FOXGLOVES are high fashion this summer. Their spectacular flower spikes took
centre stage at last month’s Chelsea Flower Show, Britain’s premier gardening
extravaganza. Modern forms of this cottage-garden favourite are fast becoming
the darlings of gardeners across the land.
Britain’s bee population may not be so keen. When botanist John Parker,
director of the Cambridge University Botanic Garden, dissected a modish new
foxglove cultivar at Chelsea, he found it contained no nectar.
With luck, the new foxgloves will not turn out to be as barren as they seem.
Flowers taken into dimly lit marquees often shut down their nectar-generating
machinery, explains bee expert Sarah Corbet, a zoology lecturer at the
University of Cambridge. In practice, the showy foxgloves may do better out in
the sunshine. But Parker’s findings reflect a trend in garden flowers. Modern
cultivars may look gorgeous but they could spell famine for bees, says Jill
Hamilton, coordinator of Flora for Fauna, a charity based at the Linnean Society
in London which has been pioneering its “nectar trials” for the past three
summers.
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Bumblebees are particularly at risk from changing fashions in the gardening
world. Many live on a knife edge. Because they are heavier than honeybees, they
use up more energy flying and so they need to find especially nectar-rich
flowers. At the same time, they have to compete with commercially managed
honeybees, which swarm all over gardens gobbling up nectar, sometimes without
even pollinating the flowers that feed them. Bumblebee colonies store very
little honey, so without a steady supply of nectar the bees will die in a matter
of days.
“Bumblebees are not just fat, incompetent honeybees,” says Corbet. They are
irreplaceable pollinators, especially in northern Europe. Because they are
better at warming up their flight muscles, bumblebees can fly when honeybees are
grounded by cool weather. They are also essential for pollinating plants with
deep flowers. Only bumblebee species with long tongues—up to twice the
length of a honeybee’s—can reach the nectar in crops such as red clover
and field bean. In New Zealand, yields of red clover seeds trebled after
colonies of the longest-tongued British bumblebee Bombus hortorum were
introduced in the early 1980s.
And only bumblebees can perform “buzz pollination”, a special technique for
collecting pollen from the distinctive swept-back flowers on plants such as
tomato, borage, cranberry and blueberry. The bees cling onto the flower with
their feet and vibrate their wings at twice the normal flying speed—you
can hear the high-pitched buzz. The flower responds with a puff of pollen, which
the bumblebee stores in pollen sacs on its legs. Buzz pollination is so
effective that Dutch tomato growers now set up bumblebee colonies in their
greenhouses. Ten years ago, growers had to rely on humans equipped with electric
vibrators, at an annual cost of some £13 000 per hectare.
Despite their economic importance, bumblebees are in decline. Only 6 of
Britain’s 19 species of bumblebee remain widespread and abundant. Intensive
agriculture and tidy-minded local authorities have put paid to hedgerows, field
boundaries and road verges that were once crowded with the wild perennials that
bumblebees depend upon for food. Gardeners could make all the difference, but
only if they choose the right plants. This is where Flora for Fauna’s “nectar
trials” come in.
With financial backing from English Nature and the RSPCA, and supervised by
Corbet, scores of students spend July and August watching bees from dawn to dusk
at the Botanic Gardens in Cambridge. They also use microcapillary tubes to
measure the changing nectar yields of flowers, showing that they peak in early
morning and evening, the times when bumblebees do most of their foraging. These
systematic observations are revealing what bees think of the flowers on offer in
specially planted beds. The results of the trials, now in their third summer,
are striking. In a nutshell, the message is that bees prefer old varieties of
flowers to new.
The structure of the flower is all-important. Plants that are native to
Britain, or which were widespread in Victorian cottage gardens, are likely to
provide rewards for pollinating insects, if only because evolution and Victorian
gardeners wanted the plants to set seed. But now, Corbet explains, plant
breeders produce plants that are propagated artificially. This means they are
free to develop floral features that are attractive to gardeners but
incompatible with healthy seed production. As a result, they produce flowers
that no longer match the needs of pollinating insects.
