With recent declines in honeybee populations, manageable wild native bees could help augment natural rates of blueberry pollination (Sampson et al., 1995). However, raising enough healthy, pest-free bees for even limited release may take years. Currently in the United States, honeybees, and bumble bees to a much smaller extent, are the only available commercial pollinators of cultivated blueberries, yet a worker honeybee typically is a reluctant floral visitor because the recessed nectaries of blueberry flowers are often out of easy reach of her proboscis (Goodman and Clayton-Greene, 1988). A longer tongue provides many native bees with greater access to blueberry nectar. A well-nourished native bee will actively harvest blueberry pollen, which is linked to greater pollination efficiency, higher fruit sets, and larger berries (Sampson and Cane, 2000). Perhaps the efficiencies of honeybees and other manageable bee species can be improved by selectively breeding blueberry cultivars with bloom morphologies that complement pollinator foraging behavior (Dafni and Neal, 1997).
Selection targeting specific floral traits might give bees greater access to floral nectaries, anthers, and pistils, thereby increasing pollinator efficiency (Lyrene, 1994a). Blueberry flowers however do not make it easy for pollinators to gather pollen and nectar rewards. In fact, pollinator visitation and pollination efficiency, particularly of honeybees, can be limited by a blueberry’s long narrow flowers and poricidal anthers (Brewer and Dobson, 1969; Danka et al., 1993; Eck, 1986; Lang and Danka, 1991). These and other floral traits of course will vary as blooms age to give bees access to a flower’s reproductive organs at the most opportune time. Even a modest change to flower structure can profoundly affect a bee’s efficiency (Eck and Mainland, 1971; Sampson et al., 2004a). For instance, a robbery slit cut into the side of corollas by a carpenter bee effectively reduces a nectary’s depth to hungry honeybees. The bees can then handle blueberry blooms more proficiently, stay longer within the crop, thereby doubling and even tripling visitation rates and pollen transfer (Sampson et al., 2004a). In addition to traits that foster bee cross-pollination, shorter styles and narrower corolla apertures may promote autogamy among self-compatible blueberry genotypes (Ritzinger and Lyrene, 1999). For commercial blueberry production, autogamy and self-compatibility may act as an insurance policy against total crop failure during acute pollinator shortages. Selfing and cross-pollination in the same blueberry cultivar can produce extraordinary fruit sets of between 70% and 80% (Sampson and Spiers, 2002).
Could intense selection and inbreeding in rare instances actually produce sterile or non-functioning blooms? Variability in corolla shape does explain how honeybees pollinate certain rabbiteye and highbush blueberry cultivars more efficiently (Davies and Buchanan, 1979; Eck and Mainland, 1971; Goodman and Clayton-Greene, 1988; Lang and Danka, 1991; Lyrene, 1994c; Martin, 1966; Sampson and Cane, 2000). Interestingly, many cultivated blueberries express a peculiar floral trait that includes a whole array of deformities that might affect flower function and viability. One cultivar in particular, Vaccinium virgatum Aiton (syn. V. ashei Reade) ‘Premier’, often bears blooms that are to varying degrees deformed. These blooms could baffle some flower-visiting bees like oligolectic Osmia ribifloris and hence impinge on the bee’s pollination efficiency (Sampson et al., 2004b). Because most blueberry plantings contain one-third or more ‘Premier’ bushes, growers are becoming increasingly concerned that deformed blooms on this cultivar are incapable of enticing sufficient pollinator visits for fruit set. Some farmers believe that the misshapen blooms of ‘Premier’ are the result of early floral bud injury inflicted by insect pests. However, these floral deformities appear to originate from a heritable trait derived from ‘Ethel’ (V. virgatum) used in early blueberry breeding programs. If these blooms are too misshapen, bees may fail to recognize them as legitimate food sources and bypass them entirely, the consequences of which would be the setting of few if any marketable berries.
Traits of blueberry blooms, even of deformed flowers, are easily measured. These traits may be used as handy selection criteria that could help predict optimal fruit set without the need for laboriously tracking fruit set for each cultivar (Eck and Mainland, 1971; Lyrene, 1994a). Floral traits and pollen viability are heritable in Vaccinium and through selection may help improve a cultivar’s rates of self-pollination, cross-pollination, or both (Ballington and Galletta, 1978; Lyrene, 1994b; Megalos and Ballington, 1987, 1988). Therefore, the objective of this study is to identify Vaccinium floral traits that promote autogamy and bee cross-pollination. The pollinator we chose to investigate is an oligolege of ericaceous plants (a blue orchard bee, Osmia ribifloris). This bee is currently under evaluation as a commercial blueberry pollinator. Although O. ribifloris are only indigenous to the western United Sates, their dietary host range includes plant species related to their western ericaceous host, Arctostaphylos. That is, O. ribifloris will readily adopt as suitable floral hosts several eastern species of blueberry (Sampson and Cane, 2000; Sampson et al., 1995; Stubbs et al., 1994; Torchio, 1990).
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