Provide set-aside areas in farmland
Two replicated trials showed that species richness of bees nesting (one study) or foraging (one study), is higher on set-aside that is annually mown and left to naturally regenerate for two years or more, relative to other set-aside management regimes or, in the nesting study, to arable crop fields.
A replicated, controlled trial with four replicates of each treatment (Gathmann et al.1994) compared cavity-nesting bees and wasps nesting on set-aside arable land managed in six different ways with crop fields and old meadows in Kraichgau, southwest Germany. The study used reed Phragmites australis stem nest boxes (described in 'Provide artificial nest sites for solitary bees'), and recorded nesting only, not foraging activity. Set-aside fields were either sown in the year of study, with a grass-clover mix or phacelia Phacelia tanacetifolia (also known as scorpion weed, lacy phacelia or tansy phacelia)or were in their first or second year of natural regeneration, with or without mowing. Overall, naturally regenerated fields had significantly more nests, and more nesting species than fields sown with fallow or arable crops.
Of the six set-aside treatments, the most species were found on two-year-old set-aside, mown in late June or early July, with a total of eight nesting bee species. This compares with four bee species found on 1-year-old unmown set-aside, and none on set-aside sown with phacelia. Twelve bee species were found on old meadows (>30 years old, with old fruit trees). Amongst 2-year-old, naturally regenerated set-aside fields, mown fields had more than twice as many species (bees and wasps) as unmown fields (average 4.8 species/field versus 1.8).
A second replicated trial in the same region (Steffan-Dewenter & Tscharntke 2001) examined the abundance and species richness of foraging bees, both solitary and social, on annually mown set-aside fields of different ages and management. The number of bee species increased with the age of set-aside fields, from 15 species on 1-year-old fields to 25 species on 5-year-old fields. Two-year-old set-aside fields had the most bee species - 29 on average, compared to 32 species for old meadows, including an average of around five oligolectic species (specialising on pollen from a small group of plant species). One-year-old set-aside fields sown with phaceliahad an average of 13 bee species, mainly common, generalised species of bumblebee Bombus and Lasioglossum.