1. Many farmers are facing high economic risks if pollinator declines continue or temporal and spatial variation in wild bee communities cause reduced pollination services. Co-flowering crops might compete for pollinators, while they also might facilitate the delivery of pollination services. This rarely studied topic is of particular interest with respect to the foraging decisions of bees from different functional groups and when more sparsely and mass-flowering crops are in bloom at the same time.

2. The abundance of honey bees, bumble bees and solitary bees in strawberry fields was quantified with transect walks along a gradient of oilseed rape (OSR) availability (product of OSR land cover and temporally changing OSR flower cover). We established a pollination experiment with pollination treatments (open-, wind- and self-pollination) to study the effects of insect pollination on strawberry fruit weight and quality.

3. Changes in OSR availability exhibited contrasting effects on social versus solitary bees in strawberry fields. Bumble bees and honey bees were less abundant in strawberry fields when OSR availability was high, whereas solitary bees were facilitated. With more strawberry flowers we found more bees in general.

4. When flowers were open-pollinated they resulted in heavier fruits with better commercial grades compared to wind-and self-pollinated flowers. A higher bee abundance enhanced the strawberry fruit weight and quality but depended on flower order and variety.

5. Synthesis and applications. Sparsely flowering crops may compete with mass-flowering crops for social bee pollinators while solitary pollinators in the field might be even facilitated. To ensure best fruit weight and quality it can be beneficial to support bee abundance in the field. While some social and solitary bee species can be managed for pollination services, wild bees, in particular solitary species, should be conserved and promoted for stable crop pollination services in dynamic agricultural landscapes.

POLLINATOR ABUNDANCE

We used standardised transect walks to sample flower-visiting bees in our study fields (strawberry fields in 8 landscapes). We had four observation periods during the strawberry flowering period in May and June. 2015 Flowering of both crops started around the end of April and co-flowering continued for four to five weeks depending on the location and weather conditions. Two transects (each 50 m x 4 strawberry rows) were established within the strawberry fields: one at the edge and one inside the field (15 m from edge) to account for edge effects. Each transect lasted 15 min and was conducted between 9 am and 6 pm while we visited each field in morning and afternoon hours. Hence, eight transects have been conducted in each strawberry field during the study period. For the observations we chose good weather conditions, i.e. days with a minimum temperature of 12°C, no rain, low wind speed and low cloud cover. Flower-visiting bees within the transect area were counted and identified to species level in the field or caught with an insect net for later identification in the laboratory. Solitary bees were identified by SB and Frank Creutzburg. Bee individuals that escaped from the insect net were not identified to species level, but their abundance was counted if they could be assigned to one functional group. Bee abundance is quantified as number of individuals per transect.

We assigned the bees to functional groups according to their sociality and level of domestication (i.e. honey bees, bumble bees and solitary bees). However, we found one individual of Halictus tumulorum which is thought to be primitively eusocial. Due to its morphological similarity with other solitary bees, we included it in the group of solitary bees for our analyses.

We quantified strawberry flower cover by counting the number of open flowers along two meters of a strawberry row within each transect area (edge and inside of the field separately).

POLLINATION EXPERIMENT

Fruit weight and commercial grades

To investigate the importance of insect pollination for strawberry fruit quantity and quality, we established a pollination experiment with three treatments (open-, wind- and self-pollination). Open-pollinated flowers were left open to allow access for all flower-visitors as well as for airborne pollen. To exclude only insects, but allow airborne pollen flow, we bagged individual flowers in bags with mesh sizes of 1 mm in the wind-pollination treatment. We used Osmolux bags (Pantek, France), which are permeable for water vapour (http://www.pantek-france.fr/agriculture.html), for the self-pollination treatment (exclude insects and airborne pollen). All bags were removed after blooming to standardise the fruit ripening.

In total, we had 40 plants per field, 20 plants at the edge and 20 plants in the inside of the strawberry field according to transect location. Five plants per transect location were assigned to the self- and wind-pollinated treatment and ten plants to the open-pollination treatment. Strawberry flowers can be assigned to different orders according to their temporal occurrence and position on the stalk. One primary flower (first order) usually occur first on the main stalk, followed by two secondary flowers which branch off the main first stalk, and followed by up to four third order flowers which branch off the second stalks. Generally, higher flower orders produce berries with a reduced size and weight. For this reason, we included only fruits from 1st to 2nd flower orders. Every strawberry fruit was weighted directly after harvest and categorized into commercial grades with respect to size, shape and colour according to the European Commission. Commercial grades were categorized as G1/E, where we pooled Grade Extra and one, G2 as Grade two and NM for non-marketable fruits.

We used a subset of three observation periods since the combined data of bee abundance and subsequent fruit yield and quality is only available for three points in time.