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Article

Urban Green Space as a Reservoir of Predatory Syrphids (Diptera, Syrphidae) for Aphid Control in Cities

by
Elżbieta Wojciechowicz-Żytko
1,* and
Maja Dobińska-Graczyk
2
1
Department of Botany, Physiology and Plant Protection, Agricultural University, al. 29-Listopada 54, 31-425 Kraków, Poland
2
Independent Researcher, 41-250 Czeladź, Poland
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(4), 953; https://doi.org/10.3390/agronomy15040953
Submission received: 11 March 2025 / Revised: 10 April 2025 / Accepted: 11 April 2025 / Published: 14 April 2025
(This article belongs to the Special Issue Advances in Beneficial Insects Research for Conservation Biological Control)
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Abstract

:
The occurrence of predatory Syrphidae (hoverflies) in green areas of cities and their role as biological control agents is determined in this work. During the study, 751 adults belonging to 21 species were captured in Moericke’s traps and with sweep nets, and 286 larvae from 10 species were reared from aphid colonies. In both cases, the dominants were Episyrphus balteatus (Deg.) Sphaerophoria scripta (L.), and Syrphus vitripennis Meig. (L.) It can be assumed that hoverflies were attracted by flowering plants and then developed in aphid colonies on trees, shrubs, and herbaceous plants, reducing the aphid population. The largest number of hoverflies was caught in the plant-rich and well-developed Wolski Forest, whose conditions were beneficial for their reproduction and survival. Studies on the voracity of hoverflies have shown that the larvae of dominant species ate from 243 to 498 individuals of Aphis fabae Scop. and from 272 to 468 specimens of Myzus cerasi (Fabr.); the efficiency depended on the syrphid and aphid species as well as the instar stage of the syrphid larva. The results indicate that urban green spaces are vital refugia for insect biodiversity and could be a reservoir of beneficial insects.

1. Introduction

The species composition and layout of vegetation is affected by increasing urbanization and changing plant–insect interactions [1]. In urban areas, plant species that are better equipped to withstand environmental stress tend to thrive, leading to a more uniform plant community. These resilient species often become the most prevalent in cities [2]. The most common species in urban environments are ornamental trees and shrubs, wild herbaceous plants growing on lawns and flower meadows, as well as ornamental plants, often exotic [3]. The composition of plants occurring in green areas is related to the transformed environment, its fragmentation, high pollution, and the urban heat island effect, as well as human preferences [1,2]. Urban green areas are very important for the insect community in cities; unfortunately, promoting and maximizing insect diversity is usually not the most important goal in parks and gardens [4,5]. Green areas in cities are designed for aesthetic, economic, or logistical reasons [6,7,8], which can threaten the local insect fauna [9].
Urban landscapes (e.g., public parks, squares, pocket parks, home gardens, even balconies) can include semi-natural and artificial habitats of varying sizes and degrees of change [10] and are considered reservoirs of biodiversity [11,12,13,14]. The survival of beneficial insects in cities depends on the presence of these reservoirs. Parks and green areas in cities are characterized by a relatively high biodiversity of plants and therefore can constitute areas that provide continuous habitats for insects, enabling them to survive in difficult urban conditions. The diversity of beneficial insects, such as hoverflies, can be influenced by the characteristics of their habitat. A higher number of flowers or greater plant diversity can lead to an increase in their population [5,15]. These refugia increase the number of beneficial insects, providing them with shelter or wintering places, as well as a source of food [10,16,17,18,19,20,21].
Contaminated urban environments create specific conditions for the development of insects in which the group of piercing–sucking arthropods, e.g., aphids, develop best. Aphids are an important factor in biotic stress, negatively affecting trees, shrubs, and herbaceous plants [22]. Their feeding results in leaf distortion, shoot twisting, and yellowing of leaves (chlorosis). Additionally, they weaken trees, making them more vulnerable to frost damage. Aphids secrete honeydew on which sooty mold fungi develop, thus reducing the decorative value of plants, which is so important for ornamental plants in green areas.
In urban areas, the use of alternative plant protection methods is particularly important due to the significant limitations in the potential for the application of chemicals due to their perceived negative ecological and human health effects [7]. Several studies have highlighted the negative impacts of pesticides on beneficial insects, such as reduced lifespan and decreased reproductive ability [23,24]. Coccinellids, chrysopids, cecidomyids, and predatory syrphids are of great importance in natural control of aphids. Together, these groups create a natural environmental resistance complex and can significantly affect the health of plants [25]. Syrphidae are an important group of insects that are able to provide many ecosystem services in urban green areas. For example, adults feed on nectar and pollen and play a large role in pollination [1,10,13,14,26,27,28] and their larvae have different food preferences: some are saprophagous, others coprophagous, whilst larvae from the subfamily Syrphinae offer several benefits as biological control agents, helping to decrease aphid populations [19,27,29,30,31,32].
While other urban insect groups like bees and butterflies have been extensively researched, there is limited knowledge about how growing urbanization affects hoverfly populations [5]. There is sparse information on the occurrence of syrphids in urban areas in Poland [19,25,33,34] and this study aims to determine the species composition of predatory syrphids (Diptera, Syrphidae) occurring in green areas (reservoirs of beneficial insects) in a city and to determine their role in aphid control, using the green space of Kraków as the study area.

