©1992 IBRA Journal of Apicultural Research 31 (1): 9-14 (1992) Cotoneaster for bumble bees and honey bees SARAH A CORBET; A WESTGARTH -SMITH Zoology Department, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK (Received 29 June 1992, accepted subject to revision 8 September 1992, accepted for publication 7 October 1992) SUMMARY Regular monitoring between May and August 1988 of a collection of 22 taxa of Cotoneaster in the UK showed that the bumble bees Bombus pratorum and B. pascuorum preferentially visited species in the section Cotoneaster. The short-tongued bumble bees B. terrestris/lucorum and honey bees visited species in both sections of the genus, concentrating on the section Cotoneaster in the dearth period of early summer and on Chaenopetalum after mid-June. The section Cotoneaster is recommended as particularly valuable for bee forage. This phenological survey should make it possible to select groups of species for amenity plantings to give a seasonal spread of flowering, enhancing the availability of nectar plants. Keywords: Cotoneaster, nectar plants, Bombus, bumble bees, Apis mellifera, honey bees, foraging, phenology, UK 10 INTRODUCTION With the loss of wild flowers from agricultural and semi-natural habitats in recent decades (Corbet ef a/., 1991; Osborneefa/., 1991), gardens and amenity plantings have increased in relative importance as forage sources for honey bees and native bees. Among wild bees, bumble bees are particularly important because of their role as pollinators of crops, and are at risk because of the documented decline of several species in arable regions of the UK (Williams, 1986; Rasmont, 1988). In a recent national survey, Cotoneaster species were highlighted as plants much visited by bumble bees, often receiving hundreds of visits per 'bee walk' (Fussell & Corbet, 1992). They are of acknowledged value as nectar sources for honey bees, particularly because they provide forage during the dearth period of May and June (Howes, 1979; Hooper & Taylor, 1988). Several species are commonly planted in gardens, parks and roadsides. It has been suggested that their value as bee forage could be extended seasonally by planting carefullychosen species that flower in sequence. In this paper we report a phenological study of flowering and usage by bees of 22 species and forms in a collection comprising one or a few bushes of each of many taxa of Cotoneaster in the Cambridge University Botanic Garden. Corbet; Westgarth-Smith divided into two sections. In section Cotoneaster (Orthopetalum Kohne) the petals are erect and the flowers are cup- or bell-shaped and often tinged with pink. The erect petals would offer the nectar some microclimatic protection, slowing evaporation in dry air and damping fluctuations in solute concentration. In section Chaenopetalum Kohne the petals are spreading and the flowers are open, saucershaped and usually white and upwards-facing. The exposed nectar would evaporate more quickly in dry air, and would be more readily diluted in humid weather (Corbet ef a/., 1979). The two sections of the genus are therefore expected to differ in the accessibility and concentration/humidity relations of their nectar, and hence in patterns of usage by long- and short-tongued bees. Flowering extended over more than 88 days. At the beginning of the survey, on 17 May, several species were already in flower, and the last flowers of C. pannosus and C. frigidus were seen on 12 August (table 1). The patterns of flowering for selected section Cotoneaster MATERIALS AND METHODS The seasonal survey was conducted between 17 May (day 1) and 19 August (day 95) in 1988 in the Cambridge University Botanic Garden. Once on each of 21 dates the observer (AW-S) walked the same standard route between 12.00 and 17.00 h recording the flower abundance score (none, 1-5, 5-50, 51-500, > 500) and bee visits on one individual mapped bush of each taxon. Ambient temperature ranged from 16 to 28°C , and was