Sow uncropped arable field margins with an agricultural nectar and pollen mix
Five replicated trials in Europe (three controlled) have documented bumblebees foraging on field margins sown with an agricultural nectar and pollen seed mix. Four replicated trials showed that field margins sown with perennial leguminous flowering plants attract significantly more foraging bumblebees than naturally regenerated (two trials), grassy (four trials) or cropped (three trials) field margins. Three replicated trials showed that a mix of agricultural forage plants including legumes (all annual plants in one trial) attracts greater numbers of bumblebees than a perennial wildflower mix, at least in the first year. Three trials in the UK found evidence that margins sown with agricultural legume plants degrade in their value to bumblebees and would need to be re-sown every few years. We have captured no evidence on the effects of field margin management on solitary bees.
Arable field margins are the focus of some specific European agri-environment measures. This section covers nectar and pollen seed mixes comprising agricultural forage plants or non-native plants, designed to provide nectar and pollen sources for bees and other flower-visiting insects. For nectar and pollen mixes exclusively composed of native wild flowers, see Sow uncropped arable field margins with a native wild flower seed mix.
In a replicated trial in central Sweden (Lagerlof et al. 1992), four 2 m wide field margins sown with a mix of legumes dominated by either red clover Trifolium pratense or white melilot Melilota alba attracted significantly more bumblebees Bombus spp. than naturally regenerated field margins and species-rich dry pasture. Red clover was the most attractive, with 72% of a total of 413 individual bees recorded on that treatment. White melilot was extremely attractive to honey bees Apis mellifera, attracting 98% of the 2,422 recorded in the study.
In a replicated trial (five plots) of field margin seed mixtures on a farm in North Yorkshire, Carvell et al. (2006) found that short-tongued bumblebees (B. terrestris, B. lucorum and B. pratorum) strongly preferred plots of annually sown cover crops including borage Borago officinalis and common melilot Melilotus officinalis over perennial wildflower seed mix. Total bumblebee abundance was higher on the annual agricultural nectar mix. On average 70% of pollen collected by buff-tailed bumblebee workers B. terrestris sampled in this study was from borage.
In a replicated controlled trial in thirty-two 10 km grid squares across England (Pywell et al. 2006), there were significantly more bumblebee species and more individuals on field margins sown 1-2 years previously with a pollen and nectar mix (average >3 species and 86 bees/transect) than on grassy margins (average 1.3-1.4 species and 6-8 bees/transect) or cropped margins (average 0.1 species and 0.2 bees/transect). There were more bumblebee individuals, but not more bumblebee species on pollen and nectar mix margins (average 86 bees/transect) than on wildflower-sown margins (43 bees/transect). The abundance of long-tongued bumblebees (mostly common carder bee B. pascuorum and garden bumblebee B. hortorum) was positively correlated with the number of pollen and nectar-mix agreements in each 10 km square.
In a replicated controlled trial at six sites across central and eastern England, Carvell et al. (2007) found that 6 m margins of cereal fields sown with a nectar flower mixture supported significantly more foraging bumblebees (species and individuals) than cropped, grassy or naturally regenerated unsown field margins. Visitors included the nationally rare long-tongued species Bombus ruderatus and B. muscorum.
The nectar flower seed mixture was based on four agricultural legumes (red clover, Alsike clover Trifolium hybridum, bird's-foot trefoil Lotus corniculatus and sainfoin Onobrychis viciifolia). Unlike a wild flower seed mixture in the same study, it supported more bumblebees than other treatments from the first year of the study. However, relative to a wild flower mixture, this mixture provided low numbers of flowers in May and June, when bumblebee queens of late-emerging species are foraging. It also showed a decline in flower numbers in year three, when it did not support significantly more bumblebees than the wild flower seed mixtures.
In a replicated, controlled study (2005-2008), across 41 farms in England and Scotland, the average number of worker bumblebees was greater on margins where legume-rich seed mix was established than on other field margins (grassy margins or track edges; Edwards 2008). No formal statistical analysis were performed on these data. There was an observed decline in the relative number of foraging worker bumblebees on these margins after they had been established for more than three years (data from five farms).
Arable margins sown with legume-grass seed mix had higher species richness of bumblebee forage plants (almost 100% cover of Alsike clover and red clover one year after establishment) over four years, compared to naturally regenerated margins on farmland at Romney Marsh, Kent, England (Gardiner et al. 2008). Bee visits were not reported in this study. Fixed-time transect walks in the clover margins are reported elsewhere (Edwards & Williams 2004) to have demonstrated a 300-fold increase in bumblebee forager numbers in the margins planted with clover, but unfortunately, no control count was carried out for comparison. However, the clover-sown plots were invaded by perennial grasses in the third and fourth years of this study, and flower numbers fell substantially.
- Lagerlöf J., Stark J. & Svensson B. (1992) Margins of agricultural fields as habitats for pollinating insects. Agriculture, Ecosystems and the Environment, 40, 117-124.
- Carvell C., Westrich P., Meek W.R., Pywell R.F. & Nowakowski M. (2006) Assessing the value of annual and perennial forage mixtures for bumblebees by direct observation and pollen analysis. Apidologie, 37, 326-340.
- Pywell R.F. et al. (2006) Effectiveness of new agri-environment schemes in providing foraging resources for bumblebees in intensively farmed landscapes. Biological Conservation, 129, 192-206.
- Carvell C., Meek W.R., Pywell R.F., Goulson D. & Nowakowski M. (2007) Comparing the efficacy of agri-environment schemes to enhance bumble bee abundance and diversity on arable field margins. Journal of Applied Ecology, 44, 29-40.
- Edwards M. (2008) Syngenta Operation Bumblebee Monitoring Report 2005-2008. Report to Syngenta (added by: Dicks L. 2010) Syngenta Operation Bumblebee Monitoring Report 2005-2008, 2005-2008.
- Gardiner T., Edwards M. & Hill J. (2008) Establishment of clover-rich field margins as a forage resource for bumblebees Bombus spp. on Romney Marsh, Kent, England. Conservation Evidence, 5, 51-57.