Biodiversity of spider mites (Acari: Tetranychidae) in Serbia: a review, new records and key to all known species

Despite the economic importance of spider mites (Acari: Tetranychidae), data on their biodiversity are scarce in some regions of Europe, such as Balkan Peninsula and particularly in Serbia. In this country, according to the Spider Mites Web database, only 17 spider mite species belonging to seven genera have been reported. This study provides a review of the Serbian literature dealing with spider mite species recorded in Serbia and presents results of a four-year faunistic survey in which spider mites were collected on cultivated plants and native vegetation throughout the country. In the survey, a total of 23 species were recorded, including six species new to Serbian acarofauna: Bryobia praetiosa, Eotetranychus aceri, E. fraxini, E. pruni, Panonychus citri and Tetranychus evansi. Together with previously reported data, it raises the number of known spider mite species in Serbia to 36. A total of 90 host plant species from 21 families that are favorable to spider mites were recorded in this study; there were 62 new host records for 20 spider mite species with 11 records of new plant species as hosts of spider mites. There were 63 new records for Serbia among host plant species, raising the number of Serbian hosts for tetranychid mites to 137. The spider mite species new to Serbian acarofauna were found on 17 newly recorded host plants from 11 families. A key to all known spider mites species from Serbia is provided.


Introduction
Spider mites (Acari: Tetranychidae) represent the most important pest mites with 1305 valid species and 3808 host plant records according to the Spider Mites Web database (Migeon and Dorkeld, 2017). This family includes species belonging to the most important mite pests of agricultural crops and ornamentals worldwide (Bolland et al., 1988;Zhang, 2003;Hoy, 2011). Despite the economic importance of spider mites, data on their biodiversity are scarce in some regions of Europe, such as Balkan Peninsula. In Serbia, which is located in the center of Balkans, 17 spider mite species belonging to six genera have been reported, according to the Spider Mites Web database (Migeon and Dorkeld, 2017). In Balkan countries located to the west and south of Serbia, 7 species have been reported from Slovenia (Bohinc and Trdan, 2013); in Bulgaria and Romania, located in the eastern part of Balkans, 8 and 12 species have been reported, respectively. On the other hand, 56 spider mite species have been reported in Greece, the southernmost Balkan country (Migeon and Dorkeld, 2017).
The first scientific data on Serbian spider mites date back to 1950-60s (reviewed by Petanović and Filipi-Matutinović, 1988), when bioecological and morphological characteristics of several recorded pest species were described. Stojnić (1993) carried out the first faunistic survey and taxonomic analysis of spider mites in Serbia collected on cultivated and ornamental plants from a wider area surrounding Serbian capital Belgrade. Recently, Mladenović et al. (2013) and Mladenović (2014) provided data on species collected in forest habitats, while Stojnić et al. (2014) analyzed species diversity on cultivated and wild apple trees in Serbia. Considering geographic and plant diversities of Serbia, as well as that a comprehensive faunistic study of Tetranychidae from the whole country area has never been carried out, we assumed the list of spider mites species inhabiting Serbia could be extended by new records. This study provides a review of the literature dealing with spider mites species recorded in Serbia and presents results of a four-year faunistic and taxonomic survey in which spider mites were collected on cultivated plants and native vegetation throughout the country. A key to all known spider mites species from Serbia is provided as well.

Materials and methods
Mite sampling was carried out during four years (2013)(2014)(2015)(2016) from April to October in 298 sampling locations. For the sake of simplicity, these locations are grouped in 24 sampling areas ( Fig. 1) in which mites were collected from various habitats (agricultural, forest, grassland, ruderal and urban). The areas C, N, L, O and U include territories of national parks. Among each location a variable number of plants were sampled. Host plants were randomly chosen with several criteria. First of all we chose plants with obvious marks of spider mites infestation, then some typical plants for that sampling location, and also some typical host plants for spider mites species. We also collected samples from crops known to be usually infested by spider mites, and also those which were obviously infested. Mites were recovered from plant samples in two different ways: i) one by one, directly from field samples using a paint brush; ii) in the laboratory, mites were extracted from leave samples following the soaking-washing-filtering method (Boller, 1984). All the specimens were temporarily preserved in 70% ethyl alcohol and then cleared in lactic acid (50%) for 24-48 hours and mounted in Hoyer's medium (Krantz and Walter 2009). Mites were examined using a Leica DMLB II phase contrast microscope and for some specimens, when needed, measurements were performed using the imaging software Perfect Image ® (Clara Vision) coupled with ProgRes ® Capture Pro 2.6 software for image acquisition.