2. Materials and Methods

2.1. Research Sites

The study was carried out during the years 2016 and 2017 in Kraków, Poland (19°56′ E 50°04′ N), in five green areas (Figure 1).
Site 1: Jordan Park is 21.5 ha, located in the city center, and established at the confluence of two rivers, the Vistula and the Rudawa; its natural community is riparian vegetation: elms (Ulmus sp.), ash trees (Fraxinus excelsior L.), alders (Alnus glutinosa (L), poplars (Populus sp.), willows (Salix sp.), lime trees (Tilia sp.), and hornbeams (Carpinus betulus L). There are also shrubs—mock orange (Philadelphus coronarius L.), forsythia (Forsythia intermedia), black elder (Sambucus nigra L.), snowberry (Symphoricarpos albus L.)—flower beds, and open grassy glades with herbaceous plants and flower meadows.
Site 2: Krakow Park is 5 ha and is located in the center of Kraków, surrounded by busy streets. In the eastern part of the park there is a pond. The park’s greenery consists of trees, including common horse chestnut (Aesculus hippocastanum L.), birches (Betula sp.), alders (Alnus glutinosa L.), and hornbeams (Carpinus betulus L.); shrubs, including barberries (Berberis L.), spirea (Spiraea L.), dogwoods (Cornus sp.), forsythias (Forsythia intermedia), currants (Ribes sp.), mock orange (Philadelphus coronarius L.), black elder (Sambucus nigra L.), and low hedges (mainly lugstr (Ligustrum vulgare L.)); and seasonal flower beds (marigolds (Tagetes sp.), begonias (Begonia L.), and perennial plantings).
Site 3: The botanical garden is 9.6 ha and is located in the center of Kraków. Containing a collection of trees and shrubs, it is composed as a landscape park, and partly in thematic groups. The collection of woody plants consists of about 1000 species and varieties; the most valuable groups are maples (Acer sp.), birches (Betula sp.), oaks (Quercus sp.), black locust (Robinia pseudoacacia L.), and coniferous trees; shrubs include lilacs (Syringa sp.), viburnums (Viburnum L.), currants (Ribes sp.), forsythias (Forsythia intermedia), roses (Rosa sp.), and snowberries (Symphoricarpos albus L.). There is a large group of ornamental plants concentrated in flower beds.
Site 4: Aviators Park is 60 ha and it stretches between busy streets. The greenery in the park consists of trees (maples (Acer sp.), oaks (Quercus sp.), larches (Larix sp.), limes (Tilia sp.), common horse chestnut (Aesculus hippocastanum L.), spruces (Picea abies (L.), bird cherry (Prunus avium L.)), groups of shrubs (barberries (Berberis L.), currants (Ribes sp.), snowberries (Symphoricarpos albus L.), mock orange (Philadelphus coronarius L.), lilacs (Syringa sp.), forsythias (Forsythia intermedia), black elder (Sambucus nigra L.); and flower plantings. Unmown lawns have been turned into meadows, where common daisies, geraniums, common yarrows, and others bloomed.
Site 5: Wolski Forest is 422 ha, located on the outskirts of Krakow, at a relatively large distance from busy roads and compact urban development. This area has a forest character, with a very rich and diverse woody vegetation including trees such as common horse chestnut (Aesculus hippocastanum L.), beeches (Fagus sylvatica L.), birches (Betula sp.), oaks (Quercus sp.), limes (Tilia sp.), hornbeams (Carpinus betulus L.), maples (Acer sp.), and bird cherry (Prunus avium L.); shrubs such as cotoneasters (Cotoneaster sp.), spindles Euonymus europaeus L.), forsythias (Forsythia intermedia), snowberries (Symphoricarpos albus L.), mock orange (Philadelphus coronarius L.), and black elder (Sambucus nigra L.); and also large areas of open space, partly wild and unmown, and partly mowed lawns, overgrown with wild herbaceous plants (with, in the wild parts, goldenrod, impatiens, small-flowered impatiens, goutweed, in the mowed parts daisies, geraniums, tufts).
On the plants of the green areas, the presence of many aphid species was recorded (e.g., Aphis fabae Scop, Aphis pomi De Geer, Aphis sambuci L., Brachycaudus cardui L., Corylobium avellana Schrk., Dysaphis crataegi Kalt., Dysaphis sorbi Kalt., Myzocallis coryli Goetze, Myzus cerasi (Fabr.), Myzus ligustri Mosl).

2.2. Method of Sampling

2.2.1. Sampling of Syrphid Larvae

To determine the species composition, larvae and pupae of syrphids were collected from aphid colonies feeding on trees, shrubs, and wild and ornamental plants, from May to September, at intervals of 10–14 days, in the morning (8 a.m.–10 a.m.), for 60 min at each site. The larvae were then individually raised in Petri dishes lined with filter paper under controlled laboratory conditions (23 °C temperature and 70% relative humidity), and were fed daily with aphids collected from natural conditions until they emerged.

2.2.2. Sampling the Adults

The syrphid adults were sampled from May to September by Moericke’s traps and with a sweeping net.
Two randomly selected transects 25 m long and 2 m wide, 20 m apart, were set at each site. Catches were carried out twice a month (with a total of 10 samples per site) using a standard sweeping net with 25 sweeps along each transect. Sampling took place between 10:00 a.m. and 3:00 p.m., under clear weather conditions, with light wind and minimum temperatures consistently above 15 °C. Syrphids were collected from flowering plants, herbaceous vegetation, trees, and shrubs. Species were identified in the field whenever possible and then released; if not, they were brought to the laboratory for identification using identification keys [35].
The traps were yellow vessels of 20 cm diameter, filled with a mixture of water and glycol [36]. An agent that reduces surface tension was added. Three traps were randomly placed in each park, spaced 20 m apart, and suspended at a height of 1.5 m above the ground. Samples were taken from May to September at 10–14-day intervals.
All collected syrphid adults were identified to species in the laboratory under the microscope, based on the keys of van Veen [35] and Rotheray [37]; the terminology used was according to Soszyński [38].

