domain: sdv sde

text:
Lola Serée
email: lolaseree@hotmail.fr

François Chiron

Muriel Valantin-Morison

Aude Barbottin

Antoine Gardarin

Flower strips, crop management and landscape composition effects on two aphid species and their natural enemies in faba bean

Keywords: Aphis fabae, Acyrthosiphon pisum, conservation biological control, Coccinellidae, Syrphidae, nectar resources

Wildflower strips and biodiversity-friendly management are promoted as ways of enhancing biodiversity and natural pest control in agroecosystems. Their efficacy for providing resources and shelters for natural enemies has been well studied, but little is known about how wildflower strip efficacy is mediated by local field management and landscape composition. In this study, we explored the effects of floral resources in perennial flower strips on faba bean aphids and their antagonists in contrasting cropping systems and landscapes. We surveyed 13 faba bean crops in northern France in 2019, and 19 in 2020. We sampled aphids (Aphis fabae and Acyrthosiphon pisum) and their antagonists (Syrphidae, Coccinellidae and mummy-forming parasitoids) on three dates during the spring, by visual observation. We assessed the floral resources provided by wildflower strips and spontaneous field margins and collected data on field management practices and land use in the surrounding landscape. We found that the percentage of flowering species in field edges providing accessible nectar to natural enemies had a positive effect on the abundance of Syrphidae and aphid mummies on faba bean plants. The positive effect of nectar resources on Syrphidae and aphid mummies decreased with increases in the proportion of semi-natural habitats in the landscape, but no interaction was observed with local management techniques. The growth rate of Aphis fabae and Acyrthosiphon pisum populations were unaffected by their antagonists. The growth rate of A. fabae populations was driven principally by local management practices (i.e. insecticide spraying and tillage). Conversely, the growth rate of A. pisum populations was driven principally by landscape composition (i.e. the proportion of semi-natural habitats and leguminous plants) and was negatively related to insecticide use. Nevertheless, insecticide spraying did not prevent increases in the growth rates of both aphid species, highlighting the need for alternative methods for controlling aphid populations.

INTRODUCTION

Chemical control to keep pest populations below damage thresholds in agriculture has its limitations. On the one hand, increasing numbers of pest species are becoming resistant to insecticides (e.g. Myzus persicae; [START_REF] Bass | The evolution of insecticide resistance in the peach potato aphid, Myzus persicae[END_REF], and on the other hand, many molecules cause (i) direct deleterious effects (mortality and sublethal effects) on natural enemies (predators and parasitoids), favouring subsequent pest outbreaks [START_REF] Cloyd | Indirect Effects of Pesticides on Natural Enemies[END_REF][START_REF] Desneux | The Sublethal Effects of Pesticides on Beneficial Arthropods[END_REF], and (ii) indirect deleterious effects on trophic networks and higher trophic levels feeding on pests, such as birds, by reducing trophic resources, and through the accumulation of pesticide residues [START_REF] Benton | Linking agricultural practice to insect and bird populations: a historical study over three decades: Farming, insect and bird populations[END_REF][START_REF] Chaiyarat | Bioaccumulation of organochlorine pesticides in the liver of birds from Boraphet wetland, Thailand[END_REF][START_REF] Hallmann | Declines in insectivorous birds are associated with high neonicotinoid concentrations[END_REF]. There is growing evidence to suggest that semi-natural habitats, by considering their proportion and heterogeneous spatial arrangement in agricultural landscapes, can enhance pest control [START_REF] Chaplin-Kramer | A meta-analysis of crop pest and natural enemy response to landscape complexity[END_REF][START_REF] Martin | The interplay of landscape composition and configuration: new pathways to manage functional biodiversity and agroecosystem services across Europe[END_REF][START_REF] Rusch | Agricultural landscape simplification reduces natural pest control: A quantitative synthesis[END_REF], although they may also favour the colonisation of fields by pests, such as aphids in winter wheat [START_REF] Alignier | The effect of semi-natural habitats on aphids and their natural enemies across spatial and temporal scales[END_REF].

Conservation biological control (CBC) aims to improve pest regulation through the conservation of natural enemies in agroecosystems [START_REF] Gontijo | Engineering natural enemy shelters to enhance conservation biological control in field crops[END_REF]. CBC involves habitat manipulation and agricultural practices designed to favour the development and survival of natural enemies [START_REF] Landis | Designing agricultural landscapes for biodiversity-based ecosystem services[END_REF][START_REF] Landis | Habitat Management to Conserve Natural Enemies of Arthropod Pests in Agriculture[END_REF]. Conflicting results have emerged from different studies on the response of natural enemies to landscape variables and the effect of natural enemies on pests. Further studies are required to determine how landscape effects can be modulated by local farm management (Karp et al., 2018). Cropping practices and field edge management are also important levers that can be used in the framework of agroecological crop protection [START_REF] Deguine | Agroecological Crop Protection[END_REF]. In this context, one alternative to chemical control is enhancing the regulation of crop pests by their natural enemies through conservation biological control.

Wildflower strips are a promising way to enhance biological control by providing nectar and pollen resources or shelters to a wide range of predators and parasitoids [START_REF] Bianchi | Sustainable pest regulation in agricultural landscapes: a review on landscape composition, biodiversity and natural pest control[END_REF][START_REF] Holland | The effectiveness of field margin enhancement for cereal aphid control by different natural enemy guilds[END_REF][START_REF] Pollier | Sowing of margin strips rich in floral resources improves herbivore control in adjacent crop fields: Floral resources improve herbivore control[END_REF]Tschumi et al., 2016a[START_REF] Tschumi | High effectiveness of tailored flower strips in reducing pests and crop plant damage[END_REF]. For instance, [START_REF] Hatt | Pest regulation and support of natural enemies in agriculture: Experimental evidence of within field wildflower strips[END_REF] reported the presence of significantly fewer aphids in wheat plots between wildflower strips than in control wheat plots. This result may reflect the larger amounts of adult hoverflies found in the presence of wildflower strips, whereas no significant difference in abundance was observed for adult lacewings, ladybirds and parasitoids [START_REF] Hatt | Pest regulation and support of natural enemies in agriculture: Experimental evidence of within field wildflower strips[END_REF]. In a recent meta-analysis, [START_REF] Albrecht | The effectiveness of flower strips and hedgerows on pest control, pollination services and crop yield: a quantitative synthesis[END_REF] found that flower strips enhanced pest control services in adjacent fields by 16% on average, but with a high degree of variability. This supports the assertion made by Begg et al. that "the greatest limitation to the development of effective CBC lies […] in its ability to deliver effective, reliable and robust biological control of pests" [START_REF] Begg | A functional overview of conservation biological control[END_REF]. There are several possible reasons for variability in the efficacy of flower strips to enhance pest control services.

First, it is the plant species composition of the flower strip, rather than its simple presence, that determines the level of provision of trophic resources and physical habitats for predators and parasitoids [START_REF] Balzan | Augmenting flower trait diversity in wildflower strips to optimise the conservation of arthropod functional groups for multiple agroecosystem services[END_REF][START_REF] Van Rijn | Nectar accessibility determines fitness, flower choice and abundance of hoverflies that provide natural pest control[END_REF]. Indeed, the effect of flower strips depends on their functional composition and structure, including the amount of available and accessible nectar they provide for predators [START_REF] Van Rijn | Nectar accessibility determines fitness, flower choice and abundance of hoverflies that provide natural pest control[END_REF]. Tschumi et al. (2016) reported that flower strips with compositions targeting particular species to favour biological control could reduce the number of aphids in adjacent potato crops by 75%.

