(Heteroptera: Aradidae). Considerations on the Local Distribution and the Habitat Preferences of This New Belgian Species Following a Nine-Months Field Survey

Belgian Journal of Entomology 111: 1–28 (2021) ISSN: 2295-0214 www.srbe-kbve.be

urn: lsid:zoobank.org:pub:7124DD6B-FFC4-4968-B423-BF30EFE3223B

Belgian Journal of Entomology

The northernmost discovery of Aradus brenskei (Reuter, 1884) (Heteroptera: Aradidae). Considerations on the local distribution and the habitat preferences of this new Belgian species following a nine-months field survey

Brecht VERKEMPINCK1, Berend AUKEMA2, Robin VAN HEGHE3 & Tim SPELEERS4

1 Erfken 5, 9340 Lede, Belgium. E-mail: [email protected] (corresponding author) 2 Naturalis Biodiversity Centre, P.O. Box 9517, 2300 RA Leiden, the Netherlands. E-mail: [email protected] 3 Regionaal Landschap Schelde-Durme, Markt 1, 9230 Wetteren, Belgium. E-mail: [email protected] 4 Essestraat 23, 9340 Lede, Belgium. E-mail: [email protected]

Published: Brussels, 23 February 2021 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

Citation: VERKEMPINCK B., AUKEMA B., VAN HEGHE R. & SPELEERS T., 2021. - The northernmost discovery of Aradus brenskei (Reuter 1884) (Heteroptera: Aradidae). Considerations on the local distribution and the habitat preferences of this new Belgian species following a nine-months field survey. Belgian Journal of Entomology, 111: 1–28.

ISSN: 1374-5514 (Print Edition) ISSN: 2295-0214 (Online Edition)

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Front cover: Male adult Aradus brenskei, collected by Robin Van Heghe on 4.VII.2020 on a Fomes fomentarius infested poplar cultivar in Erpe-Mere. © Theodoor Heijerman.

2 Belgian Journal of Entomology 111: 1–28 (2021)

Brecht VERKEMPINCK1, Berend AUKEMA2, Robin VAN HEGHE3 & Tim SPELEERS4

Abstract In May 2020, Aradus brenskei (Reuter, 1884) (Heteroptera: Aradidae) was first observed in Belgium. In nine months time, more than a thousand unique individuals were found in and around the Geelstervallei in Lede on eight different tree species, but mostly on Populus × canadensis. We found that Aradus brenskei shows a predilection for at least partially sunlit standing dead or dying trees which are infested by the tinder fungus (Fomes fomentarius). Keywords: Belgium, Fomes fomentarius, habitat destruction, large-diameter deadwood, Populus

Samenvatting In mei 2020 werd Aradus brenskei (Reuter, 1884) (Heteroptera: Aradidae) voor het eerst waargenomen in België. In een periode van negen maanden werden in en rond de Geelstervallei te Lede meer dan duizend unieke exemplaren gevonden op acht verschillende boomsoorten, maar vooral op Populus × canadensis. We hebben vastgesteld dat Aradus brenskei een voorkeur heeft voor minstens gedeeltelijk zonbeschenen dode of kwijnende bomen die aangetast zijn door echte tonderzwam (Fomes fomentarius).

Résumé En mai 2020, Aradus brenskei (Reuter, 1884) (Heteroptera: Aradidae) a été observé pour la première fois en Belgique. En neuf mois, plus d'un millier d’individus ont été trouvés dans et autour de la Geelstervallei à Lede sur huit espèces d'arbres, mais surtout sur Populus × canadensis. Nous avons constaté que Aradus brenskei préfère des arbres morts ou mourants étant au moins partiellement exposés au soleil et colonisés par l’amadouvier (Fomes fomentarius). Abstrakt

Im Mai 2020 wurde Aradus brenskei (Reuter, 1884) (Heteroptera: Aradidae) erstmals in Belgien beobachtet. Innerhalb von neun Monaten wurden mehr als tausend Individuen an acht verschiedenen Baumarten, aber besonders auf Populus × canadensis, gefunden. Wir haben festgestellt, dass Aradus brenskei eine Vorliebe hat für zumindest teilweise der Sonne ausgesetzte tote oder sterbende Bäume die vom Zunderschwamm (Fomes fomentarius) besiedelt sind.

3 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

Fig. 1 Male adult Aradus brenskei, collected by Robin Van Heghe on 4.VII.2020 on a Fomes fomentarius infested poplar cultivar in Erpe-Mere © Theodoor Heijerman.

Introduction

OCCASION For a decade, the Flemish town Lede has been home to a Buddhist temple. Out of respect for all living creatures, the monks are reluctant to remove decaying poplars surrounding their temple. Some of those trees, however, fall during storms, as happened at the beginning of 2019. With the permission of his brother-in-law, Ruben Mollij, who was a Buddhist monk for ten years in both Thailand and Belgium and who is still resident there, the first author collected a stump of one of the fallen poplars on 21.IV.2019 (Fig. 2). He put wheels under the stump so that it could serve as a play table for his four children (Fig. 3).

Fig. 2. Location 1, Lede, 21.IV.2019. © Brecht Verkempinck. Fig. 3. Location 1, Lede, 07.V.2020. © Brecht Verkempinck. After a few months, two fruitbodies of Fomes fomentarius, a wood-decaying basidiomycete attacking several tree species (VETROVSKY et al., 2011), appeared. This apparently created the suitable microhabitat for a species which has never before been observed in Belgium. In the afternoon of 7.V.2020, his five-year old daughter urged him to take a look at an insect sitting on a shell on the play table (Fig. 4). Being used to this kind of requests, he did not react immediately and nearly missed the opportunity to make the northernmost discovery of Aradus brenskei (Fig. 1) ever (AUKEMA et al., 2021).

