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Spotted Wing Drosophila is Now Active

SWD on raspberry. Photo: Charlie Rohwer

Authors: Annie Klodd, UMN Extension Educator - Fruit and Vegetable Production; and Bill Hutchison, Professor and Extension Entomologist

Spotted wing drosophila (SWD) is currently active across Minnesota. University of Minnesota researchers and MDA have a network of traps to monitor SWD throughout the state, and as of June 16, SWD has been collected in traps in Forest Lake, Rosemount, Hastings, and Chanhassen.

Once gardeners notice SWD on their ripe fruit crops, they should begin controlling them. A small amount of SWD can cause a large amount of damage on the fruit, by laying eggs in multiple berries.

Decide how you will manage SWD now

Gardeners growing strawberries, raspberries, blueberries, grapes, and other soft fruits should plan ahead to decide how they will manage SWD, and order any remaining supplies before SWD populations grow. Strawberry, honeyberry, summer-bearing raspberry, and blueberry growers should be monitoring particularly closely at this time, as those crops are either already in-season, or will be soon.

Good management of SWD involves a mix of cultural, preventative, and chemical (either organic or synthetic pesticide) practices. For home gardeners with small fruit plantings who are willing to tolerate some damage, it may be sufficient to avoid most SWD damage without pesticide application by using preventative methods.

In this article, we review the recommended practices to reduce SWD damage. However, University of Minnesota has already created many resources to aid fruit growers in developing and improving their management plans.

Because so many additional resources are available to help you build a management plan, we will keep this article brief and encourage you to use the resources at the end for further information.

Non-chemical management practices

Infested fruit being removed from the field - a common sanitation practice for reducing SWD. Photo: Annie Klodd

Harvest every 1-2 days: Based on 2019 survey data by University of Minnesota researchers, daily harvest is the most common SWD management method among Minnesota raspberry growers. Additionally, a Michigan State study has found that harvesting every 1-2 days significantly decreases berry infestation rates, compared to berries harvested every 3 days. Mary Rogers discusses this is this UMN podcast episode. 

Refrigerate the berries right away: In addition to harvesting frequently, refrigerating the berries as soon as possible after harvesting prevents the fruit from degrading, even if it has already been impacted by SWD. 

Get bad fruit out of the garden: Removing and destroying infested or dropped fruit stops the eggs and larvae from turning into adult flies, potentially reducing the SWD populations in the field.

Disrupt their habitat: Landscape fabric, mowing, and heavier pruning may, in theory, reduce SWD populations by making the garden less hospitable to them. SWD retreat to cool plant canopies and grassy areas during the hot parts of the day. Furthermore, they are poor fliers and do best under less windy conditions. Therefore, minimizing these habitats may help decrease SWD pressure. However, more research is needed to more thoroughly understand just how effective habitat disruption can be for reducing SWD. More information is available in this podcast interview with Nikki Rothwell at Michigan State University.

Exclusion netting: The use of netting structures around the crop to exclude SWD is a promising but challenging area of research. This is a technique that can be used by either gardeners or farmers, because it can be scaled up or down to fit the needs of the grower.

UMN researchers have been looking at the role of exclusion netting for the past several years. Exclusion netting is being more widely adopted on larger farms in places like California, Europe, and Canada. Structures can vary in size from a single small row of berries to an entire field. At least one Minnesota farmer, Andy Petran, has started to use exclusion netting on day neutral strawberries. Annie Klodd and Andy Petran gave a presentation about this at the 2020 MOSES Conference, and wrote a subsequent article. Read the article here, or order the presentation from MOSES.
A small exclusion structure covering one row of blueberries. Photo: Dale Ila Riggs

Organic insecticides and repellents

Spinosad is available to home gardeners, and is considered the most effective organic pesticide option for SWD at this time. Gardeners should have a couple of products on hand, and not rely heavily on Spinosad for SWD control, as resistance can develop. 

Pyrethrins like Pyganic can be used as well, however Pyganic is not as effective as spinosad. 

Essential oil repellents: University of Minnesota researchers, led by Dr. Mary Rogers and PhD student Matthew Gullickson, have been testing certain essential oil mixtures as repellents against SWD. Tests are ongoing on farms and in the laboratory. While results so far are promising, we cannot yet recommend these products without more controlled testing. 

While neem oil can be used for SWD, its efficacy is considered poor.

Because organic insecticides are limited and can be very expensive, organic gardeners are encouraged to develop more robust approaches for management of SWD, involving non-chemical (cultural and preventative) practices listed above and in the linked resources.

Synthetic Insecticides

Synthetic pesticides available to home gardeners for SWD management are limited. Sevin (carbaryl) is the most easily accessible synthetic pesticide for SWD for home gardeners. Carbaryl is a broad-spectrum insecticide, and will also kill pollinators. Those wishing to use carbaryl should follow guidelines for protecting pollinators, such as spraying in the early morning or evening.

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