Host specificity among Maculinea butterflies in Myrmica ant nests

Summary Ecological studies have been made of all 5 European species of Maculinea. These confirm that M. nausithous and M. rebeli live underground in Myrmica ant nests for 10 months of the year, as has long been known for the other 3 species. The main discovery was that each Maculinea species depends on a single, and different, host species of Myrmica. This specificity contradicts previous papers and scientific reviews of the relationship between Maculinea and ants. Therefore, early records are re-examined and 3 reasons are given to explain why most are misleading when applied to wild populations. Dependence on a single, rather than any, species of Myrmica explains why Maculinea populations exist in only a small minority of biotopes where their foodplants and Myrmica ants abound. It also explains the puzzling disappearance of Maculinea populations from apparently suitable sites. The discovery that M. alcon and M. rebeli depend on separate species of Myrmica that are not even closely related strengthens the argument that these butterflies are good species.     

The ecology of Myrmica ants in relation to the conservation of Maculinea butterflies

Maculinea butterflies in Europe, and probably most of Asia, are host specific social parasites of various species of Myrmica ants. The latest summary of field data showing the pattern of host specificity by Maculinea is presented. Myrmica ants have been well studied in the laboratory but much less is known about the ecology of their natural populations. While the former is important in understanding the adaptive evolution of Maculinea larval behaviours, the latter is of more practical importance to conservationists charged with the protection of specific populations of Maculinea. The current knowledge of habitat partition, colony growth and colony reproduction within communities of Myrmica ants is summarized in relation to the ecology of Maculinea species. Concepts used in current population simulation models are explained. A key concept is the idea that community structure (both number of species and size and abundance of nests) is controlled by the quantity and quality of suitable nest sites. Some advice is given to conservationists who might need to manipulate Myrmica ant populations in order to maintain a robust population of a Maculinea species. © Rapid Science Ltd. 1998     

Geographical variation in host–ant specificity of the parasitic butterfly Maculinea alcon in Denmark

Abstract  1. Maculinea alcon uses three different species of Myrmica host ants along a north–south gradient in Europe. Based on this geographical variation in host ant use, Elmes et al . (1994) suggested that M. alcon might consist of three or more cryptic species or host races, each using a single and different host-ant species.
2. Population-specific differences in allozyme genotypes of M. alcon in Denmark ( Gadeberg & Boomsma, 1997 ) have suggested that genetically differentiated forms may occur in a gradient across Denmark, possibly in relation to the use of different host ants.
3. It was found that two host-ant species are indeed used as hosts in Denmark, but not in a clear-cut north–south gradient. Furthermore, specificity was not complete for many M. alcon populations. Of five populations investigated in detail, one used primarily M. rubra as a host, another exclusively used M. ruginodis , while the other three populations used both ant species. No population in Denmark used M. scabrinodis as a host, although this species was present in the habitat and is known to be a host in central and southern Europe.
4. In terms of number of parasites per nest and number of nests parasitised, M. rubra seems to be a more suitable host in populations where two host species are used simultaneously. Host-ant species has an influence on caterpillar size but this varies geographically. Analyses of pupae did not, however, show size differences between M. alcon raised in M. rubra and M. ruginodis nests.
5. The geographical mosaic of host specificity and demography of M. alcon in Denmark probably reflects the co-evolution of M. alcon with two alternative host species. This system therefore provides an interesting opportunity for studying details of the evolution of parasite specificity and the dynamics of host-race formation.     

The social parasite Phengaris (Maculinea) nausithous affects genetic diversity within Myrmica rubra host ant colonies

Evolutionary theory predicts that high genetic variation maintains plasticity in a species’ response to parasite pressure. However, higher genetic diversity might also cause easier infiltration by social parasites, because odour diversity is high and nest-mate recognition poor. Here we test if the obligate myrmecophile Lycaenid butterfly Phengaris nausithous, a parasite of colonies of the highly polygynous ant Myrmica rubra causes local adaptation by enhancing genetic variance in parasitized versus non parasitized ant populations M. rubra colonies from six infested and three uninfested sites were assayed at five microsatellite loci to quantify genetic variation. Our results reveal isolation by distance and a significantly enhanced intracolonial variance due to the parasite pressure.     

