temporal polyethism (age-dependent division of labour among workers) is assumed to be weak. Few studies have investigated this phenomenon, probably because hornet nests are aggressively defended and dangerous to approach. In the present study, we propose a method for rearing nests of Vespa velutina, a species newly introduced into Europe. This method enables the handling of hornets, and thus the design of experiments. By marking all newly emerged hornets, we recorded aggressiveness in workers of different ages from three captive colonies. We observed that nest defence behaviour in V. velutina depends on the age of the workers. Nest defence appears to be mediated by the queen, probably through pheromones that promote nest organization. We also identified a previously unreported but important behaviour in V. velutina that workers are aggressive towards male hornets. This behaviour might be a strategy to avoid inbreeding. Collectively, our results provide new research perspectives for the management of social hymenopteran predators.     

Delayed sexual maturity in males of Vespa velutina

Vespa velutina var nigrithorax (Lepelletier, 1835) is an invasive predator of bees accidentally introduced in France in 2004, and it is having a serious impact on apiculture and ecosystems. Studying the reproduction of an invasive species is key to assess its population dynamic. This study explores the sexual maturation of V. velutina males and the evolution of their fertility. The main studied parameters were physiologic (spermiogenesis, spermatogenesis) and anatomic (testes size and structure, head width). Two populations of males were described based on their emergence period: early males in early summer or classic males in autumn. Each testis has an average of 108 testicular follicles. Spermatogenesis is synchronous, with only 1 sperm production wave, and completed, on average, at 10.3 d after emergence with the degeneration of the testes. The sperm counts in seminal vesicles of mature males are 3 × 10⁶ in October/November and 0.8 × 10⁶ in June. In comparison, females store 0.1 × 10⁶ sperm in their spermathecae. The early males emerged from colonies made by fertilized queens. The reproductive potential of these early males seemed limited, and their function in the colony is discussed. The sperm stock evolution in autumn males suggests the occurrence of a reproductive pattern of male competition for the access to females and a single copulation per male. The synchronicity of male and foundress emergences and sexual maturation is of primary importance for the mating success and the future colony development.     

Daily and Seasonal Extranidal Behaviour Variations in the Invasive Yellow-Legged Hornet, <Emphasis Type="Italic">Vespa velutina</Emphasis> Lepeletier (Hymenoptera: Vespidae)

In eusocial insect species, the nest represents the fundamental element of the colony. Extranidal activities (foraging, nest maintenance, defence) are fundamental for the development and the survival of the colony. Therefore, they may represent interesting targets to disrupt to limit their expansion in case of pest species such as Vespa velutina, an alien predator of domestic honeybees in Europe. An accurate knowledge of the pattern of activity of this pest’s colonies is therefore required. Due to the highly defensive nature of this hornet, a video monitoring was realized on two colonies during their growth from August to November. Three major behaviours were monitored: nest maintenance, patrolling on the nest and foraging flights. Although of different size and monitored during different years, the two colonies exhibited similar patterns of daily and seasonal variation. This work is a first step in the study of this pest especially in view of control program.     

Weak nestmate discrimination behavior in native and invasive populations of a yellowjacket wasp (<Emphasis Type="Italic">Vespula pensylvanica</Emphasis>)

In geographic regions with warm winters, invasive yellowjacket wasp colonies (genus Vespula) often exhibit polygyny (multiple queens) and persist for multiple years, despite these phenomena being rare in the native range. Here, we test the hypothesis that polygyny, caused by foreign queens being accepted into an existing colony, is the result of relaxed nestmate recognition in the invasive range, as has been observed in some supercolonial invasive ants. In bioassays with wild colonies in the field, we found that nestmate discrimination was weak in both invasive (Hawaii) and native (California) populations of Vespula pensylvanica, with significant nestmate discrimination in only ~?30% of trials. We also found that the diversity and variability of cuticular hydrocarbons, chemical compounds that mediate nestmate recognition, were not reduced in introduced populations, unlike several supercolonial invasive ant species. Our findings suggest that ancestral weak nestmate discrimination behavior of V. pensylvanica may make this species pre-adapted to transition to polygyny and extended colony lifespans when introduced into environments with benign winters that facilitate foreign queens joining existing colonies in late season.     

