Pruning for pests

By Mark Johnson
The Record-Eagle

TRAVERSE CITY (TNS) — When Jim Nugent looks out at his orchards in the summer he sees acres of tart cherry trees, but a potentially lethal bug is a little harder to spot.

The spotted wing drosophila was first discovered in Michigan in 2010 and the populations exploded between 2011 and 2013, said Michigan State University Extension District Horticulturalist Nikki Rothwell. They eventually spread to Nugent’s property.

Researchers have found the pests pose a significant threat to the region’s tart cherry trees. Rothwell said the bugs are internal feeders — females lay eggs inside the fruit with breathing tubes that hang out. Eventually the eggs hatch and the larvae emerge to feed on the fruit.

Spotted wing drosophila are known for their quick reproduction cycles, making it hard for growers to control or eliminate those populations.

“We can go from very low numbers of insects to very high, very, very quickly,” Rothwell said. “Once those numbers grow so high, it’s hard to fend them off.”

But emerging research suggests increased pruning might help.

Rothwell and other researchers in 2016 observed much of the 32,500 acres of tart cherries in Michigan were infested and appeared to be suitable reproduction sites for the bugs. The discovery prompted a trial at the Michigan State University Extension’s research station.

Officials trimmed 25 percent more than normal pruning practices on one set of trees, 25 percent less pruning on another set, and no pruning on a third. The results showed that the pruned trees likely had more airflow and sunlight breaking through the canopy, reducing humidity and spotted wing drosophila infestations, according to a report Rothwell shared.

Researchers found more larvae in the trees that did not receive pruning, the report indicates.

“It’s really promising,” Nugent said. “I’m going to be really interested to see what data (Rothwell) gets in a second year of research.”

Nugent is doing his own research at Sunblossom Orchards, the farm near Suttons Bay he owns with his wife.

He completed added trimming on about 40 percent of his tart cherry acreage in January to decongest the canopies, allow more sunlight and bring more airflow.

Different hypotheses predict the pest thrives in humid environments and prefers to fly around in the evenings or morning, not during the heat of the day. In cherry trees and even blueberries, information Nugent reviewed indicates the bugs leave the orchards or patches in the mornings for nearby woodlots and return in the evenings.

“We’re hoping if we can get more sunlight and more air flow, then they might not be attracted back,” he said.

But the added pruning could impact tart cherry yield, he said.

Rothwell continues attempts to convince growers to try the additional pruning, but they will do so knowing they likely will see lower yields while managing spotted wing drosophila populations.

Nugent hopes the pruning also will improve fruit quality.

“There is a trade-off,” he said. “We will have to see in the long run how that works.”

Giving up some fruit harvests might help farmers finally find a way to control the pest. Rothwell said growers and horticulturalists are “frantic to control it.” But fully addressing the threat likely will take more work than cutting a few more limbs from the trees.

Nugent said growers must find ways other than pruning and insecticide use to control those populations, including natural enemies to fight the bugs.

“However we achieve it, it’s really important we be able to use nature and the environment to help reduce this population,” Nugent said. “It’s a challenge. There’s no doubt about that.”

Go to www.Record-Eagle.com to see Michigan State University Extension District Horticulturalist Nikki Rothwell’s report on tart cherry pruning techniques to fight spotted wing drosophila.

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Posted by Tribune News Services

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