Researcher Enlists Honeybees as “Flying Doctors” Against Crop Disease

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Strawberry fields in Finland are plagued by grey mold, a fungus that quickly transforms scarlet berries into shaggy grey blobs, wrecking 20 percent of the country’s annual crop, on average.

But Finland’s organic fruit farmers have a swarm of new allies in the battle against grey mold. Dr. Heikki Hokkanen, a researcher at the University of Helsinki, has enlisted bees to carry biological treatments from flower to flower, warding off disease as they pollinate.

“Bees have body hairs that pollen sticks to, so it gets carried from flower to flower,” said Hokkanen. “They can also carry treatments such as microbes or fungi, providing dual ecosystem services for healthier organic crops.”

Hokkanen has studied the use of bees in fighting crop diseases at farms in Finland, Italy and Estonia. He presented his research at the international Innovations in Organic Food Systems for Sustainable Production and Enhanced Ecosystems Services conference held in Long Beach, CA, Nov. 1-2.

The biological agent used to fight grey mold in berries is a naturally-occurring soil fungus, gliocladium catenulatum. G. catenulatum works by harmlessly colonizing the strawberry flower, and preventing the grey mold from taking hold.

“Infections that begin in flowers can destroy berries and fruits,” Hokkanen said. “But if you can get there first with antagonistic microbes, it’s first-come, first-served, and the microbe that gets there first tends to be the one to successfully colonize.”

Spraying strawberry fields with chemical or biological antifungals has proved marginally useful, as 99 percent of the spray does not land where it is needed – on the just-opened strawberry blossoms – and new flowers open continuously.

“But when bees do the job, you get continuous precision application, to every flower, every day,” Hokkanen said. His research showed that spraying with G. catenulatum deposited about 400 spores per strawberry flower as a one-time treatment. In contrast, bees delivered about 10,000 spores per flower over repeated visits, using a small fraction of the inoculant material required for spraying.

The bees are brought to the field in special hives that require the creatures to crawl across a field of innoculant powder as they exit the hive. The powder sticks to their body hairs, and is left in flowers as they make their busy rounds. A trace amount of inoculant is deposited on plant leaves as the bees buzz by.

Honeybees used in the field are worker bees with a remaining life expectancy of two to three weeks. Exposure to the various inoculants used in years of studies has not harmed the creatures, or hastened their mortality, Hokkanen said.

“No farmer or beekeeper that we have worked with has reported bad effects on the bees,” Hokkanen said. “The (G. catenulatum) fungus has no effect on the bees, and in any case the hives have a sort of antibiotic effect, as they are too hot inside for funguses to survive.”

Bee-assisted biological treatment on strawberries has proved more effective than chemical anti-fungal sprays, and improves organic yields by as much as 50 percent – most likely due to improved pollination, Hokkanen said. As an added bonus, bee-treated berries last twice as long on the grocery-store shelf without spoiling.

A combination of chemical spray and bee-delivered biocontrols proved most effective in reducing grey mold in the field, Hokkanen said, but bee-vectored biological treatment resulted in the best-quality berries and longest shelf-life extension.

However, due to the vagaries of weather, “some years nothing you do works,” he said.

Finnish researchers have also worked with bumblebees, which are now commercially used in greenhouses, and other solitary and native bees. But honeybees are most frequently used in commercial and research applications.  So far, bees have been successfully deployed to fight diseases in strawberries, grapes, tree fruits and greenhouse crops.

To see bumblebees at work, check out this YouTube video:


Bee-assisted disease control has been enthusiastically embraced by Finnish strawberry farmers, Hokkanen said, and is also being used commercially in Estonia. He expects the technique to become more widespread as it gains regulatory approval in additional countries.

“There are bottlenecks in the availability of bees, in the lack of registered products to use, and lack of knowledge about bee behavior and management,” Hokkanen said. “You also need skill to steer the bees. Smaller colonies are easier to influence.”

The care and deployment of bees requires skill, precision, and respect, he said. “Bees are the third most valuable class of animal in the EU, after cows and swine, and more valuable than poultry,” Hokkanen said. “These are animals that deserve a greater level of respect.”

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