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17 March 2025

Plants for Wild Bees—Field Records in Bulgaria

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,
,
and
1
Department of Pharmacognosy, Faculty of Pharmacy, Medical University of Sofia, 1000 Sofia, Bulgaria
2
Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, 1, Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria
3
Institute of Ornamental and Medicinal Plants, Negovan, 1222 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Diversity2025, 17(3), 214;https://doi.org/10.3390/d17030214
This article belongs to the Special Issue Emerging Effects of Pollinator Loss on Biodiversity
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Abstract

One of the eight shortfalls in European wild bee data is the knowledge of the flowering plants they favor. This knowledge is of particular importance for bee gardens and wildflower strips initiatives aiming to provide forage for the wild bees. The aim of this study is to provide a list of plants that are used for food by certain taxa of wild bees in Bulgaria and to evaluate their potential for cultivation in bee gardens and wildflower strips. In relation to this, we discuss the food plant specialization of the wild bees. We summarize our observations on the pollination of certain plants and faunistic records considering the food plants in several grassland habitats on the territory of Bulgaria at altitudes between 0 and 1500 m above sea level, during the last 30 years. More than 54 taxa of wild bees are listed. They belong to the families Apidae, Andrenidae, Colletidae, Halictidae, and Megachilidae. Some of these bees are identified to the species level, and others to the genus or family level. Among the recorded wild bees are observed eight oligolectic species (22.2%). The listed bees are flower visitors of 60 plant taxa belonging to 20 families, which offer nectar and/or pollen. The wild bees’ food plants are predominantly from the families Fabaceae (15 species), Lamiaceae (14 species), Asteraceae (9 species), etc. The perennials are 67%, while annuals are 9%, annual or biennial 6%, biennials 5%, etc. We discuss the seed germination specifics of these plants. More studies are needed in this field. The conservation of wild bees may be supported by wildflower restoration activities, but the process depends on many factors, including seed germination difficulties. Therefore, the natural grassland habitats must be preserved and protected.

1. Introduction

One of the reasons for the global decline in wild bees is the insufficiency of food plants that provide pollen and/or nectar [1,2,3,4,5]. The intensive agriculture contributes considerably to this deficit [6]. The vast agricultural blocks leave no strips of entomophilous flowering plants in-between. Additionally, the farmers kill the bees’ food plants with herbicides. For example, the lavender plantations in the agricultural land in Dobruja (SE Bulgaria) are found to have very little activity and diversity of wild bees. At the same time, the ornamental plantation in the Rusalka resort, which is close to Natura 2000 steppe habitats, is characterized with a high activity and diversity of wild bees [7]. Also, in Bulgaria, in the last decade, land use has often changed from meadows to intensive agriculture. In recent years, on the Balkans, a new hazard has emerged for the floral food resources. It might turn into the next threatening factor for wild bees’ decline. Vast photovoltaic power stations (solar parks of many hectares) replace wild ecosystems, such as shrub and grasslands. The owners tend to “kill the grass” beneath the panels (using herbicides or other methods) in order to avoid the necessity of mowing [personal communication with photovoltaic power station owners]. This equals to a loss of flowering food resources. However, solar parks, managed as wildflower meadows, provide bumble bees with foraging and nesting opportunities [8]. Another hazard, related to the flowering food resources loss, is the change in the landscape. In Bulgaria, for the period of 2007–2017, a decline in livestock production was reported by more than 20% [9]. This is a significant issue concerning semi-artificial open habitats, such as pastures and meadows. In mountainous and low-mountainous regions, these habitats are often abandoned. The consequence is thriving of shrubs followed by gradual afforestation [10,11]. Where maintenance is practiced, it frequently involves intensive techniques aimed at the deliberate removal of shrubs and certain herbaceous species through mechanical treatments and pesticides and/or the enhancement in economically valuable species through fertilization. Intensive maintenance increases the economic value of pastures and meadows [11]. However, these practices simultaneously lead to a significant reduction in the biological diversity and habitats’ sustainability.
The decline in the pollination service results in agricultural loss [12,13,14]. It is shown that reducing land-use intensity on agricultural grasslands benefits bee diversity and pollination service delivery. Biodiversity-friendly management on grasslands results in improved outcomes for growers through positive effects on pollination service delivery for sunflowers and apples [15,16]. Maintaining landscape heterogeneity and improving the quality of semi-natural habitats to ensure resource diversity and continuity is fundamental to pollinator conservation [17].
Lately bee gardens [18] and wildflower strips have been implemented on farmlands to provide forage for insect pollinators [19,20,21,22,23,24,25,26]. The composition of the seed mixes and, hence, the obtained floral food resources for wild bees are of great importance. They determine the success of agri-environmental schemes, wildflower strips, and restoration activities [27,28,29,30]. Which plants species should be used for seed mix preparation is known as a result of several observational and experimental studies as well as good practices [17,29,31,32,33,34,35]. It is well demonstrated that the choice of species is important at the local level. An increase in species richness and abundance of wild bees and butterflies is recorded in wildflower strips initiated with seeds of regional origin [36]. In some European countries, phacelias are recommended among the other food plants [19,37]. However, Phacelia tanacetifolia Benth. is recognized as potentially invasive [38] as well as P. campanularia A. Gray [39]. Therefore, the composition of the seed mixes used for wildflower strips and restoration activities should be based on locally orientated research of plant–pollinators relationships. Many of the wild bees are polylectic and not that “picky” regarding the flowering food resources. However, even though the polylectic bees are flexible in their food choices, they still have preferences. For example, although bumblebees are known as polylectic, the long-tongued species have more specialized diets compared to short-tongued ones [2]. It is shown that the persisting abundant generalist pollinators buffer against the rapid loss of interaction diversity upon landscape simplification [40]. On the other hand, many species of wild bees are specialized in certain food plants. For example, Hylaeus punctulatissimus Smith, 1842, and Colletes carinatus Radoszkowski, 1891, are oligolectic on Allium sp. div., while Colletes fodiens (Geoffroy in Fourcroy, 1785) is specialized on Asteraceae (e.g., Tanacetum) [41], Eucera macroglossa uses the pollen of Malva alcea, and Osmia adunca is oligolectic on Echium vulgare [40]. The lack of the certain food plants for the oligolectic bees is hazardous for their existence. For sustainable wildflower strips establishment, it is necessary to know the local plant–pollinator networks. This topic has been the focus of scholars for many years, and considerable data on plant–pollinator networks in various regions and habitats are available [42,43,44,45,46]. Despite this, such research is still needed [26].
One of the eight shortfalls in European wild bee data is the Eltonian shortfall. It concerns the knowledge of the visitation of flowering plants to collect pollen or nectar (measured by observing interactions in situ and recording every time a bee is seen to be visiting a certain plant species) [47]. In Bulgaria, to date, the knowledge about wild bees’ flower choices based on the local flora and vegetation is scarce. Some records of certain plants and their wild bee pollinators are available [7,48,49,50,51,52,53,54,55,56,57].
The aim of this study is to provide a list of plants that are used for food by certain taxa of wild bees in Bulgaria and to evaluate their potential for cultivation in bee gardens and wildflower strips. In relation to this, the food plant specialization of the wild bees and propagation specifics of these plants are discussed.

