As part of a national team of entomologists studying the management of spotted-wing drosophila (SWD), the UC Cooperative Extension is reaching out to Central Coast Strawberry and Caneberry growers/PCAs regarding a survey on SWD Impacts. This survey will help us understand the current impacts of SWD on your farm and how these impacts may have changed in the last 10 years.
How will this information be used? This information will be used to develop new research goals as part of a USDA Specialty Crop Research Initiative proposal under development. It will also be compared to information collected in similar surveys in 2013 and 2014 to help us understand where challenges still exist for SWD management and what improvements have been made over the last 10 years and shared in extension and scholarly publications.
Who should I contact for more information? For more information about SWD management, contact Dylan Beal, djbeal@ucanr.edu, 831.294.9419, or members of our project team. For more information about this survey, contact Hannah Levenson, hklevens@ncsu.edu, 919.434.7882.
Back in 2010, the UC Davis entomology labs of integrated pest management specialist Frank Zalom and molecular geneticist and physiologist Joanna Chiu joined forces to target the spotted-wing drosophila (SWD), a serious threat to berry production in California.
Drosophila suzukii,native tosoutheast Asia and first discovered in California in 2008, lays its eggs in such soft-skinned, ripening fruits as strawberries, raspberries, cherries, blueberries, peaches, nectarines, apricot and grape.
It packs a powerful economic impact. The first year of its discovery in California, the economic loss amounted to $500 million. Latest statistics from 2015 indicate a $700 million national economic loss.
The Zalom lab discovered the first SWD field populations with insecticide resistance in 2017. As the pest continues to spread throughout much of the country, anxious growers are worried about its increased resistance to pesticides.
The team’s newly published research in Scientific Reports is the first to characterize the molecular mechanisms of insecticide resistance in D. suzukii and provide insights into how current management practices can be optimized.
“In this work, we leveraged high throughput sequencing to identify biomarkers of insecticide resistance in D. suzukii,” Tabuloc explained. “We found that different genes are responsible for resistance to different chemicals. Specifically, we found that genes involved in metabolism are highly expressed in flies resistant to pyrethroid insecticides. We also observed evidence of two different mechanisms of resistance in 2 lines generated from a single spinosad-resistant population. We found an increased expression of metabolic genes in one line and increased expression of cuticular genes in the other.”
“Therefore, we developed a diagnostic panel using these biomarker genes to differentiate between pyrethroid resistance and spinosad resistance,” Tabuloc related. “Not only can our assay now inform whether there is resistance, it can tell us which chemical the population is resistant to and how severe the resistance is. Additionally, this method is faster, enables for the testing of more populations, and is more comprehensive as compared to bioassays, the conventional way of testing for resistance.”
Tabuloc added that “our work has enabled for the detection of resistance in California populations, and we are currently doing a nationwide screening to determine whether resistance is now present in other states. Currently, we are working with the Zalom lab to use the results of our assays to try and combat resistance. There are experiments in progress trying to increase the efficacy of insecticides by blocking some of the genes involved in resistance, such that the enzymes encoded by those genes have decreased function.”
Zalom, a UC Davis distinguished professor emeritus who directed the UC Statewide Integrated Pest Management Program for 16 years, said he has been “working on spotted-wing drosophila with Dr. Chiu and her lab members since she joined the UC Davis faculty in 2010, and it has been an absolute pleasure.”
Zalom, a past president of the 7000-member Entomological Society of America (ESA) and an elected Honorary Member, ESA’s highest honor, praised Chiu, a 2019-2024 Chancellor’s Fellow professor and now chair of the Department of Entomology and Nematology, as “one of the most collaborative researchers who I have ever worked with. When our lab found the first SWD field populations with insecticide resistance in 2017, it seemed obvious to ask Dr. Chiu about identifying the mechanism of resistance to different chemical classes and if it would be possible to develop a molecular diagnostic to confirm presence of insecticide resistance in field populations without conducting the time consuming and labor-intensive bioassays that we were using.”
“Dr. Chiu and her PhD student Christine Tabuloc took on this challenge and their work culminated in the description of genes associated with the resistance and the diagnostic assay presented in this paper,” Zalom said.
