Summary:
The effects of farming system, flower resources and semi-natural habitats on bumblebees and solitary bees in intensively cultivated landscapes in Denmark were investigated in two sets of studies, in 2011 and 2012. The pan trap colour preferences of bumblebees and solitary bees were also assessed.
In 2011, bumblebees and solitary bees were trapped in road verges bordering 14 organic (organic sites) and 14 conventional (conventional sites) winter wheat fields. The quantity and quality of local flower resources in the road verge and adjacent field headland were estimated as overall density of dicotyledonous herbs in the flowering stage (quantity) and density of plants containing combined
high pollen and nectar amounts (quality). Potential flower and nesting resources (referred to as semi-natural habitats) in the surrounding landscape were assessed using up-to-date, spatially precise registers of land use as a proxy at four different scales (250, 500, 750 and 1000 m).
In 2012, the effect of a four-fold larger area of organic arable fields in simple, homogeneous landscapes on bumblebees and solitary bees was investigated in eight circular landscapes (radius 1000 m). Bumblebees and solitary bees were trapped in road verges and grassy field borders in four landscapes with a low proportion (mean 9.7%) of organic arable fields (low organic) and in four
landscapes with a high proportion (mean 41.4%) of organic arable fields (high organic). The proportion of semi-natural habitats was similar for the low and high organic landscapes (9.4% and
9.7%, respectively).
Overall density of dicotyledonous herbs was significantly higher in organic winter wheat fields, but not in the adjacent road verges, compared with their conventionally farmed counterparts. Organic farming gave significantly higher overall flower abundance of dicotyledonous herbs in both wheat fields and adjacent road verges. Its effect on flower abundance of high value bee plants was even more pronounced, with 10-fold higher mean density in organic wheat fields than in conventional wheat fields and 1.9-fold higher density in road verges bordering organic fields than in those bordering conventional fields. This was due to the absence of herbicides and to practices inherent to organic farming systems, such as use of clover (a high value bee plant) as a green manure and
fodder crop.
Solitary bees responded with significantly higher numbers of individuals and species at organic sites compared with the corresponding conventional sites and also responded positively to a fourfold larger area of organic arable fields in homogeneous landscapes. Bumblebees did not respond to farming system or to a four-fold larger area of organic arable fields.
Local flower resources and the proportion of semi-natural habitats in the surrounding landscape both contributed to increased numbers of individuals and species of bumblebees and solitary bees. Solitary bees responded positively to the proportion of semi-natural habitats at 250 m and 500 m scale, while bumblebees only responded at 1000 m scale.
The numbers of individuals and species of bumblebees increased with overall flower abundance and abundance of flowering high value bee plants in road verges, but flower abundance in wheat fields had no effect. For solitary bees, overall flower abundance and abundance of flowering high value bee plants in wheat fields increased the numbers of individuals and species, while flower abundance in road verges had no effect.
The studies on pan trap colour preferences by bumblebees and solitary bees showed that white UVcoloured traps captured on average almost twice as many individual solitary bees and 24% more species of solitary bees than yellow-coloured traps, whereas yellow traps captured on average 20% more individual bumblebees than white traps.
The positive effect of organic farming on solitary bees was the result of greater local flower resources in organically managed fields. For bumblebees, the more than three-fold increase in flower abundance in organic wheat fields compared with conventional was not sufficient to create a visible difference in numbers, nor was increasing the potential flower resource in the landscape by quadrupling the organic arable area. Instead, bumblebees responded to perennial flower resources in the road verge/grassy field border and semi-natural habitats in the landscape. Organically managed arable fields with mostly annual non-crop flowering plants in intensively cultivated landscapes probably played a minor role in bumblebee colony growth and species richness.
To counteract the negative consequences of agricultural intensification on wild bees, the differences in floral preferences and mobility between solitary bees and bumblebees need to be considered.
Solitary bees made use of the mostly annual flower resources in organic winter wheat fields and have low foraging distances, so conservation efforts should focus on increasing flower resources in the vicinity of nesting habitats, recreating lost nesting sites and protecting existing sites in field borders and other permanent linear habitats.
Bumblebees operate at much larger scales but are more dependent on abundant flower resources from perennial plants found in semi-natural habitats. Conservation efforts must thus consider appropriate management of e.g. field borders and road verges to promote the presence of abundant flowers from perennial plants instead of the many grasses often dominating these habitats.