First systematic inventory of the jumping plant lice of Luxembourg (Hemiptera, Sternorrhyncha, Psylloidea)

Abstract Psyllids (superfamily Psylloidea), also known as jumping plant lice, are a group of plant-sap sucking Hemiptera having significant pest status for crops, forest trees and ornamental plants. Only seven species of psyllids have been recorded in Luxembourg so far. An additional group of seven species has been recorded exclusively, based on the findings of their galls or specific plant deformations; but no mention exists in literature on the actual collection of the inducing insect in Luxembourg. To fill this knowledge gap, field collections were carried out during the years 2019-2020. In addition, samples from 1999-2000 stored in the wet collection of the Musée National d’Histoire Naturelle de Luxembourg were studied. This research, in combination with information coming from literature, allowed us to list 48 species of the families Aphalaridae (5 species), Liviidae (5), Psyllidae (24) and Triozidae (14), though the presence of one species within the last family (Triozarhamni) needs to be confirmed. Brief information on geographical distribution, biology and (if available) illustrations of diagnostic characters are provided on the psyllid species detected in Luxembourg so far.


Introduction
Psyllids (superfamily Psylloidea), also known as jumping plant lice, are a group of plantsap sucking Hemiptera which may have significant pest status for crops, forest trees and ornamental plants due to their copious production of honeydew, their frequent coating with waxy secretions (smearing the canopy of infested plants), the injection of toxic saliva (causing necrosis, deformations or galls) and, last but not least, their responsibility in transmitting many pathogens to plants, mainly bacteria and especially phytoplasmas (Hodkinson 1974, Hodkinson 1984, Burckhardt 1994a, Munyaneza 2010, Haapalainen 2014, Ben Othmen et al. 2019. In spite of their importance, psyllids are still poorly known in Luxembourg; this is why their species composition, diversity and distribution need to be properly assessed. According to literature, Spanioneura buxi (Linnaeus), S. fonscolombii Foerster and Trichochermes walkeri (Foerster) are the only species of Psylloidea whose presence in Luxembourg has been reported till now based on adult specimens (Baugnée 2001, O'Connor and Malumphy 2011, Eickermann et al. 2020). However, additional records of psyllids can be also considered, deriving from rich cecidological literature dealing with the presence of plants with galls or deformations probably induced by Psylloidea. Yet, these records need to be confirmed by finding the insects to which the effects on plants are attributed. In particular, Lambinon and Schneider (2004) investigated deformations caused to plants by Cacopsylla mali (Schmidberger), C. melanoneura (Foerster), Livia junci (Schrank), Spanioneura buxi (Linnaeus), Psyllopsis fraxini (Linnaeus), Lauritrioza alacris (Flor), Trioza flavipennis Foerster, T. remota Foerster and T. rhamni (Schrank), in addition to the ones caused by T. walkeri (already reported for Luxembourg). For P. fraxini, the authors reported the rear of the psyllid from leaf galls collected on Fraxinus excelsior L. from many localities in Luxembourg (Bettembourg, Bonnevoie, Bridel and Kleinbettingen). Similarly, they reported findings of eggs of T. flavipennis in pit galls found on leaves of Aegopodium podagraria L. For all remaining cecidia described, however, no indication was given on the finding of the causing insect. Deformations caused by three additional psyllid species [Camarotoscena speciosa (Flor), Trioza centranthi (Vallot) and T. urticae (Linnaeus)] were reported by Lambinon et al. (2012), who did not mention any finding of their causing insects. Deformations by L. alacris and T. centranthi and galls by T. remota were reported again by Schneider (2016) from new localities in Luxembourg. Only for T. remota, the author referred to the presence of an insect immature within each pit gall found on leaves of Quercus petraea (Matt.) Liebl. and Q. robur L. Cecidia caused by C. speciosa, L. junci and T. flavipennis were reported again by Burton et al. (2019), who explicitly mentioned for the first time the finding of nymphs of C. speciosa and their abundant wax secretion on the leaf deformations of Populus nigra L. and confirmed the occurrence of a T. flavipennis nymph in the concave leaf gall produced on A. podagraria.
Even when hypothesising that all psyllid records, based on findings of their galls, will be confirmed by collection of insect specimens, still the number of psyllid species presently known in Luxembourg is remarkably low, for example, when compared with faunas of neighbouring countries in the "Benelux" region": 69 species known in The Netherlands (den Bieman et al. 2019); at least 56 species in Belgium (Ouvrard 2021). Present knowledge on the psyllid fauna of Luxembourg appears weak also when considering the richness and composition of the regional flora (especially for plant groups that may host psyllid species), as assessed by Lambinon and Verloove (2012). The lack of a systematic inventory of the psyllids of Luxembourg, in combination with their ecological and partly agricultural significance, gave rise to the current manuscript.

