Grower Resources
Integrated Pest Management (IPM) is the standard approach for managing pests in small fruit farms. This strategy involves integrating field observations with scientific insights to make informed decisions regarding pest control. The emphasis is on employing the most gentle and least disruptive methods initially, resorting to more robust measures only when necessary. i.e. don’t use a hammer to swat a fly. SWD is no exception to this, and despite it being the biggest insect pest in small fruits, it is controlled extremely well using simple IPM strategies and principles.
SWD Float Test
Monitoring for Spotted Wing Drosophila Using Salt Water Float Tests
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SWD Control
The standard Pacific Northwest approach to SWD control consists of three steps:
Assess
Assess individual field risks and factors to get a baseline understanding of what you can expect. Evaluate forecasts, as weather conditions significantly influence SWD egg-laying and breeding. Use trapping to establish long-term comparative risk trends.
Monitor
Regular fruit sampling for larval extraction tests beginning as fruit begins to color. Focusing on high-risk areas in the field and priority fruits on the plant. Monitor at least twice a week, as increased frequency enhances early larvae detection. Sample fruit from the most susceptible field areas.
Respond
Use the information gathered from assessments and monitoring to chose the appropriate tool from the SWD management toolbox.
SWD Risk Factors
Numerous factors influence SWD risk at the field level, These ‘risk factors’ play an extremely important role in likelihood of SWD damage in a field. Assessing the field risk is important to understanding how little/much control might be necessary. Risk factors hinge on the closeness of fields to SWD population reservoirs and the field environment’s favorability for SWD movement. These factors can be categorized into landscape, crop, and cultural practices.
- Proximity to border habitats- border habitats that are prime for SWD are non-crop areas with trees, dense vegetation
- Adjacent fields that finish harvest earlier
- General air movement in the field- Is the field in an area where it seems like it is in a wind tunnel and always windier than other locations or vice versa.
- Secondary hosts nearby- Blackberry brambles in irrigation ditches and landscape fruit trees (cherries, apples, peaches, etc.) may not be large enough to provide a large population refuge
- Winter severity: Lower after winter with hard freezes (especially late/early freezes)
- In season timing: Before mid-July, low risk. After mid-July, high risk.
- Temperature: High temps into and above the mid-80’s (~29 C), lower risk. High temps staying below 80 (~26 C), higher risk.
- Humidity: Dry, low humidity, lower risk. Wet, high humidity, higher risk.
- Blueberry, Caneberry, Strawberry, etc.- Blueberries, with harder skin and generally more weather exposed fruit are less susceptible than softer caneberries which are much more susceptible.
- Cultivar seasonality- Because SWD populations build over the season, there is usually little population at the time of harvest for early season varieties which means little field pressure. As the season progresses, those populations build as does the pressure.
- Cultivar growth habit- Dense canopied cultivars with tightly packed/highly shaded fruit is more susceptible to SWD.
- High crop diversity and density of small blocks can increase risk as SWD is able to jump from different planting to planting in close proximity as fruit ripens and becomes susceptible.
- New over old/cane management (caneberries) – blackberry production = hard to penetrate with spray and increases the risk of SWD.
- Plant architecture (dense canopy vs. open canopy) – open canopy or the removal of older cranberry floricanes can reduce risk, as shaded fruit is more susceptible to damage. Good airflow reduces this risk further, while a thick layer of leaves traps moisture and cools temperatures, creating an ideal environment for SWD.
- Irrigation (drip, overhead)- using drip irrigation lowers risk of SWD, while overhead irrigation poses a greater risk because it increases humidity.
- Overhead cooling- SWD are heat-sensitive. Cooling the canopy enhances their survival and resilience during heat events.
- Weedmat – The presence of a weed mat decreases the risk of SWD larvae pupating in the soil, disrupting their lifecycle.
- Picking schedule – Late Harvest and low-frequency picky both increase the risk for SWD.
- Low yield/high yield situation- a higher density of fruit in the plant (aka when there are higher per acre yields) creates more ideal conditions for SWD.