Results and discussion
A review of spider mite studies in Serbia Table 1 provides a review of spider mites species records in Serbia composed of data from scientific literature published in Serbian (older references) and English (newer references). The first recorded spider mites were cosmopolitan and economically important species such as Tetranychus turkestani found on cotton plants (Đurkić, 1955), Tetranychus urticae found on beans and cotton, and Panonychus ulmi found on plum trees (Grujičić and Tomašević, 1956). Morphological and bioecological characteristics of another cosmopolitan species, Bryobia rubrioculus, were described by Tomašević (1965). This author also reported Eotetranychus populi, found on white poplar trees and Neotetranychus rubicola, recorded on raspberry (Tomašević, 1964(Tomašević, , 1967.
The first survey of spider mites in Serbia was carried out by Stojnić (1993) who collected tetranychid mites on cultivated and ornamental plants in various habitats from a wider area surrounding Serbian capital Belgrade (Table 1). The presence of above mentioned cosmopolitan species was confirmed and their host ranges were extended to 7 (B. rubrioculus), 10 (P. ulmi), 21 (T. turkestani) and 23 (T. urticae) plant species, belonging to Rosaceae and several other plant families. In addition to these species, 12 additional tetranychid species belonging to 9 genera were also recorded. Two of them are considered as pests of economic plants: Amphitetranychus viennensis, found on 10 host plants from the family Rosaceae and Oligonychus ununguis, recorded on several conifers. The species Neotetranychus rubi, potential pest of Rubus plants in Serbia, was recorded as well. The others were mostly oligophagous species of minor economic importance, found on ornamental and shade trees (for details see Table 1). Diapausing individuals of B. graminum were found in fallen cones of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). A total of 58 plant species from 26 families were recorded as hosts of 17 spider mite species in the survey. Mladenović et al. (2013) and Mladenović (2014) provided faunistic data on tetranychids collected mostly on wild fruit trees and shrubs in forest habitats. They found 12 species new to Serbian acarofauna (Table 1) and 14 new plant species and three new families were recorded as hosts of tetranychids in Serbia. Stojnić et al. (2014) studied spider mites diversity on cultivated and wild apple trees in Serbia and reported two wild apple species as new hosts of A. viennensis,  Tomašević (1956Tomašević ( ) (1964Tomašević ( , 1965Tomašević ( , 1967

The survey and new records
In the present study, a total of 23 spider mites species (Oligonychus sp. was identified to the generic rank only) were recorded, including 6 species new to Serbian acarofauna: B. praetiosa, E. carpini, E. fraxini, E. pruni, P. citri and T. evansi (Table 1). Together with previously reported data, a total of 36 spider mite species have been recorded in Serbia to date. Not counting Greece, this is the highest recorded number of tetranychid species in Balkans, considering that there have been 0-12 records in other countries (Migeon and Dorkeld, 2017 A total of 90 host plant species from 21 families bearing mite species were recorded in this study; there were 62 new host records in the world for 20 spider mite species with 11 first records of plant species as hosts of spider mites (Table 2).
Two species, T. urticae and T. turkestani, were found on 19 plant families and they had the highest recorded number of host plant species, 65 and 57, respectively. Host plant ranges  (Table 3), raising the number of Serbian potential hosts for tetranychid mites to 137; six plant families were newly recorded as well. The spider mite species new to Serbian acarofauna were found on 17 newly recorded host plants from 11 families.  Remarks -This species was found on 12 host plants from seven families, with nine plant species as its new hosts in the world (A. patula, E. cyparissias and R. aconitifolius are new hosts for spider mites). It is a worldwide distributed species of some economic importance, found on 269 host plants from 70 families. In Palearctic region it was recorded in 29 countries, including Greece and Romania in the Balkans (Vacante, 2016;Migeon and Dorkeld, 2017).
Remarks -This is the first record in the Balkan Peninsula. It is a Palearctic species found in Armenia, Georgia, Ukraine, Hungary and Italy on host plants from Fraxinus spp. (Migeon and Dorkeld, 2017).

Eotetranychus pruni Oudemans, 1931
Origin Remarks -In our study this species was found on 15 host plants from six families, with nine plant species as its new hosts in the world. It is a species with 32 records in Palearctic countries (including, Slovenia, Romania, Bulgaria and Greece in the Balkans) and it is considered to be a pest of economic plants, found on 30 hosts (Bohinc and Trdan, 2013;Vacante, 2016;Migeon and Dorkeld, 2017).

Genus Panonychus McGregor
Panonychus citri McGregor, 1916 Origin Remarks -This species was found on three host plants from the family Rosaceae, which are its new host records in the world. It is a worldwide distributed species with 69 records (including Albania, Bulgaria, Croatia and Greece in the Balkans), found on 108 host plants from 38 families, mostly on Rutaceae, Rosaceae and Moraceae. The species is a severe pest of Citrus plants (Vacante, 2016), also described in former Yugoslavia (Petanović and Filipi-Matutinović, 1988), but these data are not related to Serbia, which is not a citrus producing country due to unfavorable climate.
Remarks -This species has been found in 42 countries from various regions on 136 host plants from 36 families, with a preference for Solanaceae (Migeon and Dorkeld, 2017). The species is native to South America and currently an invasive pest species in Europe, found in Portugal, Spain, France, Italy, Greece and Turkey (Boubou et al., 2011;Kazak et al., 2017). According to the modeling distribution of T. evansi, as a tropical nondiapausing species (Migeon et al., 2009), its northward outdoor establishment in Eurasia appears to be mainly limited by cold stress. In this area, Mediterranean Basin corresponds to climatic borders for this mite where quite mild winters can explain its establishment, but this species could be able to survive in protected environments. In Serbia, this species was found on tomato grown in two greenhouses near Belgrade. Its possible introduction with infested plant materials would explain why this species is present in a country where the climatic conditions should not allow to this species to survive and settle.
Key to known spider mites from Serbia (based on female, unless otherwise specified)*   *Oligonychus brevipodus was not included in the key because there is no reliable data available to separate this species from other members belonging to the genus Oligonychus. As proposed by Pritchard & Baker (1955) topotype material should be collected to redescribe this species.