Data resources Study area
The Grand Duchy of Luxembourg is characterised by a temperate, semi-oceanic climate. Even though the area of the country is small (2,586 km ) with a maximal Euclidean distance of 82 km from north to south and 57 km from east to west, Luxembourg offers quite diverse physiogeography with different climatological characteristics, associated vegetation and anthropogenic land use. Traditionally, Luxembourg is divided into two main ecoregions, the Oesling (32% of total area) and the Gutland (68%) (Dohet et al. 2008). The Oesling is located in the north of the country, in the border region between Belgium and Luxembourg -and is the eastern part of woody mountains called the Ardennes with highest altitude of 450 m a.s.l. Annual precipitation ranges from 800-1000 mm and annual mean air temperature is 7.5°C (Goergen et al. 2013). The region is characterised by meadows, pastures and forests of coniferous/deciduous trees. The Gutland shows a higher level of anthropogenic disturbance (including larger cities and industrial areas) and a longer vegetational period in comparison to the Oesling (Goergen et al. 2013). Annual precipitation ranges from 700-800 mm and annual mean air temperature is 9°C (Goergen et al. 2013). It can be divided into four sub-ecoregions: the Western and the Eastern Gutland, the Moselle Valley and the Minette Basin. Land use is quite diverse in the Gutland. Pastures are still common, while the acreage of arable land is increasing. Extended deciduous forests are typical for the Gutland (Dietz and Pir 2009). For the Western Gutland, secondary sandstones and sandy soils are common (Stevens et al. 2010) and the amount of precipitation is higher compared to the eastern part (Goergen et al. 2013). The Eastern Gutland is characterised by loam-loess-based soils and a more variable topography (Stevens et al. 2010). The Minette Basin is located in the southwestern part of Luxembourg, next to the French border. It represents the former mining district, due to the rich resources of iron ore. The Moselle Valley -along the border to Germany -represents the smallest ecoregion (1% of total area of Luxembourg). It is sunnier and also drier (less than 700 mm) in comparison to all other areas of the country. Due to these very specific climatic conditions, vineyards are prevalent and agriculture shows a higher level of intensification (Dohet et al. 2008). For further information about climatological characteristics of the different regions in Luxembourg, see Eickermann et al. (2014).

Field collection of psyllids, their preparation and identification (2019/2020)
Adult psyllids were collected during one year (July 2019 to June 2020) by beating host or shelter plants over a sweeping-net, from which the specimens were captured into plastic tubes containing 70% ethanol, thereby exploiting their natural aptitude to jump. When easily visible on the plants, nymphs were also collected by picking them directly and storing them in plastic tubes as described for the adults. If necessary, plants on which samples were collected (or parts of them) were stored in plastic bags and taken to the laboratory for their specific identification, according to Lambinon and Verloove (2012).
In the lab, all specimens collected in each tube were observed using a dissecting microscope (LEICA MZ7.5); in this phase, adults were separated by sex, counted and (where possible) identified. For samples whose observation under a dissecting microscope allowed the species identification, all specimens (adults and nymphs, if present) were stored in glass tubes containing 70% ethanol, each marked with a progressive collection number, corresponding to those of the general collection register containing complete data on the date and location of collection (including the geographical coordinates) and on the plant(s) on which the sample was recorded. In case a closer examination was needed to allow species identification, a maximum of six specimens (if available, three of each sex) were mounted on permanent microscope slides. For slide preparation, selected adults were firstly cleaned in 70% ethanol under gentle heating, then left to clear overnight in 10% potassium hydroxide (KOH), rinsed in a solution of 20% glacial acetic acid, dehydrated in 95% ethanol (for 10 minutes), placed in xylene for ≥ 10 minutes, mounted in Canada Balsam and allowed to dry in an incubator for 15 days at 35°C.
Each specimen was dissected in a drop of mounting medium before being mounted on the slide, in order to mount all different parts of the body separately: head (mounted dorsal uppermost), pronotum and forelegs, mouth parts, mesoscutum and forewings (dorsally), ventral mesothorax and mid-legs, metathorax, hind legs and abdomen (laterally, with wellexposed genitalia).
Photographs of morphological details (head, forewing, male and female terminalia) were made at the Musée National d'Histoire Naturelle de Luxembourg (MNHNL), from ethanolpreserved specimens of most of the species collected, using a Keyence VHX-6000 digital microscope.
With the exception of some special cases, for which we used literature specifically concerning the taxonomic group of the species to be identified, the material was identified by using the taxonomic keys of Hodkinson and White (1979) and , following the classification and nomenclature proposed by Burckhardt et al. (2021).
All the material studied, i.e. wet and slide mounted material, is stored at the MNHNL, apart from several specimens being part of abundant field collected samples, which are stored in the collections of the museum of the University of Catania (Italy).

Study of psyllid material from Malaise and Moericke traps (1999/2000)
During the years 1999-2000, an entomological collection campaign was carried out in Luxembourg by Evelyne Carrières, focussing on the national inventory of hoverflies (Syrphidae) (Carrières 2003). The project was financed by the MNHNL. Several Malaise and yellow Moericke traps were deployed in various areas of the country and regularly controlled every 2-4 weeks after initial installation. Many of the bulk samples collected during this campaign (and stored in the MNHNL wet collection) contained psyllid specimens, which, as part of the research aimed at compiling this manuscript, were sorted out, observed under a LEICA dissecting microscope, separated by sex, counted and (where possible) identified as already reported in the previous section. Additionally, in this case, all material identified by the simple use of a dissecting microscope has been stored in glass tubes containing 70% ethanol and marked with progressive collection numbers (different from those used for the field collected material). A general register has been prepared, containing complete data on collection localities (including the geographical coordinates) and the dates of exposure and removal of the traps. In case a more sophisticated approach was necessary to identify some material, permanent slides were prepared and identifications performed as described in the previous section. All material studied, both ethanol preserved or mounted on permanent slides, is stored at the MNHNL.

