Tiny fly, big problem

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Spotted wing drosophila (view larger image)

MSU researchers lead sizeable efforts to thwart invasive fruit pest  

A tiny vinegar fly from eastern Asia called spotted wing drosophila (SWD) is fast becoming one of the most intensively studied insects at Michigan State University (MSU). That’s because the invasive pest, the size of a grain of rice, infests various fruit crops—a Michigan industry valued at more than $375 million per year.

MSU scientists began studying SWD in 2010 immediately after its first discovery in Michigan. Since then, more than 30 MSU research projects targeting the fly, which has characteristics unlike many other agriculture pests, have launched.  

Females are uniquely equipped with a serrated ovipositor that can puncture healthy fruit and deposit eggs. With the capacity to lay 100 eggs per day, SWD populations can balloon quickly. Adult flies can live for weeks under the right climatic conditions, and they do not have distinct generations, making it difficult to target and treat. Instead, populations begin to rise in the spring and peak in the autumn.

In 2008, California became the first mainland state in the U.S. to report SWD. It spread rapidly along the West Coast and then was identified the following year in Florida. That was when MSU small fruit entomologist Rufus Isaacs decided it was time to prepare for its arrival in Michigan.

“When they found SWD in Florida, that’s what really got the alarm bells ringing,” Isaacs said. “It meant that this wasn’t just a West Coast issue anymore. MSU sent me to a meeting on this topic in Oregon shortly after it was found in Florida, and when I came back it was clear we needed to get ready here in Michigan.”

In 2010, Isaacs received his first SWD grant from Project GREEEN (Generating Research and Extension to meet Economic and Environmental Needs), a partnership between MSU AgBioResearch, MSU Extension and Michigan Department of Agriculture and Rural Development. The partnership works with commodity groups to find solutions to plant agriculture challenges in Michigan. Isaacs and other fruit extension specialists and extension educators set traps to monitor for the insect. They used small plastic containers covered with holes, and filled with an attractant (at first, apple cider vinegar and now, a mixture of sugar and yeast) and a sticky trap.

Early in the growing 2010 season, the team did not find any SWD. But that changed by early fall when a few flies were collected from a monitoring site in southwest Michigan. It was later positively identified as SWD by the MSU Diagnostics Lab and then U.S. Department of Agriculture (USDA) taxonomists in Beltsville, Maryland. Since then, the invasive pest has been discovered across the Lower Peninsula.

“We’ve found it pretty much everywhere we’ve looked in the Lower Peninsula, but haven’t done much monitoring in the U.P. yet,” Isaacs said. “In the first big year we had with it, we estimated a $20 million impact in Michigan. A lot of small farmers who have you-pick farms for fall raspberries, for example, are finding it very challenging to manage. So they are either not growing those fruit anymore or are replanting with summer raspberries because the pressure from the pest is lower earlier in the year.”

Much of Isaacs’ SWD research has been in collaboration with growers of blueberries and raspberries. His early work involved testing already-registered pesticides and determining their efficacy. Other options his group has studied include physical exclusion, where growers put netting around and on top of crops as they begin to ripen.

In 2015, Isaacs and Matthew Grieshop, an organic pest management expert at MSU, were awarded a grant from USDA’s National Institute of Food and Agriculture (NIFA) to study long-term solutions with researchers at the University of Georgia and other institutions. The goal is to develop new growing practices and test organic approaches to managing SWD.

Isaacs, and Larry Gut and Ke Dong, MSU professors of entomology, are working in conjunction with North Carolina State University to study insecticides, biological and cultural control approaches for SWD, and also aim to understand how this pest might develop resistance to insecticides. This project is funded by USDA’s Specialty Crop Research Initiative (SCRI), another program of NIFA.

Biological methods may be the best long-term option because they can ease overall population pressure and reduce SWD infestation in wild berries and other non-crop plants.

“There is a community of natural enemies for SWD in Asia,” Isaacs said. “With the SCRI grant, one of the objectives is to look at biological controls. This pest can build up in wild areas where growers can’t manage them easily, but if biological controls can help to reduce the population pressure, it should allow control measures applied in the fields to work better. Risk assessments are underway to determine if the Asian biological controls are a viable option.”

Isaacs believes it will take a large group of researchers to tackle this problem. He is encouraged with the urgency expressed by funding agencies.

“We’ve gone from a lot of people getting small, local funding for research to now also being coordinated nationally through these two recent grants,” Isaacs said. “Since this is a national problem now, we need to address it with a national team of researchers.”

At the Great Lakes Fruit, Vegetable and Farm Market Expo held in Grand Rapids in December, Gerard Charlot, a researcher from Bellegarde, France, talked about the extent of SWD in his homeland. SWD was first observed in France in 2010 and is now in every region of the country, he said. In 2014, the largest population was recorded infesting fruit and in particular peaches, grapes and apricots.

