Many Pollinators Boost Cherry Yields — Not Just Honeybees

If you step into a sweet or tart cherry orchard during bloom, you’ll find bees pollinating flowers. But contrary to popular belief, according to researchers from the Integrated Crop Pollination Project, it’s more than just honeybees doing the work.

This bumble bee is one of 80 species of wild bees found visiting Michigan tart cherry during bloom. (Photo: Emily May, Xerces Society)

It’s well known that the majority of varieties grown for sweet cherry are not self-fertile and require pollen from compatible varieties to set fruit. Researchers from Germany found that, without bees, sweet cherries produce only 3% of their maximum yield. Growers typically rent honeybees to make sure that sweet cherries get pollinated.

Tart cherries, on the other hand, are self-fertile. ‘Montmorency,’ the most common tart cherry variety grown in the U.S., is planted in single-variety blocks. Even though it is self-fertile, researchers in Michigan showed that yields increase significantly when bees are present.
“We’ve shown that without pollinators, tart cherries are down to 10% of the yield compared to trees that had access to pollinators,” says Nikki Rothwell, an Extension Specialist with Michigan State University.

Sweet and tart cherry markets differ in that sweet cherries tend to be sold for fresh market, while 99% of tart cherries are processed. Rothwell adds that, in the case of Michigan cherry producers, “There is growing interest and potentially also better returns for our fresh-market products compared to processed products. I think in those situations pollination is also really important. In a processed world you’re really looking for tonnage because you get paid by the pound, and you get paid by the pound in fresh market too, but it’s all about the quality and pollination plays a big role in that.”

Pollination Insurance
Honeybees continue to be important for both sweet and tart cherry production. Even so, pollination can be a challenge for cherry growers.
“We’re facing similar issues with pollination as other places in the U.S.,” says Bob Gillespie, a faculty member at Wenatchee Valley College in Washington, who is investigating pollination of sweet cherry. “Usually the larger operations have a better opportunity of obtaining honeybees; the smaller orchardists may have more of a struggle to do that.

“There’s always a concern that there aren’t going to be enough honeybees for pollination, but most individuals seem to have been successful in obtaining the bees [this past year],” Gillespie continues. “The other issue is that when the cherries bloom, we often don’t have the best weather conditions for honeybees to forage. That can sometimes be a bit of an issue, and sometimes the sweet cherry bloom can be very, very short.”

Tart cherry bloom is even quicker than sweet cherry bloom. “In hot years we can see tart cherry bloom over in three days… . So having bees in the orchard to maximize pollination is essential,” Rothwell says.

In order to provide pollination insurance under these challenging conditions, Rothwell and Gillespie joined forces with Julianna Wilson at Michigan State University and David Biddinger at Pennsylvania State University to see which pollinators, in addition to honeybees, could be used for cherry pollination.

“We’ve been trying to characterize the wild bee community in Michigan and Pennsylvania tart cherry and also in sweet cherry in Washington… . In Michigan and Pennsylvania we were [also] looking at an alternative managed pollinator called Osmia cornifrons, which is commonly known as the hornfaced bee or Japanese orchard bee,” says Wilson.

Wild Bees Abound
Biddinger has been studying tree fruit pollinators in Pennsylvania for the past decade.

“One of the things that surprised me most going into this is how many different kinds of bees there are in Pennsylvania,” he says. “We’ve documented 370 bee species in Pennsylvania, and found at least 30 species visiting tart cherry orchards during bloom.”

In Michigan, researchers found that roughly half of the bees visiting tart cherry flowers were wild bees, even though all the orchards they studied were stocked with honeybees.

However, the story in the Washington sweet cherry orchards that Gillespie studied is a little different. He set out traps prior to and during bloom. He collected many bees, but his preliminary analysis suggests the majority of wild bees were caught before bloom started. Gillespie plans on looking into this further to see if there are ways to encourage more wild bees into the orchard, e.g. by planting wildflowers in orchards or if alternative managed bees would be a better form of pollination insurance.

Alternative Managed Bees
The Japanese orchard bee or hornfaced bee gets its two common names from where it originated and how it looks. It was imported from Japan in the 1980s and has two horn-like features, protruding from its lower face. Only a handful of growers purchase these alternative managed bees for tart cherry pollination.

The alternative managed bees, such as Osmia cornifrons, also known as the Japanese orchard bee or hornfaced bee, is being developed to supplement honey bees for tart cherry pollination. (Photo: Emily May, Xerces Society)

A study led by Biddinger back in 2013 tested the effectiveness of this bee in Pennsylvania tart cherry orchards that lacked honeybees. He found that fruit density was significantly higher in an orchard where Japanese orchard bees were released. However, given the high pollination requirements of tart cherries, Biddinger thinks these bees should be used only in combination with honeybees. In Michigan, there is some preliminary data suggesting that adding this bee to a tart cherry orchard stocked with honeybees evens out yield across the orchard.

Despite its potential, there is still a lot to learn about how best to manage this alternative bee for commercial pollination. One challenge researchers in Michigan had was getting the bees to nest in artificial nesting boxes after they were released.

“What we found this year is that binder boards are a really important artificial nesting material for this species of bee. We were using paper straws with larger holes before, but these binder boards have smaller holes,” notes Rothwell. She found that using binder boards instead of paper straws led to an 800% increase in offspring production.

Integrated Crop Pollination
Integrated Crop Pollination is the use of managed pollinator species in combination with farm management practices that support, augment, and protect pollinator populations to provide reliable and economical pollination of crops. In the case of sweet and tart cherry, this means understanding which pollinators, in addition to honeybees, can provide pollination insurance and which farm practices can best support those pollinators.

In addition to studying the different pollinators of tart cherry, Rothwell and Wilson have been investigating whether wildflowers can support cherry pollinators. Although the wildflower plantings are young, Wilson’s preliminary data suggests many tart cherry bee species visit the wildflower plantings after tart cherry bloom finishes. ●