Distribution of wild bee (Hymenoptera: Anthophila) and hoverfly (Diptera: Syrphidae) communities within farms undergoing ecological transition

Abstract Background In Havelange (Belgium), two farms are experiencing an ecological transition. We aimed to evaluate the impact of their agricultural activities on insect pollinator communities. This article depicts the situation at the very early stage of the farm transition. This study supports the fact that the maintenance of farm-level natural habitats provides environmental benefits, such as the conservation of two important pollinator communities: wild bees and hoverflies. New information Over two years (2018-2019), by using nets and coloured pan-traps, we collected 6301 bee and hoverfly specimens amongst contrasting habitats within two farmsteads undergoing ecological transition in Havelange (Belgium). We reported 101 bee species and morphospecies from 15 genera within six families and 31 hoverfly species and morphospecies from 18 genera. This list reinforces the national pollinator database by providing new distribution data for extinction-threatened species, such as Andrena schencki Morawitz 1866, Bombus campestris (Panzer 1801), Eucera longicornis (L.) and Halictus maculatus Smith 1848 or for data deficient species, such as A. semilaevis Pérez 1903, A. fulvata (Müller 1766), A. trimmerana (Kirby 1802) and Hylaeus brevicornis Nylander 1852.


Introduction
Nowadays, the greatest challenge faced by agriculture is to provide food for everyone, without altering the agro-biodiversity and the related ecosystem services (Dendoncker et al. 2018, Duru et al. 2015, Muller et al. 2017. Indeed, the worldwide intensification of agricultural systems has led to tragic biodiversity losses. During the last decades, many studies showed a strong impoverishment of insect pollinators in intensively-farmed landscapes. The depletion of these pollinators -and with them the ecosystem service of pollination -could have severe negative impacts on farmers and consumers welfare (Biesmeijer et al. 2006, Carvalheiro et al. 2013, Woodcock et al. 2019. The decrease in floral resources and the degradation of nesting sites is one of the main factors of decline (Goulson et al. 2015, Potts et al. 2010, Sánchez-Bayo and Wyckhuys 2019. In Belgium, in 2010, the insect-pollination was valuated at around 250 M€ (Jacquemin et al. 2017).
Agroecological farming systems grow crops on small areas, alongside heterogeneous habitats and complex arrangements (e.g. subdivision of plots by hedgerows, fallow areas, flower meadows etc.) that provide shelters and abundant food resources to beneficial insects (Power et al. 2012). Diversified habitats at the plot or at the farm spatial scale help to control pests, weeds and phytopathogens and provide other regulatory ecosystem services, such as pollination and preservation of nutrients and water in soils ).
The bee community (Hymenoptera: Anthophila) is amongst the most efficient pollinator groups in temperate agriculture landscapes. In Belgium, the latest inventory recorded 403 bee species, which represents almost one quarter of the European bee diversity (Drossart et al. 2019. Their morphological and behavioural traits co-evolved with flowering plants, allowing them to secure pollination (Michener 2007). The richness of bee morphologies, specialisation in pollen and nectar diets and sizes greatly supports an increase in yields in small-scale agricultural farms . Since the end of the 19th Century, Belgium has had great expertise in the monitoring of bees. Since the 70s, this survey has particularly accelerated through mapping, preservation and management of historical collections, taxonomic keys and revision of the Belgian fauna (Drossart et al. 2019).
Besides, the Diptera order represents one of the largest and most diverse groups in the pollinator community (Skevington and Dang 2002). Too often neglected, dipteran pollinators ensure the reproduction of many flowering plants (Rader et al. 2015, Ssymank et al. 2008. By consuming pollen and nectar, adult hoverflies (Syrphidae) play a pivotal role in the pollen transmission of over 70% of wildflowers (Doyle et al. 2020, Inouye et al. 2015. Hoverfly larvae exhibit a wide variety of feeding habits, including phytophagy, zoophagy, aphidophagy, saprophagy and mycophagy (Sommaggio 1999). As they cover a large spectrum of microhabitats (e.g. roots layer, herbs layer, dead wood, ponds...) (Speight et al. 2000), hoverfly larvae can be used as biological indicators to evaluate the conservation status of ecosystems (Burgio andSommaggio 2007, Sommaggio 1999). The widespread distribution of syrphids in temperate landscapes and the availability of excellent taxonomic keys for European species identification are also characteristics that promote syrphids as bio-indicators. Syrphids are very interesting organisms for studying the effects of agriculture intensification on biodiversity because they are particularly mobile (Gao et al. 2020). Moreover, hoverfly communities are strongly affected by the standardisation in landscape structures and by intensive agricultural practices (Dormann et al. 2007). In Belgium, 357 syrphid species were recorded according to the latest survey (Frank Van de Meutter, personal communication).
The impacts of agroecological transition on pollinator communities remain poorly documented. Such evaluation needs standardised and fine-scaled sampling efforts. Thus, the goal of this study is to provide a local and robust inventory of the bee and hoverfly fauna in two farms undergoing ecological transtion in Havelange County (Belgium). The general impacts of farm-scale landscape diversification on bee and hoverfly fauna are discussed. In future research, such inventory will allow an assessment of the impacts of regenerating agricultural landscapes on the pollinator community structure. Moreover, this study feeds in new records and new locations for the national repository of the wild bee and hoverfly communities, owned by the Laboratory of Functional and Evolutionary Entomology (Prof. Frédéric Francis), Gembloux Agro-Bio Tech and the Laboratory of Zoology (Prof. Pierre Rasmont), University of Mons.