Charlot said researchers in France are testing attractants such as a mixture of apple cider vinegar and wine, as well as a garlic-based repellent as means of controlling SWD. He added that dimethoate, an organophosphate insecticide, has shown the best results in field tests but he said there is speculation that it might not be available next year. The best option, however, is physical protection such as netting, but the costs are much too high, he added.

Finding natural solutions

While conventional growers have worked with researchers to test various chemical controls, among other measures, organic growers are finding fewer options at their disposal. In fact, Grieshop said that there is only one National Organic Program (NOP)-compliant insecticide that has shown any promise controlling the pest in Michigan. And there’s a catch.

“The product can only be applied in small amounts per acre per year,” Grieshop said. “Growers also have to rotate to a different insecticide to prevent resistance. Of course the problem is that if it’s the only product that works and you must rotate, you’re going to have problems controlling the pest.”

Like Isaacs, Grieshop identified protective structures and netting as a possible barrier for the fly. But the expense associated with this method can be high, making it unrealistic for most growers.

Researchers agree that spraying is the current most cost-effective approach. And lack of effective tools has overwhelmed some organic growers.

“The organic blueberry grower I work with has found in the last couple of years that early varieties do well, but the late ones don’t,” Grieshop said. “He’s losing essentially half of his crop. By Aug. 1, he’s pretty much done. He’s spraying as much as he legally can.

“The remaining challenge for organic growers is that the one product that does work best is used so much, the pest may be developing resistance. Growers are being encouraged to use less, but what are they supposed to do realistically?”

In an effort to lessen dependency on widespread pesticide application, Grieshop has received Project GREEEN funding to test non-spray control methods.

Grieshop, Gut and postdoctoral research associate Juan Huang will use attract-and-kill tactics, methods first studied with Japanese beetle and Oriental fruit moth, on SWD in a new GREEEN-funded study. The two-year project will examine the use of small nylon pouches that hang from trees and/or bushes. The pouches are treated with insecticides and filled with attractants such as pheromones or food to lure and kill the insects on contact.

Laboratory testing and fieldwork will be conducted. In the lab, researchers determine how long each species needs to be exposed to the insecticide for 100 percent mortality. Then, in the field, cameras will monitor wild insects’ interactions with the nylon bags.

The initial work isn’t compatible with organic farming because the test insecticide is not NOP-compliant. Eventually the researchers want to determine if NOP-compliant insecticides could be substituted.

“My hope is that, by expanding our attract-and-kill technique to more pests, we can identify some key insect behavioral characteristics that can predict whether this type of approach is likely to succeed for many pests,” Grieshop said. “The most exciting aspect of this pest management technique is that, by bringing the pest to the insecticide rather than broadcasting the insecticide and hoping that the insect will contact it, we are developing pest management tactics that are both economically and environmentally conscious.”

A plan for the future

Valued at nearly $100 million, Michigan’s cherry industry fulfills an important role in the state’s overall fruit production. Cherry research is conducted around the state by MSU scientists, including at the Northwest Michigan Horticultural Research Center (NWMHRC) in Traverse City and the Trevor Nichols Research Center (TNRC) in Fennville.

These two locations are where Gut and Nikki Rothwell, the center coordinator for NWMHRC and an extension specialist, have been conducting a series of pesticide efficacy trials for SWD. Soft skin on both tart and sweet cherries makes them particularly vulnerable to SWD. Rothwell had hoped cherry growers could avoid encountering an unmanageable population.Nikki Rothwell looks for SWD larvae

“Once SWD was found in Michigan, 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, but we’ve been doing research to prepare.”

Funding from Project GREEEN helped Rothwell perform laboratory tests of several insecticides at NWMHRC and TNRC in 2013 and 2014. She found varying results. Taking to the field in 2014 after the cherry harvest, Rothwell examined fruit still on the tree susceptible to infestation.

Untreated fruit was collected as a control to determine the extent of SWD damage, while other fruit was treated with various insecticides. Some of the treated fruit was gathered after seven days and some after 14 days. Rothwell’s data is promising, but she believes ongoing research is required.

Speaking at the Great Lakes Expo in Grand Rapids, Rothwell was asked by a grower about SWD thresholds and when treatment should be started.

“We haven’t determined that yet, but in my opinion – one is enough,” she said. “If you trap one fly, there are many, many others around.”

In November 2014, a summit was held at NWMHRC with researchers and growers in attendance to discuss measures that can be taken to combat SWD. A plan was devised that would incorporate further insecticide trials and other management methods, in addition to community efforts such as removing wild hosts near farms. The meeting came at just the right time.

“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.”

A 2015 SWD summit took place Nov. 20 at NWMHRC. Researchers and growers could attend in person or via webinar. Representatives from MSU included Rothwell, Isaacs, Gut and others from MSU Extension and the Department of Entomology.

“MSU is trying to stay out in front of this issue as much as possible,” Rothwell said. “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.”



  • The Nature Conservancy estimates the worldwide annual cost of invasive species to be $1.4 trillion, 5 percent of the entire global economy.
  • According to the United States Fish and Wildlife Service, U.S. agriculture loses $13 billion annually to crops from invasive insects alone.

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