The Dreaded Fruit Fly - Spotted Wing Drosphila

SWD (internet photo)

Last year, just when I was imagining the taste of a big, beautiful red raspberry, I noticed tiny white worms crawling around inside the fruit. Yuck!  I'd learned about the Spotted Wing Drosophila (SWD) a couple years earlier at a Master Gardeners conference. I was hoping to avoid it in my garden, but obviously it didn't work out that way.

Those little white worms are the larvae of the SWD, a fruit fly that is native to Asia and was imported to the United States about nine years ago. SWD was first found in California, and has since spread throughout the United States. It is more destructive than other fruit flies because the female has a firm ovipositor (the structure that the female fly uses to deposit her eggs into the host fruit) that allows her to "drill" into solid fruit to lay her eggs. Other fruit flies are only able to lay their eggs in over ripe fruits that aren't destined to be eaten anyway.

SWD larvae in raspberry (internet photo)

The worms (or maggots as they are also called) mature inside the fruit, eating into the fruit and causing it to rot. They eventually fall to the ground, pupate, and then emerge as adult flies to repeat the cycle over and over. The life cycle of the SWG is about seven to ten days, although in cooler temperatures, the life cycle may extend to about three weeks. They can destroy a lot of fruit over the course of the summer.  

SWD is only about 1/8 inch long, and looks very much like any other fruit fly with yellowish-brown color and dark colored bands on the abdomen. The males have dark spots on their clear wings which can help distinguish them from other fruit flies, but the females wings are clear with no distinguishing spots. Identification of the female SWD is made through its saw-like ovipositor. Now, keep in mind that the entire fly is 1/8 inch long. I know my eyes are not good enough to identify a microscopic ovipositor! I think it is safe to say that if you see fruit flies in your berry patch, you probably want to treat the patch for SWD.

Soft-skinned fruits are most susceptible to SWD damage. Raspberries, blackberries, strawberries, grapes, blueberries, plums, and cherries are all at high risk for SWD damage. If there has been any evidence of SWD in your area, you should begin monitoring for the presence of the SWD as soon as fruit begins to set. This can be done with a homemade plastic trap that you set out in your patch from fruit set to end of harvest.

To make your trap, take a quart size container with a top and drill or poke several small (3/16"-3/8") holes around the upper side of the cup. Leave the bottom 4 inches of the container intact to hold the vinegar. The small holes allow the SWD to enter the trap while keeping out larger flies.  

Pour one inch of apple cider vinegar and one drop of unscented dish soap in the bottom of the container. The vinegar will attract the flies and the dish soap will trap the flies in the liquid. You can also purchase the yellow sticky sheets and hang one of those from the top of the trap to catch the flies when they enter the plastic container.

Use a wire threaded through some of the top holes to hang the trap. Hang the trap in the fruit patch in a location that is protected from wind blowing it around but allows easy access for the SWD in all directions. Check the traps and change the solution every other day or so.

SWD trap (internet photo)

Once you detect SWD, spraying with an insecticide is the only really reliable way to protect your fruit. Multiple applications will be needed throughout the growing season because of the short lifecycle of the SWD. The length of time between applications depends on the product used, but is generally anywhere from every 5 to 10 days. SWD has shown itself to become resistant to insecticides if the same agent is used for each spraying, so pesticide classes should be rotated. Insecticide will kill only the adult fly and will not kill any larvae that are already in the fruit, so it is important to begin the spraying early in the fruiting season, before infestation occurs. Even if the insecticide you use is approved for organic use,you still must rotate among the insecticide classes. Always follow directions on the label for whatever product you are using and pay attention to the pre-harvest interval for safe use after spraying. 

The table below is provided by the University of Minnesota Extension service. 

Effective insecticides
Common Name	Class	Type	Residual*
Pyrethrin	Pyrethrin	Low-impact	Very short
Spinosad	Spinosyn	Low-impact	Medium
Malathion	Organophosphate	Conventional/broad spectrum	Short
Esfenvalerate	Pyrethroid	Conventional/broad spectrum	Medium
Permethrin	Pyrethroid	Conventional/broad spectrum	Medium
*Very short residual lasts less than 1 day. Short residual lasts 3-5 days. Medium residual lasts 7-10 days.

Another critical factor in controlling SWD is hygiene in your fruit patch. Always clean up any overripe or decaying fruit that has fallen to the ground or which may still be clinging to the stem. Seal infested fruit in a plastic bag and place it in the trash. Fruit in clear bags can be left outdoors where the heat from the sun will kill any flies in the bag before being disposed on in garbage. Do not compost the infested fruits as composting will not kill SWD. Don't bury it either,  as research has found that SWD can survive being buried as deep as 18 inches.

Yes, nothing beats a freshly picked raspberry or strawberry, warmed by the sun, and popped into your mouth for a nice, juicy explosion of flavor. But, those little SWD larvae take a lot of the enjoyment out of that. Now, there is no risk whatsoever to humans who happen to eat a SWD larvae, but still - yuck....Prevention is the treatment to keep those little buggers from spoiling your treat.

The Drought Tolerant Garden (slides from Dakota Garden Expo 4-22-17)