Reproductivity of early males of the temperate paper wasp Polistes rothneyi iwatai

In Polistes paper wasps, haploid early males can mate with early emerging females and leave viable offspring. In contrast, diploid early males are eventually sterile because they contribute triploid offspring via diploid sperm. Clarifying the ploidy of early males is important for determining whether early male production is a reproductive strategy for the species. We examined the mating behavior and the ploidy of early males in the Japanese paper wasp, Polistes rothneyi iwatai van der Vecht. Thirteen early males from four colonies were all diploid. Two of the nine early males (22.2%) attempted to mate with females, but only one individual (11.1%) was successful (the female's spermatheca contained spermatozoa). These results suggest that although most early males of P. rothneyi iwatai do not produce offspring, their mating may be linked to the occasional production of triploid females.     

Influence of inbreeding in the early stages of artificially reared colonies of Bombus terrestris

In the present study on the bumblebee Bombus terrestris, we investigated the influence of inbreeding on queen fitness by comparing diapause survival and egg-laying success of queens mated with nestmate and non-nestmate males. We then compared the early stage of colonies with or without diploid males and analysed colony characteristics to identify a factor predictive of colony outcome. Diapause survival was no different between queens mated with nestmates and non-nestmates, but in the latter case, egg-laying success was significantly higher. Queens mated with nestmates gave rise to a percentage of diploid male colonies (52.6%) compatible with brother–sister coupling. We obtained 18.6% of colonies with diploid males even from queens mated with non-nestmates, indicating that the colonies of origin were in some way related or homozygous at the sex determination loci. There was no difference in the early growth stage between colonies with or without diploid males, except in the number of workers emerging in the first brood, which was significantly higher in the latter. Among diploid male colonies, the number of workers and the male/worker ratio in the first brood was highly variable and was not a good predictor of subsequent colony growth. Out of 49 colonies with diploid males that reached full development, only 11 had a sufficient size to assume that they could survive in the field or, in a commercial breeding, to be suitable for pollination purposes.     

Distribution,spread, and impact of the invasive hornet Vespa velutina in South Korea

Hornets (Vespa spp) are top insect predators that can control pests, but their venomous stings and defensive behavior cause numerous human deaths throughout Asia. Hornets usually inhabit rural areas which reduces potential conflict with humans. In 2003, the invasive hornet, Vespa velutina, arrived in southern Korea (Yeongdo region) and became established. It is currently spreading northwards at a rate of 10–20 km per year. Despite originating in tropical/subtropical areas of Indo-China, its nesting biology and life cycle in South Korea are similar to those found throughout its native range, with mature colonies containing 1000–1200 adults. In 7 years, V. velutina has become the most abundant hornet species in Southern Korea by displacing native Vespa species such as V. simillima, which has a similar nesting biology. We also found a significant positive correlation between the abundance of V. velutina and the degree of urbanization, indicating that this invasive species was well adapted to urban environments. This was supported by our finding that 41% of emergency call-outs (119 Rescue Services) to deal with social wasps/hornet problems were due to V. velutina, which was twice as high as the number of calls about the next most abundant species. The rapid spread of V. velutina across southern Korea indicates that this species will continue to spread north-westward in the Korean peninsula and will become a major problem as more people and beekeepers come into contact with this aggressive invasive hornet.     