Second, the efficacy of flower strips for CBC interacts with the landscape context, resulting in contrasting results. Wildflower strips have been reported to have a stronger effect on the abundance of Syrphidae and species richness further away from woodlands or in simplified landscapes (Haenke et al. 2009;Saunders and Luck 2018). By contrast, other studies have found that the addition of flower strips most strongly affects natural enemies within landscapes of intermediate (Grab et al. 2018) or high complexity in terms of the proportion of semi-natural habitat [START_REF] Woltz | Landscape structure and habitat management differentially influence insect natural enemies in an agricultural landscape[END_REF].

Third, agricultural practices would be expected to modulate the CBC response to flower strips. The interaction between landscape and agricultural practices has been studied [START_REF] Ricci | Local pesticide use intensity conditions landscape effects on biological pest control[END_REF], but interactions between flower strips and agricultural practices, in terms of their effects on the fauna and CBC, have yet to be investigated (but see Marshall and Moonen, 2002, mostly for plants). Some Coccinellidae and Syrphidae species overwinter in the soil (e.g. Coccinella septempunctata, [START_REF] Hodek | Ecology and Behaviour of the Ladybird Beetles (Coccinellidae)[END_REF]; Episyrphus balteatus, Sphaerophoria scripta [START_REF] Raymond | Immature hoverflies overwinter in cultivated fields and may significantly control aphid populations in autumn[END_REF]) and might therefore be disadvantaged by tillage. For these species, a perennial strip would provide a less disturbed complementary habitat with resources non-substitutable from those in different habitats [START_REF] Ganser | Sown wildflower strips as overwintering habitat for arthropods: Effective measure or ecological trap? Agriculture[END_REF][START_REF] Jeanneret | Agroecology landscapes[END_REF]. Moreover, some predators and parasitoids need pollen and nectar resources to extend their longevity and fecundity [START_REF] He | The effect of floral resources on predator longevity and fecundity: A systematic review and meta-analysis[END_REF]. Weeds can provide alternative food sources that play a role in pest control, for instance for omnivorous carabid beetles that also feed on aphids and slugs [START_REF] Petit | Weeds in agricultural landscapes. A review[END_REF] or could benefit to aphidophagous predators and parasitoids by providing alternative prey, nectar and pollen as a complementary resource [START_REF] Amaral | More than weed: non-crop plants, arthropod predators and conservation biological control. Mais do que plantas daninhas: plantas espontâneas, artrópodes predadores e controle biológico conservativo[END_REF][START_REF] Norris | Interactions between weeds, arthropod pests, and their natural enemies in managed ecosystems[END_REF]. Nevertheless, such resources may be lacking in cropping systems with high rates of tillage or herbicide application [START_REF] Marshall | The role of weeds in supporting biological diversity within crop fields*[END_REF]. In this context, flower strips may provide supplementary resources (i.e., providing substitutable, additional resources in similar, nearby patches). However, flower strips may also act as an ecological trap, depending on their management, with mowing or ploughing during the summer or winter [START_REF] Ganser | Sown wildflower strips as overwintering habitat for arthropods: Effective measure or ecological trap? Agriculture[END_REF][START_REF] Timberlake | Bumblebee colony density on farmland is influenced by late-summer nectar supply and garden cover[END_REF]. Insecticide sprayed on the field may also limit the benefits of predator spill over from the flower strips into the field. Thus, compensatory effects, synergies or antagonisms may occur between local resources within the flower strip and field management over the season. In summary, the many possible interactions between field management, flower strips and landscape context limit our ability to understand and develop efficient conservation biological control strategies.

The aim of our study was to evaluate how perennial wildflower strips, agricultural practices and their interactions with landscape context affect aphid control by predators and parasitoids in faba bean crops. Leguminous crops have several agronomic advantages (e.g. crop diversification, nitrogen input; [START_REF] Kumar | Management practices of antecedent leguminous and non-leguminous crop residues in relation to winter wheat yields, nitrogen uptake, soil nitrogen mineralization and simple nitrogen balance[END_REF][START_REF] Kumar | Role of Legumes in Soil Carbon Sequestration[END_REF], but are susceptible to pests, including the aphids Acyrthosiphon pisum (Harris, 1776) and Aphis fabae (Scopoli, 1763). A. fabae can reduce the growth of the plant [START_REF] Nuessly | INSECTS ASSOCIATED WITH FABA BEAN, VICIA FABA (FABALES: FABACEAE)[END_REF]. Its sucking activity can result in yield losses and even the death of the whole plant [START_REF] Völkl | Parasitism of the black bean aphid (Aphis fabae) by Lysiphlebus fabarum (Hym., Aphidiidae): the influence of host plant and habitat1[END_REF]. A. pisum causes similar damage on pea and faba bean, through both direct feeding damage and the transmission of mosaic viruses [START_REF] Nuessly | INSECTS ASSOCIATED WITH FABA BEAN, VICIA FABA (FABALES: FABACEAE)[END_REF], such as cucumovirus in snap bean [START_REF] Nault | Seasonal and Spatial Dynamics of Alate Aphid Dispersal in Snap Bean Fields in Proximity to Alfalfa and Implications for Virus Management[END_REF], broad bean wilt virus [START_REF] Ferriol | Transmissibility of Broad bean wilt virus 1 by aphids: influence of virus accumulation in plants, virus genotype and aphid species[END_REF] or pea enation mosaic virus [START_REF] Lee | Responses to predation risk cues and alarm pheromones affect plant virus transmission by an aphid vector[END_REF]. It can affect all stages of the crop [START_REF] Stoddard | Integrated pest management in faba bean[END_REF]. Cereal aphids have been the focus of many CBC studies [START_REF] Hatt | Pest regulation and support of natural enemies in agriculture: Experimental evidence of within field wildflower strips[END_REF][START_REF] Holland | The effectiveness of field margin enhancement for cereal aphid control by different natural enemy guilds[END_REF]Tschumi et al., 2016b), but few studies have investigated legume crops, in which the effect of natural enemies is generally weaker, perhaps due to a high concentration of nitrogen in tissues leading to a more rapid growth of aphid populations [START_REF] Diehl | Effects of predator specialization, host plant and climate on biological control of aphids by natural enemies: a meta-analysis[END_REF].

In this study, we explored the effects of floral resources in perennial flower strips on faba bean aphids and their antagonists in contrasting cropping systems and landscapes. We first hypothesised that the number of aphid antagonists (Coccinellidae, Syrphidae and mummy-inducing parasitoids) would be affected positively by flower resources and negatively by insecticides and soil tillage in faba bean fields. We also expected to see a negative effect of both aphid antagonists and disturbances (insecticides and tillage) on the growth rate of aphid populations. We hypothesised that wildflower strips would have a stronger effect in less disturbed fields (i.e. those with few insecticide applications or tillage operations). Finally, we expected the effect of flower resources to be weaker in landscapes with a higher proportion of semi-natural habitats than in homogeneous landscapes with few semi-natural habitats, due to a low contrast induced by the introduction of a wildflower strip in an environment already rich in plant resources [START_REF] Scheper | Local and landscape-level floral resources explain effects of wildflower strips on wild bees across four European countries[END_REF].

METHODS

Study sites

We studied 13 faba bean fields in 2019 and 18 in 2020. All these fields were located in northern France, a region dominated by the cultivation of arable crops (mostly cereals and oilseed rape) in open field landscapes. We selected faba bean fields to cover a large gradient of resources provided by dicotyledonous species in their immediate surroundings. We chose 22 fields bordered by a spontaneous margin and we included nine fields (the maximum we found in the study area) in which farmers had sown wildflower strips in 2018 within their fields (each approximately 4 m x 500 m) to extent the gradient of floral resources. We studied this continuous gradient rather than the presence/absence of the flower strips. Each flower strip was sown with a mixture of 42 native, mostly perennial species, for a total sowing density at 13 kg/ha (See Table S2 for details per species). The plant species were selected so as to provide physical habitats and diverse trophic resources to arthropods throughout the year [START_REF] Gardarin | Understanding plant-arthropod interactions in multitrophic communities to improve conservation biological control: useful traits and metrics[END_REF]. The mixture of species sown was selected on the basis of the results of a previous analytical experiment comparing the effects of plant assemblages contrasting in terms of their specific and functional diversity (Gardarin et al., submitted). These monitored strips (sown or naturally occurring) were located at the edges of fields subjected to various combinations of tillage and insecticide use practices (See section 2.4; Figure S1; Table S1). Sixteen fields were cropped with winter faba bean varieties, and 15 were cropped with spring faba bean varieties.