2.2.3. Determination of Efficiency of Syrphid Larvae

Three common syrphid species were selected to determine the feeding efficiency of hoverflies: Episyrphus balteatus (Deg.), Syrphus vitripennis (Meig.) and Sphaerophoria scripta (L.). Syrphid eggs were gathered from aphid colonies, and the larvae that emerged were individually reared in Petri dishes lined with filter paper. The larvae were fed daily ad libitum for the total length of their development with aphids Aphis fabae Scop. or with Myzus cerasi (Fabr.). Aphid colonies were collected from the mock orange (Philadelphus coronaries L.) (A. fabae) or bird cherry (Prunus avium L.) (M. cerasi). During the initial three days, the first-stage larvae were provided with 20 aphids daily. As the larvae progressed through their development, the number of aphids increased, reaching 100 per day during the second stage and 200 per day during the third stage. Daily consumption was calculated as the difference between those delivered and those that remained alive the next day. The experiment was conducted under laboratory conditions (at a temperature of 23 °C, relative humidity of 70%, and a 14 h photo-period), with five replicates of each syrphid and aphid species.

2.3. Statistical Analysis

The species dominance, richness, and the similarity of syrphid communities were described. The dominance coefficient was accepted and five dominance classes were adopted: eudominants ≥ 10.1%, dominants 5.1–10%, subdominants 2.1–5.0%, recedents 1.1–2.0%, and subrecedents ≤ 1% [39]. Species dominance was calculated based on the formula given by Szujecki [40]:
D = s/S × 100
where D = dominance (relative abundance), s = number of individuals of a given species, S = number of individuals of all species of the studied cenotic unit.
The frequency of the examined species was determined based on the formula given by Szujecki [40]:
F = q/Q
where F = frequency, q = number of samples in which a given species occurrs, Q = number of examined samples.
Species richness was calculated based on the formula
S = (s − 1)/logN
where S = species richness, s = total number of different species, and N = total number of individuals.
The Jaccard classical index [41] was used to calculate the similarity of hoverfly associations.
Jclass = A/(A + B + C)
Jclass = Jaccard similarity index, A = number of shared species, B = number of species unique to the first assemblage, and C = number of species unique to the second assemblage.
To determine the relationship between hoverflies (number of specimens and species) caught in parks and those collected from aphid colonies, the correlation coefficient was calculated. The data were analyzed using Pearson correlation coefficients (tests on normal distribution K–S). The differences between the abundance of the syrphid species at different sites were calculated using one-way ANOVA statistics. Due to the relatively small sample sizes and unequal variances between groups, the Fisher’s LSD post hoc test was chosen as the most appropriate method for comparing group differences. A significance level of p < 0.05 was considered for all analyses. The data were analyzed using the statistical software package Statistica (Version 13, Kraków, Poland).

3. Results

3.1. The Species Compositions of Syrphidae Collected from the Aphid Colonies

During the observations, 286 syrphid larvae belonging to 10 species were collected. The number of the collected species varied from 7 (site 3) to 10 (site 5) (Table 1). No statistically significant differences were found in the number of species caught at individual sites, while differences were found in the number of individuals (Table 2).
Episyrphus balteatus (Deg.) was dominant throughout all the years of observation and across all sites (Table 1). Its larvae constituted almost 50% of all collected syrphid larvae. Epistrophe eligans (Harr.), Syrphus vitripennis (Meig.), and Sphaerophoria scripta (L.) larvae were also abundant. These four species constituted over 76% of reared larvae. The occurrence of Epistrophe eligans larvae is important because it is an early-spring species, developing in aphid colonies on trees and shrubs in early spring when the first aphid colonies are formed. It has one generation per year, and after spring feeding its larvae fall into a long diapause of up to 9 months.
The frequency of Episyrphus balteatus varied from 50% at site 4 to 90% at sites 3 and 5. Among the other species characterized by a high frequency were Syrphus vitripennis (50% at site 2) and Epistrophe eligans (30% at site 3) (Table 1). Throughout the study, the largest amount of syrphid larvae were observed at sites 3 (mean: 7.6 ± 1.4) and 5 (mean: 7.3 ± 1.9), with diverse vegetation, and where a large number of aphid colonies feeding on plants was observed; whereas the lowest number was noted at site 4 (mean: 3.0 ± 0.8) (Table 1 and Table 2). Some species were scarce and occurred only at some sites—for example, Platycheirus scutatus (Meig.) at sites 1 and 5, and Melanostoma scalare (Fabr.) at sites 2 and 5 (Table 1).
The highest species richness of hoverflies collected from the aphid colonies was noted at sites 4 and 5, whereas interestingly, and unexpectedly, the species richness at site 3 (botanic garden) was very low (3.2), although it was a place with very rich vegetation and many blooming flowers and the species richness of adult hoverflies caught in this area was one of the highest (Table 3).

