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Mechanical pollination shows promise for managing the crop load in cherry and apple orchards, preliminary research at Washington State University indicates.

With the threat of declining honeybee numbers in the United States, crop pollination has become an important issue facing agriculture. A national call for bee research resulted in projects focused on finding alternate pollinators, improving habitat for pollinators, and solving colony collapse disorder. The call also triggered the start of a small project that ultimately could make bee pollinators and pollinizers unnecessary in some tree fruit and vegetables crops.

WSU’s Dr. Matthew Whiting, cherry horticulturist, and Probir Das, WSU graduate student, spent last year working on research to prove that using an orchard sprayer to pollinate tree fruit is a viable concept.

The preliminary study showed that pollen can be suspended in solution, maintaining viability for at least an hour, and can be applied through a commercial electrostatic sprayer, with minimal modification. Moreover, the pollen reached the flower stigmas and shows potential to supplement or replace the current system that relies on pollinizers and pollinators.

The proof-of-concept study involved field and laboratory trials. Field work, conducted in several orchards with multiple apple and cherry varieties, involved using an electrostatic sprayer to make two applications of pollen at three different rates.

Netting was applied to whole trees and single limbs to exclude bees for the mechanical pollination study. Pollen was applied mechanically through the netting to flowers opening inside.  (Courtesy Matt Whiting/Washington State University)

Netting was applied to whole trees and single limbs to exclude bees for the mechanical pollination study. Pollen was applied mechanically through the netting to flowers opening inside.
(Courtesy Matt Whiting/Washington State University)

Trees were netted to keep bees out, and pollen was sprayed when 50 percent and 90 percent of the flowers were open. Scientists evaluated pollen viability of three sweet cherry pollen genotypes and developed a formulation that would suspend pollen in solution, move through the sprayer system without clogging, and maintain pollen viability long enough to stick to the flower stigmas and fertilize the ovules.

Fruit set was 15 percent higher in one cherry orchard that was mechanically pollinated than when natural pollination was used, Das reported during winter tree fruit talks in Washington. In the apple trial, which evaluated two different pollen solutions, fruit set increased 56 and 75 percent compared to natural pollination.

“We are really at the initial stage in research and are now just really getting started,” Whiting told Good Fruit Grower. “We proved that it was possible to put live tissue (pollen) through a pressurized sprayer, with its filters and nozzles, have it survive all that and still be alive, and then land on a tiny stigma and result in a viable fruitlet.”

Whiting and his research team are working this year on a full-scale project funded by the Washington Tree Fruit Research Commission. He is working with On Target Spray Systems, using one of their ATV-pulled electrostatic sprayers, and Firman Pollen, supplier of pollen.

The team is beginning to develop the concept into viable technology for pome and stone fruit.

Major research goals this year include:
—Improving pollen suspension solution to hydrate pollen faster and keep pollen viable.
—Determining optimum pollen rates.
—Evaluating effects on fruit quality and quantity.

Whiting is also seeking funding from other sources, such as specialty crop research block grants funded through the Farm Bill.

The idea of collecting pollen and applying it to fruit trees is not new. Putting pollen at the entrance of beehives and dusting pollen on trees are long established practices within the tree fruit industry. A host of companies sell pollen to tree fruit growers.

Similar approaches of mechanically spraying pollen also are being tried around the world. Pollination machines were first used in New Zealand’s kiwifruit production.

Researchers there are working to develop a “Robobee” machine that could deposit pollen where it’s needed by sensing female flowers. In Europe, scientists are studying artificial pollination by using orchard sprayers to improve fruit set of pears. Tasmanian researchers are interested in applying the concept to vegetable seed crops.

“I wanted to take the existing structure in our tree fruit industry of using supplemental pollen and see if it could be applied in solution by a sprayer,” Whiting said.

The electrostatic sprayer is an important part of the process because it positively charges the solution particles so they will adhere to the stigma. Surfaces of the stigma are negatively charged.

“Pollen grains are very small, and one of the challenges was to deposit them where they needed to be but also to have them stick on the stigma surface,” he explained.

Precise pollination

Bloom and fruit set are annual worries for orchardists because fruit set determines crop potential for the year. Whiting has studied fruit set biology for years and knows the laundry list of challenges that growers face from both bees and tree pollinizers.

For bee pollination, timely beehive placement, strength of beehives, and activity during cool weather are all worries for growers. Tree pollinizers have another set of issues, from compatibility to sufficient bloom and bloom overlap to pollinizer density. More recently, there’s concern about diseases associated with Manchurian crab apple pollinizers.

Whiting believes people around the world are recognizing the challenges of consistent cropping, from the farmer’s need for stable profitability to producing enough to feed the ever-expanding global population.

“The ultimate goal of mechanical pollination is about more than just yield security,” he said. “A major part of my motivation is to solve the problem of crop load management in pome and stone fruit production. Growers face so many challenges between pollinizer choice and density to blossom and fruit thinning. Our current crop load management is designed around setting every fruit. Then, we use chemical treatments or hand thinning to bring it down to desired levels.”

But what if growers could use what Whiting calls ‘precise pollination’ and take complete control of crop load during fruit set? Pollination from bees or pollinizing from trees wouldn’t be necessary. The precise amount of pollen needed to result in a specific crop load could be applied. The spread of pollen-borne diseases could be stopped.

In the next few years, Whiting hopes to turn precision pollination into technology that has potential to dramatically alter crop load management practices and help growers achieve more consistent cropping levels. •