Invertebrate sampling: temporal dynamics of aphids and their antagonists

Aphids and aphid mummies were counted three times fortnightly between May and June, to estimate population dynamics and biological regulation. The sampling period encompassed the period after A. fabae colonisation, and the periods of exponential colony development and regression. Counts on spring faba bean varieties took place two weeks later than those on winter faba bean varieties, according to the phenological stage of the crop. Visual observations of invertebrates were performed 5 and 30 m from the wildflower strips or the field edge, on 30 plants randomly chosen along a 50 m row parallel to the field edge. We recorded the number of each species of aphid and the number of mummies on each plant. If the number of A. fabae visibly exceeded 200, the size of the colony was estimated by counting the number of aphids per centimetre and multiplying by the length of the colony and the number of colonised stem faces. We considered as mummies of A. pisum those having a golden aspect, of the same morphology as A. pisum and located on the lower strata of the vegetation (like their host). No mummy with the morphology of A. fabae and located at the apex of the plants (like A. fabae colonies) has been observed. The numbers of Syrphidae and Coccinellidae present on the plant were counted at the same time, according to their stage of development (i.e. larva, pupa, and adult for Coccinellidae and larva and pupa for Syrphidae). We also noted the other predators, from the Araneae, Chrysopidae, Formicidae and Heteroptera, for example, seen on plants. At each date, for each predatory taxon (e.g. Coccinellidae, Syrphidae) we summed the abundances for the 30 plants per distance for each field.

For each species, the growth rate r of aphid populations between two successive counts (hereafter referred to as 'T1-T2' for growth of the population between the first two counts and 'T2-T3' for the growth of the population between the last two counts) was calculated as follows:

Where N t = the number of aphids at date t, N t+1 = the number of aphids at date t+1 and ∆t = the number of days between two observations.

Floristic survey to determine the nectar resources at the edge of the field

Botanical surveys were performed during the second aphid count, to assess the nectar resources provided to predators by sown wildflower strips and the spontaneous vegetation of the field margin (always herbaceous). The vegetation was assessed in 20 plots of 1 m 2 each, distributed along a 40 m transect (2 m between each plot), resulting in 20 m 2 of surface covered. In each plot, we attributed a cover class to each dicotyledonous species (as a putative nectar-producing resource) according to its percent cover (Class 1: 1% or less; Class 2: 2-10%; Class 3: 11-30%: Class 4: 31-75%; Class 5: 76-100%) and we noted the phenological stage of each species (vegetative, flowering, fruiting). We then calculated the cover of flowering plants (%) providing accessible nectar for predatory Syrphidae based on a morphological model coupling the size of the flower and the size of the head and mouthparts of Syrphidae, compiled for Episyrphus balteatus, Eupeodes corollae, Sphaerophoria scripta and Syrphus ribesii species (See details in Supplementary Materials Appendix B, Table S3 and Figure S2). This approach was not extended to the other predators as we lack knowledge on how they reach flower resources and we lack data on their feeding mouthparts. Coccinellidae and Chrysopidae adults, which can feed on aphids, should be though less dependent on nectar resources than Syrphidae. Similarly, we did not identify the parasitoids responsible of the mummies we observed, preventing us from extending our morphological trait-matching approach to parasitoids.

Description of cropping techniques and of the landscape

We noted the cropping techniques applied to each sampled field during interviews with farmers in 2019 and 2020. We collected detailed information about tillage operations, from soil preparation for sowing until harvest (e.g., ploughing, ploughing in of stubble, hoeing), and information about the insecticides sprayed on the crop (product name, date, dose and area treated). Based on this information, we calculated two continuous variables: the number of tillage operations carried out during the growing season and a treatment frequency index for insecticides (TFI: number of reference doses applied per hectare, [START_REF] Sattler | Assessing the intensity of pesticide use in agriculture[END_REF]. Seven fields were cropped with no insecticide and no tillage, 19 fields were cropped with no insecticide but with one to six tillage operations, four fields were cropped with both insecticides (from 1 to 2.25 applications per hectare) and tillage management (from 1 to 5 operations) and one field was cropped without tillage but with insecticide treatment (Supplementary Materials Appendix A, Table S1). All insecticide treatments have been realized before the first count.

r = "# $ % & ' &' ) "# ($ % &')

△%

The land cover and land use about 500 m from the edge of each field was digitised from the "BD-Ortho 2014" (IGN: Institut National de l'Information Géographique et Forestière) in QGIS 2.18 and coupled to landscape surveys. We focused on a relatively small scale because we were interested in shortdistance flights for foraging, the type of dispersal relevant for biocontrol in crop fields, rather than migratory flights, which take place over longer distances [START_REF] Woltz | Landscape structure and habitat management differentially influence insect natural enemies in an agricultural landscape[END_REF]. We calculated the proportion of semi-natural elements (including forest, bushes, hedges, grassland, fallow and other ecological areas, such as wildflower strips; Median=15.32% ± 16.89 SD; Min-Max=0.24 -69.48%), and the proportion of leguminous crops (faba bean, pea and lentil) around each field.

We calculated the areal proportion of fields under organic farming in the surrounding environment (500 m) based on a national database updated in 2019 by the French authorities (https://cartobio.org). Indeed, previous studies highlighted that the presence of organic fields in the neighborhood of the surveyed fields decreased the number of aphids in both organic and conventional fields [START_REF] Gosme | Local and neighbourhood effects of organic and conventional wheat management on aphids, weeds, and foliar diseases[END_REF]. In addition, organic farming enhances biodiversity and biological control at a local scale [START_REF] Muneret | Evidence that organic farming promotes pest control[END_REF] and we could expect the same effect at a landscape scale. The areal proportion of fields under organic farming in the surrounding environment varied from 0% to 74.41% (Median=21.49% ± 22.17 SD).

Data analyses

Response of aphid antagonists to local and landscape variables

We assessed the effect of local and landscape variables on the abundance of predators sensu lato, i.e. Syrphidae (i.e. larvae and pupae), Coccinellidae (i.e. larvae, pupae and adults) and mummies of A. pisum. Araneae, Chrysopidae, Formicidae and Heteroptera could not be taken into account because their abundances were too low. We ran generalised linear mixed models based on maximum likelihood estimation and assuming a negative binomial distribution, "nbinom1" for Syrphidae and mummies, and "nbinom2" for Coccinellidae, with 'Template Model Builder' (glmmTMB package, Magnusson et al., 2020). This function provides two parameterisations of the negative binomial that differ in terms of the dependence of the variance (σ2) on the mean (µ). For family = nbinom1, the variance increases linearly with the mean as σ2 = µ(1 + α), with α > 0; for family = nbinom2, the variance increases quadratically with the mean as σ2 = µ(1 + µ/θ), with θ > 0 [START_REF] Brooks | glmmTMB Balances Speed and Flexibility Among Packages for Zeroinflated Generalized Linear Mixed Modeling[END_REF]. The parameter chosen for model adjustment (nbinom1 or nbinom2) was that giving the lowest AIC. The probability of producing a structural zero was modelled as equal for all observations with ziformula = ~ 1 [START_REF] Brooks | glmmTMB Balances Speed and Flexibility Among Packages for Zeroinflated Generalized Linear Mixed Modeling[END_REF]. As explanatory fixed-effect variables, we included the cover of plants providing accessible nectar in the field edge (spontaneous vegetation or wildflower strip), the number of tillage operations, the TFI for insecticides, the distance to the field edge (5 or 30 m), the total number of aphid individuals, the date of the visit (i.e. which count), the type of variety (winter or spring faba bean), the year (2019 or 2020), the areal proportion of semi-natural habitats, leguminous and organic fields in the landscape. We investigated interactions between local resources and local management or landscape context, by testing the first-order interactions between flower resources and (i) the number of tillage operations, the TFI for insecticides, the type of variety and (ii) proportion of semi-natural habitats. We considered the spatial and temporal dependence of observations between counting dates, by introducing field name as a random-effect variable. All variables were standardized to the same scale (i.e. by subtracting the mean and dividing by the standard deviation).

