Organic Broadcaster

 

Organic farmers try innovative practices to beat spotted wing drosophila

By Annie Klodd and Andrew Petran

Spotted wing drosophila, better known as SWD, deposits its eggs on ripening fruit, resulting in fruit that disintegrates quickly or surprises the eater with wiggling larvae. Photo by Charlie Rohwer

Spotted wing drosophila (SWD), Drosophila suzukii, is an invasive fruit fly that is testing the patience of berry growers all over the globe. Organic farmers and researchers in the Midwest are playing a major role in finding ways to manage this pest successfully.

Gigi DiGiacomo, an agriculture economist at the University of Minnesota (UMN), has been surveying Minnesota berry farmers to learn how SWD has impacted their operations. Since this fly was introduced to Minnesota in 2012, growers reported median yield losses of 20%, with 79% of surveyed growers reporting infestation in 2017. DiGiacomo’s work estimated that SWD costs the state’s raspberry industry $2.2 million annually, due to yield losses and increased labor costs.

DiGiacomo’s work also underscores how SWD is changing the way we grow berries. Most Minnesota berry growers she surveyed reported using multiple tactics to combat SWD. Both organic and conventional growers reported using practices like daily harvest (clean harvests), frequent spraying, sanitation, hard pruning, and increased mowing, all to keep SWD from their fruit. 

Plan of Attack for SWD

Growers should formulate a plan for tackling SWD before the season is well underway, so they are ready to tackle this invasive pest when it shows up in mid-to-late June. 

By understanding SWD, researchers and farmers can find innovative ways to manage it. While completely eliminating SWD is not possible for most growers, they can still make their farm less hospitable for SWD to live and breed. Tools to do this include daily harvest, removing dropped fruit, mowing, hard pruning, and using landscape fabric. 

As growers pick ripe berries, they toss infected and overripe fruit in buckets to remove from the area to help control SWD. Photo by Annie Klodd

SWD relies on berries to reproduce, so clean harvests that remove all dropped or unsellable berries from the field is a critical way to keep fly populations down. Many farmers carry a “bad” bucket with them while harvesting to collect bad fruit and dispose of it in a sealed bag. Composting probably won’t kill the insect, because the eggs can turn into flies while sitting on the compost pile.

The flies localize near the ground and within the crop canopy during the day, where it is cool and humid. Recent UMN research in Dr. Bill Hutchison’s lab has indicated that flies will be active in the crop from 6-10 a.m. and 6-10 p.m., with the highest activity during the evening hours. During other periods of the day and overnight, they will leave the crop and find refuge in nearby wooded areas. 

They prefer humidity above 20% and temperatures below 86 degrees. Frequent mowing and selective pruning of perennial fruit can modify this habitat, forcing SWD to seek refuge elsewhere. Researchers at Michigan State University found that mowing every two weeks and pruning harder in the winter significantly decreased SWD on tart cherries. Trees pruned harder during the winter had 40% fewer SWD, even with no sprays applied. 

A pruning study between Dr. Mary Rogers’ lab at UMN and a Minnesota organic farm in 2017 found that pruning blueberries harder may help reduce SWD infestation. Pruning can cause the berries to ripen earlier, before the peak of SWD flight in July, and can improve spray penetration in the plant canopy so that the sprays that are applied can work better. A groundcover like landscape fabric between the rows, instead of wood mulch or grass, can also reduce habitat for SWD and make it easier to remove dropped berries.

Daily harvest: In a 2019 survey by DiGiacomo, daily harvest was the most common management practice among Minnesota raspberry growers (DiGiacomo 2019). Another study at Michigan State University found that berries harvested daily or every two days have much lower infestation than berries harvested every 3 days. Refrigerating the berries immediately after harvest can also keep the berries intact even if they have been impacted. 

Organic Insecticides

In terms of organically certified insecticides, spinosads like Entrust are so far the most effective defense against SWD. Pyganic, an organic pyrethrum, may have a moderate effect on SWD, but many farmers express disappointment with its performance. Neem oil is not considered to have acceptable efficacy on SWD. Because SWD and other insects can develop resistance to Entrust, and because it is an expensive product with limited applications per season, we highly encourage berry growers to develop more holistic management plans beyond chemical control.

Some farmers and researchers, such as Rogers at UMN, are starting to work together to test an essential oil-based repellent between Entrust applications. Preliminary results are promising. But more research must be done before we start recommending this method. We need to learn which essential oils work best, how effective they are, and how to use them most successfully. Collaborative research between universities and farmers is critical for this work.

