Norway spruce (Picea abies (L.) H. Karst) is a coniferous species with a wide distribution range across central Europe, not only in naturally colonised mountain areas but also in large areas where it has been intensively planted in the past two hundred years. Due to ongoing climate change, Norway spruce, a species highly sensitive to summer droughts and warm temperatures, is currently experiencing dieback (Kolář et al., 2017; Boczoń et al., 2018; Krejza et al., 2021) and frequent pest outbreaks (i.e., bark beetles), leading to the periodic decline of large, forested areas (Jactel et al., 2012; Marini et al., 2017; Netherer et al., 2019; Kamińska et al., 2021; Nardi et al., 2022). Stress factors such as air pollution, heat, water deficiency, and drought have even more intensive impacts on trees in urban environments (Lüttge and Buckeridge, 2023), resulting in the disappearance of many Norway spruce trees from urban plantings (Anonymous, 2023).

With the dieback of Norway spruce stands, insect communities associated with this host plant, including communities of coccinellids (Coleoptera, Coccinellidae), have also become threatened. Coccinellids play a crucial role in limiting the abundance of phytophagous insects (Hodek, 1973; Majerus, 2016), and their absence puts host plants in urban stands at greater risk. The preservation of diverse coccinellid fauna in towns and cities has both ecological and cultural significance (Soares et al., 2022). Norway spruce is a coniferous tree in which coccinellid communities have rarely been studied (Reddersen and Jensen, 2002; Selyemová et al., 2007; Kenis et al., 2020). This tree species is reported as a host of diverse coccinellid communities (Selyemová et al., 2007; Kenis et al., 2020). In central Europe, the coccinellid communities on Norway spruce include species similar to those of communities on other trees, mainly the dominant native Coccinella septempunctata Linnaeus, 1758 and invasive Harmonia axyridis (Pallas, 1773) (Stathas et al., 2011; Latibari et al., 2016; Kempkens and Gruppe, 2018; Kenis et al., 2020; Zach et al., 2020; Farrow et al., 2022), as well as coccinellids typically occurring on conifers such as Aphidecta obliterata (Linnaeus, 1758) (Burmeister et al., 2007; Timms et al., 2008; Banfield-Zanin and Leather, 2016) and Exochomus quadripustulatus (Linnaeus, 1758) (Stathas et al., 2011; Holecová et al., 2018; Nedvěd, 2020).

Therefore, in our study, we aimed to determine whether there are suitable substitute conifer species capable of replacing the role of the disappearing Norway spruce in the survival of these coccinellid communities. A suitable substitute species for Norway spruce, at least in urban habitats, may be the introduced blue spruce (Picea pungens Engelm). This species, which is native to North America (Fechner, 1990), can withstand drought better than other spruce species and is resistant to high insolation (Fechner, 1990). Since blue spruce is also resilient to high levels of air pollution (Qin et al., 2014; Bukharina et al., 2016), after its introduction to Europe, it was planted as a replacement for Norway spruce stands damaged by industrial emissions (Slodičák and Novák, 2008; Kula et al., 2016). Due to its aesthetic qualities, blue spruce can replace Norway spruce in park stands. This tree species has been widely planted in urban parks for approximately the past 160 years (Pagan and Randuška, 1988; Kulfan et al., 2010), and its entomofauna can therefore be studied in most cities in central Europe. Blue spruce is known to be a host for numerous insect groups in Europe: xylophagous beetles (Coleoptera) (Kršiak et al., 2009; Kula et al., 2012, 2013), ladybirds (Coleoptera) (Jauschová et al., 2024), moths (Lepidoptera) (Kulfan et al., 2010), sawflies (Hymenoptera, Symphyta) (Kula et al., 2016) and aphids (Homoptera) (Fryč, 2016; Wojciechowski et al., 2016).

Can blue spruce function as a host for insect communities hosted by the native Norway spruce? To answer this question, we investigated and compared the diversity of coccinellid communities on Norway spruce and blue spruce grown together in urban areas. Given their close congeneric relationships, we expected similar coccinellid communities on both spruce species in urban environments. For our study, over a two-year period, we sampled adult coccinellid communities (including Scymninae and Coccidulinae subfamilies) monthly on both spruce species in four Slovak towns representing diverse climatic regions. We then evaluated the qualitative and quantitative differences in the adult coccinellid communities occurring concurrently during the growing season on both spruce species.

The absence of qualitative differences in coccinellid communities between Norway spruce and blue spruce was mainly determined by the similar relative abundance of dominant species and the low relative abundance of rare species.

As a result, the relative abundances of coccinellid species on Norway spruce and blue spruce were similar across locations, although the dominant species across locations differed ( Table 1 ), with the exception of Aphidecta obliterata and Exochomus quadripustulatus, which were abundant across all locations. Since the relative abundance of rare species was very low, it did not affect the correlation between the relative abundance of rare coccinellid species and that of either spruce species.

The species of spruce did not affect the relative abundance of coccinellid species in the total population that were established on the species. Additionally, Norway spruce and blue spruce have different species-specific properties that affect the attraction or repulsion of particular coccinellid species.

