Camarotoscena species live across the temperate Palaearctic, from the Mediterranean Basin to the Far East, with their greatest diversity in the arid mountain regions of Asia. All species known so far are narrowly oligophagous on poplars, genus Populus, and their immatures induce leaf roll galls on the leaves, sometimes on the very same tree, and occasionally even within the same gall, as two species share a host plant. Because poplars are widely planted for timber, fuel, windbreaks and fodder, and because some Camarotoscena species are considered pests, a reliable way to identify these insects matters far beyond the walls of taxonomy. For any pest control programme, a sound taxonomic base is crucial, and until now that base has been shaky for this particular genus.

The challenge stems from the fact that Camarotoscena is morphologically what the late Russian specialist Marina Loginova once described as a monolithic genus. The male and female terminalia, which in most psyllids provide dependable diagnostic characters, are unusually uniform across the group. Adults are small, typically between 2.0 and 2.5 millimetres long, ochreous or light brown with small dark dots, and bear whitish or yellowish forewings. Species are separated by subtle features: the arrangement of surface spinules on the forewing, the density and colour of the dot pattern, body size, and fine details of the genital structures. Loginova recognised 13 species in her 1975 review, three of which have since been transferred to other genera, leaving twelve recognised species going into the new study.

The revision, based on material from museums in Bulgaria, China, Switzerland, Czechia and Basel, confirms the ten species that remain after careful re-examination. The authors propose five new synonymies, collapsing four names into existing species. Camarotoscena ujenci, erected in 1982 for just two males from Mongolia, is now considered the same as C. subrubescens, a bright orange species from the Mediterranean region. Three Chinese species described in 2011 from single females, C. wulingshanica, C. wutaishanica and C. xinjiangica, are formally synonymised with C. bianchii, a species characterised by its square female subgenital plate. A doctoral thesis had already suggested these synonymies in 2016, but under the International Code of Zoological Nomenclature such acts are only valid when formally published, which the new paper now does.

The morphological work went beyond adults. Fifth instar immatures, known for only half of the species, turned out to carry their own diagnostic characters. The researchers examined the chaetotaxy, the arrangement of setae and pointed sectasetae on the caudal plate and forewing pads, as well as the degree of sclerotisation of the ventrite bearing the circumanal ring. These immature characters independently supported the species boundaries diagnosed from adults, a satisfying convergence for a genus where adult features are so subtle. The previously unknown female of C. lauta, the largest species in the genus at 2.8 to 3.2 millimetres, is now diagnosed and illustrated, and illustrated identification keys are provided for both adults and, where known, the immatures.

Then came the molecular surprise. The team sequenced two mitochondrial markers, cytochrome c oxidase subunit I and cytochrome b, from specimens of five species collected across India, Jordan, China, Czechia and Bulgaria, using both traditional Sanger sequencing and Oxford Nanopore technology. A commonly applied 3 percent COI threshold for species delimitation failed spectacularly in two cases. Between C. fulgidipennis and C. hoberlandti, two species that coexist on black poplar in the Middle East, Central Asia and the western Himalaya, there were zero differences in COI. Between C. speciosa and C. subrubescens, the Mediterranean species pair that sometimes shares the same trees, the difference was essentially nil in both genes. And cytb additionally failed to distinguish C. trjapitzini from the fulgidipennis-hoberlandti pair, even though COI separated that species by 2.6 percent.

The phylogenetic picture was not entirely bleak. A maximum likelihood tree built from concatenated COI and cytb sequences recovered two main groups, each also defined by morphology: one containing C. speciosa and C. subrubescens, the other containing C. fulgidipennis, C. hoberlandti and C. trjapitzini. Between these groups, genetic distances ranged from about 7.3 to 9.1 percent, clearly signalling separate lineages. Within groups, however, the barcodes went silent. The authors suggest that mitochondrial introgression or incomplete lineage sorting, phenomena well documented across the animal kingdom, may explain the mismatch, and they recommend that future work include nuclear genes or genomic-scale data to resolve the conflict.

Could the species pairs actually be one species each, perhaps seasonal forms of a single lineage as known from some pear psyllids? The authors consider this unlikely, because the two forms within each pair occur simultaneously at the same localities, independent of season. The presence of two species within a single leaf roll gall has a simpler explanation: females of both species lay eggs close together on the same leaf, and the gall is induced by the sucking of the first instar immatures, so all nearby individuals end up enclosed regardless of which species initiated it. Field observations from southern France add ecological texture, with C. subrubescens favouring black poplar along watercourses while C. speciosa also colonises trees at ditch edges.

The study also redraws the map. New country records include C. fulgidipennis and C. hoberlandti from Ladakh in India, the first record of the genus and of any psyllid from that cold desert region, C. lauta from Ankara in Turkey, and C. trjapitzini from Tehran and Yazd in Iran and from Spain. Perhaps most tellingly, a specimen from Jordan previously identified as C. hoberlandti was re-examined and found to be C. trjapitzini, a correction that matters because the two species are so similar that even published DNA sequences had been attached to the wrong name.

The broader lesson is one that molecular ecologists are hearing with increasing frequency. DNA barcoding is a fast and inexpensive tool that works well for many insect groups, and it has successfully delimited species across all seven recognised psyllid families, but it is not infallible. In other psyllid genera such as Aphalara, Cacopsylla and Melanastera, mitochondrial barcodes generally identify species correctly, yet even there exceptions abound, with intraspecific divergence reaching 14 percent in some Melanastera species while morphologically distinct Aphalara species differ by less than 1 percent. The Camarotoscena results highlight the limitations of relying solely on mitochondrial DNA from a few specimens of few localities for species delimitation. Rather than risk ill-founded taxonomic upheaval, the authors retain the traditional ten-species concept and call for targeted fieldwork, fresh material from more localities, nuclear gene sequencing, and descriptions of the still-unknown immatures. For now, the poplar galls keep their secrets, and the humble microscope remains the psyllid taxonomist’s most trustworthy instrument.

Subject of Research: Species delimitation and DNA barcoding of poplar-associated psyllids in the genus Camarotoscena

Article Title: Species concepts in the psyllid genus Camarotoscena ( Hemiptera , Psylloidea ) associated with Populus ( Salicaceae )

Article References: Species concepts in the psyllid genus Camarotoscena ( Hemiptera , Psylloidea ) associated with Populus ( Salicaceae ). (n.d.). https://doi.org/10.3897/alpento.9.172655

Image Credits: AI Generated

DOI: 10.3897/alpento.9.172655

Keywords: Camarotoscena, psyllids, DNA barcoding, Populus, taxonomy, COI, cytochrome b, leaf roll galls, Hemiptera, species delimitation, Palaearctic, morphology