Results
Overall, the reliable bibliographic references (e.g. those reporting the actual collection of Psylloidea specimens), the field collections we made during 2019/2020 and the investigated Malaise trap material from 1999/2000 stored at the MNHNL allowed us to record a total of 47 species, belonging to the families Aphalaridae (5 species), Liviidae (5), Psyllidae (24) and Triozidae (13). For one additional species [T. rhamni (Schrank)], belonging to the family Triozidae, only records in literature are available relating to the presence of the galls it causes on the host plant; therefore, its presence in Luxembourg is highly probable, but needs to be confirmed by observations of psyllid specimens. The total dataset originating from the analysed 1999/2000 and 2019/2020 material can be retrieved from https://doi.org/10.15468/svfh53 (Rapisarda et al. 2021).

Aphalaridae
Aphalara sp. gr. polygoni Foerster, 1848 (Figs 1,5,9)  New record for Luxembourg: no member of the genus Aphalara Foerster has been recorded in Luxembourg so far; the identification of this taxon needs to be validated by the investigation of male specimens. Overview of head structures of psyllid species in Luxembourg (Aphalaridae, Liviidae). According to Burckhardt and Lauterer (1997b) and Cho et al. (2017b), species of this group have a Palaearctic distribution and live on plants of the family Polygonaceae, overwintering on shelter plants. Due to a high similarity of species within the genus Aphalara Foerster and the recent clarification of their taxonomic status (Burckhardt and Lauterer 1997b), many records of A. polygoni in various countries are doubtful. Hence, we decided to be more conservative by referring to the species group A. polygoni instead. Overview of head structures of psyllid species in Luxembourg (Psyllidae I).  (Figs 1,5,9,13) Findings in Luxembourg. Personal field collection by the authors: West Gutland: Arsdorf (7 ♂♂, 5 ♀♀, 8.VI.2020, on Epilobium angustifolium).
New record for Luxembourg.
Geographical distribution. Apart from North Africa, C. nebulosa is widely distributed in the Holarctic Region, from Far East Russia to nearly all north and central Europe, through Central Asia and was recorded also in North America (Canada, USA) (Hodkinson and White 1979, Burckhardt 1983, Hodkinson 1988, Tishetshkin 2007, O'Connor and Malumphy 2011, Drohojowska and Burckhardt 2014, den Bieman et al. 2019, Drohojowska and Klasa 2019. Biology. Monophagous on Epilobium angustifolium L. (Onagraceae) (Hodkinson and White 1979, Burckhardt 1983, Hodkinson 1988, Tishetshkin 2007), on which it performs one generation per Overview of head structures of psyllid species in Luxembourg (Psyllidae II). year and overwinters as a nymph (usually 4 instar) on the roots. This psyllid causes both foliar (optional) and root galls. The former consist in downward foldings of the leaf margin, which in June-July, protect the eggs (arranged in a long line along the lower margin) and subsequently the young nymphs; these very soon (already at the 1 or 2 instar) migrate to the roots, where they overwinter (Sampò 1975). Root galls are large, thread-like and tangled (Lauterer 1976 (Wagner and Franz 1961, Loginova 1968, Loginova 1974, Hodkinson and White 1979, Burckhardt 1983, Rapisarda 1985, Burckhardt and Kofler 1991, Lauterer 1994, O'Connor and Malumphy 2011, Malenovský and Lauterer 2012, Kanturski and Drohojowska 2013 (Wagner and Franz 1961, Loginova 1968, Loginova 1974, Hodkinson and White 1979, Burckhardt 1983, Burckhardt and Kofler 1991, Lauterer 1994, Hodkinson 2009, Malenovský and Lauterer 2012. It develops one generation a year and overwinters as a nymph on the roots of its host plants (Lauterer 1991).