Sampling for SWD larvae
Larvae Extraction Procedures Information
Collect a sample of marketable fruit to be tested
Fruit collected should be from the highest risk areas of the field (field edges near border habitats) and from most susceptible location within the plant (the shaded interior of bush where SWD are protected from the elements).
The number of fruit depends on the crop (given different sizes of fruit) and crop stage (how much ripe fruit is easily accessible). For blueberries, the number of fruit should ideally be between 150-300 fruit per samples, strawberries is 30-50, raspberries 60-100, blackberries 40-60. Fruit should be quick to collect and a sample shouldn’t take more than 5-10 minutes to collect. If it takes more than that, there probably isn’t enough ripe fruit to take the sample and/or take a smaller sample size of fruit.
A minimum of 3 samples should be collected from different high risk areas of the field.
Sampling should be done at least 2 times per week in order to catch very early, treatable, and non-economic SWD presence.
Put fruit in a gallon size resealable plastic bag
Lightly squeeze fruit in the bag just enough to break skins but you don’t want to smoosh the fruit to a pulp because that makes it very hard to analyze.
Optional: Leave fruit in bag in hot sun to heat to do initial larvae inspection. Heating will cause larvae, if present, to emerge from fruit and will be actively moving and easy to spot.
Pour in enough of a salt water solution to allow the fruit to float
Solution is: 1 cup of salt per gallon of water
Alternative solution is using sugar at 2.5c per 1 gal of water. Advantage of sugar is a longer survival time of larvae in the solution.
Analyze collected fruit samples
For a quick check in the field after a designated period of time (at least 15 minutes)
A final check should be done after allowing sample to soak for 1 hour or more. Use a shallow flat tray to pour bag contents into. Having a large surface area makes it easier to examine for any SWD larvae. Wire mesh can also be used to further separate fruit from solution.
Field Management
One of the basic tenets of IPM is having a toolbox of management options and utilizing the least intensive option that can address the immediate field need. For many years, the SWD management toolbox had few options outside of chemical controls, but that toolbox has and continues to expand rapidly, greatly reducing the need to rely solely on chemical controls. Here is information on some of these different management options:
- Parasitoids – Traditional biocontrol, releases of SWD specific parasitoids began in the Northwest in 2023 and the idea is that populations will establish over time and generally keep SWD populations in check. See resource.
- Mating disruption/Sterile releases – Using various gene editing technologies, sterile SWD are produced and can be released in/around the farm environment. Mating with the sterile insects produces non-viable offspring which helps to disrupt SWD population growth. Additionally, various pheromone/chemical compounds are being researched and developed which directly/idirectly disrupt SWD’s ability to reproduce. See resource
- Attract and Kill – a compound combination of SWD attractent with a chemical kill agent which can avoid have to apply chemical to the crop itself but still attain
- Traditional chemical controls
- Alternative spraying patterns – Because of SWD’s behaviour and migration patterns into a field, strategic application of chemical controls can often be applied in lower pressure situations to help control and mitigate SWD risk.
- Field perimeter spraying – As SWD moves from non-crop areas into the field, an application along the field edges can be an effective strategy to keep SWD out while not having to cover an entire field.
Trapping
Trapping is a helpful tool to track adult populations from year to year and over the course of the season. However, they are inadequate for determining field damage or risk. If trapping keep the following in mind:
- Consistency is more important than specific trap design or bait used. There are many options out there, the best solution is the one you can stick with.
- Placement is key, ensure trap is shaded and protected as that is most likely to be in SWD’s path.
- Traps in the middle of the field are unlikely to be helpful and will rarely catch SWD.
- Placing traps in adjacent border habitats can be insightful to population reservoirs. Paired closely with a field trap, seasonal border-field movement can be monitored.
- Male AND Females should be counted, ID skills crucial (ID key)
- Do not make standalone management decisions on trap counts, but do compare numbers between years to get a sense for how SWD pressure may be similar/different from past experience.
Spotted Wing Drosophila (SWD) is a major pest affecting small fruit crops in the Pacific Northwest.
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Additional SWD Resources
SWD IPM Information
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