An energetics-based honeybee nectar-foraging model used to assess the potential for landscape-level pesticide exposure dilution

Estimating the exposure of honeybees to pesticides on a landscape scale requires models of their spatial foraging behaviour. For this purpose, we developed a mechanistic, energetics-based model for a single day of nectar foraging in complex landscape mosaics. The hive population was assumed to select the resource patch with highest net energetic efficiency in perfect knowledge of its environment. During the day the optimal patch could change, due to resource depletion or related to the characteristics of the resource (i.e., the period flowers were open and provided nectar). By quantifying foraging costs and yield in terms of energy, we accounted for the impact of field distance and size, the occurrence of resource depletion and the characteristics of the resource type (crops and wild flowers), like seasonal and diurnal patterns in availability.From the model we derived simple rules for resource patch selection, e.g., for landscapes with mass-flowering crops, the net energetic efficiency would be proportional to the ratio of the energetic content of the nectar of the crop divided by distance of crop field to the hive. Also, we determined maximum distances at which resources like oilseed rape and clover could still be energetically attractive.We used the model to assess the potential for pesticide exposure dilution in landscapes of different composition and complexity. Dilution implied a lower average concentration in nectar arriving at the hive, resulting from foraging effort being diverted away from treated fields. Applying the model for all possible hive locations in a landscape, landscape-specific distributions of dilution values were obtained.For a case study area for which detailed spatial data were available, we tested three scenarios that were expected to lead to exposure dilution: the presence of i) equally attractive crop fields in the neighbourhood of a treated field, ii) highly attractive flowers strips of different widths at the edges of treated fields (off-crop in-field resources), and iii) highly attractive resources on off-field (semi-natural) habitats. The results indicated significant dilution at landscape scale only from flowers strips of more than 5 m wide and off-field habitats. On an area-base, flowers strips were approximately 10 times as effective as off-field habitats, the main reason being that the flowers strips had an optimal location, always at the edges of treated fields.