2. Materials and Methods

We summarize our observations on the pollination of certain plants and faunistic records considering the food plants during the last 30 years. These observations were performed in 50 study sites in several grassland habitats of 12 regions (Figure 1) on the territory of Bulgaria at an altitude between 0 and 1300 m above sea level. We chose the study sites in habitats that were less destroyed by agricultural activity and, at the same time, in regions with agriculture. We used the site-transect method to record the pollinators of Astragalus dasyanthus Pall., A. alopecurus Pall., Lavandula angustifolia L., Sideritis scardica Griseb., Gypsophila trichotoma Wend., Gentiana cruciata L. wild populations, and the sympatric and simultaneously flowering plants there [7,48,49,50,51,52,55,56,57] (Figure 1). We also used the transect method in the grassland communities in the rest of the study sites [50] (Kozuharova unpublished data, Trifonov unpublished data, Figure 1 (Map)). The bees were either photographed (OLYMPUS E500 with a macro-lens) and identified at least to the genus level, or captured for detailed identification. We followed Plants of the World Online and Euro+Med Plant Base [58,59] for the accepted plant taxa names (Table 1) and floristic analysis was presented by families (Figure 2). The food specialization and preferences of the bees were taken from published sources (Table 2) as well as the life forms of plants (Figure 3, Table 3). The plant propagation specifics were summarized from published sources (Table 3). Our preliminary experiments for the germination of Paeonia peregrina Mill. seeds (n = 30) include scarification and stratification.
Figure 1. Regions where the study sites are located.