The Zalom lab “selected the SWD isolines from California field populations displaying resistance to pyrethroids and spinosyns after conducting bioassays on many hundreds of SWD adults, then helped Dr. Tabuloc validate results of the molecular assay,” he said.
“This work not only represents good science; it has very practical implications,” Zalom said. “Dr. Tabuloc and I presented results of the work from both of our labs at a special berry grower seminar on insecticide resistance organized by UC Agriculture and Natural Resources (UC ANR) Farm Advisor Mark Bolda in Watsonville. The presentations were extremely well-received. The original program was targeted for about 1.5 hours, but the meeting extended to over three hours due to the extent of questions and great discussion that followed. Growers and their consultants are hungry for new information that they find interesting and potentially useful, and this work was clearly of interest to them.”
Bolda, strawberry and caneberry farm advisor in Santa Cruz, Monterey and San Benito counties, “was the first person who found the insect and asked me to come down to look at it and the problem,” Zalom remembered. “That was 2008, and we weren’t able to get an actual species identification until 2009!”
Bolda noted that the recent berry grower meeting targeted SWD resistance on the Central Coast, with the UC Davis entomologists presenting. “The research was top shelf and the need, of course, is very great,” Bolda said. “Some of the information that Frank and Christine presented has been put into immediate use in the industry.”
“What made it really special was that since we were moving only a month or so later, and this was the last Extension meeting to be held at my UC Cooperative Extension Office,” Bolda said. “After 40 some years, that’s saying a lot and it was totally apropros that Frank, given his many, many years of service, was the very last to run a meeting there….and Christine presented also…she was really great and presented fabulous information.”
Among the “iconic individuals of Central Coast strawberries” who attended, Bolda said, was retired entomologist Ed Show (Driscoll Strawberry Association, Inc.) “who has been a part of strawberries since the 1970s.”
“It was nostalgic for me since it was the last meeting that was held there because they were moving to a new office,” Zalom said. “I must have done 80 presentations at that auditorium over the years, including some of the very first ones that I did when returning to California when I was doing research on brussels sprouts and apples in that area.”
Fruitville Collaboration. Professor Chiu said the publication “culminated years of fruitful collaboration and hard work by our lab and Professor Zalom’s lab, primarily driven by Christine…This research could not have been accomplished without Christine who is uniquely qualified to successfully lead the project; she combines her knowledge in insect genomics and bioinformatics with Drosophila molecular genetics.”
Chiu pointed out that “When the Zalom lab and my lab first started our collaboration in 2010, we were already worried about the potential development of insecticide resistance in D. suzukii given the primary method for management is insecticide application and the generation time of these flies are short. Unfortunately, this became a reality in 2017.”
“We hope that our research in developing more efficient molecular diagnostics to identify resistant populations will help prevent the spread of resistance to other U.S. states by allowing them to be proactive; perhaps to adjust their management program as soon as low level of resistance is detected,” Chiu said. “We are continuing to perform research to determine if resistance, once found, can be ‘weakened’ and what are mechanisms that could drive it. We think this will be very beneficial for growers in California, who currently have to tackle D. suzukiiwith insecticide resistance.”
Tabuloc, who joined the Chiu lab as an undergraduate research assistant in 2012, received her bachelor of science degree in biochemistry and molecular biology from UC Davis in 2015, and her doctorate from UC Davis in 2023.
In addition to Chiu, Zalom and Tabuloc, the 12-member team of researchers and co-authors included Curtis Carlson, Kyle Lewald, Sergio Hidalgo, Cindy Truong and Ching-Hsuan Chen, all from the Chiu lab; Nicole Nicola and Fatemeh Ganjisaffar of the Zalom lab; and Cera Jones and Ashfaq Sial of the Department of Entomology, University of Georgia, Athens. At the time, Truong and Chen were undergraduates, and Ganjisaffar was a postdoctoral fellow, and now a senior environmental scientist with the California Department of Food and Agriculture.
Federal and state grants awarded to Chiu and Zalom funded the project: a USDA National Institute of Food and Agriculture; and a California Department of Food and Agriculture Specialty Crop Block Grant. The team credits Bloomington Drosophila Stock Center for providing D. melanogaster stocks.
