15 RUNNING HEAD

Trans. Suffolk Nat. Soc. 45 (2009)

BEES AND BOTTLEBRUSH

MICHAEL KIRBY

Bottlebrush bushes (Callistemon citrinus and C. rigidus) in my garden flowered profusely in the last week of June and July during a spell of warm settled weather. Within a day of the first flowers unfolding they were visited by honey bees and bumble bees (Bombus lucorum and B. lapidarius) which continued to work the flowers during the whole of the flowering period. There were 100–150 inflorescences per bush and at any one time there were one or two honey bees per inflorescence; bumble bees were fewer in number, about 10–15 per bush and common wasps (Vespa vulgaris) and hoverflies were also regular visitors. (The visits by the wasps were puzzling. None was caught to find out if they were new queens or workers freed from the task of feeding larvae.)

The inflorescence of bottlebrush is about 180 mm long and consists of about 80 closely packed flowers forming a tight spiral round the axis (Plate 7). Each flower has five small sepals and five petals around the rim of a deep calyx tube; each petal subtends six 15–20 mm long stiff stamens each with a small, versatile anther. The ovary is situated at the base of the cup formed by the calyx tube with a long stiff style forming, with the stamens, a dense, strong ‘thicket’. The nectary at base of each cup produces copious nectar, easily seen glistening in the sun. (Fig. 1)

Figure.1. Longitudinal section of a bottlebrush flower to show the calyx tube and ovary. Copious nectar accumulates around the ovary.

Honey bees quickly discovered the flowers and at first seemed uncertain how to get at the nectar. They approached the flowers from the base or top of the inflorescence, standing on the leaves or the exposed petals before probing for nectar (Plate 8). Soon, however, they developed a technique of landing in

Trans. Suffolk Nat. Soc. 45 (2009)

Suffolk Natural History, Vol. 45 16

the mid-part of the ‘brush’ and pushing, head first into the dense thicket of stamens. This was obviously difficult, for the bee approached the calyx tube head first and had to manoeuvre and bend its neck so that its tongue could reach the nectar. Bumble bees, with their greater bulk, forced their way into the thicket parting the stamens to reach the nectary (Plate 9). Some, both honey and bumble bees had pollen bags filled with bright yellow pollen. The behaviour of wasps was similar to that of bees, upending head first into the thicket, clinging to the filaments (Plate 10).

The genus Callistemon is almost exclusively native to Australia. It is pollinated by honey eaters and wattlebirds, small to medium sized birds adapted to pollinating Callistemon and other Myrtaceae with relatively long, curved beaks and a long tongue with a brush like tip. They land on the long flexible branches and shimmy up to the base of the inflorescence from where they probe the flowers, their heads becoming dusted with pollen.

Bees in Britain, both honey and bumble bees appear to be in decline, beset by disease and habitat loss, and gardeners in particular are exhorted to grow plants listed as being particularly suitable for bees to gather nectar and pollen. The basis upon which such plants may be selected, however, is not clear and on the basis of these lists, gardeners may neglect choice ornamental or exotic plants which are in fact good bee plants. How then to assess whether a species, not listed by, for example the Royal Horticultural Society will be good for bees? In some cases, for example the unlisted North American Penstemon spp. flower form is very similar to British natives such as foxglove and much visted by bees; The New Zealand Kowhai (Sophora microphylla) a member of the Papilionoideae with flowers rather similar to peas or beans, blooms in my garden for several weeks in March, and swarms with bumble bees throughout.

Another factor affecting flower selection by bees may be the availability of nectar producing flowers. This year, 2009, early flowering plants, particularly shrubs performed very well. As the season progressed high temperatures and low rainfall especially on free draining sandy soils reduced flowering and hastened senescence of most herbaceous garden plants.

In the case of Callistemon, which evolved in a different hemisphere and with different pollinators, British bees could have seen nothing like it; it has none of the features of an insect pollinated species such as a landing stage or honey guides which might attract bees, yet they found it within a few hours of the first flowers opening, perhaps spurred on by a shortage of more suitable flowers. In the absence of visual clues the most likely attractant seemed a smell of some sort and although most members of the family have eucalyptus like oils in the leaves, no smell could be detected from the inflorescence by a human nose.

Perhaps flower selection in bees is not a case solely of instinct. While native flowers have evolved features to attract bees, in the absence of flowers with such features, they can explore and exploit exotic species with (to them) strange flower forms.

Michael Kirby The Studio, Blythburgh Road, Westleton, Saxmundham IP17 3AS

Plate 8: Honey bee approaching an inflorescence from the distal end, landing on an exposed calyx tube and petals before probing for nectar (p. 15).

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Plate 7: Cross section of bottlebrush inflorescence showing flowers closely packed around axis (p. 15).

M.

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Plate 9: A bumble bee (subgenus Psithyrus) pushing through the thicket of stamens to get to the nectar in the calyx tube (p. 16).

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Plate 10: A wasp landing in the mid-part of an inflorescence and pushing, head first, into the thicket of stamens. A similar method was used by experienced bees (p. 16).

M.

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y