The research, published in the Journal of Hymenoptera Research, compiled data from 2017 to 2023 across the Bohemian Massif, the Carpathians, the Pannonian Basin, and three distinct Alpine systems in Austria and Italy. Sampling spanned elevations from 111 to 2,068 meters above sea level, encompassing 389 individual sampling events across 146 localities within 28 orographic units. Researchers used a standardized beating method, holding a one-square-meter sheet beneath tree and shrub branches between 30 and 250 centimeters above the ground, spending roughly twenty minutes per transect actively searching for both parasitized and unparasitized spiders. Parasitized individuals, identifiable by parasitoid larvae visibly attached to the spider’s opisthosoma, were separated immediately and reared under controlled laboratory conditions at 21 degrees Celsius and 60 to 80 percent humidity until adult wasps emerged.

The results upended expectations. Rather than following a smooth elevational gradient, parasitoidism by A. degener clustered in a geographically structured mosaic. Parasitized samples occurred at a mean elevation of 1,060 meters, compared with 751 meters for unparasitized samples, a highly significant difference. Logistic regression confirmed that the probability of parasitoidism climbed from roughly 2 percent at 200 meters to more than 35 percent at 1,600 meters. Yet when researchers tested for a classic mid-elevation peak by adding a quadratic term to their models, the improvement in fit was negligible, indicating no consistent hump-shaped response across ranges. Instead, mountain-range identity explained far more of the variation in parasitoidism than elevation alone: a model using only range as a categorical predictor reduced deviance by 25.1 percent relative to the null, while elevation alone accounted for just 10.3 percent.

This orogeographic structure manifested in stark regional contrasts. The Beskydy Mountains showed parasitoidism at five of 18 examined sites, or 27.8 percent, while the Jeseníky Mountains recorded 8 of 31 sites positive, or 25.8 percent. Meanwhile, the lowland Polabí region yielded zero detections across 20 samples at a mean elevation of approximately 180 meters, exactly as the logistic model predicted for such low elevations. Spatial scan statistics reinforced this patchiness, identifying a strong primary cluster centered in the Jeseníky region where within-cluster prevalence reached 31.7 percent against 6.3 percent outside, yielding a relative risk of 5.0 and a Monte Carlo significance of p = 0.01. Secondary clusters appeared in the Italian Alps and in the Carpathian region near the Czech-Slovak border, while one significant cold spot contained only a single parasitized sample among 64.

At the heart of this pattern lies a single spider species. Mughiphantes mughi, a small, cold-adapted linyphiid favoring shaded and humid montane environments, emerged as the principal host sustaining A. degener populations across the study area. The species exhibited a parasitoidism rate of 4.7 percent, more than double the overall mean of 1.9 percent calculated across all 4,563 examined spiders, and was parasitized at 27 percent of the sites where it occurred. Its abundance peaks in spring and autumn align neatly with the wasp’s phenology, and its consistently small adult body size may provide a suitable host throughout prolonged larval development. By contrast, spiders of the genera Entelecara and Neriene functioned as secondary or facultative hosts, exploited only under specific regional or seasonal circumstances.

The Entelecara result is particularly striking. These spiders are common and widespread in understory and shrub habitats, constructing sheet webs ecologically similar to those of Mughiphantes, yet only four parasitized individuals were observed against 30 expected under a null model of equal parasitoidism across hosts. All four came from a single mountain range, the Bukovské vrchy, suggesting intensely localized host specificity. A comparable dynamic has been documented in the related polysphinctine Zatypota percontatoria, which readily attacks the common theridiid Neottiura bimaculata in fruit orchards but rarely parasitizes the same species elsewhere. Such examples suggest that broad physiological host acceptance does not necessarily translate into widespread ecological exploitation, a distinction with major implications for how parasitoid host ranges are interpreted.