Study site and habitats description
The study was conducted in two neighbouring agricultural sites, located in the Municipality of Havelange (Fig. 1A): the Froidefontaine and Emeville farmsteads (Fig. 1B). They are located at 2 km away from each other, in the geological region of Condroz, in Wallonia (Belgium), as defined by Dufrene and Legendre (1991).

The Froidefontaine farmstead
The Froidefontaine farm (50°23'6''N, 5°8'34.799''E) covers an area of 55 hectares, with a mosaic of varied habitats. One of the management objectives is diversifying the land use by conserving natural areas (mesophilic and wet meadows, limestone slopes, ponds...) and hosting different farming projects in a collaborative way on farming areas. Thus, the farm aims at creating a rich and welcoming landscape for diversity, including biodiversity.
Within the farm, we defined four adjacent habitats ( Fig. 2A; Table 1) covering about 10 ha each: a parcel of crops (GC) including a third of the surface with vegetable crops (GC1), a meadow zone (PAT), a young apple orchard (VER) and a wetland (ZH). The parcels were surrounded by hedges principally composed of hornbeam, elderberry, dogwood, hawthorn, maple and European charcoal.

The Emeville farmstead
The Emeville farm (50°23'2.4''N 5°10'1.199''E) covers an area of just over 40 ha. In 2016, the farm managers and a committee of various partners converted conventionallymanaged fields to agroecological farming methods. To allow a complexification of the ecological network and creating an agricultural landscape enriched with biodiversity, the first actions were: laying hedges and grass strips; planting rustic apple trees; breeding Angus cattle (Bos taurus taurus L.) in an orchard; alternating temporary and permanent meadows; arranging of flowered grass strips; using no pesticides and amendments. The sampling zone covered 15 ha and was divided into seven parcels ( Fig. 2B; Table 1), which included six parcels of crops separated by flower strips and one parcel of orchard. Each flower strip (BF1, BF2 and BF3; Fig. 2B) was composed of three plant mix sequences, including a combination of one "feeder" flower patch (BFV) and one "pollinator" flower patch (BFB), separated by the cover crop patch. The cover crop patch was composed of a grass mix of Festuca arundinacea Schreb 1771 and Dactylis glomerata L. 1753 sown at 20 kg/ha. The feeder flower patch was composed of a mix of 40% of clover (Trifolium pratense L. 1753) and 60% of alfalfa (Medicago sativa L. 1753) sown at 25 kg/ ha. In order to match Agri-Environmental and Climate Measures (AECM) specifications, the pollinator flower patch was sown at 30 kg/ha and was composed of a mix including 85% of grasses (  Habitats description of the sampled parcels and flower strips.

