First Records of Eotetranychus libocedri (McGregor, 1936) and E. thujae (McGregor, 1950) (Acari: Tetranychidae) from Serbia

Abstract

During phytosanitary monitoring of ornamental conifers conducted across multiple regions of Serbia, two species of spider mites, Eotetranychus libocedri (McGregor, 1936) and E. thujae (McGregor, 1950), were recorded for the first time. Eotetranychus libocedri was found on Platycladus orientalis L. (Franco), Thuja occidentalis L., and Cupressus × leylandii A.B. Jacks & Dallim, while E. thujae was detected on T. occidentalis. These records extend the distribution area of these two allochthonous species in Europe. Monitoring alien arthropod species in Europe is crucial, as they pose a risk to native flora and fauna and can cause significant economic losses. Together with previous findings, the number of registered species of the family Tetranychidae in Serbia now totals 47 across 10 genera.

1. Introduction

Spider mites represent one of the most significant groups of pest mites in agriculture, forestry, and horticulture. The family Tetranychidae comprises 1364 species of obligate phytophages that feed on more than 4100 plant species [1]. Due to differences in host specificity, several spider mite species are closely associated with conifers; nearly 50 species from 12 spider mite genera feed on conifers of the cypress family (Cupressaceae). Within Eotetranychus, five species have been recorded on Cupressaceae plants: Eotetranychus cunninghamiae Wang, E. juniperus Tuttle & Baker, E. libocedri (McGregor), E. pueblensis (Tuttle) and E. thujae (McGregor) [1]. Eotetranychus libocedri and E. thujae are considered potential pests of coniferous plants. In contrast, Oligonychus ununguis (Jacobi) is regarded as the most harmful conifer-feeding spider mite, causing serious economic damage to ornamental conifers grown under the most unfavourable conditions of anthropogenic pressure, due to its high fecundity and rapid development. Oligonychus ununguis is a common species on conifers, and it is important to note that E. libocedri and E. thujae can also co-exist with it [2,3,4,5,6].

Eotetranychus libocedri and E. thujae belong to the same guild of spider mites, exhibit a strong trophic relationship with conifers, primarily members of the family Cupressaceae. They are mostly found on conifers in urban environments and nurseries. Both species were described from North America, where they seem to have been introduced into Europe and Asia [1].

Eotetranychus libocedri has been recorded worldwide on 16 coniferous plants across three families: Cupressaceae (Calocedrus, Cupressus, Juniperus, Platycladus and Thuja), Pinaceae (Abies and Pinus), and Tamaricaceae (Tamarix) [1]. Exceptionally, it was also found on one broad-leaved evergreen plant, Euonymus fortunei, family Celastraceae [7]. This species has been reported from seven countries worldwide: the United States, Mexico, Türkiye, Hungary, the Russian Federation, Kyrgyzstan and Uzbekistan. In Europe, its presence has been confirmed only in Hungary and the European parts of Türkiye and Russia [7,8,9,10,11,12,13,14,15,16,17].

Eotetranychus thujae has been recorded worldwide on 11 coniferous host plants, all belonging to Cupressaceae (Chamaecyparis, Cupressus, Juniperus, Platycladus, and Thuja spp.) [1]. To date, the species has been reported from seven countries globally: the United States, Canada, Georgia, Hungary, Kazakhstan, Kyrgyzstan and the Russian Federation. In Europe, its presence has been confirmed only in Georgia, Hungary, and the European part of Russia [6,9,14,16,18,19,20,21,22,23,24,25,26,27].

Data on spider mites associated with conifers in Serbia are limited to a small number of published works [28,29,30,31] and relate solely to urban environments, while no data have been published on spider mites inhabiting conifers in natural habitats. Due to insufficient taxonomic research on spider mites associated with conifers in Serbia, there are no relevant data on the significance of existing and potentially harmful spider mite species on conifers.

This study contributes to the knowledge of the Serbian acarofauna by reporting spider mite species newly recorded on coniferous plants. In the European nursery trade, alien spider mites have been repeatedly intercepted on imported ornamental plants. This trade is the primary route for the introduction of alien phytophagous arthropods into new areas. Any new potential pest species on conifers in Serbia could expand their host range and impact other European countries, particularly under changing climatic conditions. These findings provide valuable insights into the distribution of alien species of spider mites on ornamental conifers in Europe and may serve as a basis for future assessment of their potential impact on conifers in Serbia and neighbouring countries.