therefore always within the range compatible with foraging activity of honey bees and bumble bees. The nomenclature of the Cotoneaster in the Botanic Garden was checked in 1991 by Jeanette Fryer, whose account of the genus will appear in the European Garden Flora (Cullen, 1992; see also Stace, 1991). Bee names follow Prys-Jones and Corbet (1991); the two short-tongued species Bombus terrestris and B. lucorum were not distinguished in this study. There were a few honey bee hives but no hives of bumble bees in the Garden. RESULTS AND DISCUSSION The genus Cotoneaster (family Rosaceae) comprises deciduous and evergreen shrubs and trees native in cool temperate regions of Northern Asia and Europe. Its taxonomy is complicated by hybridization and apomixis (Bean, 1976; Stace, 1991). The genus is section Chaenopetalum FIG. 1. Flowering phenology of selected taxa in section Cotoneaster (above) and section Chaenopetalum (below). The survey began on 17 May 1988. Flower counts for the bush of each taxon are coded as in table 1. Upper figure: filled circles: C. horizontalis; open circles: C. franchetii; black squares: C. newryensis; open squares: C. adpressus. Lower figure: black squares: C. x watereri 'Cornubia'; open circles: C. pannosus; open squares: C. rotundifolius. 11 Cotoneaster for bees TABLE 1 . Flowering phenology of Cotoneaster taxa in Cambridge Botanic Garden between 1 7 May and 19 August 1988. Each line represents one bush. Taxa are listed in chronological sequence of peak flowering date. The maximum flower count in a given week is shown thus: 0 = zero; + = up to 5; * = 6-50; * * = 51-500; * * * = > 500. Month Week Plant species Section Cotoneaster C. lucidus Schldl C. horizontalis Decne. C. adpressus Bois C. horizontalis Decne. 'Variegatus' C. cinerascens (Rehd.) Flinck & Hylmo C. simonsii Baker C. dielsianus Pritz. C. newryensis Lem. C. sikangensis Flinck & Hylmo C. splendens Flinck & Hylmo C. insculptus Diels C. sp. nov. aff. franchetii Bois Section Chaenopetalum C. multiflorus Bunge C. monopyrenus (W.W. Smith) Flinck & Hylmo C. ignotus Klotz C. rotundifolius Lindl. C. ludlowii Klotz C. aff. microphyllus Lindl. C. frigidus Wall. C. x watereri Exell 'Cornubia' C. x watereri Exell C. lacteus W.W. Smith C. pannosus Franch. C. frigidus Wall. Bee visits Section Cotoneaster Apis mellifera Bombus terrestris /lucorum other Bombus Section Chaenopetalum Apis mellifera Bombus terrestris /lucorum other Bombus »«_.. 1 ^ 2 1 .^ 3 4 5 6 + + ^. I..I.. 7 8 ^ 9 10 11 12 A... 13 14 Q *,. *** + *** * *** *** * * + 0 + 0 + + * * * * * * 0 + 0 0 + * + + + + + + + 0 + + 0 * * * * * * 0 + 0 * * * * + * * * * * * * + + * 0 ... * „ * * * * * + * o 0 ** ** ** * * * * * 0 * + * * * * * 0 0 + + * * * + 0 0 ** + ^. + 0 * 0 * + 0 * * + * * * * + o + + 0 0 * 0 + 0 * *** *** ** 0 ** ** 0 * 0 0 1 0 0 0 * * + 0 ** * +0 * 0 0 * * *** *** *** *** 0 + * * * * 0 *** * *** 0 * 0 * * 0 0 0 0 0 0 0 0 0 0 5 2 5 1 5 1 3 0 6 2 0 2 1 1 32 5 1 3 1 0 0 9 2 0 0 1 2 0 0 11 0 2 0 9 5 9 0 94 6 26 4 29 7 1 1 0 0 0 0 0 0 0 0 1 7 0 0 0 0 0 0 1 0 0 3 3 6 1 0 3 5 2 2 0 0 12 Corbet; Westgarth-Smith TABLE 2. Total numbers of bee visits recorded on the score (see legend to table 1) and some characteristic; = semi-evergreen; E = evergreen. Leaves are classifiet Peak flowers recorded score 6 7 8 9 10 II [2 13 Section Cotoneaster C. lucidus Schldl C. horizontalis Decne. C. adpressus Bois C. horizontalis Decne. 