Spotted wing drosophila (SWD), a small fly native to Asia that first appeared in Michigan in 2010, isn’t like most pests. Populations of crop-damaging insects tend to have distinct generations. Not SWD. Several generations overlap and build through the year, attacking vulnerable agricultural crops and wild plants. The situation has created a major challenge for the Michigan berry and cherry industries.
Spotted wing drosophila in the lab.
Rufus Isaacs, a professor in the Michigan State University Department of Entomology, was one of the first to discover the pest in the state 13 years ago. While researchers didn’t know much about SWD then, they’ve had to get up to speed quickly.
Rufus Isaacs, professor of entomology at MSU.
“SWD is so destructive because it can insert eggs into ripening fruit,” said Isaacs, whose work is supported in part by MSU AgBioResearch. “Larvae can be present in harvested fruit, which can be a big problem for producers. It is also able to reproduce so quickly to build up the population, and it can infest wild fruit outside of farms, creating a reservoir population that continually invades crop fields after they’re treated.”
After hearing about SWD arriving in California in 2008 and then being detected in Florida in 2009, concerns began to mount.
“Shortly after SWD was found in Florida, I went to a conference in Oregon where we discussed measures we may need to take to get ready for the pest,” Isaacs said. “We expected it would be in Michigan before too long.”
Sure enough, Isaacs was right. He received a grant from Project GREEEN — a partnership among MSU AgBioResearch, MSU Extension and the plant agriculture industries of Michigan — in 2010 to monitor for SWD. Isaacs worked with members of the MSU Fruit Team to set small plastic traps with holes that were filled with a cider vinegar lure.
In late 2010, SWD was collected from a site in West Michigan. At that point, the difficult work began and persists today. Research in Isaacs’ lab has been performed in partnership with mostly blueberry and raspberry growers.
The team has tested already-registered pesticides to determine their efficacy, as well as non-chemical controls such as pruning, mulching and physical exclusion methods, where growers place netting around and on top of crops as they begin to ripen.
As a result of this research, online resources have been created to assist growers with SWD identification, pesticide timing and other management strategies.
“We want to ensure we’re responsive to grower needs, first and foremost,” Isaacs said. “That’s why the pairing of research and grower education programs through MSU Extension is so valuable to our team.”
Dennis Vander Kooi, a blueberry grower and owner of Woodland Enterprises Berry Farms in Zeeland, Michigan, and his family have been working with MSU researchers for many years. He is also a board member of the Michigan Blueberry Commission (MBC), helping to set research priorities for the industry.
“Rufus is one of the nation’s leading researchers on this issue, and we’ve worked with others at MSU as well,” Vander Kooi said. “He’s run several experiments on our farm, and we’ve learned a lot from them. The MBC has supported this work and will continue to in the future as we look for effective ways to manage this pest.”
Vander Kooi said the blueberry industry is undergoing a rejuvenation in Michigan, adopting newer genetics and planting varieties that meet the demands of today’s consumers. This, in conjunction with SWD, makes it a critical time for growers as they look to protect an industry that contributes nearly $132 million to the state’s economy, according to the Michigan Ag Council.
“With the creation of the MBC in 2017, we got the opportunity to start advocating for more dollars for research, and MSU has been instrumental in that,” he said. “To date, we’ve supported more than $390,000 worth of research, much of that going to MSU, and leveraged an additional $600,000 in other funding.”
In addition to the MBC, Project GREEEN has been a significant supporter of Isaacs’ SWD efforts, along with state funding through the Specialty Crop Block Grant program and national projects funded by the U.S. Department of Agriculture (USDA).
Assessing biological control options
One of the primary challenges for scientists is focusing on immediate grower needs while also investigating long-term management tactics. It’s a monumental task to balance both simultaneously.
Marianna Szucs, assistant professor of entomology at MSU.
Alongside Isaacs, Marianna Szucs, an assistant professor in the Department of Entomology, has been exploring biological control to reduce populations in the long run. As she noted, when SWD came to the U.S., its natural predators did not.
