Commercial production of northern highbush blueberry (Vaccinium corymbosum) is concentrated in the PNW, which includes Washington State, Oregon, and British Columbia. Washington State is the leading national producer, representing ≈18.6% (4452 ha) of total national cultivated blueberry production in the United States of America in 2015 (NASS, 2016). Despite the significance of the industry, production is reportedly limited by pollination deficits, particularly in western Washington (Washington Highbush Blueberry Commission, personal communication). Effective pollination is essential for optimal fruit set and large berry size in most commercial cultivars of northern highbush blueberry and is insect mediated (MacKenzie, 1997). Several tactics have been employed in the past to enhance bee pollination in crops including blueberry, where environmental conditions are often suboptimal during bloom or the crop is not attractive to bees (Sagili et al., 2015). Most of the blueberry plantings in Washington and elsewhere employ honey bees (Apis mellifera ligustica) for pollination, which are rented from commercial beekeepers and placed in fields at ≈5% bloom. These rented colonies are particularly valuable for large plantings given native bee populations and their pollination contributions are relatively low (Isaacs and Kirk, 2010).
Blueberry flowers primarily exhibit protandry, whereby flowers release pollen before stigmas are receptive (Vander Kloet, 1988). Receptivity of flowers to pollination is related to flower age, with receptivity typically limited to 5 d after anthesis (Merrill, 1936; Moore, 1964). These features limit the effective pollination period of blueberry. Pollination and fruit set may be further constrained in Washington due to unfavorable weather conditions that limit pollinator activity. Tuell and Isaacs (2010) demonstrated that foraging activity of honey bees was reduced in Michigan-grown blueberry during poor weather. Honey bee flight and foraging activity begins between 12 and 14 °C, which is frequently not achieved during the typical cool, wet springs in western Washington and the greater PNW (Winston, 1987). Additionally, high wind velocities can further reduce honey bee activity. Pollination declines over time due to poor health of both domestic and wild pollinators is also suspected to contribute to pollination deficits in blueberry. As reviewed by Potts et al. (2010), pollinator health and subsequent crop pollination declines are likely due to an array of integrated and interacting factors, including pests and pathogens [e.g., ectoparasitic varroa mites (Varroa destructor)], malnutrition, loss of habitat, decreasing genetic diversity, improper pesticide use in both hives and fields, and climate change.
Research on promoting pollination within highbush blueberry and the variable environmental conditions in which they are produced is limited. Pritts and Hancock (1992) recommended stocking densities of 0.2 to 0.8 healthy honey bee colonies per hectare in northeastern United States. Recommendations from research performed in Oregon range from 0.2 to 1.2 healthy colonies per hectare (Sagili and Burgett, 2011; Strik et al., 2006). In Washington, we have observed stocking densities ranging from 0.4 to 3.2 hives per hectare within a given cultivar. This wide range across farms reflects uncertainty in pollinator management and appears to be the result of a lack of adequate research-based information in formulating recommendations for optimizing pollination. Recommended stocking densities should vary based on cultivar, which differ in their degree of self-incompatibility, attractiveness to pollinators, and flower morphology (Courcelles et al., 2013; MacKenzie, 1997; Strik et al., 2006). Very little is known about cultivar-based honey bee stocking densities and none of the reviewed recommendations include ‘Duke’, which is one of the most widely planted cultivar in Washington. Furthermore, recommendations may need to be optimized according to climactic conditions due to the influence environmental conditions have on pollination. This observation is relevant to Washington State given the differences between the cool and humid maritime climate of western Washington and the semiarid climate of eastern Washington (DeVetter et al., 2015).
Investigating and developing a comprehensive understanding of the factors limiting pollination and fruit development would be beneficial to the blueberry industry in Washington and the PNW region in general, as it seeks to overcome these limitations and improve yields. The objective of this study was to survey honey bee activity in commercial plantings of ‘Duke’ highbush blueberry in western and eastern Washington and to assess the relationship between honey bee activity, growing region, and select yield components. Additionally, we also surveyed honey bee colony strength to monitor this variable’s relationship to honey bee activity and measured yield components. Documentation and knowledge of honey bee activity, colony strength, and potential pollination limitations would enable blueberry growers to work with researchers and other entities to refine their approaches to enhance pollination, as well as overall crop yields.
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