East Lansing, Mich. - It's a simple equation: more bees in the blueberries equals bigger berries in the basket for more money in the pocket.
Pollination is an important aspect of blueberry production and is essential for high yields, Dr. Rufus Isaacs, Michigan State University research entomologist, said during the second Integrated Crop Pollination (ICP)webinar in a series being presented by eXtension.
"Very clearly, pollinated blueberries are larger and ripen faster," Isaacs told his international audience of more than 120 people as he shared research garnered from the research program he leads that explores the pollinators and pests of economic significance to berry crop growers.
Blueberry pollen is rather heavy and is held closely within the flower, which shakes out grains of pollen like a salt shaker. Bumble bees are great blueberry pollinators, thanks to their ability to sonicate, or buzz-vibrate the flower to a certain pitch that causes a generous release of grains. Other pollinators, like wild bees and honey bees, hit the anthers with their legs.
Studies clearly show that pollination with compatible pollen is critical for more of the blueberry flowers to turn into fruit, as well as for fertilization of more of the tiny seeds inside each blueberry. "More fertilized seeds means larger berries. There is really good correlation; research shows that, as the number of bees go up, the weight goes up," he said.
A comparison of berry weights on northern highbush clusters showed the cultivar made a difference in how much impact bees and cross-pollination had on weights.
For instance, Bluecrop is considered a self-fertile variety. Self-pollinated clusters had a 33 percent increase in weight and clusters cross-pollinated by bees had a 34 percent increase in weight over un-pollinated clusters.
However, self-pollinated Jersey clusters showed a 39 percent increase in weight over un-pollinated clusters, but cross-pollination by bees showed a weight gain of 82 percent. Nelson had a 30 percent weight gain when self-pollinated, compared to 72 percent increase in weight when clusters were pollinated by bees.
So more bees mean more pollination which means more berries and more seeds which means larger berries, Isaacs summarized.
In Michigan, almost 90 percent of the pollination on large commercial blueberry farms in Michigan is provided by rented honey bee hives supplied by beekeepers.
Stocking rates vary based on the particular cultivar and flower density. Younger smaller bushes, for example, will have less flowers and need less bees than full-size, well-grown bushes.
The recommended stocking density ranges from 0.5 honey bee colonies per acre for Rubel plantings to 1.5 colonies for Bluecrop, two for Elliot and 2.5 for Jersey and EarliBlue. To lower the risk of poor pollination, some growers are investing in up to six colonies per acre.
To enhance bee effectiveness when renting hives, bear in mind that colonies from warmer regions tend to be stronger than those overwintered in the north.
And, since honey bees are most effective at pollination if colonies are strong, work with your beekeeper to get strong hives, Isaacs advised. For example, a WA Grade A colony is six frames, two-thirds covered with bees at a temperature of 65 degrees, which equates to about 15,000 bees.
The hives should be placed among the blueberries after five percent of the flowers were in bloom, in order to keep the bees focused on the target crop, and spread throughout the fields.
Wild bees and other pollinators can be good insurance, helping spread the risk and augment the work of honey bees, who don't particularly like working in overcast, cold and wet weather.
Some, like bumble bees, are efficient blueberry pollinators. Wild bees are adapted to local spring weather and more likely to fly when it's cooler. Being wild, many are also free, Isaacs pointed out.
Each blueberry stigma needs over 25 pollen grains for full pollination. A bumble bees can pollinate 12 flowers per minute, compared to five flowers a minute for a miner bee and two flowers per minute for a honey bee. The tradeoff is the multitude of honey bees at work at one time, Isaacs noted.
Bumblebees nest in insulated cavities in the ground. The queens emerge in early spring and grow their colony of workers. Ready-to-go colonies are commercially available, typically with one queen and 200 workers.
Like bumblebees, other wild bees use blueberry pollen to feed their young. Such bees are particularly important in smaller operations.
Research shows more wild bees and a greater diversity of bees when blueberry fields are near natural areas where the pollinators can find nesting sites and have access to diverse pollen sources, which is necessary for their health.
"Bees with access to diverse pollen and nectar sources live longer, have greater reproduction and lower pathogen infection and can resist pesticides better", Isaacs said.
In turn, field research shows that pollinator habitat can support increased blueberry yield.
Wild bee species need nesting habitat and flowers for food outside of the crop bloom, Isaacs said.
He advised his listeners to consider contacting local FSA and NRCS agencies for available cost-share programs to help support wild bees' need for flowering habitat and nesting sites.
When it comes to bees and other pollinators, insecticide and fungicide safety issues throughout the season is an important consideration. "Bees can pick up pesticide residues in the plants and in the soil where they are nesting," Isaacs noted.
Since toxicity levels vary by the product used, it is useful to know the LD50 rate (typical dose killing 50 percent of worker honeybees after 24 hours). The information can be found on the product or on the University of California IPM "Bee Precaution Pesticide Rating" website.
Studies on the relationship between pesticide use and the number of bees and the number of bee species show that wild bee richness declines with more insecticide use.
"Growers need pest control as well as pollination. Trying to find the balance is important," Isaacs said.
He advises growers to use IPM tactics for pest management, select the least bee toxic options and follow the label directions, which is the law. Minimize pesticide use during bloom to support honey bees, bee keepers and wild bees, and avoid pesticide contact with bees by applying products after sunset and reducing drift.
For more information on integrated crop pollination, visit www.projecticp.org. For information on floral resources and regional pollinator plant lists, visit www.xerces.org and www.michiganwildflowerfarm.com . For The Pollinator Partnership, check out www.pollinator.org.