Globally, annual crop losses attributed to stink bugs exceed those caused by many other insect pests (Karar et al. 2021; Musser et al. 2025). Stink bugs are among the most important insect pests affecting soybeans [Glycine max (L.) Merr.], corn (Zea mays L.) and they also cause serious damage to vegetables and nuts across several countries of the Americas (Sosa‐Gómez et al. 2020). In Europe, the emergence of notable stink bug-related damage has coincided with the rapid spread of the invasive brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). Over the past decade, economic damage caused by H. halys to a range of agricultural crops has been reported in many European countries (Vétek and Korányi 2017; Musolin et al. 2018 ; Damos et al. 2020; Stoeckli et al. 2020). This species has become one of the most damaging pests of soybean crops in Europe (Rolando et al. 2025) and on fruits, especially in Northern Italy (Maistrello et al. 2017), the earliest European region to report economic losses in fruit crops caused by H. halys. Italy experienced huge fruit production losses estimated of nearly EUR 740 million in 2019 (Forresi et al. 2024). Since its first detection in Slovenia, in 2017 (Rot et al. 2018), the pest has had a significant negative impact on stone and pome fruit production (Rot et al. 2023).

The primary strategies for managing stink bugs mostly rely on the use of chemical insecticides (Sosa‐Gómez et al. 2020; Bueno and Panizzi 2024; Luo et al. 2025). This approach has remained largely unchanged following the introduction of H. halys, although treatment frequency has steadily increased, largely involving broad-spectrum active ingredients (Leskey and Nielsen 2018; Bueno and Panizzi 2024). While chemical control may offer short-term suppression of pest populations, it also brings several negative side effects, including disruption of ecological balance and outbreaks of secondary pests, and does not provide a sustainable long-term solution (Rice et al. 2014; Ribeiro et al. 2021; Cingolani and Laumann 2024). In response to these challenges, alternative and more environmental-friendly pest control methods are actively being explored for the management of the brown marmorated stink bug (Morrison et al. 2016a; Rondoni et al. 2022; Fornasiero et al. 2023).

In Slovenia, research efforts have focused on evaluating the effectiveness of physical and cultural control methods, such as insect exclusion nets and trap cropping, which have been recently tested (Batistič et al. 2023; Rot et al. 2024). The potential use of mechanical approaches, including a tractor-mounted bug vacuum, has also been investigated as a non-chemical approach (Batistič et al. 2024). In addition, particular attention has been given to the assessment of natural enemies, with a special focus on egg parasitoids, which are broadly recognized as effective regulators of stink bug populations worldwide (Tillman 2011; Paz-Neto et al. 2015; Ademokoya et al. 2022). Early monitoring of egg parasitoid abundance in Slovenia, conducted during 2019–2020, revealed the presence of two native egg parasitoid species: Anastatus bifasciatus (Geofrey) (Hymenoptera: Eupelmidae) and Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae) (Rot et al. 2021), and supported their subsequent use in classical biological control programs targeting H. halys and Nezara viridula (L.) (Hemiptera: Pentatomidae). During the same period, an adventive population of Trissolcus mitsukurii (Ashmead) was identified in the western part of the country, likely introduced from neighbouring Italy, where it had previously been reported (Sabbatini-Peverieri et al. 2018; Moraglio et al. 2020).

In recent years, an increasing trend in the populations of egg parasitoids has been observed in the region (Rot et al. 2022a). Nevertheless, their abundance and effectiveness have remained insufficient to reduce H. halys population below the economic injury level. The non-native species Trissolcus japonicus (Ashmead), a multivoltine egg parasitoid of H. halys eggs whose parasitism rates in its native range in China reach up to 90% (Zhang et al. 2017), was not detected in Slovenia during the early research period.

This species was first reported outside its native range in 2014 in the United States (Talamas et al. 2015), followed by its detection in Canada (Abram et al. 2019). Subsequently, it was also detected in Europe, specifically in Switzerland and Italy (Sabbatini-Peverieri et al. 2018; Stahl et al. 2019a; Moraglio et al. 2020). Although T. japonicus has already been utilised in the classical biological control of the brown marmorated stink bug (BMSB) through augmentative releases in these countries (Sabbatini-Peverieri et al. 2020a; Conti et al. 2021), its distribution across Europe remains limited. So far, besides Italy and Switzerland, the species has only been recorded in Germany, France, and Serbia (Dieckhoff et al. 2021; Konjević et al. 2024; Martel et al. 2024). Despite CLIMEX model predictions indicating a high probability of its spread from neighbouring countries (Avila and Charles 2018), the presence of the alien species T. japonicus in Slovenia was not confirmed until 2024. Therefore, alongside monitoring the effectiveness of the existing parasitoid fauna, a primary objective of our study was the early detection of this species.