Collection methods
To assess wild bee and hoverfly diversity, we conducted standardised sampling methods by combining coloured pantraps and netting transects (Földesi and Kovács-Hostyánszki 2014, Westphal et al. 2008, Grundel et al. 2011. Sampling was performed in 2018 and 2019, from April to July. At each collection site ( Fig. 2A & B), we positioned a triplet of pantraps (FLORA model with a diameter of 26.5 cm, RINGOT, France) coloured with UV reflecting sprays in white, blue and yellow (ROCOL top tracer model, UK). The pantraps were set-up in line and spaced 3 to 5 metres apart, in order to avoid the attraction coverage bias and to reach the same probabilities of insect capture between the pantraps A. Froidefontaine farmstead map. GC, PAT, VER and ZH correspond to the sampled parcels, whose details are given in Table 1. Each numbered red dot corresponds to the position of a trio of coloured (white, yellow, blue) pantraps; B. Emeville farmstead map. PAV, FRE, EPI and DIK correspond to the sampled parcels, whose details are given in Table 1. Each numbered red dot corresponds to the position of a trio of coloured (white, yellow, blue) pantraps. BF1, BF2 and BF3 correspond to the sampled flower strips. Each blue or green numbered dot corresponds to the position of a trio of coloured (white, yellow, blue) pantraps for the "feeder" flower patch or the "pollinator" flower patch, respectively. (Amy et al. 2018, Droege et al. 2010. The pantrap triplets were separated by a minimum of 20 metres, in order to cover each parcel as homogeneously as possible (Carboni andLebuhn 2003, Eeraerts et al. 2017). Each pantrap was filled with odourless and colourless soapy water every two weeks during one day (from 9:00 AM to 5:00 PM). Every two weeks, we also conducted variable transects with an insect net for one hour in the morning and one hour in the afternoon, for each habitat in Froidefontaine and each flower strip in Emeville (Table 1; Fig. 2). We selected the sampling dates according to the following climatic conditions: temperature higher than 7°C, calm wind (< 12 km/h) and sunny and cloudless day (Westphal et al. 2008). We stocked insects in 70% ethanol for their conservation.
We followed the protocol of Mouret et al. (2007) to prepare, pin and label our collected specimens.
In 2019, we decided to let the yellow pantraps to be continuously activated from mid-May to the end of July with sampling every 10 days to maximise the capture of syrphids and considering that hoverflies have a predilection for the yellow colour , Wäckers and van Rijn 2012.

Species identification
Bee specimens were identified at the species level following identification keys of Pauly (2019) for Halictidae, Patiny and Terzo (2010) for Andrenidae and Falk (2015) for the other bee families (Apidae, Colletidae, Megachilidae and Melittidae). All Halicitidae and Andrenidae specimens were confirmed by Alain Pauly (Royal Belgian Institute of Natural Sciences) and Thomas James Wood (University of Mons), respectively. Other bee specimens were confirmed by the reference collections of Gembloux Agro-Bio Tech. Hoverfly specimens were identified at the species level using the identification key of Verlinden (1994). The specimens were then confirmed by Frédéric Francis (University of Liège) and the reference collections of Gembloux Agro-Bio Tech. We applied Belgian Red List of bees for the conservation status of identified species (Drossart et al. 2019).

Historical data of Havelange Municipality
Thanks to Data Fauna-Flora v.5.1 software (Barbier and Rasmont 2015), we queried the database of Belgian wild bees, on 26 June 2020, for the historical diversity of wild bees in the Havelange Municipality. The selected geographical quadrat was encompassed within latitude from 50°21'14.4''N to 50°24'46.8''N and in longitude from 5°7'12''E to 5°19'26.399''E. The syrphid historical data were not available for Havelange Municipality.

