If you have small fruits, including cherries, blackberries, raspberries, blueberries, and grapes that are setting fruit and ripening, be on the look out for Spotted Wing Drosophila (SWD) (Figure 1), a small vinegar fly that attacks all the delicious berries mentioned above!
If you are a small fruit grower, you’ve likely already heard of SWD, because it has wreaked havoc on the small fruit industry throughout the world. It’s ability to (1) use many different wild and cultivated host plants (moving from one to the next throughout the growing season), and (2) complete multiple generations each year, make this insect a major challenge to manage. To add insult to injury, these flies differ from the native fruit and vinegar flies in North America in a key way: rather than laying eggs in damaged or decaying fruit, they lay eggs in healthy, ripening fruits while they are still developing on the plant. This means that fruit may be destroyed or ‘wormy’ before you’re ready to harvest and enjoy it, and we don’t want that (Figure 2)!
SWD is an invasive vinegar fly that was first detected in the United States in 2008 and in Indiana in 2012. Among several diagnostic characters for identification, male SWD have spots near the tips of the wings, which may be visible to the naked eye for some people, but magnification is helpful (Figure 3A). Unlike the males, female SWD lack wing spots and can only be identified by using a magnifying glass or microscope to confirm the presence of an enlarged and hardened egg-laying organ (ovipositor) at the end of the abdomen (Figure 3B). It is this hardened ovipositor that sets SWD apart from our native fruit and vinegar flies and enables female SWD to cut through the skin of sound fruit to lay eggs.
The good news is that a lot has been learned about what can be done to help manage SWD in small fruit plantings and it boils down to the following key strategies (read more details about these in this factsheet from The Ohio State University Extension):
- Monitor the SWD population (adults and larvae) in your crops
Monitor adults with baited traps to determine when they arrive and how abundant they are. There are several commercial lures and traps available to monitor SWD adults, or you can make your own! Ideally, these traps are placed a bit early, before fruits become vulnerable, so you know when SWD are present and how populations are changing in your crop. These traps consist of a lure/bait and a liquid drowning solution at the bottom of the trap. Taken together, these elements attract adults to the trap and then capture them when they fall into the liquid at the bottom. The drowning solution should be removed weekly and examined with a microscope or magnifying glass to confirm detection. The threshold for action is 1 adult fly per trap!
Monitor larvae in fruit using the salt water test to determine how effective your spray program is at managing the SWD population and the level of larval infestation in berries. This is a pretty simple test: Add 50-100 berries to a container and pour 8 ounces of warm water + 1 tablespoon of salt and wait 15-20 minutes. If SWD larvae are present, they will leave the fruits and float to the top where you can spot them! If you see lots of larvae float to the top, this may be a sign that you need to check your spray equipment, change products, or shorten the time interval between sprays.
- Keep the field clean! Sanitation is key
Harvest fruit as soon as it is ripe and keep the field clean of any dropped or culled fruits! The flies may still lay eggs in unwanted fruit and this may contribute to growing SWD populations in your small fruit system. Destroy leftover fruits after harvest to reduce food resources available to SWD.
- Spray insecticides to manage SWD adults
If the SWD population gets out of hand, apply a synthetic or organic insecticide registered for your fruit crop, making sure to follow all label instructions and rotating between insecticide classes (to reduce the development of insecticide resistance). For a list of insecticides registered for use on small fruits, see the Midwest Fruit Pest Management Guide, 2019-2020. Please remember to be mindful of pollinators and do not spray when crops are flowering, and bees are active!
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- Figure 1. Adult Spotted wing Drosophila (SWD) on ripening raspberries. Photo: J. Obermeyer, Purdue University
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- Figure 2. Spotted wing Drosophila larvae in grape pulp. Photo: J. Obermeyer, Purdue University
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- Figure 3. Male (A) and female (B) specimens of the spotted wing Drosophila vinegar fly. Arrows indicate diagnostic characters on male and female flies. Photo: J. Obermeyer, Purdue University