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Fig. 4. Our first picture of Aradus brenskei, Lede, 07.V.2020. © Brecht Verkempinck.

OBJECTIVES Aradus brenskei is a rare South European and Ponto-Mediterranean species known in Europe from Albania, Austria, Bosnia-Herzegovina, Bulgaria, Croatia, the Czech Republic, France, Greece, Hungary, Italy (including Sardinia and Sicily), Montenegro, Romania, Slovakia, Spain and Ukraine, in the Middle East from Iran and Turkey and in the Caucasus from Azerbaijan (HEISS & PÉRICART, 2007). Aradus brenskei belongs to the Aradidae, a very old family among the Heteroptera with the oldest records found in Burmese amber dated to around 100 million years BC (HEISS & GRIMALDI, 2001), containing around 2050 species of more than 285 genera worldwide (SCHUH & WEIRAUCH, 2020). With the notable exception of Aradus depressus (Fabricius, 1794), aradid species are very rarely recorded in Belgium (BAUGNÉE et al., 2003 and www.waarnemingen.be). As AUKEMA et al., (2021) report, the Belgian list currently contains eight species: Aneurus (Aneurodus) avenius (Dufour, 1833), A. (Aneurus) laevis (Fabricius, 1775), Aradus (Aradus) betulae (Linnaeus, 1758), A. (A.) cinnamomeus Panzer, 1806, A. (A.) corticalis (Linnaeus, 1758), A. (A.) depressus (Fabricius, 1794), A. (A.) signaticornis R.F. Sahlberg, 1848 and A. (A.) versicolor Herrich- Schäffer, 1835 (BAUGNÉE et al., 2003), two of which – A. betulae and A. corticalis – can be considered as extinct in Belgium since they have not been recorded for more than a century (LETHIERRY, 1882). Aradus brenskei belongs more specifically to the Aradus betulae group, a group of nine closely related species (HEISS & PÉRICART, 2007), four of which occur in the surrounding countries of Belgium: A. betulae (Linnaeus, 1957) in France, Germany and the Netherlands, A. krueperi Reuter, 1884 in France and Germany, A. ribauti Wagner, 1956 in France and Germany, and finally A. brenskei in France (HEISS & GRIMALDI, 2001, AUKEMA et al., 2021). In France, A. brenskei has been found in nine departments, but only two of them are in Northern France: Oise and Île-de-France. The nearest locality is Les beaux Monts in Compiègne (Oise) at about 190 km, where a single female individual of A. brenskei was collected in 2009 (PARMAIN et al., 2013). This was the previous northernmost record of the species. Despite their worldwide distribution, information on the Aradidae is generally lacking (GHAHARI & HEISS, 2012). Current knowledge is based on occasional observations, with the exception of inter alia the studies of five aradid species by MARCHAL et al. (2012), of four aradid species by GOßNER et al. (2007), and of eight aradid species by MORKEL & FRIEß (2018), only the first of which

5 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae) includes information on A. brenskei. As for most other aradid species (HJÄLTÉN et al., 2006), the habitat preferences of A. brenskei are thus still largely unknown. More than three decades ago, the Council of Europe recommended its members to encourage the study of the ecology of poorly known, threatened saproxylic species, so that management practices appropriate for promoting the survival of these species could be established (COUNCIL OF EUROPE, 1988). The aim of this paper is to act on that recommendation in respect of Aradus brenskei, by shedding a light on its local distribution, its habitat preferences and its behaviour, which we studied during the nine-months period between 7 May 2020 and 7 February 2021. Material and methods

Having in mind that the poplar stump of our first location was attacked by Fomes fomentarius, we soon decided to screen its surroundings for other F. fomentarius infested structures, such as dead and dying trees, stumps, logs and lying branches. Two weeks after the first finding, the third author discovered a second A. brenskei colonised structure, which was also infested by F. fomentarius (Table 1). In parallel to the collective search for new potential habitat structures, and with a view to improving the search protocol, the first author inspected location 01 four times a day during the 20-day period between 27 May 2020 and 15 June 2020. As can be seen in Fig. 5, a clear pattern emerged as to when individuals start to leave their subcorticolous hiding places, namely after sunset.

Fig. 5. Numbers of exposed individuals during the 20 days observation period at location 01. (Only on three occasions, adults were spotted in the morning. Each time, they had not moved since the evening before. The peak on 3.VI.2020 represents the appearance of seven larvae).

In view of those results, a two-phase approach was adopted. In the first phase, we continued to map potential habitat structures by carrying out a daytime search for standing and lying deadwood structures attacked by the easily observable F. fomentarius fruitbodies in the wide surroundings of location 01 (Fig. 6). In the second phase, we inspected those structures more carefully after sunset (Fig. 7 and 8). Since flight interception traps are not suitable for assessing the abundance of Aradidae (GOßNER et al., 2007), we decided to examine those structures visually. Our survey protocol consisted of inspecting the bark of those potential host structures with a similar intensity: during around ten minutes, up to a height of two meters, and on multiple occasions in order to minimise the impact of false absences (cfr. LOOS et al., 2014).

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Fig. 6. Screening the study area for habitat structures, 6.VI.2020. © Brecht Verkempinck.

Fig. 7. Post-sunset inspection of habitat structures, Lede, 17.VIII.2020. © Robin Van Heghe.

