(Osmia spp.), a group that uses mud in nest construction. Bumble bees need grassy thatch or abandoned rodent burrows in which they build their nests. If a local bee sanctuary meets the more important general standards discussed above, appropriate nesting materials will probably follow suit, but the matter should not be disregarded. One commercial blueberry grower in south Georgia, USA set out shelled maize under sheets of plywood around his orchard in an attempt to increase the number of rodent nests in the area which, in turn, would provide future nest sites for pollinating bumble bees.
pete with the crop for pollinators. Some researchers have identified bee pasture plants for particular agricultural settings. Fussell and Corbet (1991) identified naturally-occurring bee plants in field bean and oilseed rape farmland in the UK. Patten et al.(1993) screened 21 herbaceous plants for their suitability as bee pasture near cranberry bogs in the northwestern USA. Krewer et al. (1996) identified eight candidate plants for supplemental bumble bee pasture near rabbiteye blueberry orchards in Georgia. This kind of work is necessarily local- ized because bee plants championed by one author in one region sometimes disappoint another (for example, Mayer et al., 1982, versus Ayers et al., 1987, concerning anise hyssop), and there are obvious geographic differences in plant hardiness zones and flowering times.
Finally, bee species differ in flower preferences, often based on the morphological match between flower parts and bee mouth parts (Fairey et al., 1992; Patten et al., 1993; Plowright and Plowright, 1997). We will now summarize some important principles and prac- tices for bee pasture plantings.
The importance of season-long bloom
The value of bee pastures to local bee populations is optimized in those pastures that have a season-long succession of bloom. This prin- ciple is best illustrated with bumble bees. With an annual life cycle, the top priority of a bumble bee colony is to produce a new crop of mated queens for the following season. Each colony has only a few weeks to start a nest (as a solitary queen), rear a foraging force of workers, and collect enough food to produce new queens and males.
The number of queens a colony can produce depends largely on the number of workers it can produce in the weeks leading up to the queen production period (Heinrich, 1979). Producing workers requires energy, so a colony’s reproductive success ultimately hinges on season-long availability of food. The link between good nutrition and high queen output was underscored by Bowers (1986) who showed that new queens appear earlier in those meadows with the richest flower densities.
Mid-summer nectar dearths can be disastrous. Again, bumble bees illustrate the magnitude of this problem. Unlike honey bees that store large surpluses of food, bumble bees store enough nectar for only a few days at most. This makes them vulnerable to nectar dearths.
Worker bumble bees stop incubating brood and respond lethargically to invading predators and parasites in colonies that are experimental- ly deprived of nectar for even one day (Cartar and Dill, 1991).
Therefore, a mid-season nectar dearth can be a simple matter of life or death, let alone a compromise on reproductive performance.
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These principles lead us to conclude that in planning a bee pas- ture one should choose a collection of plants so that there is an unbro- ken succession of bloom throughout the season. The first step is to identify the normally-occurring bee plants in one’s area and their his- toric bloom periods, information readily available from local honey bee-keepers. Once dearth times in the natural bloom calendar are identified, the next step is to select bee pasture plants that bloom his- torically during those dearth times. Local extension specialists, horti- culturists, nursery stock growers, and published bee plant lists are helpful in making these selections. It is important to avoid plants that bloom at the same time as the crop of interest or are invasive or other- wise noxious.
The commercialization of seed blends designed to produce a suc- cession of bloom has been realized in Europe largely in response to the EU’s land set-aside programme enacted in 1988. By 1994 the set- aside programme had resulted in 781,000 ha of arable land being removed from production in the UK alone (Carreck and Williams, 1997). This idled land represents an enormous opportunity to grow food plants for bees, and seed blends are commercially available that have been designed to provide a succession of bloom on lands desig- nated for one-year set-aside. Two of these blends are the Tübingen Mixture (Bauer and Engels, 1992; Engles et al., 1994) and the Ascot Linde SN mixture (Cebeco Zaden BV, Vlijmen, The Netherlands and Stichting Imerij Fortmond, Olst, The Netherlands). The composition of the Tübingen Mixture is as follows: 40% phacelia (Phacelia tanacetifolia), 20% buckwheat (Fagopyrum esculentum), 7% white mustard (Sinapis alba), 6% coriander (Coriandrum sativum), 5%
marigold (Calendula officinalis), 5% caraway (Nigella spp.), 5% sun- flower (Helianthus annuus), 3% red radish (Raphanus sativus), 3%
cornflower (Centaurea cyanus), 3% mallow (Malva sylvestris), 2%
anethum (Anethum graveolens), and 1% borage (Borago officinalis).