3. Results

3.1. Wild Bees and Their Food Plants in Bulgaria—Floristic Analysis

The wild bees were recorded on the flowers of 64 plant taxa, mostly from the families Fabaceae (15 species), Lamiaceae (14 species), and Asteraceae (9 species) (Figure 2, Table 1).
Table 1. Wild bees recorded in the flowers of plant species using them as a source of pollen and/or nectar.
Table 1. Wild bees recorded in the flowers of plant species using them as a source of pollen and/or nectar.
Plant FamilyPlant SpeciesBee TaxaBee Family
ApiaceaeEryngium campestre L. Colletes carinatus Radoszkowski, 1891Colletidae
Colletes hylaeformis Eversmann, 1852Colletidae
Hylaeus meridionalis Förster, 1871Colletidae
Icteranthidium grohmanni (Spinola, 1838)Megachilidae
Megachile argentata (Fabricius, 1793)Megachilidae
Scandix pecten-veneris L. Andrena danuvia E. Stoeckhert, 1950Andrenidae
AsparagaceaeOrnithogalum montanum Cirillo Osmia bischoffi Atanassov, 1938Megachilidae
AsteraceaeAchillea clypeolata Sm. Hylaeus kahri Förster, 1871Colletidae
Calendula officinalis L. Xylocopa cf. violaceaApidae
Centaurea diffusa Lam.Megachile argentata (Fabricius, 1793)Megachilidae
Centaurea salonitana Vis. Andrena albopunctata (Rossi, 1792)Andrenidae
Bombus armeniacus Radoszkowski, 1877Apidae
Lithurgus chrysurus Fonscolombe, 1834Megachilidae
Pseudoanthidium melanurum (Klug, 1832)Megachilidae
Cirsium ligulare Boiss. Bombus cf. lapidariusApidae
Bombus cf. terrestisApidae
Bombus cf. hortorumApidae
Crepis biennis L. Genus speciesMegachilidae
Leontodon biscutellifolius DC.Andrena sp. div.Andrenidae
Onopordum tauricum Willd.Xylocopa cf. violaceaApidae
Bombus cf. lapidariusApidae
Megachile cf. melanopyga Megachilidae
Xeranthemum annuum L. Osmia spinigera Latreille, 1811Megachilidae
BrassicaceaeMutarda nigra (L.) Bernh.Andrena erberi Morawitz, 1871Andrenidae
Osmia bischoffi Atanassov, 1938Megachilidae
Sisymbrium orientale L. Andrena pilipes Fabricius 1781Andrenidae
BoraginaceaeEchium vulgare L.Bombus pratorum (Linnaeus, 1761)Apidae
CampanulaceaeCampanula bononiensis L. Hylaeus sp.Colletidae
Campanula rapunculoides L. Hylaeus sp.Colletidae
Campanula trachelium L. Hylaeus sp.Colletidae
CaprifoliaceaeCephalaria transsilvanica (L.) Roem. & Schult. Megachile melanopyga (Costa, 1863)Megachilidae
Icteranthidium grohmanni (Spinola, 1838)Megachilidae
Knautia arvensis (L.) Coult. Icteranthidium grohmanni (Spinola, 1838)Megachilidae
Lomelosia argentea (L.) Greuter & Burdet Trachusa integra (Eversmann, 1852)Megachilidae
CaryophyllaceaeGypsophila glomerata Pall. ex Adams
Hylaeus meridionalis Förster, 1871Colletidae
Gypsophila trichotoma Wend.Andrena sp. div.Andrenidae
Eucera sp. div. Apidae
Ceratina cyanea (Kirby, 1802)Apidae
Hylaeus communis Nylander, 1852Colletidae
Hylaeus gibbus Saunders, 1850Colletidae
Hylaeus moricei (Friese, 1898)Colletidae
Hylaeus sp. Colletidae
Lasioglossum sp. div.Halictidae
Lasioglossum malachurum (Kirby, 1802)Halictidae
Lasioglossum duckei (Alfken, 1909)Halictidae
Lasioglossum nitidulum (Fabricius, 1804)Halictidae
Lasioglossum politum (Schenck, 1853)Halictidae
CistaceaeHelianthemum nummularium (L.) Miller Bombus pratorum (Linnaeus, 1761)Apidae
ConvolvulaceaeConvolvulus cantabrica L. Hoplitis perezi (Ferton, 1895)Megachilidae
FabaceaeAstragalus alopecurus Pall. Bombus pascuorum (Scopoli, 1763) Apidae
Bombus cf. hortorumApidae
Astragalus dasyanthus Pall. Bombus argillaceus (Scopoli, 1763) Apidae
Bombus cf. lapidariusApidae
Bombus cf. terrestris Apidae
Astragalus glycyphyllos L. Eucera sp. Apidae
Bombus cf. lapidariusApidae
Bombus pascuorum (Scopoli, 1763) Apidae
Coronilla varia L. Andrena sp. div.Andrenidae
Bombus cf. lapidariusApidae
Bombus pratorum (Linnaeus, 1761)Apidae
Halictus sp. div.Halictidae
Genus speciesMegachilidae
Megachile argentata (Fabricius, 1793)Megachilidae
Lathyrus pratensis L. Bombus cf. hortorumApidae
Lotus corniculatus L. Bombus cf. lapidariusApidae
Bombus cf. terrestris Apidae
Medicago falcata L. Andrena sp. div.Andrenidae
Onobrychis alba (Waldst. et Kit.) Desv.Andrena sp. div.Andrenidae
Genus speciesMegachilidae
Onobrychis arenaria W.D.J.KochOsmia bischoffi Atanassov, 1939Megachilidae
Osmia rufohirta Latreille, 1811Megachilidae
Trifolium alpestre Crantz Bombus argillaceus (Scopoli, 1763) Apidae
Trifolium medium L. Bombus pascuorum (Scopoli, 1763) Apidae
Trifolium pratense L. Andrena sp. div.Andrenidae
Bombus cf. veteranusApidae
Vicia dumetorum L. Andrena sp. div.Andrenidae
Bombus cf. veteranusApidae
Vicia grandiflora Scop. Eucera sp. Apidae
Vicia tenuifolia Roth. Bombus haematurus Kriechbaumer, 1870Apidae
Gentianaceae Gentiana cruciata L. Bombus argillaceus (Scopoli, 1763) Apidae
Bombus hortorum (Linnaeus 1761)Apidae
Bombus pascuorum (Scopoli, 1763) Apidae
Bombus pomorum (Panzer, 1805)Apidae
Hylaeus dilatatus (Kirby, 1802)Colletidae
Halictus sp. div.Halictidae
LamiaceaeAjuga chia Schreb. Osmia andrenoides Spinola, 1808Megachilidae
Osmia bischoffi Atanassov, 1939Megachilidae
Osmia rufohirta Latreille, 1811Megachilidae
Ballota nigra L. Andrena sp. div.Andrenidae
Bombus cf. hortorumApidae
Bombus cf. lapidariusApidae
Bombus cf. veteranusApidae
Bombus cf. veteranusApidae
Bombus pascuorum (Scopoli, 1763) Apidae
Ceratina chalibea Chevrier, 1872Apidae
Xylocopa cf. violaceaApidae
Hylaeus sp. div.Colletidae
Halictus sp. div.Halictidae
Lasioglossum sp. div.Halictidae
Anthidium cf. manicatumMegachilidae
Clinopodium vulgare L. Bombus pratorum (Linnaeus, 1761)Apidae
Lamium garganicum L. Bombus cf. hortorumApidae
Lamium maculatum (L.) L. Bombus sp.Apidae
Lavandula angustifolia L. Andrena sp. div.Andrenidae
Eucera sp. Apidae
Amegilla sp. Apidae
Bombus argillaceus (Scopoli, 1763) Apidae
Bombus cf. terrestris Apidae
Bombus haematurus (Kriechbaumer 1870) Apidae
Bombus pascuorum (Scopoli, 1763) Apidae
Thyreus sp. Apidae
Hylaeus sp. div.Colletidae
Halictus sp. div.Halictidae
Lasioglossum sp. div.Halictidae
Anthidium cf. manicatumMegachilidae
Megachile cf. centucularis Megachilidae
Megachile cf. ericetorum Megachilidae
Megachile melanopyga (Costa, 1863) Megachilidae
Osmia sp. Megachilidae
Stelis cf. nasuta Megachilidae
Mentha spicata L. Halictus sp. div.Halictidae
Lasioglossum sp. div.Halictidae
Salvia pratensis L. Andrena sp. div.Andrenidae
Genus speciesMegachilidae
Salvia virgata Jacq. Andrena sp. div.Andrenidae
Genus speciesMegachilidae
Satureja coerulea Janka Anthidium manicatum (Linnaeus, 1758)Megachilidae
Sideritis scardica Griseb. Bombus cf. terrestris Apidae
Bombus pascuorum (Scopoli, 1763) Apidae
Xylocopa cf. violaceaApidae
Stachys germanica L. Bombus cf. lapidariusApidae
Bombus pascuorum (Scopoli, 1763) Apidae
Teucrium chamaedrys L. Bombus argillaceus (Scopoli, 1763) Apidae
Bombus cf. terrestris Apidae
Rhodanthidium septemdentatum (Latreille, 1809)Megachilidae
Teucrium capitatum L. Trachusa integra (Eversmann, 1852)Megachilidae
OnagrceaeEpilobium angustifolium L. Bombus cf. lapidariusApidae
OrobanchaceaeRhinanthus wagneri Degen Bombus cf. lapidariusApidae
Bombus pratorum (Linnaeus, 1761)Apidae
PaeoniaceaePaeonia peregrina Mill. Bombus cf. lapidariusApidae
PlantaginaceaeDigitalis lanata Ehrh. Bombus argillaceus (Scopoli, 1763) Apidae
Anthidium manicatum (Linnaeus, 1758)Megachilidae
Digitalis viridiflora Lindl.Bombus cf. hortorumApidae
Bombus cf. terrestris Apidae
RanunculaceaPulsatilla vulgaris Mill. Bombus cf. terrestris Apidae
RhamnaceaePaliurus spina-christi Mill. Andrena erberi Morawitz, 1872Andrenidae
Hylaeus duckei (Alfken, 1904)Colletidae
RosaceaeRosa pendulina L Bombus cf. lapidariusApidae
RubiaceaeGalium verum L. Hylaeus sp. div.Halictidae
Lasioglossum sp. div.Halictidae
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Figure 2. Wild bees’ preferences of food plants in Bulgaria grouped by families.
Figure 2. Wild bees’ preferences of food plants in Bulgaria grouped by families.
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The plants listed in Table 1 are predominantly perennials (Figure 3). The life form of the food plants that are pollen and/or nectar sources for the wild bees is an important factor. It should be taken into account when bee gardens or wildflower strips are designed. This biological feature is strongly related to the propagation specifics of the plants.
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Figure 3. Life forms of the food plants of the wild bees in Bulgaria.
Figure 3. Life forms of the food plants of the wild bees in Bulgaria.
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3.2. Flowering Phenology and Floral Food Rewards