Sweet spurs
Flora for Fauna has practical advice for gardeners wanting to attract
bumblebees: grow your own flowers, and grow old varieties. Nasturtiums are
particularly attractive to the long-tongued B. hortorum and B.
pascuorum because other bees cannot reach deep into the flowers to collect
the nectar. Tip Top and Scarlet Jewel are good varieties. But these species will
settle for any nasturtium provided the flowers have long, elegant spurs at their
base. The nectar trials showed that varieties with spurless flowers do not
produce nectar.
Double-flowered varieties of larkspur, including Q15 Blue, also contain no
nectar. Single larkspurs, however, such as Eastern Blues, are wonderful for
long-tongued bumblebees—again, no other species can reach the nectar.
Single-flowered forms of hollyhocks, soapwort and hemp agrimony are far
preferable to the more ornate double-flowered varieties. Double hemp agrimony,
in particular, is unattractive to any insect.
Snapdragons, with their closed, deep flowers, offer exclusive dining for
long-tongued bumblebees. No other species can push open the bloom and reach the
nectar. But modern cultivars with open flowers such as Trumpet Serenade are
available to all comers. As a result, honey bees take the lion’s share of the
nectar.
The deep-flowered Salvia pratensis is also the exclusive preserve of
B. hortorum. A species of brown bumblebee with slightly shorter
tongues, B. pascuorum, enjoys S. verticillata, as well as
Cotoneaster varieties with pink bell-shaped enclosed flowers.
Anchusa, in either the annual Blue Angel or the perennial form, is highly
recommended.
Size can matter too. Horticulturally modified flowers are often larger than
the ancestral form. This may not reduce nectar supplies, says Corbet, “but if
the enlargement destroys the match between bee and flower, making the nectar
inaccessible, the new form may be valueless to insects”. Big, blowsy pansies,
the cultivated form of violas, for example, seem to present a precarious
platform for bees. Flora for Fauna’s observers saw bees repeatedly tumbling off
the highly modified flowers.
In the countryside, bumblebees rely on perennial wildflowers, but these are
becoming scarcer because of intensive land use, which repeatedly disturbs the
soil, encouraging annuals. “One-year set-aside schemes, for instance, are a
recipe for losing perennials from the agricultural landscape,” say Corbet. She
would like gardeners to redress the balance by growing deep-flowered herbaceous
perennials such as catmint, perennial cornflower, white deadnettle, woundworts
and unmodified foxgloves.
Flora for Fauna is encouraging gardeners to grow plants that are both native
and local, so this summer Corbet’s team will focus on British wildflower
species, comparing them with structurally modified garden varieties. The
researchers will also find out whether the latest forms of foxglove can attract
bumblebees, or whether they are all style and no substance.
* * *
Home grown
At The Chelsea Flower Show this year, Flora for Fauna launched the Postcode
Plants Database, developed in conjunction with the Natural History Museum in
London and sponsored by the Royal Mail.
It is the world’s first publicly available database devoted to relationships
between plants and animals in the wild. At Chelsea, people queued twenty deep to
get a chance to key in their postcode and receive a list of native plants,
butterflies and birds found in their area.
The lists are generated by software that searches through hundreds of
distribution maps of fauna and flora to come up with lists tailored to each of
Britain’s 26 million home addresses.
The database tells users which local native plants are welcoming to wildlife
and worth planting in a garden. “Never before has it been so easy for people to
find the plants truly suitable for the garden,” says Flora for Fauna
coordinator, Jill Hamilton. The group is now adding more distribution maps to
the database, and hopes to post it on the Internet and have it in the shops as a
CD-ROM by Christmas.
- Further reading: Bumblebees, by Oliver Prys-Jones and Sarah
Corbet (Richmond Publishing Co, Slough, 1991). - A handbook (£9.95) or wallcard (£1.75) for easy identification of
bumblebees are available from The Company of Biologists, Bidder Building, 140
Cowley Road, Cambridge CB4 4DL; tel 01223 426164, fax 01223 423353,
e-mail sales@thecob.demon.co.uk. - “Flower usage by bumblebees: a basis for forage plant management”, by M.
Fussell, and S. A. Corbet, Journal of Applied Ecology, vol 29, p 451
(1992).