García-Barros et al. 2004 in the Atlas of Iberian butterflies provide the first comprehensive summary of butterfly records known in Iberia up to the date of its publication (2004). For Thecla betulae, the atlas compiles 122 landscape-scale quadrats (10 × 10 km) as the Iberian distribution of the species. However, the extensive gaps in between some of the confirmed presence areas were a sign there was still much to discover on the Iberian distribution of the species. As is generally often the case after the publication of atlases, this work promoted the search for the species in many new places, as shown by the various publications since 2004, in the (mostly) Spanish scientific literature. Therefore, we have considered only the period of 2002–2021, since the publication of the Atlas, in our literature revision.

The general presence of potential food plants throughout at least the northern part of Portugal together with suitable climatic and habitat conditions, were indicative that the butterfly could be more widespread than suggested in previous works (Maravalhas et al. 2004; Vicente Arranz et al. 2013b). Likely areas to support T. betulae were opportunistically surveyed from 2004–2021, during the summer for adults (May-September) or later for eggs (August-June). We complemented these surveys with findings reported to us on social media (e.g. Facebook). In this species, eggs, larvae and adults are of straightforward visual morphological identification, and no confusion with other species is expected. The number of eggs on each host plant was then counted, and the putative food plant species was recorded, alongside location data such as coordinates and altitude.

Formally published Iberian information and newly acquired fieldwork records were complemented with distribution data available from the GBIF database (GBIF.org 2021), taking advantage of the recent development of citizen-science projects all over the world and the online availability of museum collections. All these records allowed us to build an up-to-date distribution map of the species in the Iberian Peninsula. As a means of standardisation with previous works and for viewing purposes, we converted all point coordinate and UTM area data to 10 × 10 km UTM quadrats, implementing the final dataset in QGIS v.3.10.2 (QGIS.org 2021).

In order to help increase the focus of future search campaigns, we performed species distribution modelling using maximum entropy analysis implemented in MAXENT v.3.4.1 (Phillips et al. 2006). Our Iberian dataset totals 442 single T. betulae occurence points, which we contrasted against 19 current bioclimatic variables from the WORLDCLIM database (www.worldclim.org; Fick and Hijmans 2017), clipped to the Iberian peninsula. For the estimation of the current potential distribution of the species, we ran the program at default settings for 1,000 bootstrap replicates using 75% of species localities as training data, and the remaining 25% to test the model. The result was a heat-map in shades of green, from light green (little predicted presence) to dark green (maximum predicted presence) for the whole of Iberia.

A detailed revision of the literature dealing with T. betulae in Iberia yielded nine papers focused exclusively on this species in the region: Viader (1994), Stefanescu (1997), Stefanescu (2000), Vicente Arranz et al. (2013a), Vicente Arranz et al. (2013b), Vicente Arranz and Parra-Arjona(2014),Antón-Lázaro and Beltzunegi (2015), Mortera (2015) and Salvadores Ramos and Salvadores Ramos (2018). Many others do address important ecological and/ or distribution data, albeit not exclusively about T. betulae (e.g. Gómez de Aizpúrua 1991; García-Barros et al. 2004; Maravalhas et al. 2004; Sanjurjo Franch 2007; Monasterio León et al. 2014a, 2015; Vila et al. 2018; Salvadores Ramos and Salvadores Ramos 2018). One study (Dincă et al. 2015), delves into the genetics of the species: the authors found only two COI haplotypes in Iberia, among the six scored throughout Europe (from Portugal to Russia), in a shallow and non-geographically-structured differentiation pattern.

The majority of studies combine field-detection of eggs with food plant usage in order to only produce distribution maps. A few studies describe the early stages (Gómez de Aizpúrua 1991; Stefanescu 1997; Muñoz Sariot 2011), whilst two behavioural ecology studies focus on larval interactions with ants (Stefanescu 2000) and adult selection of master trees as interaction grounds for this canopy dweller butterfly (Jubany and Stefanescu 2007).

Regarding Portugal, we found only one article dedicated to the species (Maravalhas et al. 2004), although another one (Vicente Arranz et al. 2013b) provided some observations which are unfortunately published with poor resolution to allow for checking precise locations or deeper studies other than coarse mapping. Post 2002 regional Iberian distribution works on T. betulae have been summarized in Table 1.

