An update to the distribution of invasive Ctenolepisma longicaudatum Escherich in northern Europe, with an overview of other records of Estonian synanthropic bristletails (Insecta: Zygentoma)

Abstract Background Previously, two species of Zygentoma have been reported as synanthropic in Estonia (Lepisma saccharinum Linnaeus, 1758 and Thermobia domestica (Packard, 1873)). Ctenolepisma longicaudatum Escherich, 1905 is an invasive species that is currently expanding its range in Europe, but had no published records from the northern Baltic Region. New information Ctenolepisma longicaudatum was first found in Estonia in 2018. It has currently several established populations in public buildings in Tartu and Tallinn, but has not been found in private households, nor in other places in Estonia. A brief overview of its invasion history in northern Europe is given.


Introduction
Zygentoma Börner, 1904 is a small order (with over 500 described species; Zhang 2013), of primitive insects. They are more common in warmer climates, but some species of Zygentoma are synanthropic and distributed worldwide. Two species of Zygentoma -Lepisma saccharinum Linnaeus, 1758 (= Lepisma saccharina, as widely used prior to the ICZN decision, International Commission on Zoological Nomenclature 2018) and Thermobia domestica (Packard, 1873) -have been previously recorded from Estonia (see below), both only indoors in human settlements. Ctenolepisma longicaudatum Escherich, 1905 is an invasive synanthropic species that has been rapidly expanding its range in recent years (Goddard et al. 2016, Kulma et al. 2018, Lock 2007, Meineke and Menge 2014, Pape and Wahlstedt 2002, Thomsen et al. 2019) and is considered a pest of paper and stored materials.
Ctenolepisma longicaudatum was first found in Estonia in 2018 and has since been detected at multiple locations. Here, we report for the first time these findings and provide an overview of recent knowledge of that and two other inavsive species of Zygentoma in Estonia. In addition, the invasion history of C. longicaudatum in northern Europe is summarised.

Materials and methods
All preserved material of Zygentoma in Estonian natural history collections was examined and a list of earlier literature records was compiled. Specimens were actively searched for in suitable habitats and about 30 volunteers were asked to report sightings and, if possible, collect specimens of larger than usual silverfish (in 2020). Most material is preserved in 80% ethanol and some specimens were mounted on to microscope slides after clearing in 20% potassium hydroxide (KOH). All studied material is deposited in the Entomological Collection of Estonian University of Life Sciences (IZBE) and the private insect collection of Allan Selin.

Notes
This species was recorded in Estonia as Lepisma saccharina by Remm (1966) and Vilbaste (1968). Widespread both in public buildings and private households.

Thermobia domestica (Packard, 1873) Notes
Thermobia domestica was recorded from Estonia by Martin (2007). Its only confirmed occurrence was in a rural household in south-western Estonia in 2005. The species was identified, based on dead specimens and exuvia, but live specimens were also observed. The specimens were likely brought in with imported second-hand clothes. No voucher specimens have been preserved.

Discussion
The presence of L. saccharinum in Estonia has been well known for a long time. As for Estonia, it seems unlikely that the species has been overlooked for much longer, while the initial finding localities (Estonian University of Life Sciences Entomological collection and Estonian National Archive, both in Tartu) have been constantly monitored for potential pests. All current findings are from large public buildings, whereas there are, as yet, no records from private households (but is expected to be ultimately found in the latter). Compared with other synanthropic Zygentoma species, C. longicaudatum has much lower moisture demand and thus has a good chance of surviving in archives, libraries and museums, where there is plenty of suitable food for it (Aak et al. 2019). The species, expanding its range northwards (see Fig. 2), is considered a substantial pest especially of paper (see Fig. 1E,F, Szpryngiel 2018, Kulma et al. 2018). Therefore, its monitoring and being included to IPM (Integrated Pest Management) plans of museums, libraries and archives is inevitable (Querner 2015). Extended information on efficient and safe control can be found, for example, in Aak et al. 2020a, Aak et al. 2020b, Gutsmann 2019. There are currently no known established populations of Thermobia domestica in Estonia. However, the species is known for its requirement of higher temperature in order to successfully establish. Two other Ctenolepisma species, C. lineatum (Fabricius, 1775) and C. calva (Ritter, 1910) have been recently found in Norway (Hage et al. 2020), warranting further studies on Nordic Zygentoma diversity and distribution.