7 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

Table 1. Colonised structures with details. N° (location number in chronological order), Locality, Coordinates, Date (date of first finding), Observers (BN: Bente Verkempinck, B: Brecht Verkempinck, G: Gert Van Heghe, R: Robin Van Heghe, T: Tim Speleers), Tree species (Aesculus: Aesculus hippocastanum, Alnus: Alnus glutinosa, Fagus: Fagus sylvatica, Populus: Populus × canadensis, Prunus: Prunus serrulata, Salix: Salix alba, Quercus: Quercus rubra, Tilia: Tilia tomentosa), Type (type of colonised structure; branch: fallen branch, log: lying tree, snag: standing dead or dying tree, tree: standing healthy-looking tree; the qualification of locations 74–78 as logs is open to question, since it is highly likely that A. brenskei was already present on them prior to the inspection, when they were still standing), Sun (sun exposure; S: sunny, SH: shaded, SS: semi-sunny), Decay (degree of decay of the colonised structure: dead, dying, healthy: healthy-looking but dying), Diameter (diameter of structure: 25–30cm, 30–50cm, > 50cm), Lowest fruitbody (distance of lowest fruitbody to the ground: < 5m, 5–10m, > 10m, NA: not applicable), Individuals (number of unique individuals spotted, split up into adults and larvae; A: adults; L: larvae). In red: locations that were cut down during our survey. N° Locality Coordinates Date Observ Tree Type Sun Decay Diameter Lowest Individuals ers species fruitbody 01 Lede 50.96297, 3.9615 07.V.2020 BN/B/ Populus stump S dead > 50 cm < 5m 9A/69L G/R/T 02 Lede 50.96268, 3.95571 20.V.2020 R/B Populus snag SS dead > 50 cm < 5m 6A/6L 03 Lede 50.96641, 3.95682 27.V.2020 R/B Populus snag S dying > 50 cm < 5m 20A/18L 04 Lede 50.96255, 3.95901 27.V.2020 B/R Populus snag S dead > 50 cm < 5m 35A/7L 05 Lede 50.9526, 3.9527 30.V.2020 R/B/T Salix snag SS dying > 50 cm < 5m 10A/2L 06 Lede 50.95223, 3.95373 30.V.2020 B/R/T Populus snag SH dead 30-50 cm < 5m 3A/6L 07 Aalst 50.9529, 4.0011 6.VI.2020 B/R/T Populus snag S dead > 50 cm < 5m 1A 08 Aalst 50.9521, 4.00359 6.VI.2020 B/R/T Populus snag S dead > 50 cm < 5m 1A 09 Aalst 50.95202, 4.00381 6.VI.2020 B/R/T Populus snag S dead > 50 cm < 5m 4A 10 Aalst 50.9505, 4.00469 6.VI.2020 B/R/T Populus snag SS dead 30-50 cm < 5m 1A 11 Lede 50.96255, 3.95877 6.VI.2020 B/R/T Populus branch S dead 25-30 cm NA 1A 12 Lede 50.96206, 3.95866 6.VI.2020 B/R/T Populus snag S dead > 50 cm 5-10m 24A 13 Erpe-Mere 50.952, 3.9766 9.VI.2020 T/B Populus snag S dead > 50 cm < 5m 6A 14 Lede 50.94988, 3.95679 9.VI.2020 T/B Salix snag S dead > 50 cm < 5m 8A 15 Lede 50.95048, 3.95777 9.VI.2020 B/T Populus log S dead > 50 cm NA 2A 16 Lede 50.9524, 3.95451 9.VI.2020 B/T Populus snag SH dead 30-50 cm 5-10m 2A 17 Lede 50.9494, 3.9551 9.VI.2020 B/T Populus snag SH dead 30-50 cm > 10m 1A/2L 18 Lede 50.9542, 3.953 9.VI.2020 B Populus snag SH dead > 50 cm < 5m 2A/1L 19 Berlare 51.04686, 3.99523 15.VI.2020 R/B/T Populus snag SS dead > 50 cm > 10m 7A 20 Berlare 51.047, 3.9953 15.VI.2020 R/B/T Populus snag S dead > 50 cm < 5m 8A 21 Berlare 51.0466, 3.9958 15.VI.2020 R/B/T Populus snag S dead > 50 cm > 10m 26A 22 Berlare 51.0465, 3.9958 15.VI.2020 R/B/T Populus snag S dead > 50 cm > 10m 1A