A Livilla ulicis
Craspedolepta subpunctata (Foerster, 1848) (Figs 1,5,9,13) Findings in Luxembourg. Personal field collection by the authors: West Gutland: Arsdorf (4 ♂♂, 3 ♀♀, 3 nymphs, 8.VI.2020, on Epilobium angustifolium). Overview of forewings of psyllid species in Luxembourg (Aphalaridae, Liviidae).  , Hodkinson 1988, Burckhardt 1994b, Tishetshkin 2007 Biology. Monophagous on Epilobium angustifolium L. (Onagraceae). According to Lauterer and Baudys (1968), adults fly in late May/early June and lay eggs on leaves and stems in June; the 1 instar nymphs migrate to the roots, where they cause small galls, which grow with the development of the subsequent nymphal stages (2 -4 instars); the 4 instar nymphs appear in late August -early September and spend the winter in the root galls; during the following May, these nymphs abandon the galls, climb up to the aerial part of the plant and produce the 5 instar nymphs, which in turn give rise to the new adults. Overview of forewings of psyllid species in Luxembourg (Psyllidae I).
Overview of female terminalia of psyllid species in Luxembourg (Psyllidae I).
A Biology. Oligophagous on many species of the genus Juncus L. (Juncaceae) (Hodkinson and White 1979, Burckhardt 1983, Burckhardt 1988a, Hodkinson 1986, Zeidan-Gèze and Burckhardt 1998, Hodkinson and Bird 2000, Drohojowska 2009a, Hodkinson 2009, Drohojowska 2009b, Burckhardt et al. 2018). It performs one generation per year and overwinters as adults on shelter plants (conifers). During spring, adults fly back to their host plants, on which they lay eggs. After hatching, nymphs move to the young shoots and start producing characteristic galls by transforming the inflorescences into masses of reddish small leaflets, very close to each other due to the sharp shortening of internodes (Darboux and Houard 1901, Kieffer 1901, Ross and Hedicke 1927, Buhr 1964, Buhr 1965 This psyllid produces showy galls, widely described in literature (e.g. Darboux and Houard 1901, Ross and Hedicke 1927, Buhr 1964, Buhr 1965, Sampò 1975, consisting in the downrolling of the leaf margin; the rolled up part becomes dilated, thickened, turgescent, conspicuously cross-linked in red and violet. Overview of female terminalia of psyllid species in Luxembourg (Triozidae).
A  Burckhardt 1983), but also recorded on Erica cinerea L. and Vaccinium uliginosum L. (Hodkinson and White 1979, Hodkinson 1981. Eventual other plants should be considered as occasional hosts. Winter is spent by this psyllid as a nymph on the host plants. In north England, where the biology of S. ericae has been thoroughly investigated (Hodkinson 1973a, Hodkinson 1973b, Parkinson and Whittaker 1975, this psyllid develops its cycle in one year at low altitude and in two years at higher altitudes, thus presenting two physiological "races", characterised also by very small morphological differences. Studies on population dynamics of S. ericae are reported by Hodkinson (1973b). Overview of male terminalia of psyllid species in Luxembourg (Aphalaridae, Liviidae).  Burckhardt 1983, Hodkinson and Hollis 1987, Hodkinson 1988, Syrett et al. 2007); its reports on other Genistinae (e.g. Chamaecytisus spp., Genista tinctoria L. and Ulex europaeus L.) are doubtful or need to be confirmed. This psyllid performs 2-3 (in southern Europe maybe more) generations per year, with an almost continuous development; it spends the winter in all developmental stages (especially as an adult) on its host plants.  Overview of male terminalia of psyllid species in Luxembourg (Psyllidae I).
A Biology. Arytainilla spartiophila is monophagous on Cytisus scoparius (L.) Link (Fabaceae), on which it spends its entire life cycle, performing a single generation per year and overwintering as egg. In the Northern Hemisphere, adults start to fly in mid-April and occur on the plants till the first half of June; from the second half of May, males start to decline in number and populations of this psyllid become female-biased (Wheeler 2017). For its exclusive monophagy on C. scoparius and the large populations, this psyllid can build up on Scotch broom in many European countries (especially in Great Britain) causing substantial damage; it has been artificially introduced and released for biological control in exotic habitats (such as California and New Zealand) where its host plant became invasive (Syrett et al. 2007, Hogg et al. 2015. In the absence of male specimens, it is impossible to morphologically distinguish this species from Cacopsylla melanoneura (Foerster); for this reason, collections in Luxembourg of female specimens, here attributed to the more common species C. melanoneura, could also refer to C. affinis.