3.2. The Efficiency of Syrphid Larvae

Statistically significant differences were found in the number of aphids eaten by individual hoverfly species except for M. cerasi, where Episyrphus balteatus and Syrphus. vitripennis ate similar numbers of them (Table 4). The voracity of syrphid larvae depended on the syrphid species (size of the larva), larval instar, and on the aphid species (Table 4).
During their development, lasting 11–12 days, the smallest larvae of Sphaerophoria scripta ate 190–266 (mean: 243.0 ± 19.4) A. fabae specimens and 206–318 (mean: 272.4 ± 19.5) individuals of M. cerasi. Larvae of Episyrphus balteatus and Syrphus vitripennis, both being of intermediate size, consumed about 411–541 A. fabae and 416–510 M. cerasi (Table 4, Figure 2). The efficiency of syrphid species was also compared with the cluster method (Figure 3). The red vertical line in the dendrogram was used to set a threshold of dissimilarity (319.406), and this divided the species into three groups based on their effectiveness. These groups represent the clustering of syrphid species that are more similar to each other within each group, and less similar between groups (Figure 3).
The third-instar larvae were the most aggressive feeders due to their mobility, ability to search, and high nutritional needs, consuming approximately 80% of the total eaten aphids (Figure 4 and Figure 5). The red vertical line in the dendrogram at 526.538 indicates the specific level of dissimilarity which divided the larval stages into two separate groups, where the stages in each group were more similar to one another than to those in the other group (Figure 5).

3.3. Syrphids Collected in Sweep Net and Moericke’s Traps

During the two years of the study, 751 syrphid adults (21 species belonging to the 15 genera) of the subfamily Syrphinae were caught in urban green spaces in Kraków (Table 5).
No statistically significant differences were found in the number of species caught at different sampling sites; however, when it comes to the number of individuals, site 5 was statistically significantly different (Table 2 and Table 5). The largest number of syrphid species (19) and individuals (mean: 22.5 ± 4.1) was collected at site 5 (Wolski Forest). The lowest numbers of specimens were noted at sites 1 and 2 (mean: 11.5 ± 2.0 and 11.3 ± 2.2, respectively) (Table 2 and Table 5).
Among all the species, Episyrphus balteatus was the most prevalent (eudominant), particularly at site 3 (58.5%) and site 2 (54.9%). Surprisingly, its numbers were lower at site 5 (26.2%). Overall, Episyrphus balteatus made up more than 40% of all the syrphids captured (Table 5). Its role in aphid colonies was even more significant, contributing 45.5% (Table 1), highlighting its importance as a predator bio-agent. Sphaerophoria scripta also belonged to the eudominant group; Syrphus vitripennis and Melanostoma scalare (F.) were included in the dominant species. At all sites in the city, the most numerous groups of hoverflies were subdominants. The exception was site 5, where subrecedents predominated. The frequency of Episyrphus balteatus ranged from 50% at site 1 to 90% at site 2. Other species with notably high frequencies included Sphaerophoria scripta (75% at site 1), Syrphus vitripennis (75% at site 5), and Syrphus ribesii (50% at site 5) (Table 5). Table 5 shows that other species were less numerous and species such as Chrysotoxum cautum (Harr.), Lapposyrphus (Eupeodes) lapponicus (Zett.), and Meliscaeva cinctella (Zett.) (one specimen each) were noted only at site 5, while Chrysotoxum vernale (Loew) was noted at site 1 and Didea fasciata (Macq.) at site 2. Differences in the species richness were noted across the sampling sites. The highest indicators of species richness (7.5) were noted at site 5 (with diverse vegetation) and site 3 (5.5), although surprisingly the lowest species richness was recorded in this location in relation to the larvae collected from the aphid colonies; while at site 2 the lowest species richness (4.8) was noted (Table 3). The highest similarity between syrphid species reared from aphid colonies was found at sites 1 and 4 (0.8), 1 and 5 (0.9), and 2 and 4 (0.9), while the lowest was between sites 1 and 3 and 2 and 3 (both 0.7) (Table 6). If we compared the similarity of the captured adult hoverflies (traps/sweep net), the highest was noted between sites 2 and 3 and 3 and 4, as well as 3 and 5 (in all cases 0.7) (Table 6).
In Figure 6, the occurrence of syrphids collected from aphid colonies as well as from the sweep net/traps at sampling sites is presented.
The largest number of hoverflies was caught with the sweep net/traps at site 5, while from the aphid colonies the largest number of larvae was collected at site 2; however, at the same time the smallest number of adults was recorded at this site (Figure 6).

3.4. Significant Difference and Correlations Between the Syrphidae Collected from Aphid Colonies and from Sweep Net/Traps from Sampling Sites

The results indicate that, across all sites, there were significant positive correlations between the number of syrphid larvae found in aphid colonies and the adults captured in traps or with sweep nets (Table 7). It can be assumed that hoverflies were attracted by flowering plants then dispersed, and subsequently, females laid eggs in aphid colonies. Surprisingly, in relation to the common species Episyrphus balteatus significant positive correlations were found at all sites except at site 1 (Table 8).