2. Materials and Methods

The research was conducted in August and September 2024 at 15 localities across different regions of Serbia as part of small-scale phytosanitary monitoring of ornamental conifers. A total of 35 plant samples were collected from 11 conifer species: Chamaecyparis lawsoniana (A. Murray bis) Parl., Cupressus arizonica Greene, Cupressus × leylandii A.B. Jacks & Dallim., Juniperus chinensis L., J. horizontalis Moench, J. virginiana L., Platycladus orientalis L. (Franco), Thuja occidentalis L., Picea abies (L.) H. Karst., P. glauca (Moench) Voss, and Taxus baccata L.

Two species of the genus Eotetranychus were recorded on three conifer species (C. × leylandii, P. orientalis, and T. occidentalis) in seven samples collected from six locations in urban areas.

Plant samples were collected by cutting off branches, which were subsequently packed in 5-litre plastic bags and transported to the laboratory. All samples were etherised with ethyl acetate for 30 min before examination to immobilise the mites. Quantitative assessment was performed on each sample (25 twigs, each about 25–30 cm long) under a stereomicroscope, by counting active mite stages and eggs. During this examination, mites were photographed directly on the plant scales under the stereomicroscope. Qualitative examination was performed by beating the branches over sheets of white satin paper (60 × 40 cm), enabling the collection of additional individuals that may have remained undetected during the quantitative examination. During qualitative examination, isolated mites placed in Petri dishes were photographed under the stereomicroscope.

Preliminary examination of mites was conducted using a Leica Wild M3Z stereomicroscope (Wetzlar, Germany), allowing the collection of isolated mites in Petri dishes. All collected specimens were subsequently placed in a clearing fixative, consisting of a 1:5 mixture of lactic acid (88%) and ethanol (70%). Permanent mounts were prepared using Heinze’s medium [32].

Mounted mites were examined using an Olympus BX53 phase-contrast microscope (Tokyo, Japan) equipped with cellSens Entry 2 (CS-EN-V2) imaging software for measurement and photography. Species identification was carried out using several literature sources for the family Tetranychidae [8,9,11,15,18,19,21,23,24,33,34,35,36,37]. The catalogue and worldwide database of spider mites were also consulted to verify the species distribution [1,38]. Permanent mounts are deposited in the collection of the Department of Entomology and Agricultural Zoology, Faculty of Agriculture, University of Belgrade.

As this study was observational, with a limited sample size and no intention to make statistical comparisons, the use of advanced statistical analyses was excluded.

3. Results

Two spider mite species, E. libocedri and E. thujae, were found for the first time in the fauna of Serbia.

Regarding taxonomic considerations, apart from their clear placement within the large genus Eotetranychus Oudemans, 1931 [39], both species share a number of distinctive morphological characteristics that have led to their previous assignment to smaller taxonomic groups. For instance, they have been included in the Libocedri Group within the genus Eotetranychus [9], subgenus Mononychus Wainstein, 1960, and the genus Platytetranychus Oudemans. According to Baker and Tuttle [21], the shared morphological characteristics of Platytetranychus are as follows: dorsal body setae are much shorter than the distance between their bases; prodorsal striae are longitudinal and opisthosomal striae are transverse; peritremes are distally bulbous; tibia II has five tactile setae; tarsus I bears distal and adjacent duplex setae.

• Species Eotetranychus libocedri (McGregor, 1936)

Material examined. Eleven females, six males, two nymphs collected on P. orientalis, elementary school yard, suburban settlement Rušanj, Belgrade area (44°40′57.1″ N, 20°26′21.5″ E), 22 August 2024; five females, two males, one nymph collected on P. orientalis, street ornamental row, suburban settlement Dobanovci, Belgrade area (44°49′52.8″ N, 20°13′58.5″ E), 22 August 2024; one female, two males, four nymphs collected on P. orientalis, Ušće park, city of Belgrade (44°48′54.3″ N, 20°26′44.7″ E), 22 August 2024; eight females, two males collected on P. orientalis, park, Hotel “Nais”, Niš area (43°19′45.6″ N, 21°53′20.5″ E), 13 September 2024; five females, one male, two nymphs collected on T. occidentalis, street ornamental row, suburban settlement Dobanovci, Belgrade area (44°49′52.8″ N, 20°13′58.5″ E), 22 August 2024; eleven females, four males collected on Cupressus × leylandii, school yard, village Bubanj, Niš area (43°23′00.7″ N, 21°48′32.7″ E), 13 September 2024.