A Livilla ulicis
Biology. Strictly oligophagous on hawthorns (Crataegus spp.) (Rosaceae), C. crataegi is mainly reported from Crataegus monogyna Jacq. and C. oxyacantha L. , being found also on other species, especially in Asia, such as C. coccinea L. or C.  Burckhardt 1983, Kwon 1983, Hodkinson 1988, Burckhardt and Önuçar 1993 , Burckhardt 1994a. Malus asiatica Nakai, M. baccata (L.) Borkh. and M. transitoria (Batalin) C.K. Schneid. are reported as host plants of this psyllid in Asia (Kwon 1983, Li 2011. Cacopsylla mali performs one generation per year and overwinters as egg on its host plants. It often causes leaf alterations, corrugations and distortions (Buhr 1964, Buhr 1965). Similar to other species of the genus Cacopsylla, it may show a typical summer migration: part of the adults, which fed on the host plant for 2-3 weeks after emergence, migrate to other species of trees or shrubs, to re-immigrate to the host plants in September. Adults show a summer reproductive "parapause", which ends in autumn with a reactivation with their oogenesis (Lauterer 1991).
Economic significance. Cacopsylla mali is a secondary pest of apple trees in central Europe. In spite of old reports as a very harmful species, the damage it causes to crops is usually negligible. In recent studies, 'Candidatus Phytoplasma mali', the etiological agent of the Apple Proliferation (AP) disease, has been detected also in various psyllid species, including C. mali, different from the two known vectors of this pathogen [Cacopsylla melanoneura (Foerster) and C. picta (Foerster)] (Miñarro et al. 2016); yet the real potential of C. mali to transmit the disease is still unclear and warrants further investigation. In the absence of male specimens, it is impossible to morphologically distinguish this species from Cacopsylla affinis (Löw); for this reason, collections in Luxembourg of female specimens here attributed to C. melanoneura could refer to C. affinis. Identification tools of the two species by molecular methods have been studied by Tedeschi and Nardi (2010).  (Loginova 1968, Hodkinson and White 1979, Burckhardt 1988a, Lauterer 1999, Baugnée et al. 2002 New record for Luxembourg. Geographical distribution. Widely distributed in Europe, except its most western (e.g. Great Britain, France and Iberian Peninsula) and Mediterranean parts and reported also from the Caucasian Region and eastwards to Japan (Ossiannilsson 1992, , Lauterer and Burckhardt 1997   and Turkey (Burckhardt and Önuçar 1993), to nearly all Europe and North Africa (Hodkinson and White 1979, Burckhardt 1988a, O'Connor and Malumphy 2011, Kanturski and Drohojowska 2013, Drohojowska and Klasa 2019, den Bieman et al. 2019. Recently recorded also from North America (Canada and USA; Wheeler andStoops 2001, Wheeler andHoebeke 2005), where it has been introduced.
Biology. Strictly oligophagous on hawthorns (Crataegus L., Rosaceae) ( Hodkinson and White 1979, Burckhardt 1983, Burckhardt 1988a, Wheeler and Hoebeke 2005, Hodkinson 2009. In Europe, preferably found on Crataegus monogyna Jacq. and C. oxyacantha L.; also frequent on other hawthorn species, such as on C. arnoldiana Sarg., C. laevigata (Poir.) DC or C. maximowiczii C.K. Schneid., especially in other geographical areas. On its host plants, this insect performs a single generation per year, with long-living adults during summer, when they frequently spread around also on occasional plants. Eggs are laid in late summer on the host plants (hawthorns) and overwinter. In recent studies, C. peregrina showed to be moderately associated with 'Candidatus Liberibacter europaeus' (Tedeschi et al. 2009, Camerota et al. 2012), a phloem-limited Gram-negative bacterium infecting pear plants though producing no specific symptoms; therefore, thought to be an endophyte rather than a pathogen (Raddadi et al. 2011).
Biology. Though its adults have been found by chance also on occasional plants, such as Crataegus spp. or Malus domestica Borckh. (Oettl and Schlink 2015), C. pruni is strictly oligophagous on plants of the genus Prunus L. (Rosaceae), with Prunus spinosa L. being the most frequent host plant, but it can also be observed on P. armeniaca L., P. avium (L.) L., P. cerasifera Ehrh., P. domestica L., P. insititia L., P. padus L., P. persica (L.) Stokes and P. salicina Lindl. (Hodkinson and White 1979, Burckhardt 1983, Sauvion et al. 2007, Hodkinson 2009, Steffek et al. 2012, Jarausch and Jarausch 2016. This psyllid species performs one generation per year and overwinters as an adult on shelter plants (conifers). Recent studies allowed us to understand the feeding behaviour on its winter shelter plants, through the application of electrical penetration graph (EPG) recordings and survival bioassays on different conifer species, as well as the analysis of chemical composition of their plant sap (Gallinger and Gross 2018).

