Blueberry Pollination Project

Jackie Perkins and Rufus Isaacs

During blueberry harvest, we see fields filled with blueberries weighing down the shoots of bushes. But how did those berries develop through the season, and what is the role of pollinators in berry development?

Bees are essential for helping these fruits grow into harvestable berries. During bloom, bees need to visit blueberry flowers to transfer pollen from the male part of the flower (stamen) to the female part (pistils or stigma). The pollen then germinates, and the pollen tubes grow up the stigma and into the ovaries. If the pollen is compatible and weather conditions are suitable, this results in fertilized ovaries that develop into seeds. These release hormones that trigger the swelling of the berry into a large and ripe fruit. Flowers that receive adequate pollination have more success becoming berries (we call this percent fruit set), and these berries are larger than those that received fewer pollen grains deposited by pollinators. With blueberry yield being so dependent on the movement of pollen, growers are very interested in maximizing the pollination potential on their farms.

How do we know that a field has reached its maximum potential for pollination? Our research team in the Bees and Berries project has used meticulous methods to track individual blueberry shoots and their flowers all the way from pre-bloom through to harvest to answer this question.

Prior to bloom, we randomly selected 5 bushes along 4 transects to use for our pollination experiment. On each bush, 4 similar shoots were selected and flagged, and the number of flowers on each shoot was recorded. Each shoot was given a unique code written on the flagging tape so that it could be tracked throughout the season, allowing us to measure exactly how many flowers turn into harvestable fruit. Each shoot was assigned to receive one of the following treatments:

1. Open. Flowers were able to receive pollination from bees.
2. Closed. Flowers were covered with a mesh exclusion bag prior to opening to prevent insects from visiting.
3. Full pollination. Flowers received pollination from bees, plus they were hand-pollinated 2-3 times during bloom to ensure full pollination of each flower. We collected pollen from the same fields and brushed it onto the stigmas of open flowers on these shoots.

This method allows us to measure the effect of pollination on fruit set and berry size to estimate the effects of pollination and the effects of the different management approaches to pollination. We can learn a few different things from comparison of the different treatments. For example, the difference between the closed and open treatments indicates the benefit of bees and other pollinators. The difference between open and full tells us if there is any pollination limitation occurring, where the bees available are not reaching the full pollination potential of the field.

Once bloom was complete, all shoots were covered with mesh exclusion bags to protect them from damage, pests, wildlife, or accidental harvest. Now that harvest is underway, we are returning to the fields to assess the pollination success of our different treatments using the following metrics:

1. % Fruit Set – the percent of flowers that successfully became fruit
2. Berry weight – the size of the berries
3. Seed set – number of fertilized seeds present within ripe fruits

Using these methods, we can measure how well the fields in our research trials have been pollinated and use that information to compare the different honey bee treatments and determine the economic value of different pollination strategies. This winter we will have the results from these trials in Michigan, Oregon, and Washington farms and will be reporting the results back to collaborators and our regional blueberry growers.

In today’s post, we go back to the basics of honey bees. First, we explore the honey bee colony: what it is, how it works, and who’s inside. Then, we discuss the deployment of hives on-farm.

Pollinators, such as bees, wasps, flies, and other insects and small animals are essential to crop production. Bees are considered some of the most important pollinators, of which honey bees, in particular, are widely regarded as the most economically important due to their widespread use in commercial pollination. Honey bee hives are deployed by the thousands across the US to pollinate an array of domestic crops, including blueberries.

If you are a grower, you are likely familiar with this setup. Each assembly seen above is a group of four individual hives sharing one square platform. In this image, each hive consists of 3 boxes (hive sizes and number of boxes may vary depending on beekeeper management). The large bottom boxes are known as “deeps.” These boxes contain the honey bee larvae, or brood. The small upper boxes are known as “supers.” These boxes contain the majority of the honey. Each box contains eight or ten “frames,” where the bees draw wax comb to store honey and pollen, and rear brood.

You have likely heard the terms “colony” and “hive” used interchangeably in reference to honey bees, but it is important to note that there is a distinction between the two. Think of the hive as the structure the bees live in, or in other words, their home. The colony, on the other hand, refers to the family unit of bees that live in the hive.

Honey bees are social insects, meaning they cooperate as a group in caring for their young, cleaning and protecting the nest, and feeding the colony. The colony is organized in a caste system, consisting of one reproductive queen bee, hundreds of male drones, and tens of thousands of sterile female workers. As the matriarch and only female with reproductive abilities, the queen is primarily responsible for laying eggs, sometimes over a thousand per day. Drone bees serve only one purpose, which is to mate with a queen. They do not contribute to any nest maintenance or output. The largest caste, the worker caste, is responsible for all other colony productivity, including foraging, brood care, honey production, and colony defense.

Towards the end of her lifespan, a worker bee transitions to the role of foraging, leaving the hive to collect food for the colony. Honey bees survive on a diet of nectar and pollen, both of which they gather from flowers. As a bee visits a flower to collect its resources, she becomes covered in sticky pollen thanks to the tiny hairs all over her body. When she visits the next flower, she will unintentionally leave behind some of the pollen from the previous flower. This is how insect-mediated pollination occurs.

Foragers comprise roughly one-third of the total population of the hive. Foragers are the pollination workforce. The rest of the hive population is mostly made up of younger worker bees and brood, neither of which leave the hive or contribute to foraging/pollination. It is vital to have healthy, large colonies present for pollination to ensure an adequate number of mature worker bees to forage and provide pollination. The number of boxes on a hive does not necessarily indicate more bees available for pollination services.

One of the goals of our research is to develop methods to allow growers to quickly assess the strength of the colonies they have rented for pollination. We aim to understand the relationship between forager activity in the hives used in pollination contracts and flower visitation rates by foragers observed in the fields. This research will help us determine the extent to which colony strength, size, and stocking density combine to predict both the level of bee activity on flowers during bloom and crop yield. This analysis may also reveal whether individual colony strength or colony stocking density is more important in determining the number of bees visiting blueberry flowers. These findings will help to make improved recommendations for how many hives are needed per acre to provide an adequate pollination workforce for the blueberry varieties in bloom in modern farming systems.

Bumble bees are efficient pollinators of blueberry and outperform honey bees at the individual level. So, the use of managed species in the western US is a good thing, right? This article summarizes the promises and potential pitfalls of managed bumble bees.

https://www.growingproduce.com/fruits/the-promise-and-pitfalls-of-using-managed-bumblebees-for-blueberry-pollination/

Over the last decade we have seen major advances in blueberry breeding, horticulture, and pest management, but the extension recommendations for pollination have not kept pace with these advances. This project aims to develop modern pollination recommendations and decision support tools by combining expertise from applied entomology, apiculture, horticulture, breeding, economics, and meteorology We will focus in Michigan, Washington, Oregon, and Florida to compare and conduct cost-benefit analyses of different strategies for honey bee pollination; determine pollination requirements across cultivars; determine how variable weather conditions affect blueberry pollination; develop predictive models of pollination for a Pollination Planner; and deliver information on improved blueberry pollination to the industry.

We are using The Pollination Triangle shown below to guide our approach. We aim to understand the interactions between bees, blooms, and weather so we can predict how those components predict yield. 

This knowledge will then be developed into a Pollination Planner that will allow growers to explore the implications of different honey bee stocking densities, weather conditions, and horticultural decisions on yield. We also will connect this to costs, revenues, and profit through economic analysis.

The project is just starting, so there are no results to share yet. Please connect with us online or via email to learn more about the project as it develops!

Our team is compiling information from Season 1 of this project. It will be posted soon.