4. Discussion

Studies have shown the important role played by urban green areas in maintaining the biodiversity of beneficial insects. In these areas, over a period of two years, more than thousand syrphids were collected from traps/sweep net and aphid colonies. This may not be an impressive number, but it was probably associated with the detrimental effects of urban conditions on hoverflies. Studies by several authors have reported low hoverfly diversity and density in urban environments [15,28,42], mainly due to the limited availability of food resources for predatory larvae, with food occurring less frequently in urban environments than in natural environments [42]. The lack of diversity in the urban landscape also fails to offer suitable egg-laying sites for females [15], thus demonstrating the importance and need for high-diversity green areas in cities [42].
In our study, the largest number of syrphid species (19) and individuals (225) were collected at site 5 (Wolski Forest)—the largest green area in the city, away from the city center and traffic, with a rich vegetation cover and biodiversity of trees and shrubs. Hoverfly communities, both in aphid colonies and caught in traps/sweep net, were dominated by the ubiquitous and polyvoltine species: Episyrphus balteatus, Sphaerophoria scripta, and Syrphus vitripennis. It can be assumed that they are more tolerant to selection pressure connected to anthropogenic changes such as urbanization. These species constituted more than 60% of all the collected hoverflies. The most numerous among them was Episyrphus balteatus, accounting for more than 40% (304 specimens), which highlights its great importance as a predatory bio-agent. This has been confirmed by studies by many authors, who note their presence and dominance in almost all urban environments (e.g., [15,42,43]). According to Speight et al. [43] and Jacobs et al. [44], these syrphids belong to the anthropophilic group occurring in parks, gardens, and along field hedges. They are highly migratory and have several generations per year and therefore are less sensitive to environmental disturbances [45].
Their larvae feed on aphids appearing on a wide range of plants of any size—trees, shrubs, and herbaceous plants. Kadas [46] and Passaseo et al. [28] found that half of the hoverfly species collected in the city were zoophagous in the larval stage, indicating that the larvae of these hoverflies can find sufficient food resources in green areas of cities. Faunistic studies on the diversity of hoverflies in the botanical garden in Poznań showed that 43 zoophagous species from the Syrphinae subfamily constituted 54% of all the species recorded [34]. Episyrphus balteatus, Eupeodes corollae (Fabr.), Sphaerophoria scripta, and species from the Syrphus genus, Syrphus vitripennis, Syrphus torvus O.–S., and Syrphus ribesii (L.), dominated and were present throughout the research period. These species are presented by many authors as playing an important role in limiting the number of aphids on many plants [19,25,34]. Passaseo et al. [28] collected 32 zoophagous syrphids belonging to seven species on six extensive vegetated roofs in Geneva, Switzerland, and Sphaerophoria scripta was the most abundant species, representing 61% of total catches. The authors emphasized that such a low number of collected hoverflies was related to the negative impact of urbanization on this group of beneficial insects. Jacobs et al. [44], in Antwerp (Belgium), caught 11 syrphid individuals on green roofs. These species, commonly found in urban areas, such as Episyrphus balteatus, Melanostoma mellinum, Scaeva pyrastri, and Sphaerophoria scripta, are highly migratory and are better able to overcome the habitat isolation of green roofs. Other species of hoverflies in our sampling area were less abundant and occurred only occasionally at some sites (e.g., Chrysotoxum cautum (Harr.), Lapposyrphus (Eupeodes) lapponicus (Zett.), Meliscaeva cinctella (Zett.), Chrysotoxum vernale (Loew), Didea fasciata (Macq.)), or were not found at all in our study, probably due to greater sensitivity to unfavorable urban conditions.
In our study, on all sites, significant positive correlations were found between the number of syrphid larvae collected from aphid colonies and adults caught in the traps/sweep net. It can be assumed that hoverfly adults attracted by flowering plants subsequently dispersed and the females laid eggs in aphid colonies.
Our research highlights the important role that predatory hoverfly larvae play in limiting aphid populations. During their development, larvae of Sphaerophoria scripta ate 190–266 A. fabae specimens and 206–318 individuals of M. cerasi. Larvae of Episyrphus balteatus and Syrphus vitripennis consumed about 411–541 A. fabae and 416–510 M. cerasi. Third-instar larvae were the most voracious due to their mobility, searching ability, and high nutrient demand, consuming about 80% of the total eaten aphids. The feeding capacity of syrphid larvae, as reported by other researchers [47,48,49], may differ even when the same aphid and syrphid species are studied. This is because larval voracity is influenced by various factors that affect their development, such as temperature, humidity, and photoperiod [47,48,49]. According to Tenhumberg [50], syrphid larvae consumed 396 aphids Sitobion avenae (F.) and Metopolophium dirhodum (Walk) in field conditions, which was only half of the food eaten in a laboratory experiment. The author of [50] suggested that the smaller number of prey eaten in the