Cacopsylla pyri (Linnaeus
Geographical distribution. Cacopsylla pyri is common throughout Europe, especially in the central-southern fruit growing areas of the continent and is recorded also from the Caucasian Region (Armenia, Georgia), Middle East (Iran), central Asia (Kazakhstan) eastwards up to China (Xinjiang) (Hodkinson and White 1979, Burckhardt 1983, Burckhardt 1988a, Burckhardt and Lauterer 1993, Burckhardt and Önuçar 1993, Aguiar and Martin 2001, Agusti et al. 2003 Biology. Oligophagous on plants of the genus Pyrus L., mostly Pyrus communis L. (Rosaceae) (Hodkinson and White 1979, Burckhardt 1983, Burckhardt and Hodkinson 1986, Burckhardt 1988a, Burckhardt 1994a, Aguiar and Martin 2001, Hodkinson 2009, Li 2011, Sanchez and Ortin-Angulo 2011, Civolani et al. 2013). According to Cravedi et al. (2008), it performs up to 5-7 generations per year, overwintering as adults (especially females in ovarian diapause) in bark crevices of the same pear trees, but also on other occasional plants. Egg deployment starts in early spring, with the first sunny days (2-3 consecutive days with maximum temperatures over 10°C), initially within the cracks of the twigs, then in those of smaller branches and afterwards, as soon as trees start to vegetate, on perules, petioles and leaflets. Newly-emerged nymphs tend to settle during this spring period on the newly-spread leaflets and around the calycin zone of the newly-formed small fruits. Later in the season, adults of the summer generations lay eggs on tender shoots, along the main vein of the underside of the leaves, where nymphs tend to remain and produce a huge amount of honeydew. Late summer and autumn generations have normally a reduced fertility; and population levels are usually reduced with consequent limited harm, although late summer pullulations may also occur (especially in warmer climates).
Economic significance. Cacopsylla pyri is the most common psyllid species on all cultivated pear varieties in western Europe and regionally often considered a key pest. Nymphs of the summer generation produce a large amount of honeydew, damaging both plants and fruits. In the case of heavy infestations, the honeydew can smear all green parts of the plant, reducing photosynthesis and respiration, as well as making fruits suffer a significant commercial depreciation. Cacopsylla pyri can also be dangerous, especially to young plants, as a vector of 'Candidatus Phytoplasma pyri', the agent of Pear Decline disease, which is transmitted in Europe by both nymphs and adults of this psyllid (Facundo et al. 2017, Riedle-Bauer et al. 2019a. Transmission from one year to the next can be sustained by infected overwintering adult specimens. Numerous species of entomophagous insects are known as natural antagonists of C. pyri, both parasitoids, such as Prionomitus mitratus (Dalman) or Trechnites psyllae (Ruschka) and predators, such as species of the genera Anthocoris Fallen and Orius Wolff (Hemiptera, Anthocoridae). In particular, the predation by Anthocoris nemoralis (Fabr.) is highly significant; and the integrated control of this pear psyllid must be primarily based on techniques aimed at protecting and favouring (through augmentative releases) the action of this natural antagonist. This is why, in case of strong infestation requiring chemical control, low impact applications with selective insecticides having deterrent effects on oviposition (such as plant oils) (Erler and Tosun 2017) or with products having washing properties on the honeydew (such as salts of fatty acids) can be applied in mid-spring against the second generation nymphs, if intervention thresholds are exceeded.
In order to reduce the incidence of the Pear Decline phytoplasma, especially on young plants, control should be directed against overwintering adults of C. pyri, with the main objective of reducing the vector population before the spring vegetation of plants, thus preventing the transmission on time. New record for Luxembourg.

Cacopsylla pyricola (Foerster
Geographical distribution. Before the revisions of Burckhardt and Hodkinson (1986) and Cho et al. (2017b), respectively, of the west and east Palaearctic pear psyllids, C. pyricola was reported from the Asiatic Far East, Caucasus and all of Europe; and was also indicated as introduced in North and South America. At present, its occurrence is confirmed only in Europe (including the Caucasian Region) and North America (Burckhardt 1988a, Hodkinson 1988, Burckhardt and Lauterer 1993, Burckhardt 1994a, Agusti et al. 2003 As for all other European pear psyllids, the presence of C. pyri in East Asia was also questioned, based on recent DNA barcoding studies (Cho et al. 2020).
Biology. Oligophagous on plants of the genus Pyrus L. (Rosaceae), especially on P. communis L., but also frequently recorded on P. calleryana Decne., P. pyrasater (L.) and P. ussuriensis Maxim. (Burckhardt 1983, Burckhardt 1988a, Hodkinson 1988, Burckhardt 1994a, Hodkinson 2009, Guédot et al. 2009, Cooper and Horton 2015). It performs 3-5 generations per year, overwintering as adults on the host plants (but also on other occasional fruit or forestry plants in the surrounding area). In early spring, eggs are laid in groups on the buds; later in the season and throughout all summer, they are mostly laid on the underside of the leaves, along the middle vein or in the grooves of the leaf stalks.
Economic significance. In Europe, C. pyricola is the second most important and widespread species of pear psyllids, after C. pyri, to which we refer for information on harmfulness and control methods. Apart from the negative effects on infested plants consisting in the suction of sap and production of abundant honeydew, C. pyricola is also a vector of the pathogenic microorganisms causing Pear Decline disease ('Candidatus Phytoplasma pyri') (Facundo et al. 2017, Cruz et al. 2018, Riedle-Bauer et al. 2019a); for such damage, it is particularly feared in North America. New record for Luxembourg.
Economic significance. In the past, C. pyrisuga has been reported to be harmful to pears. Current knowledge suggests that it does not cause direct damage, being apparently better adapted to wild pears or old varieties of cultivated pear trees, especially in hilly areas and in non-intensive orchards. However, it has been recently confirmed as a vector of "Ca.  (Hodkinson and White 1979, Burckhardt and Önuçar 1993, O'Connor and Malumphy 2011, den Bieman et al. 2019. Biology. Strictly oligophagous on Rhamnus spp. (Rhamnaceae), especially R. cathartica L., but also reported on R. alpinus L., R. fallax Boiss., R. imeretina Booth et al. and R. saxatilis Jacq. (Hodkinson and White 1979, Burckhardt 1983, Hodkinson 2009); records in Luxembourg refer only to occasional plants, so far. Cacopsylla rhamnicola performs only one generation per year and overwinters as an adult on shelter plants (conifers).
New record for Luxembourg.
Geographical distribution. Cacopsylla ulmi is a Eurasian chorotype, occurring in most parts of central and northern Europe, the Caucasian Region and central Asia (Hodkinson and White 1979, Burckhardt 1983, Hansen 1996, Kanturski and Drohojowska 2013, Drohojowska and Klasa 2019, den Bieman et al. 2019. Biology. Strictly oligophagous on elm trees (Ulmus spp., Ulmaceae), C. ulmi has been especially found on Ulmus effusa Willd., U. glabra Huds., U. laevis Pall., U. minor Mill. and U. pedunculata Foug. (Hodkinson and White 1979, Burckhardt 1983, Hansen 1996, Hodkinson 2009. It performs one generation per year and spends the winter as an egg on the host plants. According to Lauterer (1991), part of its population migrates during the summer months to other plant species (mainly trees), which are used for shelter.  , Hansen and Hodkinson 2006, Varga et al. 2012, Struwe et al. 2009 , Burckhardt 1983, Hansen and Hodkinson 2006, Hodkinson 2009. According to Bin (1970), it performs 2-3 generations per year, overwintering in the nymphal stage on its host plants, to which it causes showy deformations to the leaves, which are folded in a C-shape.
New record for Luxembourg.
Geographical distribution. Central European chorotype, never recorded till now in the northernmost part of the continent (Scandinavia), as well as in the "Benelux" Region and the Iberian Peninsula (Hodkinson and White 1979, Burckhardt 1983, Hodkinson and Hollis 1987, O'Connor and Malumphy 2011. Biology. Oligophagous on plants of the genus Genista L. (Fabaceae); especially recorded from G. tinctoria L. and G. germanica L. (Hodkinson and White 1979, Burckhardt 1983, Hodkinson and Hollis 1987); records on other Fabaceae, such as Cytisus scoparius (L.) Link or Ulex europaeus L., are not confirmed. Livilla ulicis performs one generation per year and probably spends the winter in the egg stage on its host plants. Ossiannilsson 1992, . It shows only one generation per year and overwinters in the egg (or perhaps in the nymphal) stage (Lauterer 1976 Geographical distribution. Widely diffused in Europe, from where it seems to be native (Hodkinson and White 1979 Burckhardt and Lauterer 1997a), but probably living also on other willows (Salix spp.) Lauterer 2012, den Bieman et al. 2019). It spends the winter as an adult on shelter plants (conifers), on which it has been collected during many of its findings; probably performs only one generation per year. Loginova, 1977 (Figs 4, 8, 16) Findings in Luxembourg. Material studied in the MNHNL collection: East Gutland: Niederanven, Aarnescht (2 ♂♂, 27.V -9.VI.1999, Malaise).