Diploid males in the bumble beeBombus terrestris

The frequency of colonies that produce diploid males after brother-sister (50%) and nephew-niece (37.5%) matings proves that inB. terrestris the sex is determined by a single multi-allelic sex locus. The diploid males which develop normally into adults make up 50% of the diploid brood. In the laboratory the growth rate of colonies with diploid males is influenced only slightly. Of 41 colony founding queens caught out of a natural population, all produced a colony without any diploid males. Therefore, the number of sex alleles in this population is estimated to be at least 24. This means that in commercial rearing systems for bumble bees, involving several generations, the occurrence of diploid males can largely be prevented by a good scheme for crossings.     

Review of the invasive yellow-legged hornet, <Emphasis Type="Italic">Vespa velutina nigrithorax</Emphasis> (Hymenoptera: Vespidae), in Japan and its possible chemical control

The distribution of the invasive yellow-legged hornet, Vespa velutina nigrithorax Buysson, is expanding in Europe and Asia. In Japan, this species was first detected on Tsushima Island, near Korea, in 2012. Because the rapid expansion of its distribution has raised public concern, the Japanese Ministry of the Environment added this hornet to the list of invasive alien species in 2015. In this paper, to contribute to the development of control techniques for this hornet, we present its phylogeny and life history. We then report its global expansion history and present status. Last, we review case studies of chemical control of vespine wasps and discuss effective techniques to control the hornet on Tsushima, where many Asian honeybee hives are kept. Successful case studies with Vespula wasps suggest the possibility of toxic baiting for the control of V. velutina nigrithorax.     

The invasion by the Yellow-legged hornet: A systematic review

The Yellow-legged hornet (Vespa velutina nigrithorax), native to regions of Southeast Asia, was accidentally introduced in Europe, South Korea, and Japan, where is has often become invasive. Due to its potential negative impacts at ecologic, economic and social levels, this hornet was included in the “Union list” of the EU legislation for invasive alien species. This means that measures are urgently needed to prevent further introductions, as well as to early-detect and control spread to avoid new populations. In this study we aim to identify the main reported drivers of distribution, ecological preferences, impacts, and methods for preventing introduction, controlling, and managing this invasive species. The supporting information was obtained from a comprehensive literature search. Then, a literature review was performed to classify the records gathered and to extract the relevant information following an adapted Drivers-Pressures-State-Impacts-Responses framework. The achieved results show a growing interest of researchers on V. velutina nigrithorax through time due to its quick spread and impacts on new ecosystems. They also revealed that there is much information on the State of invasions, whereas more knowledge is needed regarding the Drivers of those invasions. Biological traits such as life history traits, morphology, and the sting venom properties are some of the most studied topics regarding V. velutina nigrithorax. In the future, research should focus on the topics that lack information, analyse other Response solutions that meet the intended measures by the EU legislation, and use new methodology to study the impacts caused by this invader under new perspectives.     

Attack or retreat: contrasted defensive tactics used by Cyprian honeybee colonies under attack from hornets

This study describes the tactics used by Cyprian honeybees (Apis mellifera cypria) to defend their colonies against hornet (Vespa orientalis orientalis) attacks. We use simulated hornet attacks and a combination of video recordings and image analysis to reveal, for the first time, contrasted intra-subspecies defensive tactics that operate at the colony level during predation. In some colonies, when attacked, the numbers of guards at the hive entrance increases rapidly to attack, engulf, and kill invading hornets. In other colonies, guards avoid conflicts with hornets by retreating gradually and by forming a defensive line of honeybees at the hive entrance. Retreater colonies have propolis walls at the hive entrances with small apertures that are too narrow to allow the hornet to access the hive and that therefore reinforces entrance protection. On the contrary, attacker colonies have propolis walls with large openings through which the hornet can pass; these bees block the hornet's access by intensively guarding the hive entrance. We experimentally destroy propolis walls to test whether colonies consistently rebuild walls with the same intrinsic characteristics and we also monitor the survival rate of each anti-predator tactic after massive natural predation by hornets.     

Correlates of reproductive success among field colonies of Bombus lucorum: the importance of growth and parasites

Abstract.