field may be related to the additional time needed by predators to find prey. Other data on syrphid voracity can be found in the works of Wabbi [51], who noted that syrphids consumed 407–1322 A. fabae, while Cornelius, Barlow [52] determined the amount of food eaten by Syrphus corollae to be from 40.9 cal to 78.1 cal/larva. Episyrphus balteatus, the dominant species, whose voracity has been studied by many authors, ate from 202 to 235 specimens of B. brassicae [53,54], 269-468 individuals of M. persicae [54,55], and 986 of A. gossypii [55]. According to Agarwala et al. [56], the mean daily voracity of Episyrphus balteatus was 45 cabbage aphids. Wojciechowicz-Żytko [57], in their previous research, noted that Sphaerophoria scripta ate 190–306 specimens of A. fabae, whereas Episyrphus balteatus and Syrphus vitripennis consumed about 395–604 aphids during their larval development. Larvae of the last, third instar ate 80.8–89.4% of all consumed aphids. According to Jijang et al. [58], for Aphis craccivora Koch, Myzus persicae (Sulzer), and Megoura japonica (Mats) the maximum number of daily eaten aphids by second-instar larvae was 83.33, 166.67, and 47.62, respectively, and for third-instar larvae, 142.86, 200.00, and 90.91, respectively. Leir and Barlow [59] found that starved Metasyrphus corollae larvae captured aphids more rapidly than both unstarved and older larvae, and faster than younger ones as well.
It can be assumed that plant biodiversity in urban green spaces (such as public parks) would enhance hoverfly richness and diversity. These areas attracted hoverflies due to their strong reliance on flowers for pollen and nectar, which are crucial for their reproduction and survival. They also constitute a place for shelter or overwintering for beneficial insects and an alternative source of food. The study of Barahona-Segovia et al. [60] showed that high-quality green spaces in urban areas were crucial for syrphids. The authors stated that this insight can assist city planners in making more informed choices about managing and enhancing green spaces. Sharmin et al. [61] discovered that the abundance of beneficial insects was greater when trees and shrubs were planted together than when trees were planted alone. The authors noted that urban planning would benefit from including shrubs alongside urban trees (as seen in parks) to maximize the abundance and diversity of invertebrates in urban landscapes.
In natural ecosystems, a wide variety of species are observed as recedents and subrecedents, and such an arrangement was found in our study in the Wolski Forest—an area with diverse vegetation. In this area, a few rare species, such as Chrysotoxum cautum (Harr.), Lapposyrphus lapponicus (Zett.), and Meliscaeva cinctella (Zett.), were found in small numbers, a characteristic typically seen in natural plant environments. This indicates that this complex is a source of many species, which may go on to penetrate urban development and smaller green areas in the city and participate in the control of aphids. However, in the remaining sites, in small city parks, much lower species diversity was observed, with a small number of very numerous dominant species (3–5 species), which is typical for the conditions of anthropopressure. According to Bańkowska [62], in cities, a permanent, several-species group of hoverflies with large numbers is distinguished, which has managed to adapt to the unfavorable (harsher) conditions prevailing in the city. Persson et al. [63] observed that as the landscape became more urbanized and densely populated, the number of syrphid species decreased, with the species present being only a subset of the more diverse communities found in natural areas. Many larval habitats for hoverflies, are likely less common in urban parks, and syrphids require both larval habitats and flower resources for adults [61]. In urban areas, the movement between semi-natural larval habitats and parks may be restricted due to barriers created by the built environment. Thus, it was found that hoverflies are more sensitive to urbanization than other beneficial insects [41,63,64,65].
Every green area is a reservoir and refugium for beneficial insects, including hoverflies, hence we should create the right conditions for their development by sowing honey and nectar plants, leaving partially unmown meadows or sowing flower meadows, and planting trees and shrubs with a long flowering period [66,67]. Cities cannot be a starving desert for beneficial insects, we must create the right conditions for their development; hence, when planning green areas in cities, attention should be paid to those elements that are effective in promoting beneficial insects. To enhance the benefits associated with the concentration and conservation of syrphids, it is important to plant suitable flowering plants from a variety of plant families [68]. In order to allow hoverflies free access to parks, good landscape connectivity should be ensured by implementing strategies that provide a range of green spaces such as lawns, flower meadows, rows of trees, or clumps of urban crops to provide corridors of potential movement between habitats, thus ensuring the diversity and abundance of flowers that attract adult hoverflies and provide an abundance of pollen and nectar for them, while at the same time being host plants for aphids, which are food for predatory larvae. Since hoverflies are sensitive to human-induced disturbances in their habitat, the maintenance of green areas should be kept to a minimum [28].