Tetracosane on the cuticle of the parasitic butterfly Phengaris (Maculinea) nausithous triggers the first contact in the adoption process by Myrmica rubra foragers

Phengaris (Maculinea) butterflies are social parasites of Myrmica ant colonies. Larvae of the parasite are adopted by the ant workers into the colonies. Apparently, chemical signals are used by Phengaris nausithous Bergsträsser larvae to mimic those of the host brood to be recognized by the ants. In the present study, chemical extracts of ant brood and butterfly larvae using four different solvents are tested in behavioural choice assays in search of compounds involved in the adoption process. Tetracosane is the main shared compound in all brood extracts of Myrmica rubra L. and in all larvae of P. nausithous. The attractiveness of tetracosane for M. rubra workers is confirmed by testing synthetic tetracosane in behavioural choice assays, suggesting that the adoption ritual may be initiated by tetracosane.     

Chemical mimicry and host specificity in the butterfly Maculinea rebeli,a social parasite of Myrmica ant colonies

Although it has always been assumed that chemical mimicry and camouflage play a major role in the penetration of ant societies by social parasites, this paper provides the first direct evidence for such a mechanism between the larvae of the parasitic butterfly Maculinea rebeli and its ant host Myrmica schencki. In the wild, freshly moulted fourth-instar caterpillars, which have no previous contact with ants, appear to be recognized as ant larvae by foraging Myrmica workers, which return them to their nest brood chambers. Three hypotheses concerning the mechanism controlling this behaviour were tested: (i) the caterpillars produce surface chemicals that allow them to be treated as ant larvae; (ii) mimetic compounds would include hydrocarbons similar to those employed by Myrmica to recognize conspecifics and brood; and (iii) the caterpillars'' secretions would more closely mimic the profile of their main host in the wild, M. schencki, than that of other species of Myrmica. Results of behavioural bioassays and chemical analyses confirmed all three hypotheses, and explained the high degree of host specificity found in this type of highly specialized myrmecophile. Furthermore, although caterpillars biosynthesized many of the recognition pheromones of their host species (chemical mimicry), they later acquired additional hydrocarbons within the ant nest (chemical camouflage), making them near-perfect mimics of their individual host colony''s odour.     

Higher productivity at the cost of increased host-specificity when Maculinea butterfly larvae exploit ant colonies through trophallaxis rather than by predation

Abstract .1. Field studies were made of the benefits and costs of two feeding strategies in the genus Maculinea, whose final-instar larvae parasitise Myrmica ant colonies. Maculinea arion is an obligate predator of ant brood, whereas M. rebeli and M. alcon mimic ant larvae and are fed (like cuckoos) directly by the workers.
2. Samples of > 1500 Myrmica nests confirmed laboratory-based predictions that, by feeding at a lower trophic level, many (4.7-fold) more individuals of M. rebeli and M. alcon are supported per ant colony than M. arion.
3. Because of their efficient feeding, cuckoo species often occupied sites where their phytophagous early larval populations coincided to only a small extent (> 10%) with host Myrmica colonies, whereas all sites supporting M. arion had 50–100% of the phytophagous stages within foraging range of the host Myrmica species.
4. Greater host-specificity was identified as another consequence of cuckoo-feeding. The ecological cost of this is discussed .
5. The feeding of other Maculinea species had not been fully described: the data suggest that M. nausithous is a predator of ant brood and confirm that M. teleius is predacious .     

Bacterial communities within Phengaris (Maculinea) alcon caterpillars are shifted following transition from solitary living to social parasitism of Myrmica ant colonies

