Washington State leads in the production of highbush blueberry (V. corymbosum) in the United States, with 54 million kg produced from 5423 ha in 2016 [United States Department of Agriculture National Agricultural Statistics Service (USDA NASS, 2017)]. One of the most challenging production issues for blueberry cultivation in western Washington is pollination and subsequent fruit set and reduced yields.
Effective insect-mediated pollination is important for optimal fruit set and maximizing berry size in highbush blueberry (MacKenzie, 1997). Most commercial producers in Washington and the greater Pacific Northwest (PNW) rent hives of Italian honey bees (A. mellifera ligustica) for pollination services. Hives are generally placed in fields between 5% and 25% full bloom (DeVetter et al., 2016; Howell et al., 1972). Despite the use of honey bees for pollination, there are reports of insufficient pollinator activity, low fruit set, and reduced yields in western Washington and elsewhere in the PNW (DeVetter et al., 2016).
Insufficient honey bee activity and low fruit set is attributed to several factors, including weather conditions during bloom, the shape and size of blueberry flowers, and the relatively short bloom window (≈7–12 d) in highbush blueberry (Courcelles et al., 2013; Dogterom et al., 2000). Honey bees are not well adapted to pollinate Vaccinium spp. (Buchmann, 1985; Tuell et al., 2009). Honey bees are native to Europe, western Asia, and Africa, not to North America, and consequently may be less efficient pollinators of some plants native to North America, like blueberry (Garibaldi et al., 2013; Javorek et al., 2002).
Honey bees are endotherms with flight restricted by light and temperature (Tuell and Isaacs, 2010). These flight criteria make western Washington a difficult environment for insect-mediated pollination of blueberry by honey bees because of its oceanic climate and northerly latitude (≈47°N). Rainfall and cloud cover is probable between October and May with as many as six out of seven cloudy days per week, further restricting solar radiation and the ability of honey bees to offset cooler temperatures with light (Western Regional Climate Center, 2017). Honey bees exhibit reduced foraging when air temperature is below ≈12 °C and when wind speeds are above 19 km·h−1 (Delaplane et al., 2000). The close proximity of western Washington to the Puget Sound increases the likelihood of coastal breezes exceeding this threshold. Consequently, reduced honey bee foraging during key pollination periods may negatively impact fruit set and other yield components in blueberry cultivated in western Washington and elsewhere in the PNW.
In addition to the reduced foraging activity during inclement spring weather, honey bees also have a shorter average tongue length than bumble bees (Bombus spp.), a pollinator that is highly adapted to pollinate blueberry (Balfour et al., 2013). This makes access to nectar rewards more difficult for honey bees foraging on blueberry. Honey bees also lack the ability to sonicate, or “buzz pollinate,” which is the vibration of flight muscles causing dehiscence of pollen from poricidal anthers. The inability to sonicate makes collection of blueberry pollen by honey bees more difficult relative to pollinators like bumble bees, which are able to sonicate.
Berry weight/size in highbush blueberry is positively related to seed number, so maximizing ovule fertilization and seed production is important in achieving large berry size (Pritts and Hancock, 1992). Larger fruit generally have a greater number of cells; however, marketable size has reportedly been achieved with as few as 10–20 seeds/fruit in some cultivars of northern highbush blueberry (Coombe, 1976; Vorsa, 1996). Given the low honey bee visitation rates observed in a recent statewide survey in western Washington, which was below the recommended 4–8 honey bees/bush guideline, fruit set and yield enhancement through promotion of berry size may start with increasing pollination in the field (DeVetter et al., 2016; Isaacs et al., 2016).
Honey bees used for commercial pollination of blueberry in western Washington and elsewhere in the PNW are frequently stocked at a density of 10 hives/ha; however, there is some variation because of the cultivar (DeVetter et al., 2016; Sagili and Burgett, 2011). These stocking density recommendations rely on healthy hives and proper timing of hive placement for optimal pollination efficiency. This project evaluates the current recommended hive stocking density for honey bees in the PNW region compared with a higher hive stocking density in ‘Duke’ blueberries. An evaluation of honey bee hive stocking density is justified in northwestern Washington because increased honey bee foraging may promote pollination and subsequent yield components during the compressed bloom window and during spring conditions where weather is unconducive to honey bee activity.
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