5. Conclusions

Urban greenery in a city landscape heavily transformed by humans constitutes a refuge for beneficial insects, including hoverflies, providing them with a place to develop and survive. Urban agglomerations promote the development of some syrphid species (E. balteatus, S. scripta, S. vitripennis), which tolerate or even cope well with the pressure of urbanization. When green areas in cities are planted, attention should be paid to those elements that are effective in promoting beneficial insects, including syrphids.

Author Contributions

Both authors contributed to this work. E.W.-Ż. designed and performed the experiments, analyzed the data and wrote the paper; and M.D.-G. performed the experiments. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Ministry of Science and Higher Education of Poland as a part of research subsidy to the University of Agriculture in Kraków.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank Phil Murray from the Centre for Agriculture, Royal Agricultural University (England) for the English proofreading of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The distribution of the sampling sites in Kraków. Site 1—Jordan Park, site 2—Krakow Park, site 3—the botanical garden, site 4—Aviators Park, site 5—Wolski Forest.
Figure 1. The distribution of the sampling sites in Kraków. Site 1—Jordan Park, site 2—Krakow Park, site 3—the botanical garden, site 4—Aviators Park, site 5—Wolski Forest.
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Figure 2. The number of aphids consumed by each individual syrphid larva in different combinations.
Figure 2. The number of aphids consumed by each individual syrphid larva in different combinations.
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Figure 3. Cluster analysis of efficiency of different syrphid species. The cut-off red vertical line in the dendrogram indicates the level of similarity at which the dendrogram is cut (319.406). It divides the dendrogram into three groups.
Figure 3. Cluster analysis of efficiency of different syrphid species. The cut-off red vertical line in the dendrogram indicates the level of similarity at which the dendrogram is cut (319.406). It divides the dendrogram into three groups.
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Figure 4. Number of aphids eaten by syrphid larvae in various stages of development.
Figure 4. Number of aphids eaten by syrphid larvae in various stages of development.
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Figure 5. Cluster analysis of efficiency of different larval stages. Stage 1—first instar of larva, stage 2—second instar of larva, stage 3—third instar of larva. The cut-off red vertical line in the dendrogram indicates the level of similarity at which the dendrogram is cut (526.538). It divides the dendrogram into two groups.
Figure 5. Cluster analysis of efficiency of different larval stages. Stage 1—first instar of larva, stage 2—second instar of larva, stage 3—third instar of larva. The cut-off red vertical line in the dendrogram indicates the level of similarity at which the dendrogram is cut (526.538). It divides the dendrogram into two groups.
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Figure 6. The abundance of syrphids (in aphid colonies and collected with sweep net/traps) in different sites.
Figure 6. The abundance of syrphids (in aphid colonies and collected with sweep net/traps) in different sites.
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Table 1. Species composition, number, domination, and frequency of predatory Syrphidae occurring in aphid colonies at sampling sites.
Table 1. Species composition, number, domination, and frequency of predatory Syrphidae occurring in aphid colonies at sampling sites.
SpeciesSite 1 Site 2 Site 3 Site 4 Site 5 Total
No.%fNo.%fNo.%fNo.%fNo.%fNo.%
Epistrophe eligans (Harr.)610.2
Ed		2036.3
D		151418.4
Ed		30516.7
Ed		151013.7
Ed		203813.3
Ed
Episyrphus balteatus (Deg.)2440.7
Ed		851429.2
Ed		704457.9
Ed		901240.0
Ed		503649.3
Ed		9013045.5
Ed
Eupeodes corollae (Fabr.)46.8
D		1048.3
D		1079.2
D		1526.7
D		545.5
D		15217.3
D
Melanostoma scalare (Fabr.) 12.1
Sd		5 11.4520.7
Sr
Meligramma triangulifera (Zett.)35.1
D		10 33.9
Sd		1013.3
Sd		522.7
Sd		593.2
Sd
Platycheirus scutatus (Meig.)35.1
D		10 22.7
Sd		551.7
R
Scaeva pyrastri (L.)35.1
D		1024.2
Sd		5 26.7
D		1056.8
D		10124.2
Sd
Sphaerophoria scripta (L.)35.1
D		1048.3
D		1522.6
Sd		10310.0
D		1056.8
D		15175.9
D
Syrphus ribesii (L.)58.5
D		1548.3
D		1033.9
Sd		1526.7
D		1034.1
Sd		10175.9
D
Syrphus vitripennis (Meig.)813.6
Ed		251633.3
Ed		5033.9
Sd		10310.0
D		1056.8
D		153512.2
Ed
No. of specimens59100 48100 76100 30100 73100 286100
No. of species9 8 7 8 10
Eudominants (Ed) > 10%, dominants (D) 5.1–10%, subdominants (Sd) 2.1–5%, recedents (R) 1.1–2%, subrecedents (Sr) < 1%. No.—number, %—percentage, f—frequency.
Table 2. Mean number of syrphid specimens and species collected from aphid colonies and traps/sweep net at different sites.
Table 2. Mean number of syrphid specimens and species collected from aphid colonies and traps/sweep net at different sites.
HabitatSite 1Site 2Site 3Site 4Site 5
specimens
Aphid colonies5.9 ± 1.2 ab4.8 ± 0.9 ab7.6 ± 1.4 b3.0 ± 0.8 a7.3 ± 1.9 b
Traps/s.n.11.5 ± 2.0 a11.3 ± 2.2 a16.4 ± 2.3 ab13.4 ± 2.8 a22.5 ± 4.1 b
species
Aphid colonies4.8 ± 1.0 a4.8 ± 1.1 a4.8 ± 1.0 a4.2 ± 1.3 a5.4 ± 1.5 a
Traps/s.n.6.8 ± 1.4 a6.4 ± 1.5 a8.4 ± 1.3 a5.8 ± 1.7 a10.0 ± 2.0 a
Means in lines marked with different letters are significantly different from each other (Fisher multiple comparisons test, p < 0.05).
Table 3. Species richness of Syrphidae collected from aphid colonies and from traps/sweep net from sampling sites.
Table 3. Species richness of Syrphidae collected from aphid colonies and from traps/sweep net from sampling sites.
