Confusion with willow bent-wing? One species added, one removed from the list of moths in the Netherlands

Moths of the genus Phyllocnistis are inconspicuous white moths of only a few millimetres long, whose caterpillars are particularly noticeable for the feeding traces they leave behind: long, winding mines on leaves and sometimes stems of poplars and willows, which resemble the slime trail of a snail. The caterpillar only feeds on the sap from the upper cell layer, the epidermis, which gives it a silvery appearance. Furthermore, the Phyllocnistis species only mine very young leaves and shoots. 

Six species have been identified in the Netherlands to date. Two of these were only discovered in our country in recent years: Phyllocnistis valentinensis since 2017 and Phyllocnistis extrematrix since 2018. 

The shape of mines

The willow bent-wing, known as Phyllocnistis saligna, is a widespread species in the Netherlands that lives on various types of willow. Researchers have known for several years that the DNA barcodes of this leaf miner in Europe form three quite different clusters. One of these clusters was recognised about ten years ago as the species Phyllocnistis asiatica, described from Russia. This species was also reported as new to the Netherlands in 2017. At the time it was considered that this species could be distinguished from P. saligna by the shape of the leaf and stem mines, and the DNA barcode. After that discovery, Naturalis researcher Erik van Nieukerken collected caterpillars of Phyllocnistis moths at various locations in the Netherlands. Despite major differences in the shape and length of the mines, no differences in DNA could be found, and all specimens found on white willow and crack willow belonged to the cluster that had been identified as P. asiatica. The question was, what was the real P. saligna? 

Getting it right

That question was answered in 2023 by an international team of researchers (Voith and others; pdf: 4.6 MB) who described a new species from Norway, found almost exclusively on almond willow (Salix triandra). This species, Phyllocnistis triandricola, formed the third DNA cluster that had not previously been separated from P. saligna. This species was subsequently recorded from various localities in Europe, partly also on other willow species. They also identified to which DNA cluster the real P. saligna belongs. This species was described by P.C. Zeller in 1839 from what is now Poland, and seems to be mainly associated with the purple willow (Salix purpurea), something that Zeller also noted in a subsequent article in 1848 (under the name Salix helix). 

DNA barcoding clears the confusion

At the end of 2024, Erik van Nieukerken found mines in the stems and leaves of almond willow near Zevenaar for the first time, but was unable to identify them with certainty. In June 2025, another search was conducted and live caterpillars were found on fresh shoots of the almond willow, and moths were bred. DNA analysis confirmed the identification as Phyllocnistis triandricola, new to the Netherlands. In September, mines with dead pupae were found in the Brabantse Biesbosch, which were also confirmed as such by DNA barcoding. The species was also found in the Biesbosch on purple willow, on a plot directly adjacent to the almond willow. A few incomplete mines with dead caterpillars were also found on the planted exotic species Salix miyabeana. As no complete mines were found, it appears that the caterpillar cannot survive on this plant. 

Meanwhile, Steve Wullaert had collected pupae and caterpillars along the Meuse in Belgian Limburg in August. DNA analysis of one specimen showed that it also belonged to P. triandricola. 

Conclusion

In conclusion: Phyllocnistis triandricola is new to the Netherlands and Belgium and occurs at least in the river area on almond willow and (rarely?) on purple willow. The real Phyllocnistis saligna does not seem to occur in the Netherlands and Belgium, the common species here is Phyllocnistis asiatica. This species lives mainly on white willow and rack willow, but has also been observed on osier and laurel willow. 

Genitalia

Recognising these species is difficult. Voith and others found a characteristic in the female genitalia. They also noted a character in the length of the longitudinal band on the forewing: this would be shorter in P. triandricola. Although this seems to be true for the Dutch P. triandricola, P. asiatica is very variable and it seems necessary to study more material. We believe that there may be an additional characteristic in the black stripes on the front edge of the wing, see arrows in the photos: the stripe that is clearly visible in P. triandricola is (usually) absent in P. asiatica.

For the time being, it is not possible to identify captured or photographed butterflies without DNA testing or examination of the female genitalia. Mines on almond willow in the river area will usually belong to P. triandricola. For further research, we mainly need cultivated moths from different willow species, especially species other than white willow or crack willow. Observations on purple willow are particularly interesting, because the real Phyllocnistis saligna might be present there.

It seems sensible to change the name of all observations currently listed on Waarneming.nl, in NDFF and on Waarnemingen.be and other Belgian sites to 'Phyllocnistis saligna complex'. This should also be done on international platforms such as Observation.org and iNaturalist, as this applies to all of Europe. Only material with a DNA barcode can be identified with certainty for the time being.

Lifestyle and search tips

Of both species, the caterpillars start with a short, narrow mine on a fresh leaf on a newly sprouted shoot, and then continue mining into the bark of the branch. When the caterpillar is almost fully grown, it moves via the petiole to a second leaf and finally makes a fold at the edge of the leaf, in which the caterpillar spins a cocoon and then pupates. This fold is usually conspicuous. The mine is rarely limited to a single leaf. The butterfly emerges within a few weeks. The species are bivoltine and the moths that emerge in autumn hibernate. Moths are observed in most months, and the first mines usually appear in June. The occurrence of later generations depends heavily on the presence of young shoots. Sometimes caterpillars can still be found late in the year, but rarely after mid-September. 

To find fresh mines, it is useful to look for young shoots. Leaves with a chrysalis are often very noticeable and can be easily collected to rear the moth. The vast majority of observations on Waarneming.nl and Waarnemingen.be are of old, long-abandoned mines. We would like to encourage observers not to be satisfied with this, but to search for fresh mines with caterpillars or pupae. When the moth has emerged, this can be seen from the remaining pupal exuvia. Try to identify the willow species on which the mines are found. Almond willow is easy to recognise by its serrated stipules, see accompanying photo of a stem mine of Phyllocnistis triandricola on a young shoot of almond willow. 

More information

• Discovery of Phyllocnistis valentinensis in 2017 (in Dutch).
• Discovery of Phyllocnistis extrematrix in 2018 (in Dutch).
• Up for the challenge? DNA barcodes of Phyllocnistis species can be found in the barcode library BOLD.

Text: Erik J. van Nieukerken, Naturalis Biodiversity Center; Steve Wullaert; Flemish Entomological Society; Tymo Muus, Microlepidoptera.nl 
Images: Erik J. van Nieukerken (leadphoto: stem mine of Phyllocnistis triandricola); Joey Bom