It is Time to Prepare for Spotted Wing Drosophila

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. As of June 16, they have been collected in traps in Forest Lake, Rosemount, Hastings, and Chanhassen. See the figure below from the SWD Trapping Network and UMN's IPM Program on FruitEdge. This page is updated every Thursday.

Forest Lake, MN: 0.50 SWD per trap (= 1st catch of the year)
Rosemount, MN: 3 SWD per trap (raspberries)
Hastings, MN: 1 SWD per trap (grapes)
Chanhassen, MN: 5 SWD per trap (mixed crops)
Houston County, MN: 0 SWD per trap (strawberries)

Source: FruitEdge.umn.edu/swdtrap

If early-ripening fruit are present, and the trap catch is at least 1/trap/week, this is the action threshold where sprays are needed to control for SWD infestations.

Get a SWD Management Plan in Place

Farms producing strawberries, raspberries, blueberries, grapes, and other soft fruits should finalize their plans for managing SWD and order any remaining supplies. Strawberry, honeyberry, summer-bearing raspberry, and blueberry growers should be monitoring particularly closely at this time, and begin spraying for SWD once the crop is ripening and 1 fly per trap is found on their farm.

The most effective management plans integrate a mix of cultural, preventative, and chemical (either organic or synthetic pesticide) practices.

In this article, we review the recommended practices to reduce SWD damage. However, University of Minnesota has already created many resources to aid farmers 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

Daily or 2-day harvest intervals: Based on 2019 survey data, 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.

Immediate refrigeration: In addition to daily harvest, refrigerating berries as soon as possible after harvest reduces larval infestation and destruction of the berry, by preventing eggs in the berries from hatching.

Sanitation: Removing and destroying unmarketable or dropped fruit stops eggs and larvae from turning into adult flies, potentially reducing the SWD populations in the field.

Habitat disruption: Landscape fabric, more intensive pruning, and manipulating canopy air movement may, in theory, reduce SWD populations by making the field 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 the level of impact that habitat disruption can make. 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. UMN researchers have been looking at the role of exclusion netting for the past several years. Exclusion netting is being more widely adopted in larger production areas such as California, Europe, and Canada. Structures can vary in size from a single 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 (Entrust, IRAC code 5) is considered the most effective organic pesticide option for SWD at this time. In order to minimize the risk of insecticide resistance, all growers must use it wisely by rotating with other insecticide families (see IRAC code), especially in longer season crops like raspberries and day neutral strawberries. Research has confirmed SWD resistance to spinosad in California.

Refer to the label for more application guidelines.

Pyganic (pyrethrins, IRAC code 3A) has some efficacy against SWD. However, many farmers have been expressing their disappointment with its performance. The Midwest Fruit Pest Management Guide rates its efficacy on SWD as “Poor” based on controlled trials.

Essential oil repellents (Ecotrol): 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 in both laboratory and on-farm settings. Some farmers have also begun trying Ecotrol on their farms. While results so far are promising, more controlled trials have to be done with these products in order to provide recommendations for use. We must learn more about its effectiveness, proper application timing and frequency, application methods, and other factors. Further grant funding would allow for more exploration and education about these products.

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

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

Conventional Insecticides

Growers have several options for effective synthetic insecticides for use against SWD, but the labeled insecticides vary by crop. You can review what’s labelled for most MN berry crops at:

https://www.fruitedge.umn.edu/swdpestprofile.

As noted in Table 1 in the link above, be sure to note the Pre-harvest interval (PHI) and Rentry interval (REI) for workers entering the field.

Again, it is critical to rotate between modes of action (IRAC code), rather than relying heavily on one. In addition to IRAC codes, be aware of re-entry intervals, pre-harvest intervals, and application restrictions. Refer to the product labels and the Midwest Fruit Pest Management Guide before application.

IRAC codes for effective insecticides:

Mustang Maxx: 3A
Danitol: 3A
Baythroid: 3A
Proaxis: 3A
Warrior: 3A
Malathion: 1B
Imidan: 1B
Diazinon: 1B
Sevin (carbaryl): 1A
Entrust (spinosad - organic): 5
Pyganic (pyrethrins - organic): 3A

For updated insecticide control options, please refer to the annually revised “Midwest Fruit Pest Management Guide”. This guide is produced by UMN Extension and cooperating Midwest state Extension Specialists.