'Variegatus' C. cinerascens (Rehd.) Flinck & Hylmo C. simonsii Baker C. dielsianus Pritz. C. newryensis Lem. C. sikangensis Flinck & Hylmo C. splendens Flinck & Hylmo C. insculptus Diels C. sp. nov. aff. franchetii Bois Total for section 9 10 II 12 13 day 14 14 week FIG. 2. Numbers of visits allocated to taxa in section Cotoneaster (black), and section Chaenopetalum (hatched), in successive weeks of the survey (starting on May 17 1988) for honey bees, Apis mellifera (top); shorttongued bumble bees Bombus terrestris/ lucorum (middle); and other Bombus (bottom); B, pratorum and B. pascuorum, plus B, lapidarius (2 visits) and B. hortorum (1 visit). (table 1). The patterns of flowering for selected species are shown in figure 1. Mass visitation by hundreds of honey bees and bumble bees at a time on a bush or clump of Cotoneaster has often been recorded, although this survey yielded no such records (Fussell & Corbet, 1992). Honey bees and the short-tongued bumble bees B. terrestris/lucorum visited taxa in both sections of the genus, but nearly all the visits made by the long-tongued B. pascuorum and particularly by 6. pratorum were to the section Cotoneaster. B. pratorum is known to favour plant species with flowers that face downwards. It has a tongue of only mod- Section Chaenopetalum C. multiflorus Bunge C. monopyrenus (W.W. Smith) Flinck & Hylmo C. ignotus Klotz C. rotundifolius Lindl. C. ludlowii Klotz C. aff. microphyllus Lindl. C. frigidus Wall. C. x wateren Exell 'Cornubia' C. x watereri Exell C. lacteus W.W. Smith C. pannosus Franch. C. frigidus Wall. Total for section Overall total erate length, but can take nectar from deeper flowers by thrusting its narrow head into the corolla (Prys-Jones & Corbet, 1991; Fussell & Corbet, 1992). There is some indication that honey bees and shorttongued bumble bees showed a seasonal change in their relative usage of the two sections; section Cotoneaster was more important in May and early June and section Chaenopetalum in July and August (fig. 2). This trend is significant for/Ap/s (Spearman rank correlation, P < 0.01), but is heavily influenced by factors that may be peculiar to this survey, such as the size of individual bushes and the degree of shading, and it is not clear whether or not it applies more generally. In our survey the only species with more than 500 flowers for more than one week in 0 0 0 7 7 5 15 20 20 24 27 48 0 0 3 7 13 22 37 38 44 50 50 57 Cotoneaster for bees 13 jsh of each taxon summed over the whole survey (17 May-19 August 1988), with peak flower abundance af the plants (from Bean, 1976; IBRA, 1981; Gardening from Which, 1988; Stace, 1991). D = deciduous; SE as small (< 1 cm), medium (1-4 cm) or large (> 4 cm). Taxa are scored as susceptible (1), less susceptible (2) or moderately resistant (3) to fire blight. Numbers of bee visits Apis 0 7 30 0 6 0 1 0 0 12 4 0 60 B. terrestris /lucorum 0 3 11 1 B. pratorum 2 Plant characteristics B. pascuorum 0 0 0 0 0 evergreen habit /deciduous D D D berry colour leaf size black red red red red red red large small small small medium medium medium medium medium medium medium medium D D/SE D/SE D SE D D E SE shrub semi prostrate prostrate semi prostrate tall shrub tall shrub shrub shrub shrub shrub tall shrub shrub 0 0 D D tall shrub tall shrub red red large medium 0 5 0 1 0 0 0 0 0 0 6 0 2 0 0 0 0 0 0 1 0 3 D SE D E D/SE SE SE E E D tall shrub low shrub shrub semiprostrate tree tree tall shrub tall shrub tall shrub tree blue-black red red red red to yellow red to yellow orange-red red red red to yellow large small medium small large large large large medium large 64 14 2 0 0 6 0 1 1 0 25 1 8 1 10 3 6 9 2 9 5 2 58 0 4 0 5 1 0 11 0 0 0 0 0 0 0 9 2 1 8 4 4 57 84 7 176 1 2 0 1 2 3 0 12 5 1 27 236 52 1 May and early June was C. adpressus (section Cotoneaster), which received 30 of the 60 Apis visits to that section, and in July and August the only such abundantly flowering species were C. frigidus, C. x watereri 'Cornubia', C. lacteus and C. pannosus, all in the section Chaenopetalum. All these four taxa have the flowers massed in large umbels, and in all but C. pannosus an umbel may comprise more than 20 flowers. None of the other taxa we surveyed has such large umbels, but umbels of over 20 flowers occur in other species (C. salicifolius Franch., C. bacillaris Wall, and C. transens Klotz in the section Chaenopetalum and C. bullatus Bois and C. rehderi Pojark. in the section Cotoneaster (Stace, 1991)), and it may be worth exploring the potential value of orange/red