Response of aphid growth rate to antagonists, local and landscape variables

We analysed the growth rate of A. fabae and A. pisum separately, as a function of their predators and of local and landscape variables. We ran linear mixed models by maximum likelihood estimation and assuming a Gaussian distribution with the 'lme' function (nlme package, Pinheiro et al., 2017). We analysed each counting period (T1-T2 and T2-T3) separately. We introduced, as explanatory fixedeffect variables, the mean number of Syrphidae and Coccinellidae between two successive counts, the rate of parasitism for the A. pisum growth rate model, the cover of plants providing accessible nectar at the edge of the field, the TFI for insecticides, the number of tillage operations, the proportion of seminatural habitat or leguminous crops and the proportion of organic fields. We also accounted for the distance to the field edge, the number of the other aphid species (e.g., A. pisum abundance was included as explanatory variable for A. fabae growth rate, and conversely, because each species could be considered as alternative prey for aphidophagous predators and decrease predation on the focal species), the type of variety (winter or spring faba bean) and the year (2019 or 2020). We assessed the effects of local resources and management on biological control by natural enemies, by testing the first-order interactions between aphid antagonist and (i) flower resources, (ii) the number of tillage operations, (iii) the insecticide TFI and (iv) type of variety. We accounted for the spatial and temporal dependence of observations between counting dates, by introducing field name as a random-effect variable.

All variables were standardized (i.e. by subtracting the mean and dividing by the standard deviation).

Correlations between covariates in full models were assessed with variance inflation factors (all VIF < 5; [START_REF] James | An Introduction to Statistical Learning[END_REF]). All models were then simplified by a multi-model inference approach [START_REF] Grueber | Multimodel inference in ecology and evolution: challenges and solutions[END_REF][START_REF] Harrison | A brief introduction to mixed effects modelling and multi-model inference in ecology[END_REF]. Models were ranked according to Akaike's information criterion (AICc; [START_REF] Burnham | A practical information-theoretic approach. Model selection and multimodel inference[END_REF], which can be used to select the most parsimonious model from a set of candidate models. In the case of AICc differences of less than 2 with respect to the best models [START_REF] Grueber | Multimodel inference in ecology and evolution: challenges and solutions[END_REF], we used a model-averaging procedure to account for the uncertainty of model selection, and likelihood theory to produce parameter estimates with the 'MuMIn' package (Barton & Barton, 2015). Diagnostic residual plots of all full models confirmed a good fit for all. Spatial correlation in the residuals was checked with the 'bubble' function from the 'sp' package and with Correlogram (package 'ncf'), and no spatial correlation was detected. All analyses were performed with a significance threshold of 5 % in R statistical software version 3.5. 2 (2018-12-20).

RESULTS

In total, 88 dicotyledonous plant species flowered during the vegetation surveys, and the trait matching approach indicated that only 55 produced nectar accessible to Syrphidae (Table S4). Wildflower strips provided a significantly larger cover of flowering plants (mean=39.7 % ± 26.6 SD) and a larger cover of plants providing accessible nectar (31.5 % ± 25.4%) than spontaneous field margins (total resources: 17.0 ± 16.0 %; accessible nectar: 12.3 % ± 11.6 %; Fig. S3).

The abundance (mean number per field) of aphid antagonists was low at the beginning of the season. The abundance of Coccinellidae increased over time from 0.80 ± 1.32 individuals at T1 to 3.20 ± 5.05 at T2, and to 13.11 ± 22.11 at T3. The abundance of Syrphidae and mummies increased between T1 and T2, from 0.39 ± 1.11 individuals to 3.71 ± 3.86 individuals, and from 1.34 ± 3.38 mummies to 7.03 ± 14.87 mummies, decreasing slightly thereafter, between T2 and T3, to 3.03 ± 3.94 individuals for Syrphidae and 6.44 ± 9.73 mummies (Fig. S4).

The growth rates of A. fabae (0.10 ± 0.10 aphid•aphid -1 •day -1 ) and A. pisum (0.06 ± 0.12) populations were positive during the first period (between T1 and T2). During the second period (T2-T3), A. fabae populations decreased strongly in size, with a negative growth rate (-0.19 ± 0.21), whereas A. pisum populations neither grew nor decreased in size (0.00 ± 0.10) (Fig. 1).

Response of aphid antagonists to local and landscape variables

At field level, the abundances of Syrphidae, Coccinellidae and A. pisum mummies were positively related to the number of prey (Table 1). Agricultural practices had no significant effect on aphid antagonists other than a lower abundance of Syrphidae in spring faba bean than in winter faba bean crops and a higher abundance of Coccinellidae with a higher number of tillage (Table 1).

The abundances of Syrphidae and mummies were positively related to the cover of flowering species providing accessible nectar at the field edge (Table 1; Fig. 2), whereas no such relationship was observed for Coccinellidae. Interestingly, this local resource effect interacted with landscape composition. The positive effect of nectar resources on the abundance of Syrphidae and mummies weakened with increasing semi-natural habitat cover (in a 500 m landscape buffer zone; Table 1; Fig. 2). The abundance of Coccinellidae was not related to flower resources alone, but the effect of nectar resources weakened with increasing insecticides TFI (Table 1).

Response of aphid population growth rate to antagonists, local and landscape variables

Population growth rate of aphids was not related to the percent cover of plants providing accessible resources at the field edge or the number of predators, for either of the aphid species considered.

During the first period, the growth rate of A. fabae populations displayed a weak negative relationship to insecticide TFI (Table 2), with no significant effect of other variables. During the same period, the growth rate of A. pisum populations was also negatively related to insecticide TFI, but increased with the proportion of semi-natural habitat in the surrounding landscape (Table 2).

During the second period, the growth rate of A. fabae and A. pisum populations increased with the number of aphids of the other species (Table 2). The growth rate of A. fabae populations displayed a weak positive relationship to insecticide TFI and was negatively related to the number of tillage operations (Table 2), with no effect of landscape variables. The growth rate of A. pisum populations was negatively related to the proportion of semi-natural habitats and increased with the proportion of legume crops in the landscape (Table 2).

DISCUSSION

Our results show that increasing the percentage of flowering species with accessible nectar at the field scale by sowing wildflower strips in faba bean crops increases the abundances of Syrphidae and mummies. This positive effect of nectar resources decreased with the proportion of semi-natural habitats in the surrounding landscape, without interaction with local management practices. Nevertheless, this effect was not translated into greater regulation of the aphid population, because neither aphid predator levels nor aphid parasitism rates had any effect on aphid population growth rates at both sampling time.