Statistical analysis
We conducted one-way ANOVA tests to compare species richness and abundance of bee and hoverfly fauna between sampled parcels of Froidefontaine and Emeville farmsteads, separately. We also validated normal distribution of residuals of each ANOVA test. Subsequently, Tukey's post-hoc tests were used to compare each parcel pair. We separated the flower strips of Emeville farm from the parcel comparisons because they were not sampled with the same effort as those of the sampled parcels. We compared the species richness and abundance of bee and hoverfly fauna between the feeder flower patch (BFV; Fig. 2B) and the pollinator flower patch (BFB; Fig. 2B) using the Student t-test. All statistical analysis were performed using R 4.0.2 (R Development Core Team 2020) and the resulting graphs were built using ggplot2 and ggpubr packages (Kassambara 2020, Wickham 2016).

Conservation status: Least Concern
Notes: Table 2 Nomada zonata Panzer 1798 Feeds on: Cuckoo bee

Feeds on: Cuckoo bee
Notes: Table 2 Hoverfly Checklist

Conservation status: Not Applicable
Notes: Table 2 24 Noel G et al

Conservation status: Not Applicable
Notes: Table 2 Analysis Collection results Over 2 years (2018-2019) of sampling, we collected 4,303 bees and 1,998 syrphids, representing 92 species and morphospecies from 15 genera and six families for the bees and 31 species and morphospecies from 18 genera for the hoverflies (Table 2). Polylectic, oligolectic and cuckoo bee species correspond to 61%, 14% and 25% of bee richness, respectively. However, the relative proportion of specialised bee (0.9%) was low, with polylectic and cuckoo bees corresponding to 94% and 5.1% in abundance of the total sampled bees, respectively (

Statistical analysis
For Froidefontaine farmstead, bee richness in VER was significantly higher than in GC (pvalue < 0.05; Fig. 3A) and bee abundance in PAT was significantly higher than in GC, VER and ZH (p-values < 0.05; Fig. 3B). Hoverfly diversity in ZH was significantly higher than in VER (p-value < 0.05; Fig. 3C), while hoverfly abundance was homogenous amongst the Froidefontaine parcels (Fig. 3D). For Emeville farmstead, bee and hoverfly richness and bee abundance did not vary amongst parcels (Fig. 4A, B and C), while DIK parcel exhibited significantly greater hoverfly abundance than EPI, FRE and PAV parcels (p-values < 0.05; Fig. 4D). Only for bee richness and hoverfly abundance, the pollinator flower patch BFB showed significantly higher mean values than the feeder flower patch BFV (p-values; Fig.  5A and D). Mean values of species richness and abundance for bee and hoverfly fauna amongst Froidefontaine parcels GC, PAT, VER and ZH (see details given in Table 1)  Mean values of species richness and abundance for bee and hoverfly fauna amongst Emeville parcels DIK, EPI, FRE and PAV (see details given in Table 1). A. Bee richness; B. Bee abundance; C. Hoverfly richness; D. Hoverfly abundance. Letters above the boxplots represent Tukey's post-hoc comparisons.
Both farms presented suitable habitats to these polylectic species, including open wooded spaces, fallow land or lawns. The abundance of Taraxacum spp. (Asteraceae), Salix spp. (Salicaceae), Craetegus spp. (Rosaceae) and fruit trees could explain the dominance of A. cineraria, A. haemorrhoa and A. flavipes populations. Moreover, they usually nest in southexposed sites, in bare soils or in areas with sparse and short vegetation (Falk 2015). The other common polylectic bees were mainly ground-nesting species belonging to Andrena and Lasioglossum genera, such as A. nitida, A. gravida, L. calceatum or L. lativentre (Table 2).
Uncommon polylectic bee species were also collected. For example, Andrena trimmerana and Halictus maculatus (Fig. 6C) are rarely observed in the Condroz Region and more Mean values of species richness and abundance for bee and hoverfly fauna amongst flower strips BFB and BFV (see details given in Table 1 largely in Belgium. H. maculatus is a little more common in Wallonia and this species is considered as "vulnerable" in Belgium, but "least concern" in Europe (Drossart et al. 2019, Nieto et al. 2014 Rarer species were observed within the farmsteads. Collected in the orchard of Froidefontaine, Andrena schencki (Fig. 6A) had not been observed south of the Sambre and Meuse Furrow for more than 30 years (Rasmont and Haubruge 2002). Andrena semilaevis, a very rare species since 1990 in Belgium (Rasmont and Haubruge 2002), was captured in the orchard of Emeville. This polylectic species is mostly observed on the umbellifers (Falk 2015). Forty-six specimens (1.12% of total sampling) of Andrena fulvata (Fig. 6B) were collected in 2019 in all habitats of both farms, while only one observation was encoded in Atlas Hymenoptera repository for Belgium (Rasmont and Haubruge 2002). That probably means a recent installation of the population on the study sites. However, misidentification due to their morphological resemblance to A. angustior could bias its Belgian rarity (T.J. Wood, personal communication). This species nests in calcareous soils and forages principally on Asteraceae flowers, such as Taraxacum spp. (Falk 2015).
The high diversity of wild bees in the two farms could be linked to the presence of seminatural habitats around the parcels. Indeed, the implantation of hedgerows, flower strips or shrubby strips between the habitats of both farms provides sufficient floral resources during the foraging activity period of polylectic species (Albrecht et al. 2020).
Two common species, A. praecox and A. vaga and two uncommon species, A. apicata and A. mitis, were collected in different parts of both farms (  (Rasmont and Haubruge 2002).
A single specimen of Melittidae family, Melitta leporina (Fig. 6D), was sampled. The female is particurlarly related to the flowers of M. sativa and T. pratense species (Fabaceae) (Dellicour and Michez 2010), which were abundantly present around the wetland of Froidefontaine Farm. One species of Colletidae family, Colletes daviesanus, forages pollen entirely from composite flowers such as tansy, mayweeds or oxeye daisy (Asteraceae) (Falk 2015).

