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1. Pollen beetles (Brassicogethes spp.) are the main pest of oilseed rape (OSR, Brassica napus) in Europe and responsible for massive yield losses. Upcoming pesticide resistances highlight the need for other means of crop protection, such as natural pest control. Sown flower fields aim to counteract the decrease of insect biodiversity in agricultural landscapes by providing breeding and foraging sites to ecosystem service providers such as parasitoids. However, the optimal age and size of flower fields to increase natural pest control is still unclear.
2. We conducted experiments on 31 OSR fields located along a gradient of landscape-scale semi-natural habitat (SNH). OSR fields were located adjacent to flower fields which differed in age, continuity and size, or adjacent to crop fields or calcareous grasslands. In the OSR fields, pesticide free areas were established to examine interactive effects of pesticide use and flower field characteristics. The abundance of pollen beetle adults and larvae, parasitism and superparasitism rates in OSR were recorded at increasing distances to the adjacent fields.
3. OSR next to flower fields maintained continuously for at least 6 years had the lowest numbers of pollen beetle larvae. Flower fields and calcareous grasslands increased pollen beetle parasitism in adjacent OSR fields compared to OSR fields neighbouring crop fields. However, the threshold for effective natural pest control of approximately 35% could only be reached in the pesticide free areas of OSR fields adjacent to calcareous grassland and continuous flower fields.
4. In pesticide-sprayed areas, pollen beetle parasitism and superparasitism declined with increasing distance to the adjacent field, but they remained on the same level in areas without pesticides. Large flower fields (>1.5ha) increased parasitism and superparasitism more than small ones.
5. Synthesis and applications: In general, older continuous flower fields decrease infestation with pollen beetle larvae and enhance parasitism rates in OSR. Furthermore, only flower fields larger than 1.5ha were able to push pollen beetle parasitism above the threshold level of 35%. Therefore, to promote natural pest control, it would be beneficial to maintain large flower fields for several years unmanaged, to create stable habitats for natural enemies. However, pesticide use has negative effects on OSR pollen beetle parasitism and can abrogate positive effects of flower fields on natural pest control. This study highlights that sown flower fields have the potential to increase natural pest control in OSR, but this potential is depending on its age and size and can be hindered by pesticide use.

1) pollen_beetle_larvae_parasitism:

code_field: Code for the respective experimental oilseed rape (OSR) field. Adjacent sites: N: New flower field; R: Refreshed flower field; C: Continuous flower field; CG: Calcareous grassland; CC: Conventional crop field. L: large (>1.5 ha); S: small (<1.5ha).

code_plot: Code for the respective plot in the experimental oilseed rape (OSR) field. Plots are associated with a specific distance to the experimental field edge. This distance is displayed in the landscape_field_characteristics data set.

OSR_plant: Number of the oilseed rape plant (OSR). 3 plants were selected per plot and treatment.

pesticide_treatment: p: pesticide sprayed area; pf: pesticide free area.

number_open_flowers: The total number of open flowers counted on one oilseed rape (OSR) plant.

number_larvae_3mm: Number of oilseed rape (OSR) pollen beetle larvae (Brassicogethes sp.) larger than 3mm collected on one OSR plant.

number_parasitized_larvae: The number of parasitized OSR pollen beetle larvae larger than 3mm by the parasitoid wasp Tersilochus heterocerus.

number_superparasitism: The number of parasitized OSR pollen beetle larvae larger than 3mm with more than 1 egg from Tersilochus hetercerus.

2) pollen_beetle_adults:

code_field: Code for the respective experimental oilseed rape (OSR) field. Adjacent sites: N: New flower field; R: Refreshed flower field; C: Continuous flower field; CG: Calcareous grassland; CC: Conventional crop field. L: large (>1.5 ha); S: small (<1.5ha).

pesticide_treatment: p: pesticide sprayed area; pf: pesticide free area.

plot: Plot in the experimental oilseed rape (OSR) field. Plots are associated with a specific distance to the experimental field edge. This distance is displayed in the landscape_field_characteristics data set.

OSR_plant: Number of the oilseed rape plant (OSR). 3 plants were selected per plot and treatment.

number_adult_beetles: The number of adult pollen beetles (Brassicogethes sp.) found on the main raceme of the oilseed rape (OSR) plants.

3) landscape_field_characteristics:

code_field: code for the respective experimental oilseed rape (OSR) field. Adjacent sites: N: New flower field; R: Refreshed flower field; C: Continuous flower field; CG: Calcareous grassland; CC: Conventional crop field. L: large (>1.5 ha); S: small (<1.5ha).

plot: Plot in the experimental oilseed rape (OSR) field.

distance_field_edge: distance of the experimental plot to the respective OSR field edge.

size_adjacent_site: size of the adjacent site (New flower field; Refreshed flower field; Continuous flower field; Calcareous grassland; Conventional crop field) in hectare.

size_OSR_field: size of the experimental oilseed rape (OSR) fields in hectare.

SNH_amount: Amount of total SNH in the landscape in 1km radius around the focus fields.