A field survey on the presence of H. halys egg parasitoids was performed over the period 2021–2025 in western Slovenia. The investigation area included the Soča valley in the northwest, the Goriška Brda region, the Vipava Valley, the Karst region, and the entire area along the western border with Italy, extending to Slovenian Istria in the southwest, and comprised a total of 40 field sites. Detailed characteristics of the sampling sites are provided in Suppl. material 1. The monitoring of BMSB egg masses was performed on a weekly basis from end of May to mid-October each year. Locations were selected based on the presence and abundance of H. halys and preferred host plant availability. The majority of survey locations were known to contain high populations of H. halys, which led us to expect a high number of naturally-laid egg masses. During the inspections, we were focused on cultivated woody plants and wild plants growing in hedges adjacent to fruit orchards, vineyards, cultivated fields and gardens as well as focusing on woody plants growing in urban environment. Each site was surveyed for approximately 1 hour per person. The undersides of leaves of herbaceous plants were inspected throughout the plant, while tree canopies and shrubs were examined at a height between 0.5 to 2.0 m from the ground. Egg masses were collected together with leaves and stored in plastic Petri dishes. Data on location, host plant and date of collection were recorded for each egg mass. Samples were marked and transferred to the laboratory. Afterward, the eggs were examined under a stereomicroscope, counted, and classified into one of four categories based on visual assessment: hatched, unhatched, parasitized, or damaged by predation. After the first check in the lab, egg masses were reared in climatic chambers (25 ± 1 °C, 60% ± 10% r.h. and L16:D8 photoperiod) and checked every two days until the full egg hatching or parasitoid emerged. The emerged specimens were counted and removed from the Petri dishes and stored in 96% ethanol until identification. After three weeks from collection and the onset of lab rearing, parasitoid emergence was considered complete, and unhatched eggs were dissected, and eggs containing partially developed parasitoids were classified as parasitized. If the cause of mortality could not be diagnosed, eggs were counted as unhatched (dead eggs). When parasitoids had already emerged in the field, to estimate the parasitoid previously contained, shapes of exit holes and meconia were examined and compared to the descriptions of Sabbatini-Peverieri et al. (2019, 2020b). Parasitoid performance was evaluated and compared among years using three parameters: the percentage of parasitized egg masses (%), the mean percentage of parasitized eggs within parasitized egg masses (± SE), and the overall egg parasitism (%), calculated as the number of parasitized eggs divided by the total number of field-collected eggs in each year.

Morphological identification was performed on mounted specimens observing specific characters under Leica M205 C stereo microscope (Leica Microsystems, Germany) and using the identification keys of Talamas et al. (2017), Askew and Nieves-Aldrey (2004), Peng et al. (2020), Samra et al. (2018) and Triapitsyn et al. (2020).

Over the period of 2021–2025 a total of 550 egg masses of pentatomids were found in an area of approximately 3500 km², covering western Slovenia. Egg masses belonging to H. halys represented the majority of material collected (474 egg masses), with approximately 86% (Table 1) and the remaining 14% (76 egg mases in total) belonged to different pentatomid species: N. viridula, Dolycoris baccarum (L.) and Palomena prasina (L.) and Carpocoris purpureipennis (DeGeer, 1773) (Table 2). Annual parasitization rate of BMSB eggs ranged from 8.55% to 33.86%, with an average of 19.6%.

The most frequent parasitoid species from 2021–2024 were A. bifasciatus and T. mitsukurii, accounting for approximately 52% and 45% of all individuals reared from field-collected BMSB egg masses, respectively. In contrast, in 2025, T. japonicus became the most important species, accounting for 54% of all parasitoids emerging from field-collected egg masses and reared in the laboratory, followed by A. bifasciatus (34%) and T. mitsukurii (12%). Other scelionid species, including T. basalis and T. cultratus (Mayr), were recorded only sporadically throughout the study period and together parasitised less than 2% of H. halys eggs.