Diagnosis. The female E. libocedri (Figure 1a) can be recognised by specific characteristics: vertical setae (v2) are long, approximately equal to the distance between their bases; dorsocentral setae (c1, d1, e1, f1, h1) (Figure 1b,c) are smaller than other dorsal setae, very short, about half or less of the distance between their bases; dorsolateral setae (c2, d2, e2, f2) are slightly longer than dorsocentrals; both dorsocentral and dorsolateral setae are lanceolate and serrate; the spinneret of the palpal tarsus is very small, as long as it is wide; peritremes are slightly enlarged distally, not hooked (Figure 1d); pregenital striae are simple, not lobed (Figure 1e). The male (Figure 2a) can be identified by the aedeagus which is stout and simply narrows to the emarginated tip (Figure 2b).

Figure 1. Eotetranychus libocedri—Female: (a) Dorsal view; (b) Dorsocentral setae: c1, d1, e1; (c) Setae h1 and f2; (d) Peritremes; (e) Striation pattern of pregenital region.

Figure 2. Eotetranychus libocedri—Male: (a) Lateral view; (b) Aedeagus.

Host plant. This species was recorded on ornamental cypresses: P. orientalis, T. occidentalis and C. × leylandii. As the research was conducted during a prolonged period of extremely high temperatures and drought, the population density was generally low (less than 10 active stages and 20 eggs per quantitative sample), and symptoms of scale damage were not clearly expressed. Only at one locality, Rušanj, Belgrade area, on P. orientalis (156 active stages and 188 eggs per quantitative sample), there were several isolated but well-developed colonies with visible webs (Figure 3a) and numerous eggs (Figure 3b), accompanied by localised discoloration of scales. During this research, E. libocedri was not found cohabiting with other spider mites species.

Figure 3. Eotetranychus libocedri on Platicladus orientalis: (a) Mite colony in web; (b) Mite eggs in scales.

• Species Eotetranychus thujae (McGregor, 1950)

Material examined. Ten females, two males, one nymph collected on T. occidentalis, village center, Mt. Kopaonik (43°16′26.7″ N, 20°47′02.3″ E), 5 September 2024.

Diagnosis. The female E. thujae (Figure 4a) can be recognised by specific characteristics: vertical setae (v2), outer sacrals (f2), and clunals (h1) are slightly longer than the other setae (Figure 4b); other dorsal setae are small, lanceolate, and serrate (Figure 4c); the spinneret of the palpal tarsus is three times as long as it is wide; peritremes are slightly enlarged and distally bulbous (Figure 4d); pregenital striae are lobed (Figure 4e). The male (Figure 5a) can be identified by the elongated aedeagus, which gradually tapers to a rounded tip (Figure 5b).

Figure 4. Eotetranychus thujae—Female: (a) Dorsal view; (b) Setae h1 and f2; (c) Dorsocentral setae: c1, d1, e1; (d) Peritremes; (e) Striation pattern of pregenital region.

Figure 5. Eotetranychus thujae—Male: (a) Lateral view; (b) Aedeagus.

Host plant. This species was recorded only on T. occidentalis. The population density was extremely low with only rare active stages and eggs recorded (18 active stages and 13 eggs per quantitative sample). No visible symptoms of scale damage were detected. During this research, E. thujae was not found cohabiting with other species of spider mites.

4. Discussion

Together with previously published data [31,40,41], this record of E. libocedri and E. thujae increased the number of known spider mite species in Serbia to 47. Eotetranychus libocedri and E. thujae have so far been reported in Europe from only a few countries. Thus, their discovery in Serbia expands the known distribution of both species in Europe.

According to the available literature, most records of E. libocedri in Eurasia are associated with ornamental plants in urban environments, suggesting that the actual distribution may be considerably larger than currently known [15]. Therefore, further research is expected to expand the range of known host plants, including several relevant conifer families in Serbia.

During our research, E. libocedri and E. thujae were found on infested plants in single species populations, i.e., without the presence of the commonly associated species O. ununguis [3,4,5]. In mixed populations with other spider mites, O. ununguis usually reaches a higher population density, spins a stronger web, and causes more severe damage to host plants, making it difficult to objectively assess the harmfulness of other species on the same host plant [6].

The relatively low population densities recorded through quantitative assessment of the samples were consistent with the weak manifestation of damage symptoms on the host plants. However, such results do not necessarily indicate that the two species are harmless, given the prevailing environmental conditions during the study period. Dry summer months are generally not favourable for spider mite populations on conifers to reach their maximum densities [6]. Research conducted during ecologically more favourable periods for population growth in May and June may yield higher population densities.

Both species were described from the Nearctic region, and are relatively rarely recorded within their native range, which therefore remains insufficiently studied. Although their presence in the Palearctic and Europe has been confirmed, the study of their distribution in these regions is even more limited, stressing the need to collect further data on these allochthonous species. Future studies in Serbia should focus on monitoring whether E. libocedri and E. thujae expand their host range or cause damage to ornamental conifers.