Bacterial symbionts are known to facilitate a wide range of physiological processes and ecological interactions for their hosts. In spite of this, caterpillars with highly diverse life histories appear to lack resident microbiota. Gut physiology, endogenous digestive enzymes, and limited social interactions may contribute to this pattern, but the consequences of shifts in social activity and diet on caterpillar microbiota are largely unknown. Phengaris alcon caterpillars undergo particularly dramatic social and dietary shifts when they parasitize Myrmica ant colonies, rapidly transitioning from solitary herbivory to ant tending (i.e., receiving protein‐rich regurgitations through trophallaxis). This unique life history provides a model for studying interactions between social living, diet, and caterpillar microbiota. Here, we characterized and compared bacterial communities within P. alcon caterpillars before and after their association with ants, using 16S rRNA amplicon sequencing and quantitative PCR. After being adopted by ants, bacterial communities within P. alcon caterpillars shifted substantially, with a significant increase in alpha diversity and greater consistency in bacterial community composition in terms of beta dissimilarity. We also characterized the bacterial communities within their host ants (Myrmica schencki), food plant (Gentiana cruciata), and soil from ant nest chambers. These data indicated that the aforementioned patterns were influenced by bacteria derived from caterpillars’ surrounding environments, rather than through transfers from ants. Thus, while bacterial communities are substantially reorganized over the life cycle of P. alcon caterpillars, it appears that they do not rely on transfers of bacteria from host ants to complete their development.     

Distribution,host specificity,and the potential for cryptic speciation in hoverfly Microdon myrmicae (Diptera: Syrphidae), a social parasite of Myrmica ants

1. In 2002 Microdon myrmicae, a social parasite of Myrmica ants, was taxonomically separated from Microdon mutabilis. The original study in the U.K. found Microdon myrmicae to be specific to one ant species, Myrmica scabrinodis, yet it became apparent that the range of Microdon myrmicae includes at least the western Palaearctic. 2. Current knowledge of the European distributions of both Microdon myrmicae and Microdon mutabilis in Europe is reviewed. Also, in detailed studies of two Polish populations, Microdon myrmicae was found to survive equally well with two Myrmica ant species. We examine, however, the possibility that this reflects the presence of two separate Microdon species, each connected to one species of Myrmica. 3. Forty populations of Microdon myrmicae and 37 populations of Microdon mutabilis are currently known in Europe. All the populations in central and southern Europe that were visited after the separation of the two species were identified as Microdon myrmicae, while Microdon mutabilis’ recognised range is now restricted to the British Isles and Scandinavia. Myrmica scabrinodis was found to host Microdon myrmicae in 26 out of 31 populations investigated. Four other Myrmica species were identified to the host Microdon myrmicae: Myrmica gallienii (eight populations), Myrmica rubra (four), Myrmica vandeli (one), and Myrmica sabuleti (one). Microdon myrmicae occurs in waterlogged grassland habitats, mainly of the ‘Molinietum’ type, resulting in a patchy distribution relative to its host ants. 4. In two populations Myrmica scabrinodis and Myrmica gallienii are both abundant and rear Microdon myrmicae in equal proportions. Microdon myrmicae pupae from Myrmica gallienii nests were heavier and the anterior respiratory organs were of significantly different shape. In contrast, the comparisons of Microdon myrmicae pupae among all other populations showed no significant differences, suggesting only one species throughout the European range.     

Host specificity revisited: New data on Myrmica host ants of the lycaenid butterfly Maculinea rebeli

Larvae of Maculinea rebeli, one of the most endangered European butterflies, are obligatory social parasites of Myrmica ants. At present, this relationship is thought to be highly specific, with Myrmica schencki being regarded as the primary host. Here we present data on six populations from Poland and Austria, including the first record of Myrmica specioides as a host, together with published data from other central European countries, which severely questions the inference that M. schencki is the exclusive host of M. rebeli. Our results indicate that Myrmica sabuleti is the most frequently used host ant in central Europe, whereas M. scabrinodis, M. sulcinodis, M. specioides and M. schencki are used as secondary hosts. Possible explanations for this highly variable host use include (1) regional differences in semiochemicals, behaviour or social structure of the potential Myrmica host species and (2) the existence of different ecological subspecies or cryptic species of M. rebeli. Finally, we emphasize the importance of identifying local host ant species prior to further conservation strategies in order to avoid failure of management programs or even damage to populations on the edge of extinction.     