Some of the listed plants (Table 1) bloom early in spring—for example, Pulsatilla vulgaris Mill., Lamium maculatum (L.) L., and Lamium garganicum L.—and they are very important food for the bumblebee queens, which are busy to establish the colonies at that time of the season (Figure 4). Many of the listed plants (Table 1) provide nectar (Figure 5a) and pollen in early or late summer. Few plants remain flowering later in the autumn, such as Cirsium ligulare Boiss. and Epilobium angustifolium L. Late flowering plants are very important to support the bees until the end of the season.
Figure 4. Early spring food plants: (a) Bombus cf. terrestris on Pulsatilla vulgaris; (b) Bombus sp. on Lamium maculatum (photo: E. Kozuharova).
The bumblebees are mainly observed collecting nectar (Table 1, Figure 5b).
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Figure 5. Nectar-collecting bees: (a) Eucera sp. on Vicia grandiflora; (b) Bombus haematurus on Vicia tenuifolia and Astragalus dasyanthus (photo: E. Kozuharova).
Figure 5. Nectar-collecting bees: (a) Eucera sp. on Vicia grandiflora; (b) Bombus haematurus on Vicia tenuifolia and Astragalus dasyanthus (photo: E. Kozuharova).
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Many of the bumblebee workers are seen with full pollen baskets and often they are observed to transfer the pollen grains adhered to their bodies into the baskets during the flights from flower to flower (Figure 5b and Figure 6a). Megachile species are observed to collect both nectar and pollen (Table 1).
Figure 6. Nectar- and pollen-collecting bees: (a) Bombus cf. lapidaries on Onopordum tauricum; (b) Megachile cf. melanopyga on Onopordum tauricum (photo: E. Kozuharova).

3.3. Diversity of Wild Bees

In total, bees from five of the six European families were recorded (Apidae, Andrenidae, Colletidae, Halictidae, and Megachilidae) (Table 1). The list of wild bees observed on certain food plants as a result of our field studies consists of more than 54 taxa (Table 1).

