In the therapy garden of the University Psychiatric Clinics in Basel, a tiny alien beetle has been quietly building a population inside the fruitbodies of a tree fungus. Samples of the yellow bracket fungus Antrodia xantha, collected in summer 2021 from a standing trunk of the North American eastern white pine (Pinus strobus), yielded an alien species of minute tree-fungus beetle never before recorded in Switzerland: Cis chinensis Lawrence, 1991. The discovery, made during a long-running project on the invertebrate fauna of perennial wood fungi in the Basel region, triggered a series of follow-up investigations that documented the beetle’s development, its parasites, its mite passengers and its spread to other fungi nearby.
The Ciidae are among the most specialised of all beetle families, with roughly 650 described species worldwide adapted to living inside the fruitbodies of wood-decay fungi, and the genus Cis alone accounts for about 400 of them. Until 2012, only 41 ciid species had been registered for Switzerland. Cis chinensis was originally described in 1991 from specimens that reached the United States in 1988 with commercially dried fungi labelled as Ganoderma lucidum from China, and it is mainly known as a storage pest of dried fungi. Some researchers suspect it may be a younger synonym of the older Cis mikagensis, but because no type comparison has yet been made, the authors retained the familiar name.
The Basel population proved remarkably large. The three fungus samples, collected on 27 and 28 June and 11 July 2021, produced a total of 518 adult beetles by mid-February 2022, with an even sex split of 259 males and 259 females, and living adults and larvae were still present in all samples in early September 2022 even though the substrate appeared exhausted. The adults ranged from 1.55 to 2.39 millimetres in body length, with dark brown to almost black coloration typical of wild specimens, a four-pointed frontal edge on the male head, and a distinctive navel-shaped pit with brush hairs on the first abdominal sternite of the male.
One of the most intriguing findings emerged when the researchers compared body sizes across the two sampling dates. Using a linear model, they found that only 3.5 percent of the variation in body size was explained by collection date and sex, but the difference between the adults from the two June samples and those collected two weeks later in July was statistically significant (post-hoc test, P = 0.0248). June 2021 was the fifth warmest June since the Basel measurement series began in 1755, and the authors suggest that the exceptionally high average temperatures favoured the development of the larvae in the July sample, which emerged as larger adults. The sex difference in size was not statistically significant.
Measurements of ethanol-preserved larvae, 37 individuals in total, pointed to a pattern most consistent with five larval stages, although the authors caution that the sample was too small for an unambiguous result, and in some cases only four stages may apply. The final-stage larva was described and illustrated in detail, showing a milky-white cylindrical body, three pigmented stemmata on each side of the head, and robust, forward-curved urogomphi that distinguish it from native Cis species, which generally have 2.5 times rather than twice the urogomphus length relative to its basal width.
The beetles did not live undisturbed. From 8 July 2021 onwards, egg parasitoids of the wasp family Mymaridae emerged from the beetle eggs in the samples. These tiny fairyflies could be identified only to the genus Cleruchus using existing keys, and material sent to specialist Serguei V. Triapitsyn at the University of California, Riverside, revealed that the Basel wasps represent a species new to science, described in a companion paper. Remarkably, the same new species was also identified from older project material collected on Stereum fungi in the southern Black Forest of Germany in 2014 and the central Swiss Jura in 2016, where the native ciid Cis festivus occurred.
Equally striking was an unusually heavy infestation of phoretic mites. Nearly all beetles sorted during the first week and a half carried dorso-ventrally flattened deutonymphs of about 0.2 millimetres, almost exclusively on the ventral side, attached by suckers near the anal region. The researchers attribute them to the family Histiostomatidae, though the species or even genus could not be determined because the specimens were available only in a dry state. In the 11 July sample, every single beetle examined was infested, with between 4 and 31 mites per individual, while beetles sorted later in July were completely mite-free and no mites at all were found in the fungus substrate itself.
Follow-up surveys between January and March 2022 showed that Cis chinensis had not remained confined to its original Antrodia xantha colony. The species was detected in Schizophyllum commune, where it apparently displaced the native Orthocis reflexicollis during the course of the summer, and individuals were successfully reared from Pleurotus ostreatus, from which 45 adults and numerous larvae eventually emerged. A single putative fourth-instar larva was found in Meripilus giganteus, though the authors hesitate to list that fungus as a confirmed breeding host. In total, 11 ciid species were recorded on the UPK campus and its immediate surroundings, and all records of Cis chinensis beyond the original site lay less than 40 metres from where the population was first observed.
How the beetle reached Basel remains unknown, but the authors consider introduction with contaminated goods, such as dried fungi from Southeast Asia, the most plausible route. The species is now nearly cosmopolitan, with the oldest known European records being field finds from Italy in 1978, followed by Germany in 1990, France in 1998, Malta in 2000, Spain in 2010 and Hungary in 2013, and the nearest known locality to Basel lies about 110 kilometres away in the Rhine port of Kehl. The Basel find is also not the first Swiss record: in 2020 the species was caught in flight traps on the grounds of Basel’s SBB railway station, though it could not be identified at the time. The authors estimate that the Antrodia xantha population likely comprised several thousand individuals, suggesting the species had been established on the site well before 2021.
The episode underscores how urban green spaces can serve as entry points and refuges for alien species. The clinic campus, laid out as an English-style park in the 1880s and now covering 12.23 hectares with old and partly non-native trees, is managed extensively, which favours rich fungal growth and the mycetobiont insects that depend on it. The unusually broad spectrum of breeding fungi that Cis chinensis can exploit, ranging from classic polypores to agarics and russulas, together with its demonstrated ability to displace a native competitor, marks it as a species worth watching as it continues its spread across Europe.
Subject of Research: First Swiss record and ecology of the alien beetle Cis chinensis in a Basel therapy garden
Article Title: Der Fremdling im Therapiegarten – Cis chinensis ( Coleoptera , Ciidae ) und sein Umfeld
Article References: Coray, A., Marques, D. A., & Vlček, P. (2022). Der Fremdling im Therapiegarten – Cis chinensis (Coleoptera, Ciidae) und sein Umfeld. Alpine Entomology, 6, 83-95. https://doi.org/10.3897/alpento.6.86494
Image Credits: AI Generated
Keywords: Cis chinensis, Ciidae, alien species, Switzerland, Basel, Antrodia xantha, Mymaridae, Cleruchus, phoretic mites, Histiostomatidae, larval development, urban biodiversity
News Source: Glenn Wilkins. (October 28, 2026). Asian Beetle Quietly Colonises a Basel Psychiatric Clinic Garden. Scienmag.