In the period 2003–2021, the authors and collaborators carried-out fieldwork in Portugal, with a positive detection for T. betulae in 19 new sites. This presence was confirmed over the years in several of these new localities as well as in previously known ones. All currently known Portuguese records of T. betulae are shown in detail in Table 2.

Portuguese records cluster in three to four different areas. The largest majority of occurrence sites are located along the Portuguese part of the Central Iberian Mountain System (which continues into Spain to the Sierra de Ayllón, east of Madrid), in the mountain ranges of Serra da Lousã, Serra do Açor, Serra da Estrela and Serra da Malcata. Northwards, we confirm the species’ occurrence and expand it in the environs of Bragança (mountain ranges of Serra de Montesinho and Serra da Nogueira). A newly discovered but more fragmented cluster of populations is found from the northern border with Ourense (Galicia) near Boticas and Montalegre southwards to Serra do Alvão in the vicinities of the town of Vila Real. The species was also found through the observation of both adults and eggs in Serra da Peneda, but whether the latter populations are in contact needs confirmation. These recent and older observations make up about 31 known sites of occurrence of T. betulae in Portugal. A further handful of records from Serra da Malcata in Vicente Arranz et al. (2013b) could not be generally confirmed because of insufficient published data, with the exception of our own confirmation record near the town of Sabugal by the end of 2018. Our study thus provides an increase of about 428% in known Portuguese localities for Thecla betulae.

Gathering all available information regarding T. betulae in the Iberian Peninsula, including published resources since García-Barros et al. (2004), references therein (Table 1), new records from Portugal (Table 2) and data mining of online databases such as GBIF, has allowed us to produce the updated distribution map in Fig. 1. The inclusion of online sources such as GBIF and iNaturalist has allowed for the inclusion of a further 16, 10 × 10 km UTM quadrats not covered by any other source of information.

Figure 1. 

Updated Iberian distribution of Thecla betulae up to October 2021, including literature sources, online databases and fieldwork carried out during this study. In green, modelled distribution implemented in MaxEnt from 442 available presence points, under a resolution of 10km. Only a probability of occurrence above 50% is shown, darkest green shading representing areas with the highest probability of occurrence according to bioclimatic data.

From the map, it becomes clear that the actual Iberian distribution of T. betulae shows a mismatch from the one shown in most recent generalist European coverages of the species (Tshikolovets 2011; Leraut 2016), which do not extend its occurrence beyond the Pyrenees and the Cantabrian belt (but see Kudrna et al. 2015). In fact, the butterfly is found almost continuously from the pre-Pyrenean area and coastal ranges of Catalonia, in the east, through the Pyrenees, Cantabrian ranges as well as the temperate Eurosiberian areas north of these, westwards through to the Galician / Leonese ranges. As an offshoot, it colonises the temperate regions of the western Iberian Mountain System in Sierra de la Demanda and Sierra Cebollera (La Rioja, Burgos and Soria provinces) as well. In the extreme west, it is widespread in Galicia up to the coast and in northern Portugal, progressively higher up in mountain areas as going south to the Douro River near Vila Real.

As a (currently assessed) disjunct area of occurrence, T. betulae has been found in the Central Iberian ranges in several population pockets from Serra da Lousã to Serra da Estrela, in Serra da Malcata, Sierra de Gata to Candelario and in Sierra de Gredos. As yet, there are no records eastwards in the Sierra de Guadarrama or the Segovia province connecting the previously mentioned populations to the northern part of the distribution area via Spain.

The potential distribution model implemented in MaxEnt was built according to a defined set of bioclimatic variables encompassing abiotic constraints known to affect butterfly and other animal species (Fig. 1). It bears an AUC of 0.977±0.001, which is well above the 0.7 usually desired in such studies, meaning that the modelled species-distribution is very well explained by the used variables (Pearce and Ferrier 2000; Newbold et al. 2009). The projected distribution considers some currently under-sampled or poorly known areas as highly climatically suitable for the presence of T. betulae. Examples of such areas are the extreme North Aragon, the northern half of Burgos and Palencia provinces, most of Cantabria and westwards to the northern and western half of León following the more hilly and temperate landscapes. Western Zamora and the north-east of Trás os Montes in Portugal, all of Galicia but the lowlands of the south-west appear suitable too. The mountain ranges of all northern and central Portugal appear suitable too, possibly all the way southwards to the Gardunha range or even the Serra de Aire area. In the Central Mountain System of Spain, the species distribution could be connected from Sierra de Gata to Gredos and turns up as equally probable in the northern slopes of Sierra de Guadarrama, from where still no records are known.

