About spotted wing drosophila (SWD; Drosophila suzukii)

Image: A spotted wing drosophila adult male showing a dark spot on each wing.

Spotted wing drosophila (SWD; Drosophila suzukii) is an invasive fruit fly that has become a major fruit pest. It tends to impact raspberries (also known as spotted wing candy), blackberries, blueberries, and table grapes, but an also affect honeyberries. SWD is a tricky pest to manage due to a few biological advantages:

Its serrated ovipositor (a saw-like egg-laying mechanism) allows adult females to lay eggs in unripe fruits, whereas most fruit flies can only lay eggs in overripened, soft fruits.
Eggs are laid within fruits, but when larvae mature, they can either pupate within the fruit or drop out and fall to the ground if the fruit is mushy.
SWD life cycles are temperature dependent, but much shorter than other pests. This means they can have many generations per growing season if left unmanaged.
They tend to favor humid, shady environments and moderate-to-warm temperatures and recent research done at the University of Minnesota has shown that wooded edges with wild honeysuckle and other wild berries can harbor early populations of SWD before they enter a managed site.

Read more about UMN research on wild honeysuckle species and spotted wing drosophila management here:

Invasive honeysuckles: A dual threat to Minnesota’s environment and berry growers

Monitoring SWD adults

a spotted wing drosophila trap made from a deli container with liquid in it on the ground with grass in the background.

Image: A spotted wing drosophila (SWD; Drosophila suzukii) trap used to monitor and catch SWD adults . Photos taken at Blue Fruit Farm located near Winona, MN (Zone 5a).

Traps to monitor SWD can either be dry traps with purchased lures, or liquid homemade traps (see recipe below). Homemade traps tend to be less selective than ones with commercial lures, as homemade ones tend to replicate scents associated with rotting fruit (e.g., yeast, and sometimes vinegar). Adult females that are seeking fruits for egg laying are generally responding to cues related to ripening, rather than overripened, fruits.

This is why these traps are helpful for monitoring, but are generally not effective as a management option on their own. Several attract and kill options have been more recently researched and developed (see information in next section), which have their own mechanisms to attract various adult stages and are combined with an insecticide like Entrust.

Homemade trap recipe

Resources from the University of Wisconsin recommend creating homemade traps by using a 32 ounce deli cup with holes drilled into the lid, filled with the following recipe:

1 Tbsp Active dry yeast
4 Tbsp Table sugar
12 oz Water
2 drops of Dish soap

This formula works because the sugar helps the yeast to develop and the dish soap decreases the water surface tension, making it difficult for flies to escape. A small amount of vinegar can also be used on its own or incorporated into the above formulation.

A formula like this will also attract other insects, so growers using this method should also create a method to screen the trap for SWD. A mesh sieve can be used to filter out insects caught in the trap and a dissecting microscope can be used to identify SWD adults (simple models costing around $100 and can be used for identifying other pests). SWD males and females are very small. Males can be identified by the dark spot on their wing, which is more apparent than trying to find the ovipositor on the female adults.

Monitoring SWD larvae:

Fruits can additionally be checked for larvae as they grow past the point of susceptibility, which is marked by when they start to change color, which often correlates to fruit softening and sugar accumulation. Fruits can be checked for larvae twice per week through visual inspection or using the salt flotation method (also known as the salt test) where fruits are collected, mashed in a plastic bag, mixed with salt water, sieved, and then the water mixture is viewed under a dissecting microscope.

Management:

Management resources are listed on UMN's FruitEdge SWD biology and management webpage, along with the Midwest Fruit Pest Management Guide. Make sure to follow pesticide labels (the label is the law) and pay attention to FRAC numbers to prevent resistance build up in SWD populations.

Additionally, organic and conventional growers who are dealing with SWD can practice the following cultural management practices:

Frequent harvesting.
Removing and destroying infested soft fruits from the area.
Cooling down fruits quickly after harvest to slow down any SWD development within fruit.
Proper pruning and thinning to increase canopy light and air circulation.
Manage irrigation leaks.

SWD webinar recording, and UMN website and tools

Webinar recording

A lot of research has gone into learning about new methods to control SWD in fruit crops in various settings in the past decade. The UMN Extension Fruit Program delivered a winter webinar that went over SWD biology and recent research and innovative management practices, including a trap and kill option created from food-grade gum.

Video: Spotted wing drosophila management: Recent insights and innovations

UMN website and tools

The University of Minnesota has a web landing page with information about management mentioned above, the SWD trapping network, and the trap networks historical data.

UMN FruitEdge: Spotted wing drosophila webpage

For more information on organic management, check out the article below:

Organic management of spotted-wing drosophila (SWD) in small fruit production systems

There also exists a downloadable spreadsheet to calculate costs associated with management.

Calculating returns to SWD control measures