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23 Berlare 51.02385, 3.97374 15.VI.2020 R/B/T Populus snag S dead > 50 cm < 5m 10A 24 Lede 50.95056, 3.93469 7.VII.2020 B/T Fagus snag S dying > 50 cm < 5m 7A/96L 25 Lede 50.9601, 3.9749 8.VII.2020 B Prunus snag S dying > 50 cm < 5m 3A/32L 26 Lede 50.96127, 3.98012 8.VII.2020 B Prunus snag S dead > 50 cm < 5m 1A/3L 27 Erpe-Mere 50.9458, 3.953 9.VII.2020 B/T Salix tree S healthy > 50 cm < 5m 1A 28 Lede 50.9637, 3.9099 12.VII.2020 B Populus snag S dying > 50 cm < 5m 243L 29 Wetteren 50.9693, 3.878 24.VII.2020 T/B Populus snag S dying > 50 cm < 5m 5A 30 Erpe-Mere 50.9517, 3.977 25.VII.2020 B/T Populus snag S dying > 50 cm > 10m 11A 31 Erpe-Mere 50.9516, 3.9771 25.VII.2020 B/T Populus snag S dying > 50 cm 5-10m 6L 32 Erpe-Mere 50.9515, 3.9772 25.VII.2020 B/T Populus snag S dead > 50 cm < 5m 5A/24L 33 Lede 50.9655, 3.95855 26.VII.2020 B Populus snag S dead > 50 cm < 5m 2A/6L 34 Berlare 51.0356, 3.9797 30.VII.2020 R/B Populus snag SH dying > 50 cm > 10m 1A 35 Berlare 51.0327, 3.9801 30.VII.2020 R/B Populus snag S dead > 50 cm 5-10m 2A/26L 36 Berlare 51.0378, 3.9746 30.VII.2020 R/B Populus snag SH dead > 50 cm 5-10m 3L 37 Berlare 51.024, 4.0005 31.VII.2020 B Fagus stump S dead > 50 cm < 5m 1L 38 Wetteren 50.9814, 3.8819 06.VIII.2020 B Populus snag S dead > 50 cm < 5m 10L 39 Wetteren 51.0074, 3.8996 07.VIII.2020 B Quercus tree SH healthy > 50 cm < 5m 15A/10L 40 Lede 50.9632, 3.9595 07.VIII.2020 B Populus snag SS dead > 50 cm > 10m 3L 41 Lede 50.96276, 3.95922 16.VIII.2020 B Populus tree SS healthy > 50 cm > 10m 11A 42 Erpe-Mere 50.9513, 3.9935 17.VIII.2020 B Populus snag SS dead > 50 cm < 5m 1A 43 Lede 50.9519, 4.0022 17.VIII.2020 B Populus log SH dead > 50 cm NA 1L 44 Lede 50.94715, 3.95549 17.VIII.2020 B/R/T Populus log S dead > 50 cm NA 1A 45 Lede 50.94711, 3.95545 17.VIII.2020 B/R/T Populus snag S dying > 50 cm < 5m 4A 46 Lede 50.9495, 3.955 17.VIII.2020 B/R/T Populus snag SH dead > 50 cm < 5m 6A 47 Lede 50.95046, 3.95488 17.VIII.2020 B/R/T Populus snag S dead > 50 cm > 10m 1L 48 Lede 50.9523, 3.9543 17.VIII.2020 B/R/T Populus snag SH dead > 50 cm < 5m 8A 49 Lede 50.95285, 3.95442 17.VIII.2020 B/R/T Populus snag SH dead > 50 cm < 5m 1A 50 Lede 50.9537, 3.9568 17.VIII.2020 B/R/T Populus snag SH dead > 50 cm < 5m 1A/11L 51 Wichelen 51.00618, 3.95648 17.VIII.2020 R Populus snag S dead > 50 cm < 5m 15A/45L 52 Lede 50.975, 4.0085 19.VIII.2020 B Populus snag S dead 30-50 cm > 10m 2L 53 Lede 50.96199, 3.95974 20.VIII.2020 B Populus snag S dead > 50 cm 5-10m 1L 54 Aalst 50.9603, 4.0174 20.VIII.2020 B Populus snag SS dead > 50 cm 5-10m 7L 55 Aalst 50.9601, 4.01729 20.VIII.2020 B Populus snag SS dead 30-50 cm < 5m 2A/11L 56 Aalst 50.9601, 4.0174 20.VIII.2020 B Populus snag SS dead 30-50 cm < 5m 8L

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57 Aalst 50.9601, 4.0174 20.VIII.2020 B Populus snag SS dead 30-50 cm 5-10m 2L 58 Berlare 51.0396, 3.9733 22.VIII.2020 B Populus snag SS dead 30-50 cm > 10m 17A 59 Berlare 51.0389, 3.973 22.VIII.2020 B Alnus snag SS dead 30-50 cm 5-10m 1A/1L 60 Berlare 51.0241, 3.9668 23.VIII.2020 B Salix snag S dead 30-50 cm < 5m 5A/17L 61 Wetteren 51.0141, 3.8974 31.VIII.2020 B Populus snag SS dead > 50 cm < 5m 1A/2L 62 Lede 50.9626, 3.9588 31.VIII.2020 B Populus branch S dead 25-30 cm NA 1L 63 Lede 50.9678, 3.9454 02.IX.2020 B Populus snag S dying > 50 cm < 5m 2L 64 Lede 50.9681, 3.945 02.IX.2020 B Populus snag S dead > 50 cm 5-10m 1A 65 Lede 50.9675, 3.9451 02.IX.2020 B Populus snag S dead > 50 cm 5-10m 2A 66 Wichelen 50.9806, 3.9204 06.IX.2020 T/B Populus snag SS dying > 50 cm > 10m 1A 67 Lede 50.9699, 3.9023 15.IX.2020 B/T Populus snag S dead > 50 cm < 5m 15A 68 Lede 50.9697, 3.9021 15.IX.2020 B/T Populus log S dead > 50 cm < 5m 6L 69 Wetteren 50.9698, 3.8777 15.IX.2020 B/T Populus snag SS dying > 50 cm < 5m 14A 70 Wetteren 50.97, 3.8774 15.IX.2020 B/T Populus snag SS dead > 50 cm < 5m 5A/12L 71 Lede 50.9675, 3.9706 21.IX.2020 B/R/T Tilia snag S dead > 50 cm < 5m 3A/6L 72 Lede 50.9626, 3.9592 29.IX.2020 B Populus tree SS healthy > 50 cm > 10m 3A 73 Lede 50.9678, 3.9936 03.X.2020 B Aesculus snag S dying > 50 cm < 5m 8A/2L 74 Lede 50.9550, 3.9699 12.X.2020 B Populus log SS dead > 50 cm NA 3A/1L 75 Lede 50.9551, 3.9699 14.X.2020 B Populus log SS dead > 50 cm NA 12A 76 Lede 50.9550, 3.9701 14.X.2020 B Populus log SS dead > 50 cm NA 4A 77 Lede 50.9551, 3.9701 14.X.2020 B Populus log SS dead > 50 cm NA 1A 78 Lede 50.9548, 3.9700 14.X.2020 B Populus log SS dead > 50 cm NA 13A 79 Melle 50.9921, 3.8200 19.XII.2020 T/B Fagus Snag S Dead > 50 cm < 5m 3A/4L

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Fig. 8. Post-sunset inspection of habitat structures, Lede, location 44, 17.VIII.2020. © Brecht Verkempinck.

