Use of rented, purchased and native bees to pollinate highbush blueberries

Having sufficient bees present during bloom and diversifying the sources of pollination can help growers get the most from their blueberry plantings.

Bee on blueberryHighbush blueberries require pollination for ensuring that flowers present at bloom turn into large, harvestable berries later in the season. Without good pollination, highbush blueberry bushes may provide some harvestable fruit but yields will be much less than they could have been. By planning ahead for how fields will be pollinated, growers can help ensure they receive the expected return on their investments in land, bushes, and other management inputs.

Given the high per-acre input costs of blueberry production, spending money to ensure high levels of pollination makes sound business sense. Other things being equal, well-pollinated fields have larger berries, higher yields, and more even ripening than fields with sub-optimal pollination. This article focuses on pollination of northern highbush blueberry, Vaccinium corymbosum, which is the species grown in northern states and provinces.

Across Michigan’s blueberry industry, most pollination is by honey bees that are brought to fields in hives. These are arriving this week as blueberry bloom starts in southwest Michigan. Bumble bee colonies can also be purchased for placement in fields, and there are many other wild native bee species that nest in and around crop fields. By combining these pollinators into an integrated pest management (IPM) strategy, the risk of poor pollination may be minimized.

Pollen is moved by bees

For pollination to occur, sufficient blueberry pollen must be moved from the male part of flowers (anther) to the female part (stigma) while the flowers are receptive. Bees are responsible for this movement of pollen, so blueberry pollination depends on having enough bees active in the field during bloom to deliver pollen. Each flower must be visited once by bumble bees or most native bees or three times by honey bees to grow to maximum size. There can be 10 million flowers per acre, so there is a lot of work to do!

The pollen produced by blueberry flowers is relatively heavy and doesn’t waft on the wind. It is held inside the flower by salt shaker-like structures called anthers until bees visit. They may release the pollen by jiggling the flower with their legs, as is the case for honey bees. Bumble bees and some other native bees are better adapted to release the pollen using a vibration behavior. As the bees move from flower to flower, pollen grains are deposited on the tip of the stigma. Once compatible pollen is deposited on the stigma it germinates and fertilizes the ovules which produce the tiny seeds. Fertilized seeds stimulate berry growth, leading to larger berries.

Before planting: parthenocarpy, self-compatibility, and inter-planting

Northern highbush cultivars have some degree of parthenocarpy, producing berries even without pollen deposition. However, these berries will be small, slow to ripen, may drop off, and most would not be considered marketable (see photos below).

Northern highbush cultivars 

Many popular northern highbush blueberry cultivars are self-fruitful, meaning they can be fertilized by pollen of the same cultivar. This is one reason why solid blocks of Bluecrop are highly productive. In other cultivars, such as Nelson, cross pollination (from another cultivar) is essential for full pollination and yield, achieved by bees moving pollen between cultivars as they fly from row to row. In this situation, planting fields with alternating blocks of co-blooming and compatible cultivars ensures cross-pollination. While alternate rows of two compatible cultivars would be the best for cross-pollination in this situation, alternating blocks of up to 10 rows will allow exchange of cross-compatible pollen. There is a range of dependence on cross pollination across highbush blueberry cultivars, so before selecting cultivars and their planting arrangement growers should check the level of self-fruitfulness with the nursery.

Using honey bees

Wait until bloom has started to bring in bees. Flowers of blueberries are generally less attractive to honey bees than other flowers due to the relatively low nectar reward. Because of this, it is best to bring in bees once the crop has started to bloom so that bees forage more on blueberries than other flowers. If brought in too early, bees may learn to forage elsewhere reducing their focus on your crop fields. Under warm spring conditions, highbush blueberry flowers are viable for three to four days after the flowers open, so move bees into blueberry fields after 5 percent bloom but before 25 percent percent of full bloom.

Renting healthy colonies. If you are renting honey bee hives, you should expect to receive healthy and vigorous bees. A healthy colony contains around 30,000 worker honey bees and will have six frames of brood. Having weak hives will affect how much pollination the fields receive, so it is worth taking time to ensure you have strong hives. If you suspect weak colonies, talk to your beekeeper about getting additional hives or replacing them. One strong hive of 40,000 bees will provide better pollination than two 20,000 bee hives. One way for growers to ensure they receive strong colonies is to establish a pollination agreement that lays out the grower’s expectations. This can include the strength of the colonies and how quickly the colonies will be taken out of the field after bloom.

Stocking densities. Feral colonies of honey bees and abundant native bee populations used to contribute to blueberry pollination. However, mite pests have decimated the numbers of feral honey bee colonies, and many farms do not provide enough habitat for native bees to survive in high abundance. This makes fruit production more dependent than ever on managed bees, so it is important to stock fields with sufficient bees to supply enough visits to flowers. Research and experience in blueberries has shown variation across northern highbush cultivars in their needs for bee pollination (Table 1). If fields are managed for maximum production and have higher flower densities and yield, increased levels of honey bee stocking may be needed.