4. Discussion

4.1. Specialization and Food Preferences of the Wild Bees

Some of the flower visitors are identified to the family or genus level (Table 1), and their specialization to the food sources cannot be specified. Most of the European wild bees are polylectic as follows in terms of families (Figure 7): Apidae—78.9%, Andrenidae—48.6%, Colletidae—74.0%, Halictidae—88.9%, and Megachilidae—50.6% [60], while almost all European species from the family Melittidae are oligolectic [61]. This aligns with the findings of previous studies, showing that the majority of wild bees are generalists [62]. Polylectic species are important for maintaining pollination services across diverse ecosystems, as they provide flexibility in response to the availability of different plants.
Figure 7. Food socialization of the wild bees—percentage of taxa in each category (polylectic, oligolectic, or monolectic), according to Michez and co-authors [61]. Legend: x-axis—percent values, y-axis—wild bee families.
Amongst Apidae, bumblebees are known to be polylectic [63]. However, they also have food plant preferences. The short-tongued bumblebee species that emerge early in the season have less specialized diets compared to long-tongued ones [2] and they remain abundant.
Similar to the European pattern, we found that the Bulgarian wild bees listed in this study are preliminary polylectic. Some species of Andrena are polylectic, while others are oligolectic (Table 2). Polylectic are 28 wild bee species (77.8%). Eight wild bee species (22.2%) are oligolectic to certain plant genera, broadly oligolectic or polylectic with strong preferences to certain plant genera (Table 2). The food preferences of few of them are not known (Table 2). The majority of the observed oligolectic species are broad specialists on the family Asteraceae or forage specifically on certain genera from the family, such as Lithurgus chrysurus, Osmia spinigera, and Hylaeus duckei (Table 2).
Table 2. Food preferences and specialization of the wild bees recorded in the study sites in Bulgaria.
Table 2. Food preferences and specialization of the wild bees recorded in the study sites in Bulgaria.
Bee TaxaFood Preferences References
Amegilla sp. Deficient data
Andrena albopunctata (Rossi, 1792)Deficient data
Andrena erberi Morawitz, 1871Deficient data
Andrena danuvia E. Stoeckhert, 1950Polylectic[41]
Andrena pilipes Fabricius 1781Polylectic[64]
Andrena sp. div.Polylectic/oligolectic
Anthidium cf. manicatumPolylectic[65]
Anthidium manicatum (Linnaeus, 1758)Polylectic[65]
Bombus argillaceus (Scopoli, 1763) Polylectic[66]
Bombus armeniacus Radoszkowski, 1877Polylectic[2,63]
Bombus cf. hortorumPolylectic[2,63]
Bombus cf. lapidariusPolylectic[2,63]
Bombus cf. terrestris Polylectic[2,63]
Bombus cf. veteranusPolylectic[2,63]
Bombus haematurus (Kriechbaumer 1870) Polylectic[2,63]
Bombus pascuorum (Scopoli, 1763) Polylectic[2,63]
Bombus pratorum (Linnaeus, 1761)Polylectic[2,63]
Ceratina chalybea Chevrier, 1872Polylectic[67]
Ceratina cyanea (Kirby, 1802)Polylectic[67]
Colletes carinatus Radoszkowski, 1891Polylectic with a strong preference
for Allium		[68]
Colletes hylaeformis Eversmann, 1852Narrowly oligolectic (Eryngium)[68]
Eucera sp. div. Polylectic/oligolectic
Halictus sp. div.Deficient data
Hoplitis perezi (Ferton, 1895)Oligolectic on Convolvulaceae[69]
Hylaeus communis Nylander, 1852Polylectic[70]
Hylaeus duckei (Alfken, 1904)Broadly oligolectic on Asteraceae[70]
Hylaeus gibbus Saunders, 1850Polylectic[70]
Hylaeus meridionalis Förster, 1871Probably polylectic[71]
Hylaeus moricei (Friese, 1898)Polylectic[70]
Hylaeus sp. div.Deficient data
Icteranthidium grohmanni (Spinola, 1838)Polylectic[65]
Lasioglossum duckei (Alfken, 1909)Deficient data
Lasioglossum malachurum (Kirby, 1802)Polylectic[72]
Lasioglossum nitidulum (Fabricius, 1804)Deficient data
Lasioglossum politum (Schenck, 1853)Deficient data
Lasioglossum sp. div.Deficient data
Lithurgus chrysurus Fonscolombe, 1834Oligolectic on Centaurea[73]
Megachile argentata (Fabricius, 1793)Polylectic[74]
Megachile cf. centucularis Polylectic[67]
Megachile cf. ericetorum Probably oligolectic on Fabaceae[67]
Megachile melanopyga (Costa, 1863) Polylectic pollen harvested from Centaurea and Cardueae, Atraceae[67,75]
Megachilidae Genus speciesPolylectic/oligolectic[75]
Osmia andrenoides Spinola, 1808Polylectic[76]
Osmia bischoffi Atanassov, 1938Polylectic[77]
Osmia rufohirta Latreille, 1811Polylectic[77]
Osmia sp. Polylectic/oligolectic
Osmia spinigera Latreille, 1811Oligolectic on Asteraceae[78]
Pseudoanthidium melanurum (Klug, 1832)Oligolectic on Carduoideae[65]
Rhodanthidium septemdentatum (Latreille, 1809)Polylectic[79]
Stelis cf. nasuta Parasitic, nectar visits
on Lamiaceae		[80]
Thyreus sp.
Trachusa integra (Eversmann, 1852)Observed on Jasione
(Campanulaceae)		[81]
Xylocopa cf. violaceaPolylectic[67]