For each structure on which we found individuals of Aradus brenskei (hereafter referred to as “colonised structures” or “locations”), we noted the tree species and the deadwood type. We followed the distinction made by MORKEL (2017) between standing structures (in our case: standing trees, which are also called “snags” when they are dead or dying, and stumps) and lying structures (in our case: lying trees or “logs”, and fallen branches). Large-diameter deadwood structures are known to provide a more stable environment for organisms (JAGERS OP AKKERHUIS et al., 2006; MORAAL, 2014). By having a longer decay time, they provide a long-term source of nutrients (VÍTKOVÁ et al., 2018). This encourages researchers focusing on flat bugs to examine their occurrence in the light of the diameter of colonised structures (GOßNER et al., 2007; MORKEL, 2017; MARCHAL et al., 2012). We followed that model by estimating the diameter of each structure and by classifying them in four classes, with diameters of less than 25 cm, 25–30 cm, 30–50 cm and more than 50 cm. Their decomposition spectrum was scored in four classes, ranging from healthy-looking, dying and dead, to dead but entirely rotten. The sunlight exposure was classified as sunny, semi-sunny or shady.

11 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

Taking into account that enzyme activities measured in wood located close to fungal fruitbodies are likely to reflect the activity of F. fomentarius (VETROVSKY et al., 2011), we estimated the position of the lowest fruitbody above the ground (less than 5 m, 5–10 m, and more than 10 m) in order to assess whether there was a positive correlation between that position and the occurrence and abundance of A. brenskei. Information on the numbers and the behaviour of both adults and larvae was noted at each inspection. Since the attraction by artificial light could provide indirect evidence of flight (LARIVIÈRE et al., 2006), we regularly used a moth trap (lamp type HPL-N 125W/542) near location 01 to catch dispersing specimens. Results

DISTRIBUTION Central East Flanders During our nine-months field survey, we found 79 colonised structures with 401 adults and 717 larvae of A. brenskei in central East Flanders: in Lede, Berlare, Aalst, Wetteren, Erpe-Mere, Wichelen and Melle (Fig. 9). As appears from Table 1, most colonised structures were found in the Geelstervallei, a lowland landscape stretching along the banks of the Molenbeek (a tributary of the Scheldt), with multiple plantations and rows of Populus x canadensis and grasslands bordered with pollarded Salix alba. We also confirmed its presence in Honegem, a neighbouring nature reserve with similar conditions, and Berlarebroek, a nature reserve where a meander of the Scheldt formed a landscape also characterised by poplar plantations (MAERTENS, 2019). Nevertheless, we also spotted individuals in different environments, namely in the river dune system of Den Blakken (Wetteren) and even near busy roads (locations 24–26).

Fig. 9. Map with location numbers and tree species of the colonised structures.

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Study area We never found individuals on F. fomentarius infested structures in other localities, such as in parts of Aalst (Kapellemeersen, Osbroek, Gerstjens and Kravaalbos), Sint-Lievens-Houtem (Vlierzele), Ghent (Vinderhoutse bossen), Lokeren (Molsbroek), Denderleeuw (Wellemeersen), Ninove (Neigembos), Geraardsbergen (De Gavers) and Asse (Fig. 10). Those absences on potential habitat structures enabled us to delimit our study area, i.e. the area in which we found (and examined) colonised structures. With the distance between our northernmost and southernmost colonised structures (locations 20 and 27) being around 11 km and the distance between our westernmost and easternmost colonised structures (locations 79 and 54) being about 13 km, our study area covers approximately 130 km².

Fig. 10. Belgian distribution of Aradus brenskei (5x5 km squares). Grey squares: investigated squares with structures infested by Fomes fomentarius. Red dots: squares with structures colonised by Aradus brenskei. © André van Loon, EIS Kenniscentrum voor Insecten en andere ongewervelden, Leiden.

COLONISED STRUCTURES Tree species We found Aradus brenskei on structures of eight tree genera: Populus, Salix, Fagus, Prunus, Quercus, Alnus, Tilia and Aesculus (Fig. 11). The colonised structures consisted for 84 % of Populus x canadensis (66 structures), for 5 % of Salix alba (four structures), for 4 % of Fagus sylvatica (three structures) and for 3 % of Prunus serrulata (two structures). We also found individuals on one Quercus rubra, on one Alnus glutinosa, on one Tilia tomentosa and on one Aesculus hippocastanum.

13 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

Contrary to Aradus betulae (Linnaeus, 1758), which was found by the first author on a birch log in Germany (Dwergter Sand, 52.8904, 7.9268, 1.VII.2020), we never found A. brenskei on Betula, even though we examined inter alia five F. fomentarius infested birches near colonised structures.

Fig. 11. Tree species of the colonised structures.

Deadwood type In 87 % of the cases, A. brenskei was found on standing structures (Fig. 12). In this category of standing colonised structures, in two occasions we also found individuals on stumps: the poplar stump of location 01 and the beech stump of location 37.

Fig. 12. Deadwood type of the colonised structures.

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The remaining 13 % of the colonised structures were lying structures: two lying branches and nine lying trees (“logs”). On the two lying branches (locations 11 and 62), which were only ten meters away from the standing structure of location 04, we found each time one single individual. In October, we discovered 34 individuals on five newly felled poplars (locations 74–78). Their qualification as lying trees is however open to question, since it is highly likely that A. brenskei was already present on those poplars when they were still standing a few days before inspection. In conclusion, if we exclude the newly felled poplars of locations 74–78, we only found twelve individuals on six lying structures (two branches and four lying trees). Hence, merely 1 % of all individuals were found on lying structures and many lying structures did not harbour any individual. For instance, in August alone, we inspected 47 F. fomentarius infested logs near colonised structures, without finding any individual. Deadwood volume We found A. brenskei in three diameter classes: 25–30 cm, 30–50 cm and > 50 cm. Of the colonised structures, 84 % (i.e. 66 structures) had a diameter exceeding 50 cm (Fig. 13). A diameter between 30 and 50 cm was established for 14 % of the colonised structures (i.e. 11 structures). Only two colonised structures fell within the category of 25–30 cm: the lying branches of locations 11 and 62, having a diameter of respectively 26 and 27 cm. Moreover, we only found one individual on each of them (one adult and one larva). We never documented individuals on F. fomentarius infested structures with a diameter of less than 25 cm.

