DOI: 10.5061/dryad.3xsj3txn0
title: Flower visitors and fruit set of sweet cherry in Germany

description: Mason bees (Osmia spp.) are efficient fruit tree pollinators that can be
 encouraged to occupy and breed in artificial nesting material. In sweet
 cherry orchards, they are occasionally used as an alternative managed
 pollinator as a replacement for or in addition to honey bees (Apis
 mellifera). Yet, the lack of practical guidelines on management practices,
 e.g., optimal stocking rates, for both mason bee nesting material and
 honey bees might compromise pollination service provision. In this study,
 we assessed the relationship between stocking rates (honey bee hives and
 mason bee nesting material) and the abundance of honey bees and mason bees
 in 17 sweet cherry (Prunus avium) orchards in Central Germany. We
 furthermore performed a pollination experiment
 to explore the interactive effect of mason bees and honey bees on
 sweet cherry fruit set. In the orchards, both honey bee and mason bee
 abundance increased with increasing stocking rates of hives or nesting
 material, respectively. Honey bee abundance increased linearly with
 stocking rates. In contrast, mason bee abundance asymptoted at 2-3 nesting
 boxes per ha, beyond which more boxes resulted in little increase
 in visitation rate. Our pollination experiment demonstrated that the
 orchards were pollen limited, with only 28% of insect-pollinated flowers
 setting fruit versus 39% of optimally hand-pollinated flowers. Honey bees
 and mason bees enhanced sweet cherry fruit set, but only when both were
 present and not when either was present alone in an orchard. Our findings
 demonstrate that offering nesting material for mason bees and employing
 honey bee hives can enhance bee abundance in sweet cherry orchards. By
 increasing honey bee abundance in combination with enhanced mason bee
 abundance, farmers can substantially boost fruit set and
 potentially sweet cherry yield. To enhance pollination services, farmers
 should consider the benefits of increasing pollinator biodiversity as an
 immediate benefit to improve crop yields.
description: Cherry flower visitors and cherry pollination Fieldwork was carried out in
 spring 2020 in orchards within the federal states Sachsen-Anhalt and
 Thuringia, Germany (Figure 1). Both federal states are dominated by
 agriculture (&gt;60% of land cover). We selected 17 sites, of which two
 were experimental orchards and 15 were commercial mixed fruit orchards
 (Table S.1). The experimental orchards are owned by research institutions
 for conducting research experiments but are managed as commercial orchards
 in order to simulate agriculturally field-realistic conditions. The size
 of the orchards devoted to sweet cherry (Prunus avium) cultivation ranged
 between 0.2 ha and 36 ha (6.6 8.6; mean SD). Distances between field sites
 were &gt;2 km (47.9 28.9 km; mean SD) to ensure spatial independence.
 Pollination management consisted of providing honey bee hives and nesting
 material for mason bees (Osmia cornuta). Though Osmia bicornis is common
 in the study region, its flight period does commence until after cherry
 bloom has ceased and therefore, we did not consider it further in this
 study. Commercial bumble bee nests were not used by cherry farmers in this
 study. To investigate the effect of bee management on bee abundance and
 subsequently on sweet cherry fruit set, we selected sites with varying
 pollinator management. Mason bee management ranged from zero (i.e., no
 nesting material provided) to 8.6 nesting boxes per ha. On-farm bee
 management was decided prior to the study by the farmer and therefore the
 size of the nesting boxes varied across sites. To make them comparable, we
 set the standard size of a nesting material box to be 100 x 54 cm
 containing approximately 500 tubes made of paper, bamboo or drilled wood,
 each of a length of 13-16 cm and a diameter of 4-10 mm. Farmers initiated
 mason bee management in different years, ranging from 2002 to 2020
 (General_info_orchards). In the two sites at which farmers started
 providing nesting material in the same year as the study (2020), Osmia
 cornuta cocoons were provided in addition to the nesting material to
 ensure similar conditions (General_info_orchards). The number of honey bee
 colonies per site (General_info_orchards) varied from zero to 20 hives per
 ha. We quantified the abundance of insect flower-visitors during the peak
 bloom of sweet cherry (16.04.2020 23.04.2020) at each site for one day.
 For each site, two transect walks of 90 minutes each (in total 180
 minutes) were performed alongside cherry trees on a sunny day, one in the
 morning and one in the afternoon on the same day. Transect walks were
 performed &gt;50 m from the edge of the orchard and included trees of the
 cultivar `Regina` used for a pollination experiment (described below). One
 observer continuously walked alongside the `Regina` cultivar, while noting
 all visible flower visitors. Flower visitors that touched the reproductive
 parts of a cherry flower were counted and identified to morphological
 groups: honey bees, bumble bees, mason bees (Osmia cornuta), other wild
 bees, butterflies, flies, beetles, and ants. Though flower-visitor
 observations performed only for one day might be a limiting factor,
 assessments in cherries with a very limited flowering period of around 5-8
 days restrict observations over several days in multiple orchards. Ambient
 (shade) temperatures during the transect walks were recorded with a
 digital thermometer; ranging from 9C to 22C. Data can be found in
 Abundance_Transectwalk_Cherry. In order to quantify provision of
 pollination services, we studied one of the most common sweet cherry
 cultivars, Regina, which was present in all 17 study sites. This cultivar
 is self-sterile (S-alleles: S1S3) and requires cross-pollination for
 successful fruit development (Lech et al., 2008; Holzschuh et al., 2012).
 We chose at least one row of Regina trees in each orchard, which was
 planted either next to a cross-compatible pollinizer cultivar or which was
 interspersed with a pollinizer cultivar in the same row. Pollinizer
 cultivars varied across sites (General_info_orchards). We selected 20
 Regina trees in each orchard that were at least 50 m from the orchard
 edge. On each tree, we applied three flower treatments: insect exclusion
 (bagged: B), hand pollination (hand: H), and open insect pollination
 (open: O). For each treatment, we chose a flower bundle, which we marked
 with coloured ribbons, cord, and barrier tape to later locate the
 treatments. In some cases, several flower bundles were used for one
 treatment if one flower bundle contained less than three flowers. During
 full bloom and on the same day as observations of flower visitors, we
 counted all open and receptive flowers per bundle and removed old
 (over-flowered) or young (still closed) flowers. The insect exclusion
 treatment (B) was bagged in fine netting (1 mm PVC mesh) prior to cherry
 bloom (6.04.2020 12.04.2020) to prevent insect pollination and remained
 bagged throughout cherry bloom; it enabled us to estimate the contribution
 of wind pollination and autonomous self-pollination to fruit set. Flower
 bundles of treatment H were manually pollinated with pollen from at least
 two flowers of an adjacent pollinizer (see Table S.1 for pollinizer
 cultivars per site) as a measure of maximal fruit set. Treatment O
 remained unmanipulated as a measure of current pollination service
 provision at each fruit orchard accessible to all flower-visiting insect
 including honey bees and mason bees. Sweet cherry fruit set was counted
 three times, once in May to assess initial fruit set (ca. four weeks after
 flowering), once in June after the so-called June fall (ca. eight weeks
 after flowering), and once prior to harvest to assess final fruit set,
 approximately in the beginning of July (ca. 12 weeks after the
 experimental manipulations). We divided the number of developed fruits per
 bundle by the number of flowers per bundle for each fruit count period
 (i.e., May, June, July) to give the percentage fruit set per treatment. In
 addition, we recorded the weight of each cherry fruit during the final
 fruit set count in July. Data can be found in
 Pollination_experiment_cherry. For all data entries entered as NA, data
 was not possible to collect.