Managed and Wild Bee Flower Visitors and Their Potential Contribution to Pollination Services of Low-Chill Highbush Blueberry (Vaccinium corymbosum L.; Ericales: Ericaceae)

Toward evidence-based decision support systems to optimize pollination and yields in highbush blueberry

Highbush blueberry (Vaccinium spp.) is a globally important fruit crop that depends on insect-mediated pollination to produce quality fruit and commercially viable yields. Pollination success in blueberry is complex and impacted by multiple interacting factors including flower density, bee diversity and abundance, and weather conditions. Other factors, including floral traits, bee traits, and economics also contribute to pollination success at the farm level but are less well understood. As blueberry production continues to expand globally, decision-aid technologies are needed to optimize and enhance the sustainability of pollination strategies. The objective of this review is to highlight our current knowledge about blueberry pollination, where current research efforts are focused, and where future research should be directed to successfully implement a comprehensive blueberry pollination decision-making framework for modern production systems. Important knowledge gaps remain, including how to integrate wild and managed pollinators to optimize pollination, and how to provide predictable and stable crop pollination across variable environmental conditions. In addition, continued advances in pesticide stewardship are required to optimize pollinator health and crop outcomes. Integration of on- and off-farm data, statistical models, and software tools could distill complex scientific information into decision-aid systems that support sustainable, evidence-based pollination decisions at the farm level. Utility of these tools will require multi-disciplinary research and strategic deployment through effective extension and information-sharing networks of growers, beekeepers, and extension/crop advisors.

Precision management of pollination services to blueberry crops

While the cultivated area of pollinator-dependent crops is increasing, pollinator availability is decreasing, leading to problems in many agroecosystems. For this reason, pollinator-dependent crop growers often rent beehives to support their pollination requirements to sustain fruit productivity. However, the efficiency of those pollination systems has not been extensively studied. Here, we compared the effect of “precision” pollination (i.e., application of pesticides coordinated with growers, audit of hives, dietary supplementation and individual distribution of hives) with conventional practices (i.e., pesticides applications without coordination with growers and no audit of hives, low maintenance of hives and hives distributed in large groups) on the mean level of pollination and fruit production and quality in blueberry crops. In nine blueberry fields, we measured bee visitation rate to flowers, fruit set, fruit firmness and fruit weight. On average, precision-pollinated plots had 70% more bee visits to flowers and produced 13% more fruits that were 12% heavier and 12% firmer than those obtained through conventional practices. These results showed that pollination efficiency could be improved if key management related to bee strength, distribution and health care are taken into account. Due to these results, we encourage growers and beekeepers to include precision pollination practices to both increase the productivity of blueberry fields and the wellbeing of honey bees within agroecosystems.

Identities, concentrations, and sources of pesticide exposure in pollen collected by managed bees during blueberry pollination

Bees are critical for crop pollination, but there is limited information on levels and sources of pesticide exposure in commercial agriculture. We collected pollen from foraging honey bees and bumble bees returning to colonies placed in blooming blueberry fields with different management approaches (conventional, organic, unmanaged) and located across different landscape settings to determine how these factors affect pesticide exposure. We also identified the pollen and analyzed whether pesticide exposure was correlated with corbicular load composition. Across 188 samples collected in 2 years, we detected 80 of the 259 pesticide active ingredients (AIs) screened for using a modified QuEChERS method. Detections included 28 fungicides, 26 insecticides, and 21 herbicides. All samples contained pesticides (mean = 22 AIs per pollen sample), with pollen collected from bees on conventional fields having significantly higher average concentrations (2019 mean = 882.0 ppb) than those on unmanaged fields (2019 mean = 279.6 ppb). Pollen collected by honey bees had more AIs than pollen collected by bumble bees (mean = 35 vs. 19 AIs detected at each farm, respectively), whereas samples from bumble bees had higher average concentrations, likely reflecting differences in foraging behavior. Blueberry pollen was more common in pollen samples collected by bumble bees (25.9% per sample) than honey bees (1.8%), though pesticide concentrations were only correlated with blueberry pollen for honey bees. Pollen collected at farms with more blueberry in the surrounding landscape had higher pesticide concentrations, mostly AIs applied for control of blueberry pathogens and pests during bloom. However, for honey bees, the majority of AIs detected at each farm are not registered for use on blueberry at any time (55.2% of AIs detected), including several highly toxic insecticides. These AIs therefore came from outside the fields and farms they are expected to pollinate. For bumble bees, the majority of AIs detected in their pollen are registered for use on blueberry during bloom (56.9% of AIs detected), though far fewer AIs were sprayed at the focal farm (16.7%). Our results highlight the need for integrated farm and landscape-scale stewardship of pesticides to reduce exposure to pollinators during crop pollination.

Blueberry Yields Increase With Bee Visitation Rates, but Bee Visitation Rates are not Consistently Predicted by Colony Stocking Densities

Pollinator-dependent crops rely on the activity of managed and wild pollinators. While farm management and surrounding landscape can influence wild pollinator contributions, managed pollinator contributions may be primarily driven by their stocking densities, though this is not well studied across crops. We selected 20 southern highbush blueberry farms along two independent gradients of honey bee Apis mellifera L. (Hymenoptera: Apidae) stocking density (~1-11 hives/acre) and bumble bee Bombus impatiens Cresson (Hymenoptera: Apidae) stocking density (0 - 3 colonies/acre) ensuring that stocking densities were not correlated with farm or landscape attributes. Across farms, we observed managed and wild bee visitation rates, and measured yield estimates. Farms with greater bumble bee stocking densities had higher bumble bee visitation rates and yield estimates, but farms with higher honey bee stocking densities only received higher honey bee visitation rates at the end of bloom and did not have higher yield estimates. The main wild pollinator, the southeastern blueberry bee Habropoda laboriosa (Fabricius) (Hymenoptera: Apidae), showed higher visitation rates on organic farms and in late bloom. In general, higher visitation rates by honey bees, bumble bees, and H. laboriosa were correlated with higher yields. Our results suggest that yields are limited by bee visitation rates, and that within the stocking density ranges studied, increasing managed bumble bees, but not honey bees, increases their visitation rates. While H. laboriosa had the greatest effect on yield estimates, its activity appears to be limited by both a phenological mismatch with crop bloom and farm management.

Complementary Contribution of Wild Bumblebees and Managed Honeybee to the Pollination Niche of an Introduced Blueberry Crop

The entomophilous pollination niche (abundance, phenotypic traits, foraging behaviours and environmental tolerances of insect pollinators) helps to understand and better manage crop pollination. We apply this niche approach to assess how an entomophilous crop (blueberry, Vaccinium ashei) can be expanded into new territories (i.e., northern Spain) far from their original area of domestication (North America). Insect visits to blueberry flowers were monitored in a plantation on 12 different days, at 8 different times during day and covering various weather conditions. Abundance, visitation rate, pollen gathering behaviour, and frequency of inter-plant and inter-row movements were recorded. The pollinator assemblage was basically composed of one managed honeybee species (50.8% of visits) and three native bumblebee species (48.3%). There was a marked pattern of seasonal segregation throughout bloom, with bumblebees dominating the early bloom and honeybee the late bloom. Pollinators also segregated along gradients of daily temperature and relative humidity. Finally, the two pollinator types differed in foraging behaviour, with bumblebees having a visitation rate double that of honeybee, collecting pollen more frequently and changing plant and row more frequently. The spatio-temporal and functional complementarity between honeybee and bumblebees suggested here encourages the consideration of an integrated crop pollination strategy for blueberries, based on the concurrence of both wild and managed bees.