Fig. 13. Deadwood volume (diameter) of the colonised structures.

Decomposition spectrum Aradus brenskei was found on structures on a wide spectrum of wood-decay stages. However, 95 % of the colonised structures were clearly dead or dying (Fig. 14). Only four F. fomentarius infested structures were healthy-looking, having still many foliated branches. We never found any individual on an entirely rotten F. fomentarius infested structure. Most of the colonised structures had already detached bark (51 structures) or had easily removable bark (20 structures). Only five colonised structures had an unremovable bark, at least

15 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

Fig. 14. Degree of decay of the colonised structures. within our reach (locations 11, 41, 53, 62 and 72). Moreover, many F. fomentarius infested structures which tested negative for A. brenskei had a bark that was impossible to remove. Sunlight exposure Of our colonised structures, 56 % were in rather sunny places (Fig. 15), being in the middle of a meadow (e.g. location 08), on a terrace (location 01), at a playground (location 37) or alongside a path (e.g. location 04). Almost 30 % were in a semi-sunny position, receiving sunlight for part of the day. Being situated in the middle of a forest area, only 15 % of the colonised structures received lower levels of solar radiation (Fig. 16). The 47 F. fomentarius infested lying structures that we

Fig. 15. Sunlight exposure of the colonised structures.

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Fig. 16. Sparse canopy of poplars, location 61, Wetteren, 31.VIII.2020. © Brecht Verkempinck. examined in August and on which we did not find any A. brenskei were all surrounded by lower vegetation. By contrast, the branches on which we found A. brenskei (locations 11 and 62) were exposed to the sun all day long.

Presence of Fomes fomentarius We only encountered A. brenskei on structures with F. fomentarius fruitbodies. Moreover, each time we encountered fruitbodies of other fungi (such as Ganoderma lipsiense, Laetiporus sulphureus and Piptoporus betulinus) on or near colonised structures (e.g. near location 79), we checked those fruitbodies as well, but we never found individuals on or under them. This suggests that, at least in our study area, A. brenskei is monophagous, feeding only on the tinder fungus.

17 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

Fig. 17. Destroyed fruitbody, location 19, Berlare, 15.VI.2020. © Brecht Verkempinck.

Fig. 18. Well camouflaged larva, location 79, Melle, 19.XII.2020. © Brecht Verkempinck.

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Fig. 19. Larvae feeding on fruitbody, location 1, Lede, 30.VII.2020. © Brecht Verkempinck.

Fig. 20. Nutritional competition with Bolitophagus reticulatus, location 32, Erpe-Mere, 25.VII.2020. © Brecht Verkempinck.

19 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

We could not find any positive correlation between the visible fungal biomass of F. fomentarius and the abundance of A. brenskei. Since high numbers of individuals were found on deadwood structures with only one fruitbody (e.g. 7 adults and 96 larvae at location 24), the number of fruitbodies does not seem to be decisive. As to the freshness of the fruitbodies, on several occasions we found A. brenskei on broken fruitbodies (Fig. 17). Nevertheless, we never encountered, even on or near colonised structures, any individual on rotten or hollow fruitbodies, even when they were colonised by Bolitophagus reticulatus (Linnaeus, 1767), which we encountered by the way on most (A. brenskei) colonised structures.

BEHAVIOUR OF ARADUS BRENSKEI Under the bark As the English name for its family (flat bugs) already indicates, Aradus brenskei has a flattened body. The synonym bark bugs refers to the fact that its flattened body enables it to live under the bark. As such, A. brenskei is perfectly adapted to its subcorticolous lifestyle. Individuals were, however, often spotted on the bark and on the F. fomentarius fruitbodies, in temperatures ranging from 8,7°C (11.X.2020) to 36,2°C (30.VII.2020). Individuals seen during the day were mostly larvae, who were well camouflaged both on the fruitbodies and on the bark (Fig. 18 and 21). During the day, adults were only very rarely spotted on the bark. The best way to find individuals then, was to remove bark. Nutrition We have often seen A. brenskei feeding on F. fomentarius fruitbodies (Fig. 19). Even during the day, the gregarious larvae were observed feeding on them. Adults of Bolitophagus reticulatus were, however, in a nutritional competition with A. brenskei, and often pushed A. brenskei away towards other parts of the colonised structures (Fig. 20). Around 35 % of the standing colonised structures had their lowest fruitbodies more than five meters from the ground (24 out of 69 standing colonised structures; Table 1). This confirms that A. brenskei also feeds on the lower parts of the fungal hyphal cords under the bark. In fact, we saw individuals piercing through bark crevices to extract liquids from fungal hyphae (Fig. 21). On two occasions in May, we even spotted individuals hanging from their stylets under a fruitbody (Fig. 22). They probably fell down when feeding, and were unable to climb up the fruitbody by withdrawing the stylet into their head capsule. Life cycle Since we spotted young larvae from May until the end of December (e.g. on 20.V.2020 at location 02, but also, under the bark of location 79, on 19.XII.2020), we estimate that mating starts in springtime and lasts until the late summer. We witnessed mating for instance on 3.VI.2020 (Fig. 23). Since we also found larvae on lying trees (locations 43 and 68) and even on a lying branch (location 62), we presume that A. brenskei reproduces not only on standing (Fig. 23), but also on lying structures. We frequently witnessed adults being near or even on top of their brood, which could indicate parental care (Fig. 24; TAYLOR, 1988). We spotted fewer individuals on the bark from October onwards. The almost daily post-sunset inspections of several locations during the last fourteen weeks of our survey resulted in only four records: three in the first two weeks of November and one in December (1♀ on 18.XII.2020, at location 01 at 9,1°C). In that period we still found, however, both adults (e.g. 2♀♀ on 17.XII.2020 at location 04 and 1♀ on 30.I.2021 at location 18), and larvae (e.g. 19.XII.2020 at location 79), under the bark. A. brenskei seems thus to start hibernation, both at the larval and the adult stage, somewhere between October and November. On 19 February