Seasonality added another dimension to the story. Mughiphantes mughi was the only host recorded in spring, while Entelecara, Kaestneria, Neriene, and Pityohyphantes were parasitized predominantly in autumn, with Entelecara and Kaestneria peaking in September and Neriene and Pityohyphantes peaking in October. Many of the spring-parasitized M. mughi likely carried larvae acquired the previous autumn, which had overwintered on their hosts before emerging as adults the following year. This pattern points to a predominantly univoltine life cycle in montane regions: parasitoidism occurs in late summer and autumn, larvae survive winter on the host, and adult wasps emerge in spring. At lower elevations and in warmer habitats, bivoltinism may occur, consistent with historical British records reporting adult flight activity from June through October.

Molecular analyses using CO1 DNA barcoding resolved a long-standing taxonomic ambiguity. Acrodactyla similis, a cryptic congener morphologically nearly indistinguishable from A. degener, was confirmed as a distinct species through both ASAP and bPTP species delimitation methods. The single sequenced A. similis specimen, collected from the Pannonian lowland parasitizing Tenuiphantes flavipes, formed a well-supported separate lineage. This finding raises an intriguing hypothesis: if A. similis occupies lowland habitats while A. degener dominates montane ones, partial elevational niche partitioning between the two species could contribute to the observed distribution patterns. Within A. degener itself, however, specimens reared from different host genera were intermingled across the phylogeny, providing no evidence of cryptic host-specialized lineages.

Yet the genetic data also hinted at unresolved complexity. Mean uncorrected pairwise p-distances within A. degener ranged from 0.4 percent in the Bohemian Massif to 2.6 percent in the Alps, with the most divergent pairwise comparisons reaching 6.3 percent. The single sequenced individual reared from Pityohyphantes phrygianus in the Italian Alps showed only 95 percent similarity to all other samples, making it the most genetically distinct specimen in the study. While intraspecific CO1 distances can reach 10 percent in parasitoid Hymenoptera, the possibility that such divergence points to cryptic species cannot be excluded with current sampling. Broader geographic coverage and additional genetic markers will be needed to determine whether this structure reflects geography, host use, or incipient ecological differentiation.

The study’s authors acknowledge limitations, including uneven seasonal coverage, with autumn sampling missing from key mountain systems, and underrepresentation of lowland habitats where structurally suitable conifer stands are rare. Preliminary observations also suggest host body size may constrain parasitoid development: larvae were recorded only on juvenile Neriene whose body size approximated that of large M. mughi, hinting that successful development requires hosts within a specific size range. Nevertheless, the overall picture is clear. Acrodactyla degener occupies an intermediate position between strict specialist and true generalist, combining broad physiological host acceptance with geographically and seasonally structured ecological specialization. As a model system for investigating host shifts and the evolution of host use in polysphinctine wasps, it offers a window into how ecological context, not just evolutionary capability, shapes the realized niches of parasitoids in fragmented montane landscapes.

Subject of Research: Ecology and host specificity of the parasitoid Darwin wasp Acrodactyla degener across Central and Southern European mountain systems

Article Title: When generalists become picky: Regional and seasonal specialization in a pimpline Darwin wasp ( Hymenoptera , Ichneumonidae ) with a supposedly broad host range

Article References: Korenko, S., Solar, F., Rothová, H., Janšta, P., Šimečková, A., Černecká, Ľ., Gajdoš, P., Purgat, P., Opatová, V., & Heneberg, P. (2026). When generalists become picky: Regional and seasonal specialization in a pimpline Darwin wasp (Hymenoptera, Ichneumonidae) with a supposedly broad host range. Journal of Hymenoptera Research, 99, 753-778. https://doi.org/10.3897/jhr.99.186686

Image Credits: AI Generated

DOI: 10.3897/jhr.99.186686

Keywords: Acrodactyla degener, parasitoid wasps, Ichneumonidae, Linyphiidae, host specificity, elevation gradients, montane ecology, seasonality, DNA barcoding, cryptic species, spider parasitoids, Central Europe