Interspecific relationships in co‐occurring populations of social parasites and their host ants

Myrmica ant colonies host numerous insect species, including the larvae of Maculinea butterflies and Microdon myrmicae hoverflies. Little is known about the interspecific relationships among these social parasites and their host ants occurring in sympatric populations. We investigated communities of social parasites to assess the strategies allowing them to share the same pool of resources (i.e. Myrmica colonies). The present study was carried out at five sites inhabited by different social parasite communities, each comprising varying proportions of Maculinea teleius, Maculinea nausithous, Maculinea alcon, and Microdon myrmicae. We investigated their spatial distributions, host segregation, the degree of chemical similarity between social parasites and hosts, and temporal overlaps in colony resource exploitation. Spatial segregation among social parasites was found in two populations and it arises from microhabitat preferences and biological interactions. Local conditions can drive selection on one social parasite to use a Myrmica host species that is not exploited by other social parasites. Myrmica scabrinodis and Myrmica rubra nests infested by larvae of two social parasite species were found and the most common co‐occurrence was between Ma. teleius and Mi. myrmicae. The successful coexistence of these two species derives from their exploitation of the host colony resources at different times of the year. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 699–709.     

Performance of Myrmica ant colonies is correlated with the presence of social parasites

1. The performance of ant colonies depends on different factors such as nest site, colony structure or the presence of pathogens and social parasites. Myrmica ants host various types of social parasites, including the larvae of Maculinea butterflies and Microdonmyrmicae (Schönrogge) hoverfly. How these social parasites affect host colony performance is still unexplored. 2. It was examined how the presence of Maculinea teleius Bergsträsser, Maculinea alcon (Denis & Schiffermüller), and M. myrmicae larvae, representing different feeding and growth strategies inside host colonies, is associated with worker survival, the number of foragers, and colony productivity parameters such as growth and reproduction. 3. It was found that the presence of social parasites is negatively associated with total colony production and the production of ant larvae and gynes. Male production was lower only in nests infested by M. teleius, whereas the number of worker pupae was significantly higher in all types of infested colonies than in uninfested colonies. Laboratory observations indicated that nests infested by Maculinea larvae are characterised by a higher number of foragers compared to uninfested nests but we did not find differences in worker survival among nest types. 4. The observed pattern of social parasite influence on colony productivity can be explained by the feeding strategies of parasitic larvae. The most negative effect was found for M. teleius, which feeds on the largest host brood and eliminates a high number of sexual forms. The strong, adverse influence of all studied parasite species on gyne production may result in low queen production in Myrmica populations exposed to these social parasites.     

Mimetic host shifts in an endangered social parasite of ants

An emerging problem in conservation is whether listed morpho-species with broad distributions, yet specialized lifestyles, consist of more than one cryptic species or functionally distinct forms that have different ecological requirements. We describe extreme regional divergence within an iconic endangered butterfly, whose socially parasitic young stages use non-visual, non-tactile cues to infiltrate and supplant the brood in ant societies. Although indistinguishable morphologically or when using current mitochondrial and nuclear sequence-, or microsatellite data, Maculinea rebeli from Spain and southeast Poland exploit different Myrmica ant species and experience 100 per cent mortality with each other''s hosts. This reflects major differences in the hydrocarbons synthesized from each region by the larvae, which so closely mimic the recognition profiles of their respective hosts that nurse ants afford each parasite a social status above that of their own kin larvae. The two host ants occupy separate niches within grassland; thus, conservation management must differ in each region. Similar cryptic differentiation may be common, yet equally hard to detect, among the approximately 10 000 unstudied morpho-species of social parasite that are estimated to exist, many of which are Red Data Book listed.     

Food stress causes differential survival of socially parasitic caterpillars of Maculinea rebeli integrated in colonies of host and non-host Myrmica ant species

Final instar larvae of Maculinea rebeli Hirschke (Lepidoptera, Lycaenidae) are social parasites of Myrmica Latreille (Hymenoptera, Formicidae) nests. In the populations of the southern French Alps and Spanish Pyrenees, >95% adult M. rebeli emerge from colonies of Myrmica schencki Emery, despite >60% caterpillars being adopted by other Myrmica species (non‐hosts). However, in laboratory culture caterpillars can be reared successfully by many of the non‐host species. This contradiction, which has led some to question the existence of host specificity, has been explained by the lack of stress, particularly food stress, in laboratory cultures compared to wild conditions. Here, we report the results of an experiment that tested the survival of M. rebeli caterpillars that had been growing well, after being socially integrated into a series of host and non‐host cultures, and were then subjected to a 4‐week period of stress induced by a ‘starvation diet’ estimated to be less than the minimum for ant survival. Significantly more M. rebeli survived in M. schencki cultures than with any of the other Myrmica species (all died in most non‐host cultures). Under a starvation diet, caterpillars are killed and eaten along with dead workers – this never happens under an ample diet – rather than simply starving to death. It was noted that the proportion of young M. rebeli caterpillars that survived initial integration into an ant colony (including some M. schencki colonies) was a good predictor of subsequent survival under starvation conditions. We concluded that two key phases of host specificity exist in the life of this social parasite: initial integration, in which the caterpillar simply has to be accepted into a host society, followed by full integration, when a relatively high hierarchical status within the host society becomes essential for a caterpillar's survival during periods when the host colony is stressed, e.g., by food shortage. This experimental regime provides a useful bioassay for testing host specificity in other populations of Maculinea.     