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2021 (9,9°C), we found two female adults on the bark of location 4 and 1 male adult on the bark of location 12, which indicates the end of their hibernation period.

Flight We observed two flight attempts: on 25.VI.2020 at location 30, after shining on an adult with a torchlight, and on 15.VIII.2020 at location 06, after removing some bark. Nevertheless, we never observed any real flight activity, neither direct from our field observations during the day or after dark, nor by catching specimens at light during the night.

Discussion

GENERAL Some saproxylic heteropterans show a predilection for a single tree species (VOGEL et al., 2020a). This is not the case for A. brenskei. In the past, this flat bug has been found on Quercus (WAGNER, 1955), Populus (HEISS, 1990), Tilia (HEISS et al., 2007) and Fagus (MARCHAL et al., 2012). We found it additionally on Salix, Prunus, Alnus and Aesculus. Given that all the colonised structures were also infested by Fomes fomentarius and since we have very often seen A. brenskei feed on the fruitbodies and mycelia of that fungus, A. brenskei is not only deadwood-dependent (“saproxylic”), but also mycetophagous. It is even reasonable to assume that its occurrence is related more to the presence of that fungal species than to the tree species. The tree species only seems relevant to the extent to which it is attacked by that fungus. As regards the colonised tree species, the predominance of poplars is most probably influenced by the fact that our study area is most densely populated by poplars (WATERINCKX, 2001). After World War II, the moist meadows on the alluvial soils formed by floodings of the Scheldt were massively planted with the fast-growing Populus x canadensis with the aim of meeting a shortage of wood in the economy (STEENACKERS et al., 2018), in particular for the match industry (STROOBANT & ROBBEN, 2003). In the last two decades, those plantations have become increasingly neglected, which has resulted in an increased volume of standing deadwood (GOVAERE, 2020), which, in turn, has benefited F. fomentarius and thus A. brenskei. Like the majority of flat bugs (MORKEL & FRIEß, 2018), A. brenskei depends on dead or dying wood structures. A. brenskei was mostly observed on standing dead or dying trees, but was to a minor extent also discovered on lying structures. In the category of lying structures, A. brenskei was more often recorded on logs, probably thanks to their greater durability (VOGEL et al., 2020b). Branches possibly also have a reduced buffering for heat and drought, which challenge the survival and larval development of saproxylic beetles (VOGEL et al., 2020a). If we exclude the newly felled poplars of locations 74–78 (cfr. legend to Table 1), 99 % of the individuals were found on standing structures. As a result, we can conclude that A. brenskei is far more likely to be found on standing structures (cfr. BOUGET et al., 2012 for saproxylic beetles). An explanation could be that lying deadwood is more humid and colder than standing deadwood (MORAAL et al., 2007), and thus has a faster rate of decay (VÍTKOVÁ et al., 2018). We presume, however, that A. brenskei also reproduces, exceptionally, on lying structures, since we discovered young larvae on them. Whereas Aradus depressus can be found on small branches and twigs (AUKEMA & HERMES, 2016), A. brenskei seems to prefer large-diameter deadwood, as does A. betulae (MORKEL, 2017). Just as MARCHAL et al. (2012) only found A. brenskei on structures with a diameter exceeding 20 cm in Fontainebleau, we only recorded A. brenskei on structures with a diameter exceeding 25 cm. Individuals were found on structures on a wide decomposition spectrum, ranging from dead to healthy-looking. Just as has been reported for Aradidae in Germany

21 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

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(MORKEL, 2017), entirely rotten trees were not suitable as habitat structures. A major indicator for the global degree of decomposition seems to be the looseness of the bark. It is commonly thought that dehiscent bark provides a more adequate habitat for flat bugs, in terms of forage, nesting and shelter (MARCHAL et al., 2012). Light is an important abiotic factor influencing the diversity of heteropterans (GOßNER, 2009). Also, within the communities of saproxylic beetles the highest species numbers are typically reached in sun-exposed deadwood (VOGEL et al., 2020b). Just as Aradus betulinus (Fallén, 1807) and A. betulae are known to prefer sun-exposed habitats (SEIBOLD et al., 2014), we found that most of our colonised structures were in rather sunny places. Moreover, the sparse canopy and the ensuing low shade-casting ability of our poplar cultivars (THOMAES & DE KEERSEMAEKER, 2011) enabled the sunrays to reach the bark of many colonised structures. This is consistent with an earlier finding that higher canopy openness leads to an increase of saproxylic beetles (MÜLLER et al., 2010). Given that our few colonised lying structures were also sun-exposed, sun exposure seems to be a key factor to the occurrence of A. brenskei. A. brenskei only occurs on wood structures infested by the wood-decaying basidiomycete Fomes fomentarius. Whereas aradid species are rarely observed feeding on fruitbodies (GOßNER et al., 2007), we have often seen A. brenskei doing that. Larvae can be seen feeding on F. fomentarius fruitbodies during the day, even in direct sunlight. The fact that 35 % of our standing colonised structures had their lowest fruitbodies more than five meters from the ground confirms that A. brenskei also feeds on the lower parts of the fungal hyphal cords.