4.2. Floral Rewards

Many plants from the families Lamiaceae, Asteraceae, Fabaceae, and others are good sources of nectar [31,82,83,84]. Basically, plants with numerous stamens are considered a good source of pollen. For example, Paeonia peregrina has plenty of stamens, and we have observed honeybees and Bombus cf. lapidarius worker (Table 1) to gather pollen in its flowers. This species is among the peonies with the least quantity of nectar [85]. Interestingly, most authors consider Pulsatilla vulgaris to be a polleniferous plant due to the large number of stamens, which do not produce nectar; however, it has been shown that the flowers have staminodial nectaries [86]. Although rich in protein and thus an important food for the larvae, the pollen appears to be a tricky resource. Many flowers offer small pollen quantities [87]. Also, plants have adaptations to protect their pollen. They develop mechanical and chemical barriers that reduce the pollen exploitation. Although dandelions are listed amongst important pollen source for wild bees [29], Taraxacum pollen is pointed out as an example of chemical defense, which explains why Asteraceae pollen is rare in generalist bee diets [68,88]. This is interesting given the fact that Taraxacum is known for the high level of apomixis [89], meaning most dandelions do not need pollen for their propagation. On the other hand, the members of tribe Cardueae (knapweeds and thistles) are pointed out as the pollen sources for Megachille pilicrus, M. flabellipes, M. melanogaster, and M. picicornis [75].