Trissolcus japonicus was first detected in Slovenia in 2024 during routine monitoring, when only a few individuals were recorded. In late August 2024, a single parasitised H. halys egg mass laid on Ginkgo biloba L. was found in Šempeter pri Gorici (45°55'54.19"N, 13°38'43.45"E), Goriška region, western Slovenia. By detailed examination of the egg masses the in laboratory we found that some parasitoids had already emerged in the field. Characteristic exit holes of two different species were present. According to descriptions of Sabbatini-Peverieri et al. (2020b), six of them belonged to A. bifasciatus, the other two showed characteristics consistent with Trissolcus spp., including the crescent-shaped creamy brownish to dark-grey meconium typical of this genus. In line with Morrison et al. (2016b), 14 showed signs of predation. After a few days, three parasitoid wasps emerged from the remaining unhatched eggs. Morphological features of all three female specimens matched the description for T. japonicus by Talamas et al. (2017, 2019). The morphological identification was supported also by molecular analysis. Molecular sequences obtained from the three samples resulted identical and matches with the species T. japonicus. Specifically, the 5’ fragment of the COX I mitochondrial gene was a 100% match with T. japonicus isolate BIIRU HR Beijing 2009 (OP136000).

By 2025, T. japonicus had expanded its distribution and was detected at four new locations, all situated within an approximately 5 km radius of the initial discovery site in 2024 (Fig. 2).

Between 2021 and 2025, a total of 55 egg masses of N. viridula were collected, comprising 4475 eggs, representing the highest number among the surveyed Pentatomidae. The mean egg parasitism rate for N. viridula over the five-year period was approximately 24%. Parasitism was attributed to three primary parasitoid species. Trissolcus basalis was the most abundant species, accounting for 43% of all individuals emerging from field-collected egg masses reared in the laboratory. Anastatus bifasciatus represented 28% of emerged parasitoids, followed by T. mitsukurii with 27%. Ooencyrtus telenomicida (Vassiliev) (Hymenoptera: Encyrtidae) was recorded only sporadically, comprising less than 2% of emerged parasitoids. Anastatus bifasciatus exhibited a broad host range, in addition to N. viridula, it also parasitized D. baccarum and C. purpureipennis.

A 5-year study of the presence of stink bug egg parasitoids confirmed a rich diversity of species in the region. The egg parasitoid community consisted of seven parasitoid species belonging to three Hymenopteran families: Encyrtidae (O. telenomicida), Eupelmidae (A. bifasciatus) and Scelionidae (T. basalis, T. cultratus, T. mitsukurii, T. japonicus, Telenomus sp.).

The native A. bifasciatus was the most widespread species, inhabiting the entire area of western Slovenia, from the cooler parts of the upper Soča Valley in the northwest to the warmer areas of Slovenian Istria in the southwest. Its ability to parasitize several other stink bug species has been documented, including N. viridula, the second most abundant and economically damaging species in the region after the brown marmorated stink bug. According to our observations, A. bifasciatus was the most common species of the parasitoid complex associated with H. halys during the period 2021–2024. Conversely, it less frequently parasitized N. viridula eggs, even though it was the third most frequent parasitoid of this species, right after T. basalis and T. mitsukurii. Although the diameter of N. viridula eggs falls within the size range that A. bifasciatus is capable of parasitizing, we observed, consistent with previous studies (Stahl et al. 2018; Rebora et al. 2022), that the sex ratio of the offspring was affected by host size, with only males emerging from N. viridula eggs.

Multiparasitism involving A. bifasciatus was also recorded. Because a single female has a limited egg load and cannot exploit an entire BMSB egg mass (Stahl et al. 2018), remaining stink bug eggs are often available to other parasitoids. During the early phase of our study, A. bifasciatus co-occurred with T. mitsukurii, as previously reported (Zapponi et al. 2020). After the detection of T. japonicus, this species was also observed co-parasitising BMSB egg masses with A. bifasciatus. These two parasitoids can coexist and may exhibit synergistic interactions that enhance the control of H. halys populations. However, competition outcomes cannot be excluded (Konopka et al. 2017).

The first detection of T. japonicus in low numbers in western Slovenia in 2024, followed by subsequent records in 2025, most likely reflects an early phase of regional spread rather an isolated introduction event. Given the geographic closeness to Friuli Venezia Giulia, where T. japonicus has become established following both accidental introductions and repeated releases, its presence in western Slovenia could be explained by natural dispersal across the border (Sabbatini-Peverieri et al. 2018, 2020a; Conti et al. 2021; Malossini et al. 2024). A similar expansion pattern was previously documented for T. mitsukurii, which spread from northern Italy into Slovenia within a few years of its initial detection (Scaccini et al. 2020; Rot et al. 2021).