Eryngiofaga lautereri
New record for Luxembourg.
Geographical distribution. Known till now only from a few countries in central-eastern Europe, E. lautereri has been described on material from the Czech Republic (Moravia) and Germany (Thuringia) (Loginova 1977), although the German material has been erroneously indicated in the original description as originating from Austria (according to . Later, it was reported from other localities in the Czech Republic and Germany, as well as from Slovakia (Lauterer 1979, Malenovský and. It seems to be a rare species. Biology. Trophically linked to plants of the genus Bupleurum L. (Apiaceae) (Hodkinson, 2009), it is reported exclusively to live on B. falcatum L. (Loginova 1977, Malenovský and Lauterer 2012. Life cycle of this psyllid has been studied by , who reports two generations per year, with adults emerging in May-June and in August-September, respectively (only one generation may be shown by part of the population, which spends summer as diapausing nymphs of the first offspring); winter is spent by the nymphs (of both generations) on the host plant.
Lauritrioza alacris (Flor, 1861) (Figs 4,8,12) Findings in Luxembourg. No explicit mention exists in literature on findings of this insect in Luxembourg, though its galls produced on Laurus nobilis are reported by Lambinon and Schneider (2004) and by Schneider (2016) (West Gutland: Bonnevoie; Moselle: Remich), who did not report, however, any finding of this psyllid in or nearby the galls. Therefore, the present record is the first direct finding of L. alacris in Luxembourg.
Geographical distribution. Most probably native from the Mediterranean Region on wild laurel and widely distributed in this area (Halperin et al. 1982, Burckhardt 1988a, Zeidan-Gèze and Burckhardt 1998 ), it has spread (and has been introduced) on cultivated L. nobilis in nurseries, gardens and Economic significance. In case of a strong infestation, leaf deformations and galls caused by this psyllid, which dry up and become black as soon as they are abandoned by the nymphs, may cause serious aesthetic damage to cultivated laurel in gardens and parks and direct control of this insect may be occasionally necessary, especially in nurseries and on young plants.

Trichochermes walkeri (Foerster, 1848)
Findings in Luxembourg. Trichochermes walkeri was reported from Luxembourg by Baugnée (2001). In addition, the galls it causes on Rhamnus cathartica L. were reported for the country by Lambinon and Schneider (2004) Burckhardt 1983), but also frequently recorded on R. saxatilis Jacq. ( Cobau 1912 Halperin et al. 1982; distributed eastwards to Turkey and the Caucasian Region , Burckhardt and Önuçar 1993. Biology. Widely oligophagous on several species of the genera Centranthus DC.  , Burckhardt 1988a, Hodkinson 2009, Spooner 2016, Badmin 2017. It usually performs one generation per year, overwintering as an adult on shelter plants (conifers), though in warmer areas, it likely shows an almost continuous life-cycle (with an undefined number of yearly generations), spending the winter in all developmental stages.
On its host plants, T. centranthi causes showy leaf deformations, by rolling up the margins and forming irregular and turgid galls, which are initially pale green and later turn red; flowers and inflorescences may be also deformed by this psyllid, through hypertrophies or atrophies of the stamens or distortion of apical flowers (which are reduced to subglobular processes) , Sampò 1975. New record for Luxembourg.

