Influence of woody vegetation on pollinator densities in oilseed Brassica fields in an Australian temperate landscape

Wild pollinators may benefit Brassica oilseed production in temperate Australia, yet it is not known how the density of potential pollinators varies in these landscapes. In this study we assessed whether the density of feral honeybees, hoverflies (probably 2 species) and native bees (multiple species) in temperate Australian Brassica oilseed crops was related to the composition of the landscape. The density of pollinators was measured at multiple points in six different Brassica oilseed paddocks (20-80 ha) at least 1.75 km apart. Landscape composition at multiple scales (radii 100-2000 m) was determined from GIS layers of Brassica paddocks, woody vegetation and non-woody vegetation, and a derived layer expected to reflect the condition of woody vegetation remnants (the 'Link' score). Densities of feral honeybees were higher near the edges of Brassica fields than towards the middle. Densities of feral honeybees were strongly positively associated with the summed 'Link' score within 300 m and with the amount of woody vegetation. Densities of native bees and hoverflies were not strongly associated with woody vegetation or with woody vegetation with a high 'Link' score. Our results suggest that maximising feral honeybee abundance within paddocks in these landscapes may require smaller paddocks than those typically used, interspersed with habitat beneficial to feral honeybees such as woody vegetation in good condition.