This article was first published in the Summer 1998 issue of Plantlife magazine.
Living larders for bumblebees
Stock your garden with bee-friendly plants
and you can help to safeguard these vital
insect pollinators, as Dr Livio Comba and
Dr Sarah Corbet report
URBANISATION AND AGRICULTURE have redesigned the landscape, destroying much of the native perennial vegetation that once provided nectar and pollen for bees, butterflies and other flower-feeding insects. This may be one reason why several species of bumblebee have declined or become locally extinct in Britain in recent decades. If the decline continues, wild flowers that depend on such pollinators may be lost.
As the numbers and diversity of wild flowers in the countryside decline, gardeners can help by growing plants that sustain valuable pollinators - particularly the vulnerable long-tongued bumblebees. Long-tongued bumblebee species such as Bombus pascuorum and B. hortorum depend on deep flowers with abundant nectar, and are their irreplaceable pollinators.
Bumblebee colonies store only a few days' worth of energy reserves, and so are much more vulnerable than honey bees to food shortages caused by a scarcity of flowers or poor foraging weather. They therefore need constant access to nectar-rich plants throughout spring, summer and autumn.
Early flowering nectar-rich plants are a particularly vital resource for the queen bumblebee as she emerges in March from her overwinter hibernation. She must single-handedly find enough food to mature her eggs, find and establish a nest - often a disused mouse burrow - and rear the first batch of workers. These workers forage for provisions that are used to rear more and more workers and then, towards the end of the season, male bees and new queens. If a colony is to produce large numbers of these daughter queens that will found colonies next year, its workers need a succession of nectar-rich flowers throughout the summer.
A "nectar trial" project was set up in 1995 under the auspices of Flora-for-Fauna, an organisation that encourages gardeners to grow plants valuable to wildlife. Funded by English Nature, the RSPCA and Rio-Tinto, students working in the Cambridge University Botanic Garden counted bees on flowers from dawn to dusk during three summers. They measured the floral reward available to insects, focusing on nectar because it is more easily quantified than pollen. Their findings reveal which garden plants and native species are pollinator-friendly.
|Table 1 Some plants used by bumblebees.|
As a rule, old-fashioned cottage-garden varieties proved best for bees. Garden plants of a simple, original form were compared with more elaborate, horticulturally modified forms. These may be more attractive to some gardeners, but they often proved less valuable to bees.
For example, unmodified nasturtuims such as "Tip Top" secrete nectar in a long spur and so provide a food reserve for long-tongued bumblebees. But the double form "Whirlybird" is spurless and provides no nectar. Short-tongued bees visit it for pollen, but it is useless for nectar-collecting long-tongued bees.
Similarly, single larkspurs offer nectar to long-tongued bumblebees, but double larkspurs have neither spur nor nectar. Bumblebees straddle the small flowers of violets to take nectar, but they have difficulty on the larger flowers of pansies.
In the ancestral form of snapdragon, only the long-tongued bumblebees can push past the inflated lip to reach the nectar. Their competitors - the weaker, shorter-tongued honeybees - rarely do so. But voracious honeybees can easily reach into the more open flowers of the modified form "Trumpet Serenade." These flowers secrete nectar at about the same rate as the unmodified form, but more of it goes to honeybees, leaving less for long-tongued bumblebees.
Sympathetic gardeners who grow insect-worthy flowers (see Table 1) may help to sustain populations of pollinating insects, particularly the valuable long-tongued bumblebees.
Dr Livio Comba studied the foraging ecology of bees for his doctorate at the University of Rome. Funded by the Stanley Smith Horticultural Trust, he is now working for Flora-for-Fauna at the University of Cambridge.
Dr Sarah Corbet is a lecturer in Zoology at the University of Cambridge. She works on pollination ecology and pollinator conservation.
Bumblebees by O.E. Prys-Jones and S.A. Corbet gives more information about bumblebees. This book and a bumblebee identification card are available from the Richmond Publishing Co. Ltd, PO Box 963, Slough SL2 3RS (tel 01753 643104). For a list of suppliers of British native seeds and plants please contact Plantlife.