Responses of aphid antagonists to local and landscape variables

Contrary to our initial hypothesis, insecticide use and tillage frequency had no significant effect on the abundance of Syrphidae or on the number of A. pisum mummies. The abundance of Coccinellidae was however positively affected by the tillage frequency and not affected by insecticide use. Ladybird first and second instars are known to be very sensitive to insecticides, but this susceptibility depends on species and type of insecticide [START_REF] Afza | Effect of Intra-Guild Predation and Sub Lethal Concentrations of Insecticides on the Predation of Coccinellids[END_REF]. The insecticides used in these fields were mostly pyrethroids, rapid-action contact pesticides that are unlikely to prevent recolonization by insects after treatment. The positive effect of tillage frequency on the mean number of Coccinellidae per field could be due to an effect of organic farming. Indeed, in our study (Figure S1), and more generally [START_REF] Puech | Organic vs. conventional farming dichotomy: Does it make sense for natural enemies? Agriculture[END_REF], organic farming relies on a higher frequency of ploughing and other tillage methods than conventional ones. The lack of synthetic pesticides seems to predominate over the increased tillage intensity, explaining why organic cropping systems harbour a wide range of natural enemies, such as Coccinellidae [START_REF] Makwela | Biodiversity of predatory beetle groups, carabidae and coccinellidae and their role as bioindicators in wheat agroecosystems[END_REF][START_REF] Poupelin | Coccinellidae)[END_REF]. The absence of a negative effect of tillage on Syrphidae and number of mummies suggests that these insects did not overwinter in the field and that their populations colonised the crop mostly from the surrounding landscape.

The abundances of Coccinellidae, Syrphidae and mummies increased with increasing aphid abundance, suggesting a prevailing bottom-up effect. Indeed, the principal natural enemies were aphidophagous and gravid Syrphidae, which usually oviposit close to their prey because first-instar larvae have a limited dispersal ability [START_REF] Almohamad | Searching and oviposition behavior of aphidophagous hoverflies (Diptera: Syrphidae): a review[END_REF]. Plants providing accessible nectar resources increased the abundance of Syrphidae and mummies, but were not related to the abundance of Coccinellidae, as shown in several other studies [START_REF] Hatt | Pest regulation and support of natural enemies in agriculture: Experimental evidence of within field wildflower strips[END_REF][START_REF] Ramsden | Optimizing field margins for biocontrol services: The relative role of aphid abundance, annual floral resources, and overwinter habitat in enhancing aphid natural enemies[END_REF]. Adult insects require nectar, in addition to pollen, to enhance survival and reproduction [START_REF] Lundgren | Nutritional aspects of non-prey foods in the life histories of predaceous Coccinellidae[END_REF][START_REF] Rodríguez-Gasol | The ecology of predatory hoverflies as ecosystem-service providers in agricultural systems[END_REF].

Hoverflies can make use of other resources (e.g. the extra-floral nectar of faba bean crops, aphid honeydew, flower pollen), but we have shown that nectar is generally a limiting resource that can be increased by planting wildflower strips. The lack of effect on the abundance of Coccinellidae may reflect a lower requirement for flower resources at the adult stage, because adults can also feed on prey, such as aphids [START_REF] Weber | Assessing the trophic ecology of the Coccinellidae: Their roles as predators and as prey[END_REF]. Floral resources may become important for ladybirds when prey is scarce [START_REF] Lundgren | Nutritional aspects of non-prey foods in the life histories of predaceous Coccinellidae[END_REF], notably when individuals emerge and search habitats for mating and ovipositing. Flower resources alone are not nutritionally and metabolically sufficient for ovigenesis of most predatory ladybirds. However, combined with prey food (which is essential food for ladybird as they allow to reproduce them; Hodek, 1996), they accelerate oviposition [START_REF] Hatt | The role of Perilla frutescens flowers on fitness traits of the ladybird beetle Harmonia axyridis[END_REF]. In this study, aphids were abundant (mean of 607 aphids per ladybird), potentially accounting for the lack of attractiveness of flowers to these insects. Our results showed that the number of mummies increased with the amount of available nectar resources, both at 5 m and 30 m. First of all, it is interesting to find a relationship to both distances in the field, since previous studies found decreasing levels of aphid parasitism with longer distance to flower strip [START_REF] Tylianakis | Improved Fitness of Aphid Parasitoids Receiving Resource Subsidies[END_REF]. In addition, like hoverflies, parasitoids can also feed on other sugar resources such as honeydew [START_REF] Gillespie | Beyond nectar provision: the other resource requirements of parasitoid biological control agents[END_REF]. Therefore, the positive effect of the amount of nectar resources on the number of aphid mummies reinforces the importance of nectar provided by spontaneous or sown vegetation in the strips.

There was no interaction between the effect of flower resources and of local management on Syrphidae and mummies, suggesting that neither antagonism nor synergy occurred. For Coccinellidae however, a negative interaction was found between nectar resources and insecticide treatments. We do not think this result is robust because these variables had no significant effects alone (with additive effects only). Wildflower strips may, therefore, be valuable for predators and parasitoids in all cropping systems.

Interestingly, the positive effect of flower resources on the abundance of mummies and Syrphidae was smaller in landscapes containing higher proportions of semi-natural habitats than those with a low proportion. Highly complex landscapes may indeed benefit little from the addition of perennial habitats, because these areas already contain rich and abundant communities of beneficial insects such as natural enemies [START_REF] Isaacs | Maximizing arthropod-mediated ecosystem services in agricultural landscapes: the role of native plants[END_REF]. Contrary to the intermediate landscape-complexity hypothesis [START_REF] Tscharntke | Landscape moderation of biodiversity patterns and processes -eight hypotheses[END_REF], the effect of nectar resources was the strongest in simple landscapes in our study, where flower created the largest ecological contrast with the surrounding landscape [START_REF] Scheper | Local and landscape-level floral resources explain effects of wildflower strips on wild bees across four European countries[END_REF]. Moreover, we studied arthropods being mobile enough to recolonise quickly the studied fields, even in simple landscapes.

Effects of cropping techniques, aphid antagonists and landscape variables on the growth rate of aphid populations

As expected, the growth rate of A. fabae and A. pisum populations were influenced by cropping techniques, but A. pisum populations also responded to landscape composition. In addition, their responses to local and landscape variables change between sampling time.

Insecticides, which were all applied early, before the first count, had a negative effect on the growth rate of A. fabae and A. pisum populations during the first sampling time, although this growth rate remained positive (i.e. population size increased). However, the effect of insecticide treatment frequency index (TFI) on A. fabae populations was positive during the second period. Thus, insecticide treatments controlled aphid populations just after their application, but did not prevent the aphid population from growing again afterwards. The negative effect of tillage frequency on the growth rate of A. fabae populations could be linked to an organic cropping system effect. Indeed, biological control is often higher in organic than in conventional cropping systems [START_REF] Muneret | Evidence that organic farming promotes pest control[END_REF] despite soil disturbance caused by a higher tillage frequency in organic cropping systems [START_REF] Puech | Organic vs. conventional farming dichotomy: Does it make sense for natural enemies? Agriculture[END_REF]. We moreover found that a low insecticide TFI was associated with a reduced aphid growth rate during the second sampling time (T2-T3).

However, contrary to our expectations, we detected no direct top-down effect of the abundance of Coccinellidae or Syrphidae on the dynamics of A. fabae or A. pisum populations, despite the many reports of effective biological control by these aphidophagous predators [START_REF] Thies | The relationship between agricultural intensification and biological control: experimental tests across Europe[END_REF]Tschumi et al., 2016b). We observed a bottom-up effect of aphids on their predators, but the numbers of these specialist predators over the growing season probably remained too small to prevent aphid population outbreaks (mean of 408 aphids per predator; [START_REF] Hasken | Effects of different intensities of fertilisers and pesticides on aphids and aphid predators in winter wheat[END_REF]. Indeed, mostly studies have reported effective biological control of aphids by aphidophagous predators on wheat [START_REF] Thies | The relationship between agricultural intensification and biological control: experimental tests across Europe[END_REF], on which the number of aphids is generally smaller than on legume crops because A. fabae, unlike Rhopalosiphum padi or Sitobion avenae on cereals, forms colonies of numerous aphids in a sleeve around the stems [START_REF] Ighil | Les pucerons des grandes cultures: cycles biologiques et activités de[END_REF]). In addition, aphids feeding on legume crops achieve a higher nutritional status than those on other crops, and this may enhance the growth and survival of aphid populations [START_REF] Emden | Aphids as crop pests[END_REF]. We detected no effect of the rate of parasitism on the growth rate of A. pisum populations. According to Thies et al. (2005), parasitoids would be able to control aphid populations only at low aphid densities, which was not the case here.

