Maximizing Pollination Services for Blueberry Production in Pennsylvania

Final report for ONE19-340

Project Type: Partnership
Funds awarded in 2019: $29,990.00
Projected End Date: 03/31/2022
Grant Recipient: The Pennsylvania State University
Region: Northeast
State: Pennsylvania
Project Leader:
Dr. Margarita Lopez-Uribe
Penn State University
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Project Information


Blueberries are a high-value crop in the United States and one of the fastest-growing crops in Pennsylvania. This crop depends on bee pollination to maximize yields. While managed honey bees are regularly rented to achieve optimal pollination in commercial farms, there are two disadvantages with this approach to guarantee pollination: (1) honey bee rentals are costly, and (2) honey bees are less efficient pollinators of blueberries than wild bee species. The goal of this project was to partner with blueberry growers in Pennsylvania to generate information that will improve their production practices by maximizing pollination services. Specifically, we planned to (1) identify what bees are providing the greatest pollination, and (2) quantify pollination limitation. Results from this study show that the pollinator community of blueberries in central Pennsylvania is diverse and that there is little evidence of pollination limitation. However, growers must be aware of the current status (high value of wild bee pollination) and implement practices that support the conservation and maintenance of wild bee pollinators.

Project Objectives:

This proposal seeks to understand three aspects of blueberry pollination that will directly benefit our partner growers and others in Pennsylvania:

* Objective 1: Quantify the level of free pollination services provided by wild bee species. This information will benefit farmers by indicating the degree to which they could exclusively rely on free pollination services or need to augment this input with honey bee rentals.

* Objective 2: Determine what bee species are providing pollination services. By identifying the key wild pollinators of blueberry farms, farmers will be able to design informed management practices to enhance bee populations of the key pollinators (e.g., through pollinator plantings, additional nesting habitat, timing of crop protection practices on other flowering species in their farm, etc.)

* Objective 3: Determine whether blueberry farms in Pennsylvania are pollination limited. Quantifying the level of pollination limitation will allow farmers to determine whether or not they would improve productivity and receive economic gains by increasing pollination services.

The combination of these pieces of information will allow growers to understand their pollination needs, make informed decisions about how to maximize pollination services and focus their bee-conservation efforts for their farms.


Pollinator-dependent crops are increasingly at risk of reduced yields and less marketable fruit because of poor pollination. Declines in wild bee populations, the high rental costs of honey bees and climate unpredictability have led to less stable pollination services across the United States. Therefore, appropriate farm management practices to ensure optimal pollination services are necessary to maximize crop yields.

Blueberries are an economically important fruit crop that requires insect-mediated pollination to obtain profitable yield levels. Blueberry bushes produce more seeds, larger fruit, and higher yields with greater bee pollination services (Isaacs and Kirk 2010). Wild bees are on average three times more effective pollinators of blueberries than honey bees (Apis mellifera) on a per visit basis (Rogers et al 2014; Javorek et al 2002). In addition, blueberry flowers need to be sonicated (through buzz pollination) to release the pollen from flowers. Honey bees cannot perform buzz pollination, making them effective pollinators for blueberry only when wild bees are also present to release the pollen. Therefore, optimal management practices that support wild bee pollination can improve crop production while reducing honey bee rental costs, which would translate into a significant net gain for farmers.

* Problem 1: Honey bee colonies are regularly rented to achieve optimal pollination in blueberry farms due to the perception that native bee population sizes are unreliable and hard to monitor (Hanes et al 2015). While managed honey bees can be reliably rented for blueberry pollination every year, their numbers have declined in the past 15 years which has significantly increased honey bee rental costs (currently at ~$100/hive). This increase in honey bee rental costs translates into increased production costs for farmers and prices for consumers.

* Problem 2: Even with honey bee rentals, blueberries can be pollination limited. Blueberries require buzz pollination to release pollen from flowers, and honey bees are not capable of buzz pollination. Therefore, unless quantified, growers may not be aware that their yields could increase if pollination services improved in their farms.

* Problem 3: Blueberries are one of the fastest growing crops in Pennsylvania and there is not a single study assessing pollination services to blueberries in the region. Given the nature of diversified farms nested within complex landscapes in Pennsylvania, the need for pollination services in blueberries may not be comparable to systems of different scales and landscape contexts that have been heavily studied (see below).

Our collaborating farmer partners and results of surveys to blueberry farmers in Pennsylvania indicate the importance of understanding who pollinates their crops to better manage and protect these pollinator populations while reducing pollination limitation and ensuring fruit set and maximum fruit size. We will work with our partners to provide this information that will directly improve productivity and economic gains while facilitating future efforts across multiple diversified direct-retail farms in our region.



Click linked name(s) to expand/collapse or show everyone's info
  • Mark and Laura McDonald - Producer
  • Jarod & Robyn Scheiber - Producer


Materials and methods:

We worked with our two farmer partners to achieve the goals of the project. We used the Elliot variety in year 1 and Bluecrop in year 2. We visited each farm at least 5 times to accomplish the following activities: (1) mark branches for the 3 pollination treatments: (1) open pollination, (2) selfing, and (3) hand pollination. We chose a total of 10 shrubs on each farm and marked 3 flower clusters with flagging tape (about 30 unopened flowers per cluster) in each individual shrub. During the first visit, the flower clusters for treatment #2 were bagged to avoid biotic pollination.