red fire blight resistance 3 2 3 2 3 2 2 1 2 2 these as bee forage. B. pratorum and the longtongued B. pascuorum concentrated on section Cotoneaster throughout the season. Cotoneaster species are valuable not only for their attractive habit and foliage and their colourful berries, but also for their wildlife value, providing berries for birds as well as nectar for bees and other insects. The abundant and accessible nectar of species of the section Cotoneaster is taken by longtongued and short-tongued bees, as well as by other organisms including wasps (Buzzard, 1936; Pflumm, 1982) and a marsh or willow tit (Parus palustris or P. montanus) on C. simonsii (personal observation, SAC). Their value for bees can be 14 greatly increased if species are selected to provide a seasonal succession of forage. Our survey indicates the flowering sequence only in the broadest terms, because it involved only 22 taxa, generally only one bush of each type, and only one year. Nevertheless, it shows that a selection of Cotoneaster species can offer bee forage for several months, and it indicates that the two sections of the genus differ in their value to bees of different species, in ways that may change seasonally. With their bell-shaped flowers, species of the section Cotoneaster are almost the only members of the genus visited by B. pratorum and B. pascuorum. During the critical period in May and early June, when there may be a dearth of other forage (Howes, 1979), the section Cotoneaster received more visits than the section Chaenopetalum from honey bees and the short-tongued bumble bees B. terrestris/ lucorum. A mixture of Cotoneaster species, including several in the section Cotoneaster, could provide a seasonal succession of forage for bumble bees and honey bees from May to August, as well as an ornamental diversity with respect to habit, foliage and the colour and season of the berries (fig. 1, table 2). For example, the semi-prostrate C. horizontalis and the prostrate C. adpressus might be planted with the shrubs C. newryensis or C. splendens and C. franchetii. A few species from the section Chaenopetalum could be included, such as the small-leaved semi-evergreen C. rotundifolius or the prostrate evergreen C. microphyllus, with the large-leaved semi-evergreen tree Cotoneaster x watereri 'Cornubia' and the medium-leaved C. pannosus or the large-leaved evergreen shrub C. lacteus. Unfortunately some members of the genus Cotoneaster are susceptible to fire blight, a bacterial disease that can be transmitted by flower-visiting insects to rosaceous crops such as apple or pear. Taxa of Cotoneaster are classified as very susceptible to fire blight (score 1 in table 2), less susceptible (score 2) or moderately resistant (score 3). It is unwise to plant susceptible Cotoneaster species near orchards or nurseries, especially if the flowering period of the Cotoneaster species overlaps the primary or secondary flowering of susceptible crops. ACKNOWLEDGEMENTS We thank Mrs Julia MacNeill of Westhill, Inverness for suggesting the idea of a seasonal sequence of Cotoneaster species, Dr Peter Yeo for taxonomic help and for suggesting that the sections of the genus differed with respect to bee visits, Mrs D Fletcher for advice about fire blight and for the list of susceptibility scores given in table 2, Mike Fussell for comments on the manuscript, the director and staff of the Cambridge University Botanic Garden for Corbet; Westgarth-Smith allowing us to work there and Professor Sir James Beament for facilities in the Department of Applied Biology. REFERENCES The numbers given at the end of references denote entries in Apicultural Abstracts. BEAN, W J (1976) Trees and shrubs hardy in the British Isles. Vol. 1. John Murray; London, UK; pp 729-759. BUZZARD, C N (1936) De ('organisation du travail chez les abeilles. Bulletin de la Societe d'Apiculture des Alpes-Maritimes 15(116): 65-70. 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