We also found that the growth rates of A. pisum and A. fabae populations late in the season increased with the number of aphids of the other species. This simultaneous increase in both populations may dilute the biological control effect due to the large increase in prey numbers, with any increase in one population favouring a decrease in predation on the other aphid population [START_REF] Sánchez-Hernández | Alternative extraguild prey modifies focal extraguild prey consumption and parasitism but not intraguild predation intensity[END_REF].

Only A. pisum responded to landscape composition in our study. The growth rate of populations of this species increased with the proportion of semi-natural habitat, including fallow and grassland in the early part of the season, and with the proportion of legumes (i.e. mostly lentil, faba bean and pea) later in the season. Fallow and grassland may contain leguminous species (e.g. Trifolium spp., Medicago spp.) that are host plants for A. pisum, like cultivated legume species [START_REF] Kordan | Susceptibility of forage legumes to infestation by the pea aphid Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae)[END_REF][START_REF] Wale | Biology of the Pea Aphid, Acyrthosiphon Pisum (Harris) (Homoptera: Aphididae) on Cool-Season Legumes[END_REF]. Nevertheless, later in the season, the growth rate of A. pisum populations decreased with the proportion of semi-natural habitat, possibly due to an indirect positive effect of their natural enemies, leading to biological control. We found no relationship between landscape composition and aphid antagonist populations, but previous studies have reported beneficial effects of the proportion of semi-natural habitat on the biodiversity of predators (vertebrates or invertebrates) and pest control in the field [START_REF] Chaplin-Kramer | A meta-analysis of crop pest and natural enemy response to landscape complexity[END_REF][START_REF] Veres | Does landscape composition affect pest abundance and their control by natural enemies? A review[END_REF][START_REF] Rusch | Agricultural landscape simplification reduces natural pest control: A quantitative synthesis[END_REF][START_REF] Holland | Semi-natural habitats support biological control, pollination and soil conservation in Europe. A review[END_REF]; but see Karp et al. 2018).

Although our field sampling extended over a relatively large number of farms, a limit of our study is the number and frequency of count that probably was too low to capture the accurate temporal dynamics of aphids and how it is affected by the predators and by parasitoids. We also did not take into account the very beginning of the dynamics of potential aphids and the potential effects of generalist predators such as carabids and spiders [START_REF] Thies | The relationship between agricultural intensification and biological control: experimental tests across Europe[END_REF], which also respond to flower strips and cropping systems [START_REF] Ganser | Sown wildflower strips as overwintering habitat for arthropods: Effective measure or ecological trap? Agriculture[END_REF][START_REF] Lys | Augmentation of beneficial arthropods by strip-management[END_REF][START_REF] Tamburini | Conservation tillage mitigates the negative effect of landscape simplification on biological control[END_REF][START_REF] Tuck | Land-use intensity and the effects of organic farming on biodiversity: a hierarchical meta-analysis[END_REF]. We thus recommend increasing the number of visits and starting earlier the sampling in next studies.

Application for management

Our results support the idea that, over and above the existing effects of landscape composition, the biological control of aphids can be managed through cropping practices in the field (e.g. by managing insecticide applications) and by planting flower strips [START_REF] Begg | A functional overview of conservation biological control[END_REF]. We found that it was not so much the simple presence of a flower strip, as the proportion of the plant cover within it providing accessible nectar that was strongly related to the presence of hoverfly larvae in the adjacent crop, as already reported for adult hoverflies [START_REF] Van Rijn | Nectar accessibility determines fitness, flower choice and abundance of hoverflies that provide natural pest control[END_REF]. We also found that flower strips had a greater impact in simplified landscapes. These results provide a basis for the deployment of biological control in various landscape and agricultural contexts. However, the different responses of the twoaphid species studied to these levers limits the generic scope of our results and should encourage additional studies on a diversity of pests with contrasting patterns of behaviour.

Figure 1 :

 1 Figure 1: Growth rate of Aphis fabae (a) and Acyrthosiphon pisum (b) populations between two successive sampling time in faba bean crops.

Figure 2 :

 2 Figure 2: The abundance of (a) Syrphidae (larvae and pupae) and (b) Acyrthosiphon pisum mummies as a function of the percentage cover of plants providing accessible nectar (determined for Syrphidae) in interaction with the proportion of semi-natural habitats (500 m). The slope of the interaction is represented for the mean and the standard deviation value of semi-natural habitat (18 ± 17 %).

Figure S3 :

 S3 Figure S3: Percent cover of all dicotyledonous flowering species with or without the presence of a wildflower strip for a) all species recorded and b) for species providing accessible nectar to Syrphidae.

Figure S4 :

 S4 Figure S4: Temporal dynamics of the abundance of a) Syrphidae (larvae and pupae), b) Coccinellidae (adult, larvae and pupae) and c) Mummies of Acyrtosiphon pisum per counting date (T1, T2 and T3) in faba bean.

Table 1 :

 1 Effect of local and landscape variables on the abundances of Syrphidae, Coccinellidae and parasitized aphids (mummies). Significant effects (P<0.05) are indicated in bold typeface. All variables were standardised.

	Response variable	Explanatory variable	Estimate	Std. error	Z value	P-value
	Syrphidae	Intercept (Ref= 2019, 5 m, winter faba bean, Count T1)	-0.59	0.37	-1.62	0.106
	(larvae and	Year 2020	-0.21	0.19	-1.09	0.277
	pupae)	Tillage	-0.09	0.11	-0.81	0.419
		Insecticide TFI	0.22	0.16	1.33	0.184
		Distance 30 m	0.21	0.15	1.44	0.151
		Variety (spring faba bean)	-0.63	0.28	-2.23	0.026*
		Number of aphids	0.13	0.06	2.09	0.038*
		Flower resources	0.90	0.25	3.64	<0.001***
		% Legumes	-0.12	0.10	-1.22	0.224
		% Organic fields	-0.21	0.13	-1.59	0.111
		% Semi-natural habitat	0.05	0.09	0.50	0.619
		Count T2	2.25	0.33	6.90	<10 -4 ***
		Count T3	2.07	0.32	6.45	<10 -4 ***
		Flower resources x tillage	-0.01	0.10	-0.08	0.933
		Flower resources x insecticide TFI	0.45	0.35	1.29	0.198
		Flower resources x crop (spring faba bean)	-0.66	0.33	-2.00	0.046*
		Flower resources x % semi-natural habitat	-0.37	0.14	-2.65	0.008**
	Coccinellidae	Intercept (Ref= 2019, 5 m, winter faba bean, Count T1)	-0.07	0.40	0.16	0.870
	(adult, larvae	Year 2020	-0.45	0.33	-1.34	0.179
	and pupae)	Tillage	0.39	0.18	2.13	0.033*
		Insecticide TFI	-0.16	0.21	-0.77	0.443
		Distance 30 m	0.07	0.18	0.41	0.686
		Variety (spring faba bean)	-0.14	0.47	-0.29	0.770
		Number of aphids	0.31	0.13	2.40	0.016*
		Flower resources	-0.05	0.34	-0.15	0.885
		% Legumes	-0.08	0.15	-0.55	0.583
		% Organic fields	-0.24	0.21	-1.12	0.261
		% Semi-natural habitat	0.04	0.15	0.24	0.807
		Count T2	1.04	0.30	3.45	<0.001***
		Count T3	2.67	0.26	10.25	<10 -4 ***
		Flower resources x tillage	0.17	0.19	0.87	0.384
		Flower resources x insecticide TFI	-0.69	0.34	-0.77	0.040*
		Flower resources x crop (spring faba bean)	0.53	0.45	1.17	0.243
		Flower resources x % semi-natural habitat	-0.15	0.20	-0.77	0.442
	Parasitized aphids	Intercept (Ref= 2019, 5 m, winter faba bean, Count T1)	0.11	0.38	0.29	0.772
	(mummies)	Year 2020	0.14	0.24	0.58	0.561
		Tillage	0.04	0.14	0.27	0.788
		Insecticide TFI	0.04	0.17	0.22	0.827
		Distance 30 m	0.22	0.17	1.29	0.196
		Variety (spring faba bean)	0.21	0.39	0.55	0.584
		Number of Acyrthosiphon pisum	0.35	0.05	6.44	<10 -4 ***
		Flower resources	0.59	0.30	1.98	0.048*
		% Legumes	0.07	0.10	0.70	0.486
		% Organic fields	-0.29	0.19	-1.51	0.130
		% Semi-natural habitat	-0.07	0.13	-0.56	0.579
		Count T2	1.19	0.26	4.49	<10 -4 ***
		Count T3	1.09	0.27	4.05	<10 -4 ***
		Flower resources x tillage	0.14	0.12	1.19	0.233
		Flower resources x insecticide TFI	-0.12	0.33	-0.38	0.708
		Flower resources x crop (spring faba bean)	-0.51	0.40	-1.28	0.201
		Flower resources x % semi-natural habitat	-0.47	0.17	-2.75	0.006**