Fig. 22. Adult hanging under a fruitbody, location 4, Lede, 28.V.2020. © Brecht Verkempinck. Fig. 23. Mating, location 4, Lede, 3.VI.2020. © Brecht Verkempinck.

23 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

For A. brenskei, we agree with HEISS & PÉRICART (2007) that generations of aradid species may develop asynchronously, resulting in the presence of both larvae and adults throughout the year at the same locality. They also point out that those species develop acyclically over a period of two years, with in the first year larvae hatching from eggs laid in spring and hibernating as older larvae of stage IV and V and in the second year becoming adult, mating in autumn, and overwintering for the second time before laying eggs the next spring. For A. brenskei, however, we observed mating as early as the beginning of June (Fig. 23). We agree, however, with their finding that larvae also (start to) hibernate. A. brenskei seems to start hibernation between October and November (cfr. STEHLÍK & HEISS, 2001, for the related betulae-group).

Long dispersal flights are common among the Aradidae, due to the ephemeral occurrence of suitable structures (WACHMANN et al., 2007). For instance, it is presumed that A. betulae disperses after winter in order to colonise new trees (GOßNER et al., 2007). Aradus obtectus (Vásárhelyi, 1988) uses olfactory cues emitted by dead wood to locate potential resources over long distances (KOBAN et al., 2016). Flights have also been reported for Aradus depressus depressus (AUKEMA & HERMES, 2016). Even though there was no direct evidence of flight by

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A. brenskei, indirect evidence of flight was provided by the observation of two flight attempts, and by the distributional pattern (cfr. RANIUS, 2006), with colonised structures being separated by the river Scheldt. The fact that we found adults on a healthy-looking poplar with only one (fresh) fruitbody, more than 20 meters above the ground (location 72), could mean that A. brenskei colonises trees at early stages of decay (cfr. SEIBOLD et al., 2014 for saproxylic heteropterans in general). This would, of course, require some further research.

CONSERVATION Contemporary forest management still often adversely affects the availability of deadwood and the associated species (MORKEL, 2017 for A. betulae). Feeding exclusively on Fomes fomentarius, A. brenskei indirectly depends on a continuous supply of deadwood. This importance of deadwood for flat bugs was already recognised nearly twelve decades ago, when OSBORN (1903) assumed that the occurrence of flat bugs had decreased rapidly because of the removal of dead wood (see also SIITONEN & MARTIKAINEN, 1994 for Finnish Karelia). Decaying trees were historically removed as they were considered a waste of resources (THORN et al., 2020) and a hatchery of forest pathogens (VOGEL et al., 2020a). Bearing in mind that 84 % of the colonised structures in our study area consist of Populus x canadensis, a significant threat to A. brenskei is therefore the way some former poplar plantations are dealt with. Location 29 was cut down in September, together with the 15.000 m² of surrounding poplars. Exactly the same happened in February 2021 to location 53, and in October to locations 74–78 and the hundreds of other potential habitat structures surrounding them (Fig. 25). Those colonized structures were removed on the grounds that, being poplar cultivars, they are an exotic species (MOREAU, 2020) having “no ecological value at all”, that are thus to be replaced by indigenous species, such as Populus nigra (VERBAERE, 2020). From the viewpoint of saproxylic beetle diversity alone, however, non-native tree species (VOGEL et al., 2020a), as was demonstrated also for Populus x canadensis in particular (ANDRINGA et al., 2019), are not that different from native species, and thus definitely have an ecological value.

25 VERKEMPINCK B. et al. The northernmost discovery of Aradus brenskei (Heteroptera: Aradidae)

In our study area, even though deadwood has reached its highest level in Flanders at any time over the last five to ten centuries (VANDEKERKHOVE et al., 2013), a further massive removal of poplar cultivars might lead to a habitat discontinuity for A. brenskei. We would therefore recommend deadwood enrichment (e.g. VOGEL et al., 2020b), focusing in our study area at least for the coming years on the omnipresent decaying poplar cultivars. A. brenskei could benefit from retaining sufficient numbers of at least partially sunlit large-diameter deadwood structures (COUNCIL OF EUROPE, 1988). That deadwood enrichment approach could be implemented at low economic efforts by reducing the removal of poplar cultivars in order to allow them to undergo natural senescence – except if the aim is to restore for example valuable grasslands (VERSTRAETEN et al., 2003) – or by retaining decaying trees during final harvesting (HÄGGLUND et al., 2015). More than three decades ago, the Council of Europe recognised that saproxylic organisms are a fundamental part of the European natural heritage and recommended avoiding the removal of deadwood (COUNCIL OF EUROPE, 1988). In this light, it is astonishing that the European Commission recently neglected those living dead in its EU Biodiversity Strategy for 2030 (EUROPEAN COMMISSION, 2020). If further survey would establish that A. brenskei is threatened, which does not seem unlikely considering the destruction of its habitat as occurred in our study area and given the low records worldwide, could protection at Union level as the one granted to Aradus angularis (Sahlberg, 1886) by the Habitats Directive, then be a reasonable option?

Acknowledgements

We would like to thank the daughter of the first author (Bente Verkempinck) for discovering the first individual of A. brenskei in Belgium. We are also grateful to the anonymous reviewers for their helpful suggestions on the structure and methodology, to Dik Hermes for his valuable suggestions on the content and terminology, to Stephen Hobbs for critically reviewing the English grammar, to Tine Lassuyt for proofreading the text, to Theodoor Heijerman for delivering the detailed picture of a male adult A. brenskei, and to André van Loon for delivering the map on the Belgian distribution of Aradus brenskei.

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