Site 1 Site 2 Site 3 Site 4 Site 5
Trap/
s.n.		a.c.Trap/
s.n		a.c.Trap/
s.n		a.c.Trap/
s.n		a.c.Trap/
s.n		a.c.
No. of species1191181371281910
Species richness4.94.44.84.15.53.25.24.77.54.7
a.c.—aphid colonies, s.n.—trap/sweep net.
Table 4. The efficiency of E. balteatus, S. vitripennis, and Sph. scripta on A. fabae and M. cerasi.
Table 4. The efficiency of E. balteatus, S. vitripennis, and Sph. scripta on A. fabae and M. cerasi.
E. balteatusS. scriptaS. vitripennis
A. fabae430.2 ± 6.2 b243.0 ± 19.4 a498.6 ± 12.3 c
M. cerasi441.2 ± 10.8 b272.4 ± 19.5 a468.0 ± 12.2 b
Means in lines marked with different letters are significantly different from each other (Fisher multiple comparisons test, p < 0.05).
Table 5. Species composition, number, domination, and frequency of predatory Syrphidae collected in traps and sweep net at the sampling sites.
Table 5. Species composition, number, domination, and frequency of predatory Syrphidae collected in traps and sweep net at the sampling sites.
GatunekSite 1 Site 2 Site 3 Site 4 Site 5 Total
No.%fNo.%fNo.%fNo.%fNo.%fNo.%
Baccha elongate (Fabr.) 43.4
Sd		1075.2
D		15167.1
D		25273.6
Sd
Chrysotoxum cautum (Harr.) 10.4
Sr		510.1
Sr
Chrysotoxum vernale (Loew)21.7
R		10 20.3
Sr
Dasysyrphus tricinctus (F.)10.9
Sr		5 10.6
Sr		5 20.9
Sr		540.5
Sr
Didea fasciata (Macq.) 32.7
Sd		10 30.4
Sr
Episyrphus balteatus (Deg.)2420.9
Ed		506254.9
Ed		909658.5
Ed		856347.0
Ed		805926.2
Ed		7530440.5
Ed
Epistrophe eligans (Harr.) 31.8
Sr		1010.7
Sr		5104.4
Sd		20141.9
R
Eupeodes corollae (F.)32.6
Sd		1010.9
Sr		521.2
R		521.5
R		10156.7
D		40233.1
Sd
Eupeodes latifasciatus (Macq.) 54.4
Sd		15 20.9
Sr		570.9
Sr
Lapposyrphus (Eupeodes) lapponicus (Zett.) 10.4
Sr		510.1
Sr
Melanostoma mellinum (L.) 43.5
Sd		1063.7
Sd		2032.2
Sd		1041.8
R		10172.3
Sd
Melanostoma scalare (F.)87.0
D		2521.8
R		521.2
Sr		102014.9
Ed		3073.1
Sd		15395.2
D
Meligramma triangulifera (Zett.)21.7
R		5 42.4
Sd		10 41.8
R		10101.3
R
Meliscaeva cinctella (Zett.) 10.4
Sr		 10.1
Sr
Platycheirus albimanus (F.)32.6
Sd		15 42.4
Sd		15129.0
D		2531.3
R		10222.9
Sd
Platycheirus scutatus (Meig.)43.5
Sd		1065.3
D		15106.1
D		3043.0
Sd		 20.9
Sr		5263.5
Sd
Sphaerophoria scripta (L.)5951.3
Ed		751614.2
Ed		501911.6
Ed		5596.7
D		25219.3
D		6512416.5
Ed
Syrphus ribesii (L.)43.5
Sd		1032.7
Sd		10 32.2
Sd		152511.1
Ed		50354.7
Sd
Syrphus torvus (O.-S.) 10.6
Sr		5 135.8
D		25141.9
R
Syrphus vitripennis (Meig.)54.3
Sd		10108.8
D		30127.3
D		4096.7
D		252410.7
Ed		75608.0
D
Xanthogramma pedissequum (Harris) 10.9
Sr		5 10.7
Sr		5156.7
D		25172.3
Sd
No. of specimens115100 113100 164100 134100 225100 751100
No. of species11 11 13 12 19
Eudominants (Ed) > 10%, dominants (D) 5.1–10%, subdominants (Sd) 2.1–5%, recedents (R) 1.1–2%, subrecedents (Sr) < 1%. No.—number, %—percentage, f—frequency.
Table 6. Species similarity between sites—syrphids collected in traps/sweep net and from aphid colonies at sampling sites.
Table 6. Species similarity between sites—syrphids collected in traps/sweep net and from aphid colonies at sampling sites.
Site 1Site 2Site 3Site 4Site 5
Site 1x0.50.60.40.5
Site 20.8x0.70.60.5
Site 30.70.7x0.70.7
Site 40.90.90.8x0.6
Site 50.90.70.80.8x
(Black) traps/sweep net; (red) aphid colonies.
Table 7. Pearson correlation coefficients (p) between the number of adult syrphid specimens collected in traps/sweep net and syrphid larvae found in aphid colonies in different sites. Red color: significant positive correlation.
Table 7. Pearson correlation coefficients (p) between the number of adult syrphid specimens collected in traps/sweep net and syrphid larvae found in aphid colonies in different sites. Red color: significant positive correlation.
Site 1Site 2Site 3Site 4Site 5
a.c.Trapsa.c.Trapsa.c.Trapsa.c.Trapsa.c.Traps
Mean ± SD (N = 5)5.9 ± 1.211.5 ± 2.04.8 ± 0.911.3 ± 2.27.6 ± 1.416.4 ± 2.33.0 ± 0.813.4 ± 2.87.3 ± 1.922.5 ± 4.1
r20.2472410.2954750.3796070.4234260.405910
p0.0257080.0132410.0038200.0018910.002518
Table 8. Pearson correlation coefficients (p) between the number of adult syrphid specimens of E. balteatus collected in traps/sweep net and syrphid larvae found in aphid colonies in different sites. Red color: significant positive correlation.
Table 8. Pearson correlation coefficients (p) between the number of adult syrphid specimens of E. balteatus collected in traps/sweep net and syrphid larvae found in aphid colonies in different sites. Red color: significant positive correlation.
Site 1Site 2Site 3Site 4Site 5
a.c.Trapsa.c.Trapsa.c.Trapsa.c.Trapsa.c.Traps
Mean ± SD (N = 5)4.8 ± 3.14.8 ± 2.92.8 ± 1.412.4 ± 5.88.8 ± 6.519.2 ± 7.72.4 ± 2.112.6 ± 9.07.2 ± 6.0511.8 ± 5.5
r20.7454110.8539700.9646770.8269130.828358
p0.0593660.0248110.0028480.0323150.031896
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Wojciechowicz-Żytko, E.; Dobińska-Graczyk, M. Urban Green Space as a Reservoir of Predatory Syrphids (Diptera, Syrphidae) for Aphid Control in Cities. Agronomy 2025, 15, 953. https://doi.org/10.3390/agronomy15040953

AMA Style

Wojciechowicz-Żytko E, Dobińska-Graczyk M. Urban Green Space as a Reservoir of Predatory Syrphids (Diptera, Syrphidae) for Aphid Control in Cities. Agronomy. 2025; 15(4):953. https://doi.org/10.3390/agronomy15040953

Chicago/Turabian Style

Wojciechowicz-Żytko, Elżbieta, and Maja Dobińska-Graczyk. 2025. "Urban Green Space as a Reservoir of Predatory Syrphids (Diptera, Syrphidae) for Aphid Control in Cities" Agronomy 15, no. 4: 953. https://doi.org/10.3390/agronomy15040953

APA Style

Wojciechowicz-Żytko, E., & Dobińska-Graczyk, M. (2025). Urban Green Space as a Reservoir of Predatory Syrphids (Diptera, Syrphidae) for Aphid Control in Cities. Agronomy, 15(4), 953. https://doi.org/10.3390/agronomy15040953

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