Spotted Wing Drosophila Threat Looming Large

Cherry growers were hit hard in 2016 by the invasive pest, spotted wing drosophila.

Nikki Rothwell (left) examines SWD larvae

Michigan’s cherry industry is valued at nearly $100 million, ranking No. 1 in the U.S. in tart cherry production and close to the top for sweet cherries. Michigan State University (MSU) cherry research spans the Great Lakes State, including at the Northwest Michigan Horticultural Research Center (NWMHRC) in Traverse City and the Trevor Nichols Research Center (TNRC) in Fennville.

Nikki Rothwell, the center coordinator for NWMHRC and an extension specialist, has used these two locations in particular to conduct a series of pesticide efficacy trials for spotted wing drosophila (SWD). The small fly is an invasive pest from Asia and is ravaging fruit crops across the country. Soft skin on both tart and sweet cherries makes them particularly vulnerable to SWD, but there was little concern until recently.

“Once SWD was found in Michigan (in 2010), we started to monitor the situation for cherries but weren’t extremely worried,” Rothwell said. “We thought the cold winters would help, and cherry harvest was over before the SWD population got out of control late in the summer and into fall. That seemed to hold true until 2015.”

Financial loss estimates to the cherry industry are unavailable for 2015, according to Rothwell, but the damage was vast. Many growers had fruit rejected at the processor, and some didn’t bother trying to sell.

Starting in 2013, funding from Project GREEEN has helped Rothwell perform laboratory tests of several insecticides at NWMHRC and TNRC. She has found varying results thus far.

After the cherry harvest Rothwell examines fruit still on the tree, which is susceptible to infestation. Untreated fruit is collected as a control to determine the extent of SWD damage, while other fruit is treated with various insecticides. Some of the treated fruit is gathered after seven days and some after 14 days. Rothwell has seen promising results, but more research is needed.

In 2014 and 2015, SWD summits were held at NWMHRC with researchers and growers to discuss measures that can be taken. A plan was devised that would incorporate further insecticide trials and other management methods, such as removing mulberries and other wild hosts near farms.

Rothwell is also analyzing the impact on tart cherry orchards from neighboring sweet cherries. She postulates that sweet cherries, which have experienced a large amount of drop in 2016 (falling to the ground), are creating the ideal scenario for tart cherry infestation. If growers see even one fly, she believes they should start their management program. Where there’s one SWD, there are many more.

“We’re fortunate that people in the cherry industry are early adopters of new integrated pest management strategies,” Rothwell said. “2015 was the first big year for SWD in cherries in Michigan, and although it’s still a substantial problem, we have a whole host of individuals working on finding appropriate answers.

“MSU is trying to stay out in front of this issue as much as possible. We have experts doing a lot of great work, of course, but we are also making ourselves available to address the growers’ concerns. The continued partnership among universities, commodity groups and growers will be essential.”

In addition to Project GREEEN, Rothwell’s work is funded by the Michigan Cherry Committee.

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