Among the nineteen bioclimatic variables used, four were especially relevant for the model outcome. Hence, these four variables frame well the currently known distribution of T. betulae in Iberia. In decreasing order of importance these four variables are: bio 4 (Temperature seasonality), bio 9 (Mean temperature of driest quarter), bio 7 (Temperature annual range) and bio 12 (Annual precipitation) (Table 3). For a detailed explanation of all bioclimatic variables we refer to O’Donnell and Ignizio (2012). In short, T. betulae is positively affected by relatively stable temperatures over the year, particularly not too high during the summer and relatively high annual precipitation for the Iberian territory (Table 3). This translates into a temperate climate being essential for the presence of the species.

There are some works dealing with the ecology and habitat resource preferences of T. betulae throughout Europe, much less is known from Iberia though.

In the whole of Eurasia, the species is known to select a number of shrub and tree species in the genus Prunus as hosts. It shows a preference for P. spinosa (sloe) but many other species have been recorded too, even in the allied genera Crataegus and Chaenomeles. Other recorded species are: P. domestica, P. insititia, P. avium, P. padus, P. persica, P. mahaleb, P. serotina, P. armeniaca, P. asiatica, P. mandchurica and P. cerasifera (Dantchenko et al. 1995; Tolman and Lewington 1997; Tshikolovets 2011; Mortera 2015; Leraut 2016).

In Iberia, until very recently the only recorded food plant was P. spinosa but the burst of recent publications on the species has increased the number of known species. Iberian populations are now known to also include, in decreasing importance: 1) cultivated P. domestica or the wild and difficult-to-differentiate P. insititia (in Asturias, Catalonia, Extremadura and Castilla y León and in central Portugal); 2) P. cerasifera (Asturias); 3) P. persica (Catalonia); 4) P. avium (Extremadura) and P. armeniaca (Asturias). Other species have been repeatedly suggested as possible local sources, such as P. mahaleb, P. padus or P. cerasus but their use is still in need of confirmation.

Sequeira et al. (2011) list six native species of Prunus in Portugal: P. avium, P. mahaleb, P. lusitanica, P. padus, P. spinosa and P. insititia. Other species, such as almond (P. dulcis), peach (P. persica) and damascus (P. armeniaca) are widely present in orchards across the country. We retrieved the distribution of native species likely to be food plants in Portugal, according to Porto et al. (2021) (P. spinosa), Aguiar et al. 2021 (P. insititia) and Lourenço et al. (2021) (P. avium) and have represented their known distribution in Fig. 2. According to literature and our new records, the most recorded T. betulae food plants in Portugal are plum (Prunus domestica) and its wild relative Prunus insititia, and P. spinosa. P. avium and other less common host plants are yet to be confirmed in this territory, although one of the authors (PL) has witnessed egg-laying behaviour on Crataegus monogyna in Central Portugal. Furthermore, the butterfly has yet to be located in many areas where the potential food plants are more widespread such as Estremadura, Alentejo or most of Trás-os-Montes (Fig. 2). P. spinosa is particularly widespread in the limestone areas of central Portugal and in the north-east of the country but opportunistic searches for eggs of T. betulae in these areas have so far proven unsuccessful.

Figure 2. 

Distribution of potential native food plants in Portugal: P. spinosa, P. insititia and P. avium according to and adapted from Porto et al. (2021), Aguiar et al. (2021) and Lourenço et al. (2021), respectively.

Positive results are mostly associated with hedges, old orchards and woodland margins under a temperate environment. Eggs were mainly found as singles in forks or near developing buds which would burst in spring time. However, clusters of up to nine eggs were found too (Fig. 3).

Figure 3. 

T. betulae and its hosts in the field. A. Single egg on P. domestica, site 9, 1-XI-2009; B. Single egg on P. dulcis, site 20, 1-XI-2018; C. Same as previous; D. Same as previous. Arrow points to egg; E. Cluster of 8+1 eggs on P. insititia, site 12, 21-VIII-2014; F. L4 larva from site 1, reared from egg on P. spinosa collected IV-2008; G. One of the almond trees where eggs of T. betulae were located, site 20, 1-XI-2018.