After the flowers are in full bloom (one week after our first visit), we visited the farms again (at least 3 more times per farm) to collect the bee visitation data. To estimate visitation, we walked two rows of the crop for 10 minutes and we counted the number of flower-visiting insects. We also distinguished between insects that are pollinating flowers and those that are nectar robbing. The pollinating species were recorded in five categories that we can accurately identify on the wing: (1) honey bees; (2) bumble bees (queens & workers); (3) carpenter bee; (4) mid-size bee; (5) green bee; (6) small black bee. At the end of the visit of the 3rd visit to each farm, we bagged the flowers from the open pollination treatment (treatment #1). We hand pollinated the flowers in treatment #3, and bagged them as well. Three months after this 3rd pollination visit, we visited the farms again to harvest all the fruit from the branches included in the 3 treatments. We collected data on the percent fruit set by counting the number of berries on each cluster excluding the unpollinated berries drop. Berries were brought to the laboratory and weighted to estimate yield for the 3 treatments. 

In addition to our two farmer partners (Green Berry Farm and Bee Tree Farm), we also worked at the Penn State Experimental Ag Station (Rock Springs). We kept in close communication with the farmers during the duration of the experiment and we shared with them the annual reports that we obtained from all the data collection of the two years of the experiment. 


Research results and discussion:
  • Pollinator diversity at each site varied, with the smallest non-commercial planting having the greatest diversity of pollinators. While managed honey bee colonies were present at all sites, they were not commonly observed pollinating blueberries, compared to other wild bees such as bumblebees (54 visits by honey bees compared to 161 visits by bumblebee queens and workers in 2021). One major pollinators at all sites were mining bees in the genus Andrena.
  • In Central Pennsylvania, bagging flowers to keep out pollinators and forcing self-pollination resulted in a 71% reduction in the total number of ripe berries per flower cluster, and a 62% reduction in total harvest weight per flower cluster. This indicates that, at least for Blue Crop, pollinators are crucial for proper fruit production.
  • There was a 17% decrease in the average weight of each berry in the open-pollinated berries compared to hand-pollinated berries. Decreased berry weight is one sign of pollen limitation, so this may reflect slight signs of insufficient pollination at these sites
  • On average, we detect 27 bees visiting blueberry flowers per farm. Pollinator abundance from these three sites were linked with berry weight data and show that sites with higher pollinator abundance show less of this increase in berry weight in the pollen addition treatment.
Research conclusions:

The three farms in this study all had a variety of wild and supplemented bees that were overall effective at providing sufficient pollination services. There was limited evidence for pollen limitation reflected by the supplemental pollen additions that resulted in slightly larger berries. Therefore, growers in Pennsylvania should be aware of the critical importance of wild bees for production of blueberries, and in order to reach maximum crop yield growers should follow best practices to attract and maintain a diverse array of wild pollinators. 

Participation Summary
2 Farmers participating in research

Education & Outreach Activities and Participation Summary

1 Curricula, factsheets or educational tools
1 Published press articles, newsletters
2 Webinars / talks / presentations

Participation Summary:

2 Farmers participated
60 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

Education outputs include:

Learning Outcomes

2 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

Farmers in central Pennsylvania are aware of the need to optimize pollination services for blueberry crops. They acknowledge that their production could be over 15% more profitable if pollination was managed and optimized in their farms.

Project Outcomes

2 Farmers changed or adopted a practice
1 New working collaboration
Project outcomes:
  • This project demonstrated that the blueberry cultivar Bluecrop is highly pollinator-dependent. When flowers were bagged, we found a reduction of 62% in the weight of ripe berries. Thus, farmers need to provide pollinators to guarantee crop yields in this popular highbush blueberry cultivar.
  • In central PA, we found evidence of signs of pollination limitation with a 17% increase in berry weight in flowers where flowers were hand-pollinated (pollen was transferred manually to stigmas). These results suggest that farmers in central PA could increase their yields by over 15% if pollination was maximized on these farms.
  • We observed over 30 bee species visiting blueberry blossoms. About 50% of the species were solitary mining bees (genus Andrena). On average, we detect 27 bees visiting blueberry flowers per farm. Farms with higher bee abundance showed higher fruit weight, further emphasizing the importance of pollination to increase crop yields.
Assessment of Project Approach and Areas of Further Study:

This project provided critical information on a first step to understanding the pollination systems and needs of blueberry crops in central Pennsylvania where farms are small-scale and highly diversified. Even though we found high wild bee diversity and higher bee pollination abundance in the farms of our partners, we detected signs of pollination limitation. This is a call to farmers in the region about some existing problems with pollination and areas where farmers could change practices to maximize pollination and yields. Areas of further study include developing a better understanding of specific practices that farmers can use to increase wild pollinators. In our fact sheet, we give general recommendations, but further research is necessary.

Information Products

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.