Determining the foraging potential of oilseed rape to honey bees using aerial surveys and simulations

The modern agricultural landscape is characterised by large areas of monoculture, including fields of mass-flowering crops. These may benefit bees by providing abundant forage, but may cause harm if the fields are treated with insecticides. Here, we investigated the potential exposure of honeybee colonies to pesticides in flowering oilseed rape (OSR) in England using a combination of aerial photographs and geographical modelling. Aerial photographs were used to detect and map fields of blooming OSR in eight 20 x 20 kilometers(2) areas across England where OSR is grown. We determined the distance to the nearest OSR field for randomly chosen positions (i.e., potential hive locations) using a digital map made of each area. The proportion of colonies within foraging range to the nearest OSR field in each area was then calculated using published data on honeybee foraging distances obtained by waggle dance decoding. We found that the distribution of distances to the nearest OSR field varied greatly among study areas, ranging from almost all colonies being within range to almost all being out of range (6-79% were within median foraging distance). In one study area, we examined changes over a 4-year period and found little year-to-year variation due to crop rotation (23-36% within median foraging distance). Our specific results show that exposure in England to OSR and to any chemicals used in treatment varies greatly among areas. Overall, our methodology provides a novel way of quantifying the exposure of bee colonies or other central place foragers to a particular crop or land-type at a landscape level.