Table 2 :

 2 Effect of aphid antagonists, local and landscape variables on the growth rate of aphid populations. We only show the best models and explanatory variables, selected after multi-model inference for each species and each period. Estimates from the full average of best models are presented. Significant variables are shown in bold typeface (P<0.05). All variables were standardized. 'Imp' is the sum of Akaike weights associated with each variable in all selected models.

	Response variables	Period	Explanatory variables	Estimate	Std. error	Adjusted SEM	Z value	P-value	Imp
	Growth rate of Aphis fabae	T1-T2	Intercept (Ref= 2019, 5 m)	0.10	0.02	0.02	5.88	<0.001
	populations		Year 2020	-9.47•10 -8	7.64•10 -5	7.83•10 -5	1.00•10 -3	0.999	<0.01
			Insecticide TFI	-0.04	0.02	0.02	2.37	0.018*	1.00
			Tillage	-3.00•10 -5	1.03•10 -3	1.05•10 -3	0.03	0.977	<0.01
			Distance 30 m	3.00•10 -8	3.16•10 -5	3.22•10 -5	1.00•10 -3	0.999	<0.01
			Abundance of Coccinellidae	8.91•10 -9	1.42•10 -5	1.46•10 -5	1.00•10 -3	1.000	<0.01
			Flower resources	3.13•10 -5	1.05•10 -3	1.06•10 -3	0.03	0.977	<0.01
			% Organic fields	9.67•10 -4	5.84•10 -3	5.93•10 -3	0.16	0.870	0.04
			% Semi-natural habitat	-6.09•10 -5	1.46•10 -3	1.48•10 -3	0.04	0.967	<0.01
		T2-T3	Intercept (Ref= 2019)	-0.20	0.03	0.03	7.28	<10 -4
			Year 2020	8.06•10 -3	0.03	0.03	0.27	0.790	0.10
			Insecticide TFI	0.08	0.03	0.03	2.94	0.003**	1.00
			Tillage	-0.06	0.03	0.03	2.22	0.026*	1.00
			Abundance of
			Acyrthosiphon	0.07	0.02	0.03	2.84	0.005**	1.00
			pisum
			Abundance of Syrphidae	3.93•10 -4	4.41•10 -3	4.53•10 -3	0.09	0.931	0.02
			Flower resources	-5.04•10 -4	5.27•10 -3	5.42•10 -3	0.10	0.926	0.02
			% Organic fields	6.05•10 -4	6.14•10 -3	6.33•10 -3	0.10	0.924	0.03
	Growth rate of Acyrthosiphon	T1-T2	Intercept (Ref= 2019)	0.105	0.027	0.082	3.70	2.15•10 -3
	pisum		Year 2020	-0.08	0.03	0.04	2.19	0.028*	1.00
	populations		Insecticide TFI	-0.04	0.02	0.02	2.16	0.031*	1.00
			Abundance of Syrphidae	1.19•10 -4	1.51•10 -3	1.54•10 -3	0.08	0.938	0.01
			% Semi-natural habitat	0.05	0.02	0.02	2.83	0.005**	1.00
			% Organic fields	2.64•10 -4	3.07•10 -3	3.16•10 -3	0.08	0.934	0.02
		T2-T3	Intercept (Ref= 2019)	0.04	0.02	0.02	1.52	0.129
			Year 2020	0.06	0.03	0.03	1.92	0.054.	1.00
			Insecticide TFI	1.59•10 -3	6.68•10 -3	6.75•10 -3	0.24	0.813	0.07
			Abundance of Aphis fabae	0.03	0.01	0.01	2.16	0.031 *	1.00
			Abundance of Coccinellidae	-8.68•10 -4	4.47•10 -4	4.56•10 -4	0.02	0.985	0.98
			Abundance of Syrphidae	4.72•10 -4	3.69•10 -3	3.72•10 -4	0.13	0.889	0.02
			Flower resources	1.27•10 -9	5.81•10 -6	5.99•10 -6	0.00	0.999	<0.01
			% Semi-natural habitat	-0.03	0.01	0.01	2.23	0.026*	1.00
			% Legumes	0.03	0.01	0.01	2.59	0.010**	1.00
			Insecticide TFI x
			abundance of	-1.41•10 -5	5.77•10 -4	5.82•10 -4	0.02	0.981	<0.01
			Syrphidae
			Flower resources x
			abundance of	1.54•10 -9	4.90•10 -6	4.95•10 -6	0.00	0.999	<0.01
			Syrphidae

Table S4 :

 S4 List of flowering species sampled with nectar accessible or not to Syrphidae according to the previous morphological model. The mean cover (with standard deviation SD) of each plant species per field and its cumulated sum among all fields sampled is given (No standard deviation is given when the specie was found only once across fields).