Cuckoo bee species
We only collected two specimens of cuckoo bumble bees (subgenus Psithyrus Lepeletier), Bombus campestris and B. vestalis, in Froidefontaine wetland and in Froidefontaine orchard (

Hoverfly species
Within both farmsteads, Sphaerophoria scripta was, by far, the most abundant hoverfly species, followed by Eristalis tenax and Episyrphus balteatus, corresponding together to almost three quarters of the total number of collected specimens (Table 2). These species are the most common syrphids encountered in Central Europe (Alhmedi et al. 2010, Francuski et al. 2013, Nengel and Drescher 1990. Aphidophagous larvae of S. scripta and E. balteatus are important for pest control in agricultural systems, while E. tenax larvae recycle the organic matter in wet manures, muds or ponds (Sommaggio 1999). We also emphasised the presence of Melanostoma mellinum, which occured in almost each habitat and particularly in flower strips. Adults M. mellinum are specialised in the floral visitation of anemophilous plants ( Van der Groot and Grabandt 1970).
Beside these ubiquitous species, rarer species were found in only a few habitats: Xanthogramma pedissequum, Myathropa florea and Ferdinandea cuprea (Fig. 7). Unlike S. scripta and E. tenax, these species do not migrate. The larvae of X. pedissequum feed on aphids reared on the anthills of some Lasius sp. Fabricius 1804 (Hymenoptera: Formicidae) (Speight 2020). The species M. florea and F. cuprea present a microphagous larval stage. In intensified agricultural landscapes, it is conceivable that the environmental requirements of such species are scarcely fulfilled. Notably, microphagous species appear to be particularly sensitive to pesticides ). On the contrary, agricultural landscapes of Froidefontaine and Emeville Farms are suitable for these specialist species, because they include semi-natural ecosystems and organic orchards where cattle or sheep are grazing. We also identified two specimens of Platycheirus immarginatus that are specialist foragers on Bolboschoenus maritimus (L.) (Table 2) (Speight 2020). We did not find this plant species in Froidefontaine farmstead, meaning that P. immarginatus might forage on other plant species.
Continuous sampling represented only 4.33% of the total hoverfly specimens. However, it allowed us to reveal two more hoverfly species, in Emeville flower strips: Xylota sylvarum and X. segnis, whose larvae are saproxylic and live close to roots and dead wood (Speight et al. 2000).

