Spotted wing drosophila proves formidable fruit foe
When a tiny invasive fly — the size of a grain of rice — called spotted wing drosophila (SWD) first arrived in California from Asia in 2008, researchers weren’t overly concerned. Entomologists were confident that management programs could be developed quickly and the risk to the soft-fleshed fruit that the pest covets could be mitigated.
But that outlook changed the following year when the insect was reported more than 2,000 miles away in Florida. This revelation gave researchers, including Michigan State University (MSU) small fruit entomologist Rufus Isaacs, great pause.
“When they found SWD in Florida in 2009, 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 about this pest 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.”
MSU researchers didn’t want to take any chances, especially with a statewide fruit industry valued at more than $375 million per year. In 2010, Isaacs received his first SWD grant from Project GREEEN (Generating Research and Extension to meet Economic and Environmental Needs), a partnership among MSU AgBioResearch, MSU Extension and the Michigan Department of Agriculture and Rural Development. Project GREEEN works with commodity groups to find solutions to plant agriculture challenges in Michigan.
Isaacs, along with other fruit extension specialists and extension educators, set traps to monitor for the insect. Researchers 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. In early fall 2010, a few flies were collected from a monitoring site in southwest Michigan. They were later positively identified as SWD by the MSU Diagnostics Services 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.
Most of the SWD research led by Isaacs has been in collaboration with blueberry and raspberry growers. His early work involved testing already-registered pesticides and determining their efficacy. Other options his group has studied include physical exclusion, where growers place 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 the 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, as well as MSU entomologists Larry Gut and Ke Dong, is working in conjunction with North Carolina State University to study insecticides, biological controls and cultural control approaches. The group also aims to understand how this pest might develop resistance to insecticides. This project is funded by USDA’s Specialty Crop Research Initiative (SCRI), a program of NIFA.
“There is a community of natural enemy insects that attack 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 much better. Risk assessments are underway to determine if the Asian biological controls are a viable option here in the U.S.”
It will take a large group of researchers to tackle this problem, Isaacs says, but 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.”
The biggest obstacle to finding a control mechanism is that SWD is very different from other pests. Females are equipped with a serrated ovipositor that can puncture healthy fruit and deposit up to 100 eggs per day. Adult flies may live for weeks under the right climatic conditions and there are no distinct generations, making targeted treatments difficult. Instead, populations begin to rise in the spring and peak in the fall. Additionally, researchers say little is known about SWD behavior.
Gut and doctoral student Danielle Kirkpatrick are attempting to develop better monitoring tools by learning about how far SWD travel.
“A primary focus of Danielle’s research is studying how far the flies move and at what distance traps attract flies,” Gut said. “In other words, what’s the trapping area of a trap? Right now we can’t really use the tools as effectively as possible if we don’t really know how far apart to put them. So far, it appears they are moving pretty good distances, much farther than we thought.”
Researchers agree that spraying is the most cost-effective approach currently, but a lack of options has overwhelmed some growers. In an effort to lessen dependency on pesticide application, Gut and Grieshop have received Project GREEEN funding to test non-spray control methods.
Gut, Grieshop and postdoctoral research associate Juan Huang are using attract-and-kill tactics that were first studied with Japanese beetle and oriental fruit moth. The two-year project examines the use of small nylon pouches that hang from trees and bushes. The pouches are treated with insecticides and filled with attractants such as pheromones or food to lure and kill the insects on contact. In the lab, researchers determine how long the pest needs to be exposed to the insecticide for 100 percent mortality. Then, in the field, cameras monitor wild insect interactions with the nylon bags.
This year, the Michigan Tree Fruit Commission, with support from the peach and plum industries, awarded Gut a grant to observe SWD with multiple peach and plum varieties. After collection, Gut traps the fruit with several flies to monitor behavior. While the industry wants to get out in front of the pest to prevent major threats, Gut said he believes that adding to the knowledge base on host selection and insect behavior will be most important.
“Treating is also difficult because we don’t know a lot about what SWD are doing in the winter,” Gut said. “It’s a critical piece of information that we’re trying to find. Once it gets colder, they change their morphology and physiology and become a winter morph. It’s a much larger, darker, cold-tolerant form of the insect. We’re hoping that the population shrinks at this time to a level that, if we find out where they are, we can treat these low levels of the winter morph and end up creating a situation where the population doesn’t build as high in the summer and fall.”
THE NO. 1 CHERRY PRIORITY
Cherries occupy a critical space in Michigan’s fruit production portfolio with an industry valued at nearly $100 million per year. Cherry research is conducted around the state by MSU scientists, particularly at the Northwest Michigan Horticulture Research Center (NWMHRC) in Traverse City, Michigan, and the Trevor Nichols Research Center (TNRC) in Fennville, Michigan.
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. Thin skin on tart and sweet cherries makes them particularly vulnerable. When SWD found its way to Michigan, Rothwell hoped cherry growers could avoid an unmanageable population.
“Once SWD came to 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. This year, we found them even earlier.”
Funding from Project GREEEN and the Michigan Cherry Committee has helped Rothwell and Gut perform ongoing laboratory tests of several insecticides at NWMHRC and TNRC — with mixed results thus far. Although a breakthrough hasn’t happened yet, Phil Korson, the president of the Cherry Marketing Institute, is grateful to have a team of researchers dedicated to the cause.
“Half of the supply of tart cherries in the U.S. are grown in northwest Michigan, so we are fortunate as a state to have excellent researchers working with growers on SWD,” Korson said. “The industry has supported research, and MSU has received several national grants. I’m confident that, with the resources we have, our team will lead the way in developing a holistic plan for this pest.”
In addition to encountering SWD earlier in 2016, a July 8 hailstorm complicated matters further by damaging fruit in the Traverse City area. Growers were left with injured cherries on trees or on the ground, a haven for SWD breeding. Rothwell and her team dealt with this challenge in various ways, including physically destroying the fruit by shaking it to the ground and using equipment such as a truck to crush it. Ideally, this would make the cherries less appealing to SWD. She saw positive results from the tests but is interested in long-term control.
“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.”
“SWD is really the No. 1 priority of the cherry industry at this point,” Gut added. “This pest threatens raspberries, blueberries and has now gotten into cherries, so we need to find a management strategy that works as quickly as possible. There is no one-size-fits-all solution. We’ve gotten more resources through grants, and now we need to continue the work.”
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