Exclusion Netting 

Frustrated by the time-consuming, labor-intensive, and costly nature of SWD management, some farmers are surrounding their berry fields and high tunnels with exclusion netting in order to keep out insect pests altogether. Exclusion netting is already being adopted by berry farms of all scales in places like California, New York, Canada, and Europe. Researchers at UMN, and a small handful of Midwest farms, are experimenting with it, too.

Andrew Petran of Twin Cities Berry Company in Farmington, Minnesota, used exclusion netting over ¼ acre of day-neutral strawberries in 2019, and plans to expand it to another ½ acre in 2020. His structure consists of 80g netting, PVC and steel support poles, and polyamide wires; this setup meets one of his key requirements, that it must be moveable to fit with his crop rotation. For perennial fruit crops such as blueberry or raspberry, installing a more permanent support structure may be more suitable. 

Netting helps prevent SWD from getting at ripening strawberries. Photo by Andrew Petran

Dale Ila Riggs, the owner of Berry Protection Solutions and The Berry Patch Farm in New York, has several high tunnels with netting covering the side openings, and caterpillar tunnels covered in exclusion netting. She has helped many growers learn how to implement exclusion netting on their farms, emphasizing that it is not a one-size-fits-all method. Rather, the specific setup of the exclusion structure comes down to the scale, needs, and existing infrastructure of each farm. Riggs also designed a vestibule for her side-netted high tunnels, to help ensure that no flies enter the tunnels with her when she goes in to harvest.

What about pollinators? Naturally, the question of pollinators comes up in each presentation we give on exclusion netting. Raspberries and strawberries are both wind-pollinated, and recent research at UMN with pollinators in high tunnels has shown that the impact of pollinators on strawberry and raspberry yield and quality is not as clear-cut as one may think. 

However, many farmers using exclusion netting often order bees and place the hive boxes inside their exclusion for the period when the crop is being pollinated. We are often asked if the exclusion netting can wait to be put up until after the crop has been pollinated. The main barrier to this is that crops like raspberries and strawberries continue flowering and pollination throughout the harvest period, when SWD are targeting the berries. So, if farmers wish to use pollinators for raspberries and strawberries, the netting would need to go up prior to the start of flowering.

Moving forward: It can be easy to get discouraged by SWD, and many growers really are struggling with this insect. However, in times like these we all need hope, and the good news is that organic farmers and researchers are working together to discover feasible, innovative methods to manage this pest. Management of invasive pests does present a challenge, but it is a challenge we can overcome if we are willing to think outside the box and think holistically. 

Learn More

Annie Klodd and Andrew Petran presented a workshop on organic management of SWD at the 2020 MOSES Organic Farming Conference. See slides from their presentation here: bit.ly/SWDorganic. To pair the slides with the audio recording of their workshop ($5 for an MP3 download), see bit.ly/mosesorganicSWD.

Annie Klodd, Gigi DiGiacomo, and Mary Rogers discuss SWD on two recent episodes of the UMN podcast What’s Killing My Kale? See www.fruitedge.umn.edu/kalepodcast. 

Annie Klodd is a University of Minnesota Extension Educator for Fruit. She welcomes your questions about SWD at kloddann@umn.edu. 

Andrew Petran is a fruit researcher-turned-grower who owns Twin Cities Berry Company.

References:

1. Klodd, A., N. Hoidal, M. Rogers, University of Minnesota. What’s Killing My Kale? Season 3, Episode 6: Organic Management of Spotted Wing Drosophila.

2. Klodd, A., N. Hoidal, G. DiGiacomo, University of Minnesota. What’s Killing My Kale? Season 3, Episode 5: The economic impact of SWD on the Minnesota berry industry.

3. Leach, H. et al. 2017. Rapid Harvest Schedules and Fruit Removal as NonChemical Approaches for Managing Spotted Wing Drosophila. Journal of Pest Sciences 91. 219-226.

4. Petran, A. and M. Rogers. 2017. Pruning and Mulching for Control of Spotted Wing Drosophila: Effects on Fruit Marketability and Infestation. ASHS Annual Conference.

5. Poizner, S. and N. Rothwell. Orchard People. Urban Forestry Radio Show and Podcast. Episode 46: Fruit Trees and Spotted Wing Drosophila. June 25, 2019.

 

From the May| June 2020 Issue

 

Back to Current Issue

Comments are closed.