Impact of agroecological practices on wild bees and hoverflies communities at the farm scale
By in-depth sampling, we documented new occurrences of almost 1/4 of Belgian bee fauna in two farms in ecological transition. For the historical region of the Municipality of Havelange, we have almost quintupled the richness of wild bees community despite high quality monitoring of these populations in Belgium (Drossart et al. 2019). There are few studies of this type in a close environment and with comparable methodology. Therefore, comparing our results with other studies seems to be of little relevance. This study leads us to consider that, on small areas undergoing ecological transition, an important richness of pollinators is easily reached. Moreover, it is possible that the conducted survey underestimates the real diversity per plot, even if the pattern of dominance rarity should be maintained. We also lack data at the end of the season, especially for late summer bees, such as Colletes hederae (Schmidt and Westrich 1993). For hoverflies, we still lack inventory data on the scale of the Belgian territory (Frank Van de Meutter, pers. comm.).
The practices on and around the studied farms seemed favourable to pollinators (Fig. 8) and especially to the polylectic species. In Froidefontaine Farm, the land tenure showed strong impact on bee richness and abundance by an alternation of floral bee-feeding parcels, like the Froidefontaine pasture (PAT; Fig. 3B) and bee-nesting parcels, like the Froidefontaine orchard (VER; Fig. 3A & Fig. 8B). On the one hand, late mowing permits the keeping of abundant floral resources throughout the bee activity period (Meyer et al. 2017) and, on the other hand, sheep grazing permits theconservation of some bare soil sites that favour ground-nesting bees (Cane 1991). Landscape micro-habitats, such as ponds, hedgerows or groves, are important to the survival of many pollinator species, especially by providing habitats for hoverfly larvae (Sommaggio 1999). The wetland of Froidefontaine (ZH) (Fig. 8A) harboured higher hoverfly diversity than the other parcels (Fig. 3C), with species like S. scripta, Cheilosia sp. and E. tenax, whose larvae have different diets (i.e. aphidophagous, phytophagous and microphagous, respectively) (Sommaggio 1999, Speight 2020). The cultivated parcel of Froidefontaine (GC) ( Table 2) and the pea crop of Emeville (DIK) (Fig. 4D) showed high abundances of aphidophagous hoverflies, likely caused by the high prevalence of aphids on crops. The flower strips separating the parcels of Emeville Farm consisted of a floral mix especially designed to fill the ecological requirements of bees and hoverflies (Fig. 8D). The floral composition of these flower strips attracted more hoverfly specimens than bees, which were mainly represented by A. mellifera (Table 2). Moreover, they were combined with belatedly-mowed hedges that support floral resources for pollinators throughout their activity season. Similarly, the hedgerows bordering the parcels of Froidefontaine (Fig. 8C), coupled with ecological crop management practices (i.e. no-till, no chemical inputs...), promoted the establishment of wild bee populations (Albrecht et al. 2020). Indeed, hedgerows and other semi-natural habitats usually represent superior floral richness and abundance compared to intensive agricultural land use (Hannon and Sisk 2009). According to the Belgian Red List of bees (Drossart et al. 2019), we have collected several species indexed in threatened categories from diverse habitats of both farms, especially in the orchard and in the wetland of Froidefontaine. These species were represented by one specimen of A. schencki, one specimen of B. campestris, one specimen of E. longicornis and nine specimens of H. maculatus. We also mitigated the data deficiency in Belgium for a few records of bee species, such as A. semilaevis, A. trimmerana and Hylaeus brevicornis (Fig. 6E). Taxonomically recent recognition, split from species complex and morphological similarity with widespread taxa or less studied genera (e.g. Hylaeus sp.) reflect current taxonomic impediments for 9.4% of the Belgian bee richness (Drossart et al. 2019).
Pollinator composition of each farmstead harboured both common and rare species, which indicates that on-farm diversification and organic practices may be an important refuge for rare, Red-Listed or oligolectic pollinator species (Guzman et al. 2019). Restoring or incorporating diverse habitats in agro-ecosystems is therefore a long-term solution for the conservation of pollinating species (Saint Clair et al. 2020).