	Leucanthemum vulgare	yes	9.62 ± 8.02	48.08
	Lotus corniculatus	no	1.00	1.00
	Lythrum hyssopifolia	no	0.30	0.30
	Malva sylvestris	no	0.72 ± 0.81	2.15
	Species Matricaria perforata Matricaria recutita	yes Accessible yes	Mean cover ± SD 0.77 ± 1.04 (%) 14.48 ± 25.08	Cumulated sum 3.85 (%) 159.30
	Achillea millefolium Medicago lupulina	yes no	0.14 ± 0.05 0.86 ± 1.32	0.55 4.30
	Agrimonia eupatoria Medicago sativa Althaea hirsuta Melilotus officinalis	no yes no no	0.65 14.95 ± 28.53 0.35 0.68 ± 0.55	0.65 59.80 0.35 2.05
	Ammi majus Myosotis arvensis Anagallis arvensis Onobrychis viciifolia	no yes no no	0.05 0.38 ± 0.04 2.99 ± 3.40 1.30 ± 1.02	0.05 0.75 17.95 10.40
	Anthemis tinctoria Papaver rhoeas Anthriscus sylvestris Pastinaca sativa Aphanes arvensis Picris echioides Barbarea sp. Plantago lanceolata Barbarea vulgaris Polygonum persicaria Bellis perennis Potentilla reptans Capsella bursa-pastoris Ranunculus acris Carduus nutans Ranunculus arvensis Centaurea cyanus Ranunculus bulbosus Centaurea scabiosa Reseda lutea Cerastium glomeratum Reseda luteola Cicuta virosa Rumex crispus Cirsium arvense Senecio jacobaea Cirsium vulgare Senecio vulgaris Convolvulus arvensis Sherardia arvensis Cornus sanguinea Silene inflata Crepis setosa Silene latifolia Crepis sp. Sinapis arvensis Echium vulgare Sisymbrium officinale Erigeron acris Sonchus asper Euphorbia helioscopia Stellaria media Fallopia convolvulus Taraxacum officinale Filago vulgaris Tordylium maximum Fumaria officinalis Torilis arvensis Galium aparine Trifolium campestre Galium mollugo Trifolium pratense Geranium dissectum Trifolium repens Geranium molle Veronica arvensis Geranium pusillum Veronica persica Geum urbanum Vicia hirsuta Hesperis matronalis Vicia sativa Hypericum perforatum Viola arvensis Hypochaeris radicata Vicia faba	no yes yes yes yes yes no yes no yes yes yes yes yes no yes yes yes no no yes no yes yes yes no yes yes yes no yes no yes yes yes yes yes no yes yes yes yes yes no yes yes no yes no yes no yes yes yes yes yes yes yes no no no no no yes no	5.26 ± 6.01 1.49 ± 1.91 0.05 1.75 ± 2.85 1 3.61 ± 5.04 2.35 3.83 ± 4.06 0.63 ± 0.60 0.95 ± 1.13 2.30 ± 1.98 1.51 ± 1.44 0.20 ± 0.22 1.45 0.05 0.40 3.77 ± 6.28 1.10 ± 0.71 0.12 ± 0.03 0.94 ± 0.77 0.1 0.05 0.10 0.05 ± 0.07 0.3 0.10 0.08 ± 0.04 0.63 ± 0.69 1.23 ± 1.33 0.35 0.05 0.05 0.53 ± 0.64 0.29 ± 0.24 0.10 0.98 ± 1.60 0.43 ± 0.53 0.55 ± 0.42 0.30 0.41 ± 0.58 0.40 0.15 9.25 0.92 ± 1.13 2.25 3.65 0.05 5.83 ± 7.81 0.83 ± 1.16 7.43 ± 10.43 4.68 ± 6.11 0.06 ± 0.03 1.18 ± 1.06 3.56 ± 4.70 0.32 ± 0.36 1.55 ± 2.35 8.04 ± 11.30 0.86 ± 1.17 0.20 1.42 ± 2.24 0.15 ± 0.14 2.70 ± 3.22 0.38 ± 0.04 0.65 ± 0.49 0.05 ± 0.00 0.10	31.58 10.40 0.05 7.00 1 7.23 2.35 11.48 3.15 1.90 25.30 7.53 0.60 1.45 0.05 0.40 33.90 2.20 3.75 0.35 0.10 0.05 0.10 0.10 0.30 0.10 0.15 1.90 9.85 0.35 0.05 0.05 1.60 1.75 0.10 4.90 1.10 1.70 5.30 0.30 0.15 0.40 9.20 9.25 3.65 2.25 0.05 11.65 14.85 6.60 0.25 18.70 67.73 5.90 10.85 1.90 7.70 16.08 4.25 0.20 40.55 0.60 1.30 0.75 0.10 0.10
	Knautia arvensis	no	0.55 ± 0.52	1.65
	Lactuca serriola	yes	0.10	0.10
	Lamium album	yes	0.50	0.50
	Lapsana communis	no	3.95	3.95
	Lepidium draba	yes	6.93	6.93

ACKNOWLEDGEMENTS

This work was funded by INRAE (metaprogram EcoServ), the François Sommer Foundation, AgroParisTech, Office Français de la Biodiversité, Fondation pour la Recherche sur la Biodiversité, and LabEx BASC. We would like to thank Eleonore Courteau, Herminie Meynard, Audrey Fabarez, Léna Jégo, Alexandre Morisseau, Dominique Le Floch, Emmanuel Frankenreiter and Arnaud Butier for their technical assistance in the collection of field data. We thank all the farmers involved in the study.

AUTHOR CONTRIBUTIONS

LS, FC, MVM, and AG conceived the ideas and designed the methodology; LS, AG and AB collected the data; LS analysed the data; AG and LS wrote the manuscript. All authors contributed critically to the drafts and approved the final version for publication.

Declaration of Competing Interest

We declare no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A: Overview of the cropping techniques implemented in the fields survey.

Figure S1: Mean number of tillage operations and Insecticide Treatment Frequency Index (TFI) depending on cropping system (organic, conservation or conventional). Organic farming was characterized by a high number of tillage operations but no insecticide sprayed compared to conventional and conservation cropping systems, while conservation systems do not perform ploughing and only a few employed superficial tillage. Appendix B: Estimation of nectar accessibility to Syrphidae.

We estimated the amount of nectar provided by the plants by summing, in each field margin (spontaneous vegetation or flower strip), the percent cover of, assessed in vegetation surveys, providing available and accessible nectar to Syrphidae. We focused on Syrphidae because (1) they were dominant aphid predators in our study, (2) the adults require nectar as a source of energy and (3) they have a limited access to floral nectar depending on morphological constraints. Plant species producing floral or extrafloral nectar were selected from vegetation surveys. Nectar was considered to be available when it was produced during the period of predator observations, by selecting species at flower stage or producing extrafloral nectar between the first and the third aphid count. Nectar accessibility depended on morphological matching between plants and predators. Extrafloral nectar, which is not enclosed in a perianth but produced on bracts or stipules, was considered to be always accessible. We determined the accessibility of floral nectar, with a mechanistic trait-based approach, by adapting the geometric model proposed by [START_REF] Van Rijn | Nectar accessibility determines fitness, flower choice and abundance of hoverflies that provide natural pest control[END_REF].

Flower traits related to nectar availability (presence of floral and extrafloral nectar) and accessibility (flower opening diameter, corolla height, nectar depth and nectar tube diameter) were measured in a previous study [START_REF] Gardarin | The hump-shaped effect of plant functional diversity on the biological control of a multi-species pest community[END_REF].

To determine the access of aphidophagous Syrphidae to the nectar provided by plants, we retrieved from the literature the size of their mouthparts and head, which limit corolla penetration. We focused on the four dominant species (Table S3) that were found in malaise traps in a complementary unpublished experiment. Altogether, these four species represented 91 % of all Syrphidae trapped individuals.

Table S3: Morphological measurements of the head and mouthpart size of the Syrphidae, retrieved from the literature (Gilbert, 1985a;Gilbert, 1985b). A decision tree [START_REF] Gardarin | The hump-shaped effect of plant functional diversity on the biological control of a multi-species pest community[END_REF] was built to take into account the three constraints limiting nectar accessibility: (1) ability of the insect to penetrate the flower, which is dependent on head size and flower opening, (2) ability to reach the nectar, which depends on proboscis length, nectar depth and corolla height, and (3) proboscis width and nectar tube diameter in the presence of nectar (Figure S2). Here, we hypothesized that there was no constraint related to the proboscis width (width of the labral sucking tube) with regards to nectar tube diameter, and that sucking was always possible, even in narrow tubes [START_REF] Gilbert | Foraging ecology of hoverflies. Morphology of the mouthparts in relation to feeding on nectar and pollen in some common urban species[END_REF].

Head width (mm)

Using the decision tree, we matched flower and insect traits for all four Syrphidae species. In general, floral nectar accessibility was the same for all four species. We considered that nectar was accessible in general to Syrphidae when it was the case for the four selected species. If results on nectar accessibility were not congruent across Syrphidae species (situation met in only a few instances), we considered that nectar was not accessible in general to these predators.