Endangered Blue Butterfly Breaks the Ant Rules of Its Own Species

In the sandy heathlands of the northern Upper Rhine Valley, one of Germany’s most endangered butterflies is quietly rewriting a textbook rule about its own species. The Idas Blue (Plebejus idas), a small lycaenid butterfly whose caterpillars are famous for striking up intimate partnerships with ants, has long been classified as an obligate myrmecophile — a species whose larvae cannot survive without the attendance of particular ant partners. Now a detailed field study, published in Nota Lepidopterologica, has shown that the last remnant population in Baden-Württemberg behaves in a fundamentally different way, engaging ants only opportunistically. It is the first time such a profound intraspecific difference in the degree of myrmecophily has been demonstrated for any lycaenid butterfly, and the finding carries urgent weight for a population hovering on the brink of extinction.

Myrmecophily — the close association between caterpillars and ants — is one of the most celebrated stories in butterfly biology. In many lycaenid species, caterpillars possess specialized glands that secrete sugary fluids and nutrient-rich offerings, and in exchange for these rewards, ant workers guard the larvae against predators and parasitoid wasps. German researcher Konrad Fiedler, whose extensive reviews underpin much of the field, distinguished between facultative and obligatory relationships. In facultative associations, caterpillars may be tended by whatever ants happen to encounter them, gaining partial protection but depending on no one species. In obligatory myrmecophily, by contrast, larvae rely on the presence of workers of one ant species, or a few closely related ones, while feeding on their host plants — and females of these butterflies typically lay their eggs only where they can assure themselves of the right ants. Among Europe’s non-parasitic lycaenids, only two species, the Silver-studded Blue (Plebejus argus) and the Idas Blue, were previously considered to show obligatory ant associations.

The Idas Blue itself is a butterfly of enormous geographic reach. Its range spans the holarctic, from North America across the Palearctic to the North Cape, and its populations occupy habitats from sea level to over 2000 meters in the Alps. Roughly 50 host plants from four plant families have been reported across its range, reflecting pronounced regional variation in diet. Yet in the northern Upper Rhine Valley, the picture is starkly different: the species is monophagous there, feeding exclusively on Broom (Cytisus scoparius), a shrubby legume, at both study sites and at numerous surrounding localities of former occurrence. This dietary narrowness, combined with the population’s precipitous decline, made the region’s Idas Blues a compelling puzzle for researchers Paul Schullerer, Ralf Güsten and Matthias Sanetra, who undertook the field study at two remaining localities about 30 kilometers apart.

The southern study site lay in the Stollhofener Platte nature reserve at Rheinmünster, while the northern site was a decommissioned army firing range in the Mörscher Hardt forest at Rheinstetten. A third locality, a disused gravel pit at Malsch, where the butterfly had last been recorded in 2019, could not be reconfirmed during the study. The state species protection programme of Baden-Württemberg has monitored the Idas Blue at these sites since the 1990s, and the monitoring data tell a grim story: roughly half of the occupied sites were lost between 1995 and 2009, and today only two localities in the state still harbour the species. The researchers searched intensively for caterpillars across both generations in 2022 and 2023, recording for every larva found the accompanying ants, the height of the caterpillar above ground, and the total height of its host plant.

Sixteen caterpillars were located after intense searching — eight at each site, nine belonging to the first generation and seven to the second. All larvae were green with a darker dorsal stripe and white markings, and most were in their final fourth instar. What the researchers found attending them was remarkable in its diversity. Five species of formicine ants were recorded as symbionts: Formica pratensis, F. rufibarbis, F. cunicularia, Lasius niger and L. psammophilus. At Mörscher Hardt, all attending ants were F. pratensis, apart from a single worker of F. cunicularia found on one caterpillar. At Stollhofener Platte, the dominant partner was L. psammophilus, with one F. rufibarbis record and one pupa attended by F. pratensis. At Malsch, earlier high-quality photographs allowed the identification of L. niger attending two caterpillars. The ant partners recorded reflected the dominance patterns of the respective biotopes — exactly what one expects from a facultative myrmecophile.

The phylogenetic breadth of these associations is what makes the finding so striking. Although all five ant species belong to the subfamily Formicinae, the genera Formica and Lasius are quite distant within that subfamily, classified in different tribes. Moreover, the subgenera within Formica itself diverged early in their evolutionary history. In Central European populations of the Idas Blue studied previously — in Bavaria and the Swiss lowlands — only three closely related species of the Formica cinerea-group served as ant partners, and pupation occurred in close proximity to nest entrances, sometimes even within the nest galleries. In the Upper Rhine Valley, by contrast, the researchers found no trace of ant-dependent oviposition. Eggs were deposited quickly, typically without ants present, and without careful inspection of the substrate by the females. One pupa was found attached at a height of 85 centimeters on a broom shrub, indicating the larva had made no effort to descend to the base of the plant or into the leaf litter to pupate — a behavior utterly at odds with the nest-oriented pupation of obligate populations.