While Table 1 shows 2.5 hives per acre for Jersey and Earliblue, some growers are using even greater stocking to ensure good pollination even if spring weather is cool and there are only a few good days for honey bee activity. A good rule of thumb is that you’ll need four to eight honey bees per bush in the warmest part of the day during bloom to get blueberries pollinated.

Table 1. Recommended stocking density of honey bees for highbush blueberry pollination
(from Pritts & Hancock)

Variety

Honeybee hives/acre

Rubel, Rancocas

0.5

Weymouth, Bluetta, Blueray

1.0

Bluecrop

1.5

Elliot, Coville,   Berkeley, Stanley

2.0

Jersey, Earliblue

2.5

Hive placement. If possible, place the colonies in sheltered locations with the entrances facing east. This will encourage earlier activity as the hive warms in the morning sun. Hives should be spread out around the farm to maximize floral visitation, with a maximum of 300 yards between hives.

Using bumblebees

Bumble bees are very efficient at pollinating blueberry, with activity at lower temperatures than honey bees, faster visits to flowers, and higher rates of pollen transfer per flower visit. A bumble bee species native to eastern North America, Bombus impatiens, has been reared for use as a crop pollinator. Our recent evaluations with this species in commercial Jersey fields found they provided comparable yield and fruit set when compared with honey bees. These insects are available commercially and can be shipped directly to the farm. Koppert is one supplier based in Michigan that provides the bees in Quads, each containing four colonies housed within a weather-proof box. One Quad per acre is a good starting density if using bumblebees alone, but growers may also purchase bumble bees to integrate with honey bees, thereby diversifying pollination sources. This approach should help ensure movement of pollen between flowers during conditions that are unsuitable for honey bees.

Rearing bumble bees takes time so orders should be made 14 to 16 weeks in advance to guarantee delivery. Place Quads through the farm and away from honey bee hives. A door on the box of the Quads can be used to collect the bees and move them during spraying, plus they can be moved to later blooming cultivars and crops providing greater flexibility than honey bee colonies.

Native pollinators

In our recent studies at MSU, over 150 native (wild) bee species were found in Michigan blueberry fields. About 10 of these were sufficiently abundant during bloom and carried enough blueberry pollen to be considered valuable crop pollinators. Most of these are solitary ground-nesting digger bees in which the female bee tunnels into the soil, lays an egg in a side tunnel then collects pollen that is placed in a ball next to the egg as food for the larva. This process is repeated multiple times for the many eggs that the female bee lays. These bees need undisturbed soil and have been seen nesting underneath blueberry bushes in the weed-free strip. Other native bees such as bumble bees are active through the whole season. These species also need undisturbed soil to nest in abandoned rodent burrows or grass tussocks, but they will also use old mattresses, compost piles, and other protected sites with small entrances.

In small blueberry fields surrounded by diverse landscapes, native bees can provide the majority of pollination. However, as blueberry farm size and intensity increase, the high abundance of flowers and the small amount of natural area results in too few native bees for full pollination, and so growers rent honey bees. Still, by creating bee habitat that includes a mix of plants that bloom before and after blueberries growers can help support native bees as part of an IPM strategy.

For more on native plants to support pollinators, visit MSU’s Native Plants and Ecosystem Services website. Every little bit of habitat will help, so consider this a long-term process of building bee habitat back into the farm landscape. Currently, the Farm Service Agency (FSA) is providing a generous cost share for growers in West Michigan counties interested in establishing pollinator habitat in their farms. See your local FSA or NRCS office for details of the SAFE Program for Pollinators. Other programs are available that can also help support improvement of habitat for bees on farmland.

Pest management during pollination

To protect pollinators, do not apply broad-spectrum insecticides when blueberry flowers are open. By monitoring for pest problems carefully before and during bloom, growers can help minimize the need for pest control. If an insecticide application is necessary during bloom, the compounds that are least toxic to bees should be used, with careful observation of the pollinator restrictions on the label. Two insecticides that can both be applied during bloom for control of moth larvae in blueberry are products containing Bacillus thuringensis or Bt (i.e., Dipel, Javelin), and the insect growth regulators Intrepid and Confirm. It is best to have an agreement with the beekeeper in advance about what to do if you need to make pesticide applications during bloom, and be sure to inform the beekeeper two to three days before an application so that precautions can be taken to minimize bee exposure.

Make applications when bees are not foraging; late evening application is better than morning application because the insecticide has time to be absorbed and for the residue to dry before bees are active. Dust formulations should be avoided because particles can be picked up easily by the bees’ hairy bodies.

More information and a list of chemicals with their toxicity to bees is available from a recently-updated extension bulletin, “How to Reduce Bee Poisoning from Pesticides,” from Oregon State University.

Summary

Pollination is an essential component of growing blueberries, and high levels of fruit set and getting large, evenly ripening berries requires bees to deposit enough pollen on stigmas during bloom. Most of this is done by honey bees currently, while there are other managed and wild bees that can contribute to pollination. As with pest management, reliance on one strategy may not be the most sustainable approach, so diversifying pollination sources can spread risk to ensure consistent pollination and profitable yields every year.

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