Effects of latitude, altitude and climate on the habitat and conservation of the endangered butterfly Maculinea arion and its Myrmica ant hosts

Analyses were made of the habitats of the endangered myrmecophilous butterfly, Maculinea arion, and its host ant, Myrmica sabuleti, in five regions, spanning most of the climatic range of the butterfly in west Europe. Near their altitudinal or north latitudinal limits, both species are confined to warm south-facing slopes where the turf is grazed <3 cm tall, but at low altitudes further south, where the summer climate is 4°C warmer, the butterfly and ant inhabit ground with any aspect other than south-facing, and the butterfly is restricted to swards >20 cm tall. Intermediate types of habitat were used in regions with intermediate summer climates. The implications for conservation management are discussed. The fact that M. arion has a narrow niche and occupies very early successional stages near its range-edge makes it difficult to conserve in regions with cool climates. A successful example is described from one site in the UK. During 20 years of intensive management the sward structure was altered from being tall and dense to that predicted as optimal for M. arion at its range-edge. The species of Myrmica changed greatly during this period, with thermophilous ants, including M. sabuleti, supplanting cool-loving non-host species. A population of M. arion now inhabits this site. We predict that the conservation of M. arion will be easier and cheaper to achieve under the warm climates of central lowland Europe where different, less intensive, management is required. © Rapid Science Ltd. 1998     

An evolutionarily stable strategy model for the evolution of dimorphic development in the butterfly Maculinea rebeli, a social parasite of Myrmica ant colonies

Caterpillars of the butterfly Maculinea rebeli develop as parasites inside ant colonies. In intensively studied French populations, about 25% of caterpillars mature within 1 year (fast-developing larvae [FDL]) and the others after 2 years (slow-developing larvae [SDL]); all available evidence indicates that this ratio is under the control of egg-laying females. We present an analytical model to predict the evolutionarily stable fraction of FDL (pESS). The model accounts for added winter mortality of SDL, general and kin competition among caterpillars, a competitive advantage of SDL over newly entering FDL (priority effect), and the avoidance of renewed infection of ant nests by butterflies in the coming season (segregation). We come to the following conclusions: (1) all factors listed above can promote the evolution of delayed development; (2) kin competition and segregation stabilize pESS near 0.5; and (3) a priority effect is the only mechanism potentially selecting for pESS < 0.5. However, given the empirical data, pESS is predicted to fall closer to 0.5 than to the 0.25 that has been observed. In this particular system, bet hedging cannot explain why more than 50% of larvae postpone growth. Presumably, other fitness benefits for SDL, for example, higher fertility or longevity, also contribute to the evolution of delayed development. The model presented here may be of general applicability for systems where maturing individuals compete in small subgroups.     

Reconciling genetic expectations from host specificity with historical population dynamics in an avian brood parasite, Horsfield's Bronze-Cuckoo Chalcites basalis of Australia