Whilst many potential areas with P. spinosa gave negative results for T. betulae, one of the most interesting outcomes of our field work was the detection of eggs on almond trees (Prunus dulcis). This Prunus species was hitherto unknown to be used by T. betulae (Fig. 3). The site where this observation comes from lies in the south-western part of Serra da Estrela, under a thermophilic situation allowing for the dozen 5–6 years old almond trees to grow, a condition not usually met in the remaining of T. betulae potential sites.

Iberian adult T. betulae records are scarce, as elsewhere, and most data derives from the observation of the early instars through direct search on the potential food plants. We now know that across Europe, females choose food plants with a certain profile: usually a fairly young or younger-than-average branch of an isolated Prunus located on a sun-exposed forest-edge or hedgerow. It then descends to less than 2 m above ground where eggs are laid preferably singly in forks or near buds (Fartmann and Timmermann 2006; Merckx and Berwaerts 2010). In Iberia, the situation seems analogous but our own observations suggest a predominance of Portuguese records on older shrubs or even plum trees in perfectly established orchards, near villages where the eggs are often found in clusters of up to 9 (Fig. 3). However, these may be the result of several oviposition events, forced by the low availability of food plant resources in areas where the matrix is composed of Eucalyptus or Pinus agro-forestry.

Eggs or their remains may be found through the year but are the overwintering stage, from the end of the flight period until leaf burst in February or March. Upon hatching, young larvae eat the young leaves or flowers. The early-stages have been described in detail by Gómez de Aizpúrua (1991) and Muñoz Sariot (2011), while the larval ecology has been studied chiefly by Stefanescu (2000). Between feeding periods, the green dome-shaped larva rests on the underside of a leaf until mid-spring, reaching L3–L4 by the end of April in Catalonia (Stefanescu 2000). Caterpillars do not possess Newcomer’s exocrine gland but the presence of perforated cupola organs (Malicky 1970) allow for them to be attended by at least two species of ants – Formica rufibarbis and Lasius grandis – in exchange for an amino acid solution reward (Stefanescu 2000; Muñoz Sariot 2011). Completion of the larval stage is reached by May or June and pupation happens among withered leaves on branches or down near the soil (Thomas and Emmet 1989; Muñoz Sariot 2011). Portuguese data agree with such cycle, with the greatest majority of eggs being found during Autumn and Winter, and larvae being found from February to May. The pupal stage has been found only once in July (Fig. 4 – dark shading).

Figure 4. 

T. betulae life-cycle based upon Portuguese observations of the species.

In the rest of Europe, adults show an extended but univoltine flight-period from the end of June to October, varying with latitude and altitude, with a peak in August. Iberian data roughly agree, but observations from the Catalan Monitoring Scheme highlight a slight protandry with a male peak in early August and females extending their flight well into October when they are the only sex to be seen (Jubany and Stefanescu 2007). In the rest of Spain the very few records are centred in August to October. Portuguese adult observations, although representative of this general flight curve, seem to happen earlier than expected, from late May to September, peaking in August (Table 2, Fig. 4).

Butterfly behaviour is largely unknown because it takes place mostly at canopy level near the tree tops in light woodland. However, it seems that both sexes but particularly females do come down to soil level in the early morning to feed on nectar or dew-drops (Jubany and Stefanescu 2007). Later in the day, adult males congregate in particular trees, called ‘master trees’ (Thomas and Emmet 1989) which do not need to be food plants but are usually prominent features in the landscape. Here, they wait for passing females when courtship is initiated. In Iberia, this behaviour has been observed in Catalonia (Jubany and Stefanescu 2007) using Sorbus aria and Quercus ilex as master trees but is yet to be ascertained in the rest of the territory. According to the various references, the most observed behaviour of this butterfly is still oviposition by females at the end of the season, descending down the top branches backwards until about eye height to oviposit in rather secluded parts of the food plant. In Portugal, this type of behaviour accounts for many observations as well as just perching on sunlit portions of the canopy of the host plant (usually Prunus spinosa) or nearby shrubs and trees like Quercus robur, Castanaea sativa, Salix atrocinerea or Betula pubescens.