The researchers carefully weighed alternative explanations for the deviation. Could earlier classifications of the Idas Blue as an obligate myrmecophile have been erroneous? The evidence from Switzerland, including convincing demonstrations of ant-dependent oviposition, argues strongly against this. Could the Upper Rhine situation be artificial, caused by the local disappearance of the obligatory ant partners? There is no known example of an obligately myrmecophilous lycaenid adapting to different ant species when its original hosts vanish, and historical records of the region’s habitats do not indicate a fundamental change in vegetation cover that would have favoured a different ant fauna. Could the populations in question not truly be conspecific? The researchers consider a taxonomic split premature, given how little is known about life histories across the species’ many regional variants, though DNA analyses might one day clarify phylogenetic relationships among populations. Instead, they invoke the geographic mosaic theory of coevolution — the idea that a species may adapt differently in separate regions regarding its interactions with other species, with shifts in the nature of ant-lycaenid interactions potentially occurring multiple times within the species.

Building on this framework, the authors propose defining ecotypes of the Idas Blue according to the type of myrmecophily, host-plant usage and habitat structure. The Upper Rhine population, together with populations across the northern half of France, constitutes a broom ecotype displaying facultative myrmecophily and using shrubby legumes as larval host plants. In Brittany, oviposition has been recorded on Gorse species closely related to Broom, and large ant mounds probably belonging to F. pratensis were reported near host plants, with one record of caterpillars on broom growing in a F. pratensis nest. A river gravel ecotype, by contrast, is obligatorily associated with ants of the F. cinerea-group, uses numerous herbaceous legumes and Sea Buckthorn as host plants, and lives in alpine riverbeds and anthropogenic secondary habitats with very little vegetation. An alpine ecotype may exist at high elevations in the Swiss Alps, apparently monophagous on Helianthemum species. Populations feeding on Common Heather in German heathlands, the Calluna-type of earlier authors, cannot be characterized on host-plant usage alone, given the fundamental importance of different types of myrmecophily.

For conservation, the findings carry immediate practical relevance. The Upper Rhine population should be regarded as an evolutionarily significant unit with high conservation value, given its adaptive divergence from other Idas Blue populations. The preservation of heathland with broom and interspersed nutrient-poor grassland on sand is an increasing struggle, driven by rising nitrogen deposition and neophyte pressure from Canada Goldenrod, American Black Cherry and Tree of Heaven. Increased ground cover by low vegetation likely reduces ant density in broom heathland, and reversing this trend may require soil disturbance through choppering or sod-cutting, ideally in a mosaic of rotating plots so ant colonies are not impaired all at once. The study also overturns a long-held assumption about the importance of small host plants: larvae can thrive in the upper half of large shrubs, where F. pratensis is available as a partner, and the current structure of broom patches is by itself an insufficient explanation for the species’ decline. The associated ants are thermophilous species that vanish under closed shrub cover, and since ant presence reduces larval mortality, the interior of dense scrubland can hardly be suitable for larval development.

The reasons for the extraordinarily rapid range contraction of the Idas Blue in the Upper Rhine Valley remain far from understood, and the unknown factors evidently affect other populations of the broom ecotype as well, but not those of the river gravel ecotype. What the study establishes beyond doubt is that, in this species, a large amount of biodiversity involves variation at the population level. Facultatively myrmecophilous populations of the broom ecotype coexist with obligately myrmecophilous populations of the river gravel ecotype within Central and Western Europe, and such fundamental differences within a single lycaenid species regarding its association with ants have never been reported before. For the preservation of genetic diversity, this intraspecific variation must be taken into account in practical conservation management — and the critically endangered population of the Upper Rhine Valley, the last of its kind in Baden-Württemberg, now stands revealed as a highly important conservation unit whose peculiar life history may be its greatest scientific legacy.

Subject of Research: Facultative myrmecophily in a highly endangered population of Plebejus idas (Linnaeus, 1761) in the northern Upper Rhine Valley ( Lepidoptera , Lycaenidae )

Article Title: Facultative myrmecophily in a highly endangered population of Plebejus idas (Linnaeus, 1761) in the northern Upper Rhine Valley ( Lepidoptera , Lycaenidae )

Article References: Güsten, R., Sanetra, M., & Trusch, R. (2025). Facultative myrmecophily in a highly endangered population of Plebejus idas (Linnaeus, 1761) in the northern Upper Rhine Valley (Lepidoptera, Lycaenidae). Nota Lepidopterologica, 48, 185-202. https://doi.org/10.3897/nl.48.146868

Image Credits: AI Generated

DOI: 10.3897/nl.48.146868

Keywords: Facultative, myrmecophily, highly, endangered, population, Plebejus, idas, Linnaeus, northern, Upper, Rhine, Valley