4.3. Propagation Potential of the Food Plant Species

Almost all known botanical species successfully reproduce through seeds. Sexual reproduction via seeds generates genetically diverse individuals, which supports the maintenance of genetically rich populations with high adaptive potential. However, a significant challenge in the seed propagation of wild plant species lies in the inherent nature of their seeds. Wild plant seeds are typically adapted to ensure both reproductive success and the long-term survival of the species. These adaptations, beneficial in their natural environments, often become obstacles to intentional propagation efforts. The most notable characteristic of seeds from wild species (as opposed to cultivated ones) is dormancy, which can result in delayed or sporadic germination over extended periods [90,91,92,93,94]. Additionally, certain species exhibit highly specific requirements for seed activation and germination, such as exposure to periodic high temperatures, smoke, light, or interactions with specific biological agents, which are often difficult to replicate artificially [90,91]. Breaking seed dormancy is often a challenge. It can be achieved through various techniques depending on the plant species specifics. These methods include stratification, scarification, chemical or physical treatments of seeds, the application of growth regulators, inoculation with symbiotic organisms, and more [90,91,92,94,95,96].
One of the main drawbacks of seed propagation, especially when performed directly in the field, is the low survival rate of young plants. Plants are most sensitive and vulnerable to various adverse factors and environmental impacts during the early stages of development. For this reason, species should be sown in seasons and locations that closely resemble their natural habitats. Alternatively, young plants can be cultivated until they are robust enough to be reintroduced into their natural environment. For example, Sideritis scardica is propagated industrially by producing seedlings under artificially created conditions and later on transplanted in the field [97,98,99,100]. Astragalus dasyanthus and A. alopecurus are problematic for cultivation even if seedlings are produced under artificially created conditions; therefore, their native populations need to be protected efficiently [101].
Vegetative propagation methods are not widely used for wild plants. The primary limitation of vegetative propagation is that the resulting plants are genetically identical to the parent plants, leading to populations with low genetic diversity. However, vegetative propagation offers several advantages. It shortens the juvenile period in some perennial species, which can span several years [102]. Another significant benefit is the ability to propagate species for which seed propagation is challenging, inefficient, or excessively time-consuming. Vegetative propagation methods are particularly valuable for species with infrequent or poor seed production or in cases where seeds are extensively damaged by diseases and pests. These methods provide an effective alternative for conserving and propagating such species [90,92,95,103,104,105]. To mitigate the negative aspects of vegetative propagation, it is essential to use as many parent plants as possible. Additionally, planting vegetatively propagated individuals near natural populations of the same species can promote increased genetic diversity in subsequent generations.
When establishing wildflower strips, the interactions among the selected plant species and the environmental conditions in which they are introduced are of crucial significance. Some species are more competitive in dry and nutrient-poor soils, while others thrive in mesophilic or moist environments. Similarly, certain species perform well in open habitats, whereas others prefer proximity to trees and shrubs or grow as understory plants in forests. Some species are diffusely distributed within plant communities, while others tend to form dense clusters or groups. It is also important to consider that many wild plants naturally reproduce in stable plant communities, whereas others (ruderal species) require periodic habitat disturbances for successful reproduction [93]. Furthermore, the flowering phenology of different plant species should be taken into account to ensure that the strips provide a continuous food source for pollinators over the longest possible period.
All these factors highlight the necessity of carefully designing the plant communities to be included in the strips. Such a design ensures that the strips are as stable and effective as possible for maintaining the natural biodiversity of pollinators [106,107,108,109].
Table 3. Life forms of the food plant species and their propagation potential.
Table 3. Life forms of the food plant species and their propagation potential.
Plant FamilyPlant SpeciesLife FormPropagationSource of Information
ApiaceaeEryngium campestre L. perennialSowing seeds in autumn or springUnpublished data from Sokolov
Scandix pecten-veneris L. annualSowing seeds in springUnpublished data from Sokolov
AsparagaceaeOrnithogalum montanum Cirillo perennialSowing seeds in autumn, divide abundance of small bulbs in summer[110,111,112]
AsteraceaeAchillea clypeolata Sm. perennialSowing seeds in spring; divide in winter or spring[110,111,112]
Calendula officinalis L. annual or biennialSowing seeds in autumn or spring[111,112]
Centaurea diffusa Lam. biennialSowing seeds in autumn or spring[110,111,112]
Centaurea salonitana Vis. perennial
Cirsium ligulare Boiss. annual or biennialSowing seeds in springUnpublished data from Sokolov
Crepis biennis L. biennialSowing seeds in spring to autumn. Unpublished data from Sokolov
Leontodon biscutellifolius DC.perennialSowing seeds in autumn to springUnpublished data from Sokolov
Onopordum tauricum Willd. annual or biennialSowing seeds in springUnpublished data from Sokolov
Xeranthemum annuum L. annualSowing seeds in spring[110,111,112]
BoraginaceaeEchium vulgare L. biennialSowing seeds in autumn[111,112]
BrassicaceaeMutarda nigra (L.) Bernh. annualSowing seeds in spring[111,112]
Sisymbrium orientale L. annualSowing seeds in spring[112]
CampanulaceaeCampanula bononiensis L. perennialSowing seeds in spring[110,111,112]
Campanula rapunculoides L. perennialSowing seeds in spring
Campanula trachelium L. perennialSowing seeds in spring
CaprifoliaceaeCephalaria transsilvanica (L.) Roem. & Schult. annualSowing seeds in autumnUnpublished data from Sokolov
Knautia arvensis (L.) Coult. biennial or perennialSowing seeds in autumnUnpublished data from Sokolov
Lomelosia argentea (L.) Greuter & Burdet biennial or perennial Sowing seeds in autumnUnpublished data from Sokolov
CaryophyllaceaeGypsophila glomerata Pall. ex Adams
perennialSowing seeds in autumnUnpublished data from Sokolov
Gypsophila trichotoma Wend. perennialSowing seeds in autumn
Temperature alternation or treatment with gibberelic acid promote germination 		Unpublished data from Sokolov
[113]
CistaceaeHelianthemum nummularium (L.) Miller perennialSowing seeds in spring, rooting cuttings[110,111,112]
ConvolvulaceaeConvolvulus cantabrica L. perennialSowing seeds in spring[110,112]
FabaceaeAstragalus alopecurus Pall. perennialSowing seeds in spring
Scarification is needed, but survival of the seedlings is very low		[101,110]
Astragalus dasyanthus Pall. perennialSowing seeds in spring Scarification is needed, but survival of the seedlings is very low[101,110]
Astragalus glycyphyllos L. perennialSowing seeds in spring[110]
Coronilla varia L. perennialSowing seeds in autumn[110]
Lathyrus pratensis L. climbing annual or perennialSowing seeds in spring[110,111]
Lotus corniculatus L. perennial or annualSowing seeds in spring or autumn[110,111,114]
Medicago falcata L. perennialSowing seeds in early springWell-established technology of cultivation of its close relative M. sativa L.
Onobrychis alba (Waldst. et Kit.) Desv. perennialSowing seeds in autumnUnpublished data from Sokolov
Onobrychis arenaria W.D.J.Koch perennialSowing seeds in autumnUnpublished data from Sokolov
Trifolium alpestre Crantz perennialSowing seeds in spring or autumn[110,111,114]
Trifolium medium L. perennialSowing seeds in spring or autumn[110,111,114]
Trifolium pratense L. perennialSowing seeds in spring or autumn[110,111,114]
Vicia dumetorum L. perennialSowing seeds in autumnUnpublished data from Sokolov
Vicia grandiflora Scop. annual or biennialSowing seeds in autumnUnpublished data from Sokolov
Vicia tenuifolia Roth. annual or perennialSowing seeds in autumnUnpublished data from Sokolov
Gentianaceae Gentiana cruciata L. perennialSowing seeds in spring[110,111]
LamiaceaeAjuga chia Schreb. annual or subshrubSowing seeds in autumn or spring -
Ballota nigra L. perennialSowing seeds in spring, rooting cuttingsUnpublished data from Sokolov
Clinopodium vulgare L. perennialSowing seeds in spring, Separation of rooted shoots[110,114]
Lamium garganicum L. perennialSowing seeds in springUnpublished data from Sokolov
Lavandula angustifolia L. subshrubCultivated, vegetative propagation (cuttings), adventive (by seed)[111]
Mentha spicata L. perennialSowing seeds in spring, rooting cuttings[111]
Salvia pratensis L. perennialSowing seeds in spring, rooting cuttings[111]
Salvia virgata Jacq. perennialSowing seeds in springUnpublished data from Sokolov
Satureja coerulea Janka perennialSowing seeds in spring, rooting cuttings[115]
Sideritis scardica Griseb. perennial (short living)Seedlings need to be prepared and transplanted. Sowing seeds in February for stratification, giberelic acid treatment promotes germination if the seeds are not stratified
divide clump, rooting cuttings		[97,98,99,100,110,116]
Unpublished data from Kozuharova
 
 
Unpublished data from Sokolov
Stachys germanica L. perennialSowing seeds in spring, divide clump, rooting cuttingsUnpublished data from Sokolov
Teucrium chamaedrys L. perennialSowing seeds in spring, divide clump, rooting cuttings[115]
Teucrium capitatum L. perennialSowing seeds in spring, rooting cuttingsUnpublished data from Sokolov
OnagrceaeEpilobium angustifolium L. perennialSowing seeds in spring, root cuttings[110,111,114]
OrobanchaceaeRhinanthus wagneri Degen annualSowing seeds in autumnUnpublished data from Sokolov
PaeoniaceaePaeonia peregrina Mill. perennialSeed sowing—germination occurs after 16 months of outdoor stratification, division of the rhizome at the end of summerUnpublished data from Sokolov and Stoycheva; [111,117,118]
PlantaginaceaeDigitalis lanata Ehrh. biennial or perennialSowing seeds in spring or early autumn[110,111,112]
Digitalis viridiflora Lindl.perennialSowing seeds in spring
RhamnaceaePaliurus spina-christi Mill. shrub or treeSowing seeds extracted from the fruits in autumnUnpublished data from Sokolov
 