However, studies on other introduced scelionid parasitoids suggest that their spread across a region after initial establishment is often limited. In Slovenia, T. mitsukurii remains largely confined to the Goriška region several years after its first detection, despite high host densities, consistent with Simaz et al. (2023), who reported that adventive natural enemies often spread and increase slowly in locations distant from their original introduction.

The discovery of T. japonicus in 2025 in the same places where it was first detected in 2024 indicate its reproductive activity and overwintering success. Previous studies have shown that the sub-Mediterranean climate of western Slovenia is favourable for the overwintering and reproduction of H. halys (Rot et al. 2022b). Therefore, it is reasonable to expect that these conditions will also support the establishment of T. japonicus, a species known to tolerate even lower temperatures than its host (Nystrom Santacruz et al. 2017; Lowenstein et al. 2019). Our findings support the model predictions of Tortorici et al. (2023), which predicted a high probability of habitat suitability in this region for the establishment of both T. japonicus and T. mitsukurii.

Over the five-year study period, overall egg parasitism of BMSB was 19.6%, which is in in line with previous findings (Moraglio et al. 2020) and comparable to or exceeding rates reported in similar studies worldwide (Dieckhoff et al. 2017; Abram et al. 2017; Andreadis et al. 2021; Ozdemir et al. 2025). However, overall parasitism varied considerably between years, ranging from 8.5% in 2022 to 33% in 2025.

The increase in parasitism rate observed in 2025 coincided with the higher proportion of T. japonicus among emerged parasitoids in our samples. This was also supported by the mean parasitism rate within parasitized egg masses (± SE), which was highest in 2025 (76.22 ± 4.56%) compared with previous years before the detection of T. japonicus (36.62–52.93%). Previous studies have shown that T. japonicus can become an important and highly effective egg parasitoid of H. halys following establishment (Donahoo et al. 2023; Falagiarda et al. 2023). However, in our case, this relationship will need to be further monitored and confirmed in future years. Conversely, the lower overall parasitism rate recorded in 2022, despite relatively high egg mass availability, may indicate temporal disruption of host–parasitoid dynamics, that could be driven by adverse climatic conditions. Previous studies have shown that climatic factors, particularly temperature, can affect parasitoid performance and may influence parasitoid–host interactions (Furlong and Zalucki 2017; Spahn and Lill 2022).

Based on meteorological data from western Slovenia for 2021–2025, there is an indication that the exceptionally high summer temperatures in 2022 may have negatively affected parasitoid reproductive activity and survival. Average temperatures during the summer months (June–August) of 2022 were the highest recorded within the five-year study period and exceeded even those of 2024, which is considered the warmest summer at the European scale (C3S & WMO 2024).

Differences in thermal tolerance among the main parasitoid species occurring in the region may largely explain their responses to extreme thermal conditions and, consequently, their ability to parasitize stink bug eggs. Anastatus bifasciatus performs best within a temperature range of 20–30 °C (Stahl et al. 2019b), while exposure to temperatures above 32 °C is associated with increased mortality risk, indicating sensitivity to heatwaves. In comparison, T. mitsukurii exhibits a lower thermal optimum than T. japonicus and is therefore more disadvantaged during extreme heat events, whereas T. japonicus maintains relatively high performance at elevated temperatures (Simaz and Szűcs 2021; Yonow et al. 2021; Mele et al. 2022).

Over five years, our research has revealed a rich and diverse community of egg parasitoids associated with H. halys in western Slovenia. The native A. bifasciatus dominates in the region, while the recently detected scelionids T. mitsukurii and T. japonicus track the invasion of H. halys, reflecting a broader regional expansion likely supported by intentional releases in other Western European countries. The occurrence of both native and non-native parasitoids highlights their potential importance in future biological control strategies; however, their actual contribution to the regulation of H. halys populations under field conditions requires further investigation. Future studies should focus on parasitoid population dynamics, impact on pest populations, and interactions with other management practices, including insecticide applications. However, the effectiveness of these control strategies may be challenged by environmental conditions, which should be considered when evaluating the long-term potential of biological control approaches.