Mitochondrial DNA (mtDNA) is being used increasingly to explore the evolution of host specificity in avian brood parasites. A stable coevolutionary equilibrium between multiple phylogenetically unrelated hosts and a brood parasitic species predicts that mtDNA diversity in the parasite should be relatively deep and phylogenetically structured. Also, the different intraspecific clades resulting from parasitism to multiple sympatric hosts should themselves occur sympatrically. However, mtDNA diversity in brood parasites is as susceptible to effects of historical population dynamics as in any species. We demonstrate the relevance of these dynamics to the use of mtDNA in understanding coevolution between an Australian brood-parasite, Horsfield's Bronze-Cuckoo Chalcites basalis and its hosts, Malurus fairy-wrens and Acanthiza thornbills. Previous ecological and behavioural analyses argue that Malurus- and Acanthiza-specific host races exist in C. basalis. Yet mtDNA diversity in C. basalis is low and phylogenetically unstructured (mean sequence divergence 0.15 +/- 0.07%, range 0.00%-0.31%) and tests of mtDNA neutrality and range expansion vs. population stability (Tajima's D, Fu & Li's F* and D*, Fu's F(S), mismatch analyses) all indicate that C. basalis has expanded its range very recently, probably within the last few tens of thousands of years following climatic amelioration after a peak of aridity in the late Pleistocene. The low mtDNA diversity and its lack of phylogenetic structure in C. basalis deny the existence of evolutionarily long-term stable host races in C. basalis but not the possibility of recently evolved ones. They highlight the need for renewed behavioural and ecological study of the relationship between C. basalis and its hosts. Our findings illustrate the need to understand the evolutionary context in which a brood parasite and its hosts have evolved if mtDNA data are to be used in testing hypotheses concerning the origin and maintenance of host specificity. They also add to the growing body of work illustrating the use of mismatch analyses and Fu's F(S) in detecting range expansions.     

Interspecific differences in cuticular hydrocarbon profiles of Myrmica ants are sufficiently consistent to explain host specificity by Maculinea (large blue) butterflies

The chemical signatures on the cuticles of five common Myrmica ant species were analysed (49 colonies of M. rubra, M. ruginodis, M. sabuleti, M. scabrinodis and M. schencki), each ant being the specific host of one of the five threatened European species of Maculinea butterfly. The cuticular hydrocarbon profile (based on the relative abundance of each chemical) of each ant species was highly distinctive, even between the morphologically similar species M. sabuleti and M. scabrinodis. There was no significant difference in the chemical profiles of workers and larvae from any colony. Nor was there much pattern in the intraspecific variation: colonies from the same populations were significantly, but only slightly, more similar to each other than to colonies from distant populations. M. rubra showed remarkably little variation between populations sampled widely from northern Russia, Ukraine, Scotland and southern England. The data were compared with published profiles of M. rubra and two North American Myrmica species, and with a quantitative reanalysis of data for Maculinea rebeli caterpillars. We conclude that the hydrocarbon profiles of Myrmica species are sufficiently and consistently different for chemical mimicry to explain the pattern of host specificity recorded for the European Maculinea butterflies. The optimum strategy for chemical mimicry in each of the two life-styles of Maculinea larvae is discussed: we suggest that predatory species might benefit from mimicking the median profile of their model whereas the "cuckoo" species would benefit when variation between siblings encompasses a large range of the variation recorded within a local population of the model species.     

Microsatellite analysis reveals strong but differential impact of a social parasite on its two host species

The speed and the dynamics of the co-evolutionary process strongly depend on the relative strengths of reciprocal selection pressures exerted by the interacting species. Here, we investigate the influence of an obligate social parasite, the slave-making ant Harpagoxenus sublaevis, on populations of the two main host species Leptothorax acervorum and Leptothorax muscorum from a German ant community. A combination of genetic and demographic data allowed us to analyse the consequences of raiding pressure on the hosts' life history and possible host preferences of the parasite. We can demonstrate that slave raids during which the social parasite pillages brood from neighbouring host colonies are both frequent and extremely destructive for both host species. Microsatellite analysis showed that, on average, a single slave-maker colony conducts more than three raids per year and that host colonies mostly perish in the aftermath of these parasite attacks. Only in few cases, surviving nests of previously raided host colonies were found in the surroundings of slave-maker colonies. As a consequence of the high prevalence of parasites and their recurrent and devastating slave raids on host colonies, the life expectancy of host colonies was severely reduced. Combining our results on host-specific parasitic colony founding and raiding frequencies with the post-raid survival rate, we can demonstrate an overall higher mortality rate for the smaller host species L. muscorum. This might be caused by a preference of H. sublaevis for this secondary host species as demographic data on host species usage indicate.