Action: Plant parks and gardens with appropriate flowers
Two replicated trials in the USA and Canada have found more wild bees (either more species or more individuals) in gardens planted with bee forage or native plants, relative to conventionally managed gardens. Another USA trial found more bee species after the addition of bee forage plants to a community garden. Three trials in the UK or USA have shown that native flowering plants or bee forage plants are well used by wild bees when planted in gardens. A UK trial demonstrated that some popular non-native or horticulturally modified garden flowers are not frequently visited by insects, despite providing nectar in some cases.
Supporting evidence from individual studies
Natural shaped, rather than horticulturally modified varieties of garden plants are recommended for foraging insects. A trial of nearly natural and horticulturally modified varieties of six popular garden plants in the Cambridge University Botanic Gardens, Cambridgeshire, England (Comba et al. 1999a) found that bumblebee visits to hollyhock Alcea rosea and larkspur Consolida sp. were more frequent on natural, single-petalled forms than on horticulturally modified, double-petalled varieties. Bee visits to four of the flower types - nasturtium Tropaeolum majus, pansy Viola x wittrockiana, marigold Tagetes patula and snapdragon Antirrhinum majus were infrequent despite ample nectar provision from some varieties. There was a tendency for wild bees to prefer natural flower shapes in pansy, marigold and snapdragon, but not in nasturtium.
A trial of 25 native flowering herb species planted in the Cambridge University Botanic Gardens, UK, identified 16 species frequently visited by wild bees (Comba et al. 1999b). Ten species (seven of which were frequently visited by wild bees) were shown to provide abundant nectar in the garden environment.
A trial of six native plant species (marsh woundwort Stachys palustris, wood betony S. officinalis, purple loosestrife Lythrum salicaria, common toadflax Linaria vulgaris, bird's-foot trefoil Lotus corniculatus and meadow clary Salvia pratensis) recommended for pollinator-friendly gardens in the Cambridge University Botanic Gardens, UK, found all six were nectar-rich and frequently visited by wild bees (Corbet et al. 2001). A double-flowered variant of bird's-foot trefoil tested in the same study produced no nectar and attracted no insects.
A replicated trial in the Phoenix metropolitan area, in the Sonoran Desert of the USA, found that eight gardens planted with dry-loving plants (xeric landscaping) supported a greater diversity of bees than eight gardens planted with non-native plants such as grasses that needed to be irrigated (McIntyre & Hostetler 2001). In September, xeric gardens had approximately 10 bee species, compared to less than five species/garden in ordinary gardens.
Tommasi et al. (2004) measured bee abundance and diversity in wild areas, and gardens managed for wildlife or managed traditionally, in Vancouver, British Columbia, Canada (five to eight sites of each type). They found that gardens managed for wildlife under the 'Naturescape' programme, sown with native plant species and infrequently mown grass areas, had significantly more bee individuals than traditionally managed gardens with mown lawns and non-native plants such as petunia Petunia sp., tulip Tulipa sp., pansy Viola sp. and rhododendron Rhododendron sp. (approximately 45 wild bee (non-Apis mellifera) individuals caught/hour of sampling on average in Naturescape gardens, compared to less than 20 bees/hour in traditional gardens). Naturescape gardens did not have significantly more bee species than traditional gardens.
Wojcik et al. (2008) planted a 180 m2 urban plot at the University of California, Berkeley, USA with 78 garden plants chosen to provide a consistent floral resource throughout the season, and monitored bee visits the following summer. The plot provided pollen and nectar from spring to early autumn, and attracted 32 bee species, from 17 genera and five families, demonstrating the potential of newly planted urban gardens to provide resources for native bees.
Pawelek et al. (2009) added 41 types of 'bee attractive' plants, both native and non-native, to a 4,000 m2 community garden in San Luis Obispo, California and monitored the abundance and diversity of native bees over three years from 2007 to 2009. Plants were planted in 1 ´ 1.5 m patches, in 19 of the 29 plots within the garden, and bees were recorded in 3-minute counts on selected patches, regularly between July and October. The number of bee species recorded rose from five in 2007 (less sampling effort than subsequent years) to 21 in 2008 and 31 (including four non-native species) in 2009. The added plants that attracted the greatest number of wild bee species were blanketflower Gaillardia x grandiflora and bog sage Salvia uliginosa (both non-native, 11 species recorded on each).
- Comba L., Corbet S.A., Barron A., Bird A., Collinge S., Miyazaki N. & Powell M. (1999) Garden flowers: Insect visits and the floral reward of horticulturally-modified variants. Annals of Botany, 83, 73-86
- Comba L., Corbet S.A., Hunt L.V. & Warren B. (1999) Flower, nectar and insect visits: evaluating British plant species for pollinator-friendly gardens. Annals of Botany, 83, 369-383
- Corbet S.A., Bee J., Dasmahapatra K., Gale S., Gorringe E., La Ferla B., Moorhouse T., Trevail A. & Van Bergen Y. and Vorontsova M. (2001) Native or exotic? Double or single? Evaluating plants for pollinator-friendly gardens. Annals of Botany, 87, 219-232
- McIntyre N.E. & Hostetler M.E. (2001) Effect of urban land use on pollinator (Hymenoptera: Apoidea) communities in a desert metropolis. Basic and Applied Ecology, 2, 209-218
- Tommasi D., Miro A., Higo H. & Winston M.L. (2004) Bee diversity and abundance in an urban setting. The Canadian Entomologist, 136, 851-869
- Wojcik V.A., Frankie G.W., Thorp R.W. & Hernandez J.L. (2008) Seasonality in bees and their floral resource plants at a constructed urban bee habitat in Berkeley, California. Journal of the Kansas Entomological Society, 81, 15-28
- Pawelek J.C., Frankie G.W., Thorp R.W. & Przybylski M. (2009) Modification of a community garden to attract native bee pollinators in urban San Luis Obispo, California. Cities and the Environment, 2