• 1 In natural populations, colonies of bumble bees vary in many important life history traits, such as colony size and age at maturity, or the number and sex of reproductives produced. We investigated how the presence of parasites in field populations of the bumble bee Bombus lucorum L. relates to variation in life history traits and reproductive performance. A total of thirty-six colonies was placed in accessible nest sites in the field and monitored at regular intervals throughout a season.
• 2 Among the life history correlates, early nest foundation was strongly associated with large maximum colony size, old age and large size at maturity, and this in turn with successful production of males and queens, as well as with the number of sexuals produced. Overall, reproductive success was highly skewed with only five colonies producing all the queens. Sixteen colonies failed to reproduce altogether.
• 3 The social parasite Psithyrus was abundant early in the Bombus colony cycle and preferentially invaded host nests with many first brood workers and thus disproportionately large size, i.e. those colonies that would otherwise be more likely to reproduce or produce (daughter) queens rather than males. To prevent nest loss, Psithyrus had to be removed soon after invasion. Therefore, the effects reported here can only be crude estimates.
• 4 Parasitoid conopid flies are likely to cause heavy worker mortality when sexuals are reared by the colonies. Their inferred effect was a reduction in biomass that could be invested in sexuals as well as a shift in the sex ratio at the population level resulting from failure to produce queens. We suggest to group the inferred correlates into ‘early events’ surrounding colony initiation and social parasitism, and ‘late events’ surrounding attained colony size in summer and parasitism by conopid flies. Our evidence thus provides a heuristic approach to understand the factors that affect reproductive success of Bombus colonies.

     

How <Emphasis Type="Italic">Apis mellifera</Emphasis> Behaves with its Invasive Hornet Predator <Emphasis Type="Italic">Vespa velutina</Emphasis>?

Invasive species are now recognized as a major cause of native biodiversity loss worldwide. In the current deleterious context for pollinators, the invasive yellow-legged hornet, Vespa velutina, represents an additional threat to the domestic honeybee, Apis mellifera, in Europe. Therefore, understanding the impact of this predator on honeybee colonies is of major importance. In the present study, we tried to assess the impact of V. velutina on the honeybee foraging and defence behaviour based on the video monitoring of two hives. Balling behaviour is reported here for the first time under natural conditions in A. mellifera against V. velutina in Europe. Although these results are preliminary and should be carefully considered, we found that the number of hornets impacted honeybee foraging and defence behaviours. More defensive behaviours were notified in the hive, which survives slightly longer. This may suggest that selecting for more defensive colonies may provide an interesting perspective.     

Competition between the native and the introduced hornets Vespa crabro and Vespa velutina: a comparison of potentially relevant life‐history traits

1. Invasive alien species are a major threat to biodiversity. In addition to predation and parasitism, native species might suffer from competition when invasive alien species occupy a similar ecological niche. 2. This study focused on the potential interspecific interaction between two hornets: the Asian yellow‐legged hornet, Vespa velutina, a high‐concern invasive alien species recently arrived in Europe; and the native European hornet, Vespa crabro. The two species share a similar ecological niche and V. velutina is rapidly expanding across Europe, which suggests that V. crabro might suffer from competition. 3. Under laboratory‐controlled conditions, two life‐history traits that might cause the two species to compete were investigated: (i) the ability of workers to find food sources and their flexibility in exploiting them (through individual food item choice tests and exploration assays); and (ii) the worker resistance to pathogens (through immune challenge tests). 4. The results show that trophic preference of both species highly overlaps, with a marked dietary preference for honeybees compared with other insect prey and non‐prey protein items. No differences were observed in the exploratory behaviour of both species. Finally, constitutive antibacterial activity was greater in workers of the native species than in workers of the invasive hornet. 5. This laboratory study provides a first assessment under controlled conditions of the factors affecting competition between workers of two hornet species and proposes a framework to assess, in wild contexts, the magnitude of the competition and the impact of the introduced V. velutina on the native V. crabro.