Trioza flavipennis Foerster, 1848
Findings in Luxembourg. This species has not been found by the authors in Luxembourg so far. Nevertheless, findings of its leaf deformations produced on Aegopodium podagraria are reported by Lambinon and Schneider (2004) and by Burton et al. (2019) (West Ripka 2008, Hodkinson 2009). According to Lauterer (1991), it performs only one generation per year, overwintering as an adult on shelter plants (conifers) and with a very slow nymphal development during summer (from May to late August), which is spent as a 2 instar nymph in a sort of "parapause"; aestivating nymphs soon resume their development in September and produce adults already in October (the latter overwinter).
This psyllid produces little pit galls on the leaves of its host plants, protruding to the upper face; the nymph causing each gall settles on the corresponding concavity occurring on the lower face , Ross and Hedicke 1927, Buhr 1964, Buhr 1965.

Trioza rhamni (Schrank, 1801)
Findings in Luxembourg. No explicit mention exists in literature on findings of this insect in Luxembourg, though its galls produced on Rhamnus cathartica L. are reported by Lambinon and Schneider (2004) (Moselle: Ahn), who did not report, however, any finding of this psyllid in or nearby the galls.
This species has not been found by us during our research in 2019-2020 nor in the investigated Malaise and Moericke trap samples; therefore, based on finding of its galls, it likely occurs in Luxembourg, but its presence needs to be confirmed by direct observations.
Geographical distribution. Trioza rhamni is a European chorotype, distributed and recorded over nearly the whole continent eastwards to Turkey and the Caucasian Region (Hodkinson and White 1979, Burckhardt and Önuçar 1993, Hansen and Greve 1999, Kanturski and Drohojowska 2013. Biology. Strictly oligophagous on Rhamnus spp., with R. cathartica L. being its most frequent host plant, but also reported on Rhamnus pallasii Fisch. & C.A. Mey. (Hodkinson and White 1979, Burckhardt 1983, Hansen and Greve 1999, Kanturski and Drohojowska 2013. It probably performs two generations per year ( Lauterer 1991) and overwinters as an adult on shelter plants (conifers). As a result of egg deposition, T. rhamni causes small pit-galls on its host plant leaves , Cobau 1912, Ross and Hedicke 1927, Buhr 1964, Buhr 1965 Biology. Strictly oligophagous on Urtica spp. (Urticaceae), with many records especially from U. dioica L. and U. urens L., but also from other plant species having a more restricted geographical distribution within the wide distribution area of this psyllid. Trioza urticae has a relatively rapid life cycle (depending on environmental conditions) and performs various generations per year (even more than four) on its host plants; it overwinters as an adult on various shelter plants, especially conifers.

Final remarks
The research here presented, realised through field collections and the study of historical collection material stored at the MNHNL, allowed us to significantly increase the number of species of Psylloidea known for Luxembourg. Based on data available in literature so far and considering also the species already known in the country only by the records of their galls or deformations, the Luxembourg psyllid fauna has more than tripled as a result of the present work. The increase is even greater (almost 7-fold) if the number of species previously known for the territory is strictly limited to only those reported through the direct finding of specimens. As to methodological aspects, it is interesting to note that the contribution of passive collection -in particular Malaise trapping -added seven new species to our assessment.
Despite the new records here presented, our knowledge of the psyllid fauna of Luxembourg is still incomplete. Further sampling is required on potential host plants on which psyllids have not been collected so far. Thus, for example, research on plants of the genus Sorbus L. must be intensified, as well as on numerous herbaceous plants that host in Europe psyllid species of the genera Craspedolepta Enderlein, Bactericera Puton and Trioza Foerster; on willows ( Salix spp.), further research could also allow us to find additional psyllid species. Considering the floristic richness occurring in Luxembourg and also the psyllid fauna of neighbouring or close countries which are better explored, despite its small size, it is likely that further research could lead to a significant increase of up to about 20% of the psyllid species found so far in Luxembourg.
Finally, on the applied level, all species living on agricultural crops deserve appropriate future attention, especially those belonging to species complexes recognised as vectors of phytopathogenic organisms and whose real distribution and harmfulness in Luxembourg should be suitably monitored. This can be realised by involving local plant protection services, thus to further investigate possible impacts of changing environments on biology, vectoring activity, pest importance and spread of individual species.

Funding program
The work was carried out within the project "Risk assessment for psyllids and whiteflies under current and future climate conditions in Luxembourg", financed by the Fonds National de la Recherche de Luxembourg (FNR) in the framework of the INTER MOBILITY programme (INTER/Mobility/18/12958064/RAPID-IN/).

Conflicts of interest
The authors have no conflicts of interest.