The invasive eurytopic coccinellid Harmonia axyridis (Roy et al., 2016) is known to dominate coccinellid communities on broadleaved trees (Honek et al., 2015; Viglášová et al., 2017). This ladybird species occurs on Norway spruce as well but usually in lower abundances (Kenis et al., 2020), which is in accordance with our results. Among conifers, Harmonia axyridis seems to prefer Scots pine (Pinus sylvestris) (Holecová et al., 2018; Kenis et al., 2020).

While no qualitative differences were found in the coccinellid communities on either spruce species, there were significant quantitative differences. All coccinellids (the community of all species), species of the tribe Coccinellini, species of other tribes (with the exception of the Žilina location) and some of abundant species were significantly more abundant on Norway spruce than on blue spruce. We can speculate about the reasons for these differences in the abundance of coccinellids between both spruce species. Possible causes include trophic factors, such as differences in the abundance or species composition of the arthropod complex serving as the prey for coccinellids. The aphid communities on Norway spruce and blue spruce seem to consist of similar aphid species (see, for example, Fryč, 2016; Wojciechowski et al., 2016). Trophic differences can be quantitative – the abundance of aphids on blue spruce seems to be lower than that on Norway spruce (pers. observation TJ, PZ). Other groups of phytophagous insects, as possible alternative food sources for coccinellids (Hodek and Evans, 2012; Majerus, 2016), seem to occupy both spruce species similarly, such as the larvae of moths (Kulfan et al., 2010) and sawflies (Kula et al., 2016).

Another biotic difference is the quality of the host plant (Goßner and Ammer, 2006; Bezemer et al., 2014). Differences in plant architecture, structural heterogeneity and plant surface, e.g., foliage shape and density, may aid or hinder coccinellid movement and searching and foraging behaviour (Shah, 1982; Grevstad and Klepetka, 1992; Pervez and Yadav, 2018). Little is known about whether there are differences in needle chemicals between blue spruce and Norway spruce (Soukupová et al., 2001). A comparison of the needles of Norway spruce and blue spruce with regard to their resistance to emissions revealed similar concentrations of chlorophyll and carotenoids but showed differences in phenolic compound accumulation between the two spruce species (Soukupová et al., 2001). The effects of chemical differences in the needles of both spruce species on coccinellids and their prey are not known.

How would coccinellid communities change if Norway spruce trees completely disappeared? For example, we can consider the replacement of the spruce by deciduous trees in the Děčínský Sněžník locality (Czech Republic, 50.79 N, 14.11E, 700 m a.s.l.), which has resulted in a shift in the composition of insect communities (Kula and Tryner, 2003a, b), including those of aphidophagous syrphids (Diptera) (Kula, 1997). A similar shift in the species composition of the local coccinellid fauna could also occur in the urban habitats in which this study was conducted.

Unlike the native Norway spruce, blue spruce is expected to be able to withstand unfavourable environmental conditions (Fechner, 1990; Qin et al., 2014; Bukharina et al., 2016). Our results suggest that in the absence of Norway spruce in urban environments, the coccinellid community inhabiting congeneric blue spruce will likely survive with no changes in composition, i.e., a decrease in species richness will not occur. In our study, the abundance of most coccinellid species was greater on Norway spruce than on blue spruce when coccinellids had the option to select Norway spruce and selection did not require migration to distant habitats. When such an option is not available, an increase in the abundance of coccinellids on blue spruce can be expected since the community of coccinellids adapted to conifers may find refuge there. However, confirmation of this assumption requires a more detailed study.

In conclusion, both native Norway spruce and introduced blue spruce host similar coccinellid communities in urban areas. Blue spruce was introduced to Slovakia more than 150 years ago (Pagan and Randuška, 1988), and in the time period since its introduction, it has recruited a coccinellid community with a species composition similar to that of the native Norway spruce. Closely related nonnative blue spruce seems to be a useful substitute host for spruce-associated insect fauna in the case of Norway spruce extinction in urban areas. A long-term study of coccinellid communities on spruces would be beneficial since the abundance of this insect is known to fluctuate interannually (this study; Selyemová et al., 2007).

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The manuscript presents research on animals that do not require ethical approval for their study.

TJ: Conceptualization, Data curation, Investigation, Methodology, Writing – original draft, Writing – review & editing. LS: Conceptualization, Project administration, Writing – original draft, Writing – review & editing. PZ: Conceptualization, Investigation, Methodology, Supervision, Writing – original draft. MS: Funding acquisition, Investigation, Project administration, Writing – review & editing. ZM: Funding acquisition, Methodology, Project administration, Writing – review & editing. AH: Software, Supervision, Validation, Writing – original draft, Writing – review & editing. JS: Formal analysis, Methodology, Software, Writing – review & editing. MH: Funding acquisition, Investigation, Project administration, Writing – review & editing. JK: Investigation, Methodology, Supervision, Writing – original draft, Writing – review & editing.