The availability of seed blends of annuals such as these is certainly applaudable and is conducive to the adoption of bee conservation practices by farmers. However, prepared blends may not replace local- ly designed plant assemblies. Working with the Tübingen and Ascot Linde SN mixtures in the UK, Carreck and Williams (1997) found that phacelia dominated in terms of establishing, flowering, and attracting insects and that the other plants contributed little to bee diversity.
The importance of perennials as bee pasture plants
Another principle important in planning bee pastures is that perenni- al plants are generally to be preferred over annuals. Although some annuals provide rapid and relatively abundant bee forage, perennial
32 Chapter 4
herbs and shrubs are generally superior forages and preferred by bees (Parrish and Bazzaz, 1979; Fussell and Corbet, 1992; Dramstad and Fry, 1995; Petanidou and Smets, 1995). Compared to annuals, peren- nials are richer nectar sources owing, in part, to their ability to store and secrete sugars from the previous season. Perennials provide bee populations with a more-or-less dependable food source year after year and encourage repeated bee nesting in the area. This is an impor- tant factor explaining why the number of bee species and plant species tends to increase together over time in undisturbed meadows.
Thus, we can generalize that whenever possible bee pastures should be stocked with perennial bee plants. Considering the repeated labour and inputs required for annuals, perennials are a cost-effective, low- maintenance choice for growers installing bee pasture.
It is important to remember that bee nesting and foraging activities centre on flower-rich habitats. There is evidence that bumble bee queens prefer to nest in flower-rich meadows (Bowers, 1985). The for- aging range of most non-honey bees is probably smaller than that of honey bees (Osborne et al., 1991), although this may not always be the case with bumble bees (Dramstad, 1996; Saville et al., 1997; Osborne et al., 1999). All things considered, it seems most advisable to place bee pastures as near as possible to the crop of interest. This increases the chance of bees nesting near to, and foraging on, the crop. Working with candidate bee pasture plants on cranberry farms, Patten et al. (1993) recommended ‘a sequence of early blooming forage plants adjacent to [cranberry] bogs’ to encourage bumble bee nesting near the crop.
In most cases, installed bee pastures should be designed so that pasture plants do not bloom at the same time as the crop of interest and thereby compete with it for pollinators. However, not all bloom overlaps are harmful. Some highly-attractive flowering plants, called magnet species, can attract large numbers of pollinators to a plant (crop) community (Thompson, 1978). With facilitation or spill-over pollination (Laverty, 1992), the rate of pollinator visitation in a crop is augmented due to the presence of nearby blooming magnet species, as shown by Brookes et al.(1994) in Canadian lucerne.
Not all experts agree on the value of establishing bee pastures, or bee sanctuaries in general. Torchio (1990a) questions whether such
‘reconstituted habitats’ are cost-effective in areas of intense agriculture and high land values. He further points out that habitat management programmes rarely address the problems of reintroducing and main- taining entire bee communities in areas from which they have become extinct. These are valid questions and must be addressed before bee habitat management programmes gain wide acceptance.
Nevertheless, we believe that bee conservation has the potential to be an important component of an overall agricultural land use policy.
Conservation efforts are well grounded in ecologic theory and supported
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by studies that consistently link high bee diversity with large, undis- turbed, florally-rich, long-blooming habitats. It remains to see if conser- vation programmes will practically and profitably translate into large and sustainable pollinator populations at the local level.