[112]
RosaceaeRosa pendulina L. shrubSowing partially mature seeds in autumnVastly practiced method for all cultivars of Rosa spp.
[111]
RubiaceaeGalium verum L. perennialSowing seeds in early autumn; divide clump in winter[112]

4.4. Principle Recommendations for the Composition of Pollinator-Friendly Mixes of Plants Seeds

Although this study lists certain plants and bees, it has some limitations. The seasonal variations, sample size, and time of observations on chosen plant species (some plants were observed longer than others) affect the records of wild bee diversity. Nevertheless, we documented certain food choices of wild bees. This study demonstrates that pollinator-friendly wildflower strips should be tailored considering a number of factors and principles. It is important to include suitable plant taxa in the pollinator-friendly mixes. The most important principle is to select locally specific entomophilous plants. Since we documented the bee visitors of only 64 plan taxa, while the flora of Bulgaria comprises of at least 2000 entomophilous plants, other local members of the same genera could be conditionally recommended for the restoration activities and pollinator-friendly wildflower strips. The first step to consider is to design pollinator-friendly wildflower strips, using plant species with a broader spectrum of visitors. These are the plants favored by most polylectic bees. The families with the largest number of observed associations are Fabaceae, Lamiaceae, and Asteraceae. They should be presented with locally sourced species. It is also essential to include plant species that support specialized oligolectic bees, such as most members of Melittidae and certain species from the families Apidae, Andrenidae, Colletidae, Halictidae, and Megachilidae. For example, Eryngium campestre and Coronilla varia are of utmost importance in potential plant mixes. Such key plant species should be prioritized in pollinator habitat restoration alongside other local species to support a large and diverse bee community.
The flowering period and the ability of plants to withstand drought conditions are increasingly important factors, especially as climate change continues to alter the timing and intensity of seasonal patterns. A well-balanced seed mix for wildflower strips should include species that bloom at different times of the year, ensuring a continuous food supply for wild bees throughout the season. Furthermore, the inclusion of drought-resistant plants is critical for ensuring habitat resilience under changing climatic conditions. These plants can maintain their floral resources even during periods of water scarcity, thereby supporting bee populations during stressful conditions.
Given the complex interactions between wild bees and their plant hosts, the best strategy for supporting these pollinators in agricultural and disturbed landscapes may be to sample plant communities from fragmented habitats, such as roadsides, field margins, and urban green spaces. These habitats often contain a diverse mix of generalist plants that can support a potentially wide range of bee species, as observed during our field studies. The restoration of natural habitats, however, would require specific sampling of local communities. The systematic, long-term monitoring of pollinators is needed. It is highly beneficial for ecological research, decision-making, and conservation action [119]. The EuPPollNet (European Plant- Pollinator Networks) database contains harmonized taxonomic data on plant–pollinator interactions. The implications for further research should address data gaps in plant–pollinator interactions and guide future efforts in conservation planning [120].

5. Conclusions

The necessity of pollinator-friendly habitat restoration is increasing as is the need for wildflower strips along large agricultural fields and roadsides. Furthermore, the efforts for natural habitats restoration and designed wildflower strips should focus on local native plant communities as models. The target should be specific plant species that are native to the region and are essential for maintaining the ecological integrity of these systems. Seed mixes for pollinator-friendly habitats’ restoration or flower strips should be composed to provide a diverse array of floral resources that are available throughout the whole growing season.
We provide a basic list of plants that offer food resources to certain wild bee taxa native to Bulgaria. This study also highlights the necessity of further and detailed research on the plants and their wild bee pollinators in Bulgaria. This will ensure comprehensive coverage in conservation and habitat restoration efforts. Future research should aim also to test the effectiveness of wildflower strips in different regions and habitats. The goal should be to determine the best combinations of plants for supporting both generalist and specialist wild bee species that also have the benefit of supporting other pollinator groups, such as flies, beetles, butterflies, etc.
The wild bees’ conservation may be supported by wildflower restoration activities, but the process depends on many factors including seed germination specifics. There are difficulties in the seed germination of many entomophilous plants. In order to overcome this barrier, more studies are needed for practical solutions in each specific case. The pollinator-friendly habitats’ restoration and wildflower strips creation activities should be aligned with initiatives and measures for the protection and preservation of natural grassland habitats.

Author Contributions

Conceptualization, E.K. and T.T.; methodology, E.K.; data collection, E.K., T.T. and R.S.S.; writing—original draft preparation, E.K.; writing—review and editing, E.K., T.T., R.S.S., N.Z. and. C.S.; visualization, E.K.; supervision, E.K. All authors have read and agreed to the published version of the manuscript.

Funding

Ekaterina Kozuharova is grateful for the financial support to the European Union—NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, Project No. BG-RRP-2.004-0004-C01.

Institutional Review Board Statement

Not applicable.

Acknowledgments

Special thank you to Nicolaos Vereeken, Justyna Kierat, and Mohamed Shebl for the help with the identification of some bees.

Conflicts of Interest

The authors declare no conflicts of interest.

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