“Anytime there is introduction of an invasive pest, one of the main issues is that the new landscape doesn’t have natural enemies for it,” she said. “They don’t come with the pest, and the native insects in the new place aren’t equipped to deal with it right away. What we wanted to know is if we exposed parasitoid wasps native to Michigan to SWD, would they begin to see them as a food source?”
A parasitoid insect lays its eggs in or on a host insect, and the developing larvae consume the host until it eventually dies.
Szucs and her team chose two parasitoid wasp species commonly found throughout North America. Researchers initially saw that attack rates in the wild were quite low, and the ability of native species to rapidly adapt to a new food source was unknown. In the lab, the parasitoids were exposed to SWD and forced to prey on them.
Within three generations of selection from a small number of parasitoids — 30 wasps or fewer — successful parasitism improved by 259% for one species and 88% for the other.
“Obviously this is an artificial scenario because we only gave them one option, and in the wild they only attack something if they know to do it,” Szucs said. “But this was a novel way to show that there is some potential to raise native parasitoids and influence their fitness to attack SWD.”
Besides improving native species, the other biological control option is to introduce natural predators. That’s what an MSU team including Isaacs has done with the samba wasp, which is native to Asia. After years of testing and applying for permits, the USDA Animal Plant Health Inspection Service and Michigan Department of Agriculture and Rural Development (MDARD) approved the release of the samba wasp in SWD-threatened locations.
The samba wasp detects already-infested fruit and targets the smallest stages of SWD larvae. Laying its eggs inside its host, the growing samba wasp feeds and ultimately kills the larvae, emerging as a wasp in roughly a month.
The team will continue to evaluate the wasps’ success in the coming growing season, as well as assessing the ability to survive Michigan winters.
“Hopefully they’re able to survive the winter and the populations are robust enough to be successful next year,” Isaacs said. “We don’t expect results right away, but this is a long-term project for us to determine if this can help reduce the need for insecticides and other management strategies moving forward.”
‘Perfect conditions for SWD’
The northwest portion of Michigan’s Lower Peninsula is world renowned for its cherry production. Growers in the Great Lakes State are responsible for 70% of the U.S. supply of tart cherries, about 80% of which are grown in the northwest Lower Peninsula. But the industry is under siege from a variety of sources, from invasive pests and climate change to competition abroad.
SWD, however, has hit the industry particularly hard. Nikki Rothwell, the coordinator of the NWMHRC and fruit specialist with MSU Extension, said this challenge affects every Michigan cherry grower.
Nikki Rothwell, coordinator of the Northwest Michigan Horticulture Research Center.
“If an orchard starts with 100 flies in mid-June, and each female can lay 300 eggs each week, even the best growers with the best programs have a hard time controlling millions of flies,” she said. “Plus, cherries are so attractive that it’s hard to monitor the populations once the fruit get ripe, which makes it hard to make good management decisions based on traditional trapping and integrated pest management strategies we’ve successfully used with other pests.”
One of Rothwell’s initial studies showed that tart cherries create the optimal conditions for SWD. While the flies leave most cropping systems during the day to avoid the hot sun, tart cherry canopies provide a cool layer of protection for them to mate and infest fruit. She found that simply pruning the trees and keeping grass clipped helps immensely.
“We did a pruning study in which we removed different numbers of branches and found that if you remove six to 10 branches in a tart cherry canopy, you can reduce SWD infestation by 40% even without insecticides,” Rothwell said. “We also showed that mowing grass short between tree rows reduced infestation.”
Recent research includes modeling SWD in cherries. Rothwell hypothesized that flies seem to be in the orchard at all times during the summer and begin to lay eggs suddenly. She believed flies were present and waiting for the right time to infest ripening fruit.
To better understand this timing, Rothwell and her team have monitored tart cherries as they ripen over the course of the growing season and brought fruit back to the lab. There, they look at when flies begin infestation. Using this information, the team is producing a model for growers that will more precisely indicate ideal timing for pesticides and other management techniques.
“We’re refining the model now, but we’re hoping to have something soon that growers can use,” Rothwell said. “The industry really depends on MSU to help work through this problem.”
Rothwell’s work has been supported by the Michigan Cherry Committee, MDARD, Project GREEEN and the Foundation for Food and Agriculture Research. — By Cameron Rudolph, Michigan State University
