The Practical Gardener: Spotted Wing Drosophila

May 21, 2016

Spotted wing drosophila, a fruit fly pest not native to California, has been around these parts since 2008. They can absolutely ruin fruit crops if not taken seriously.
Credit Oregon State University

Hi.  I'm Pam Geisel, Your Practical Gardener.  Today we’ going to talk about a horrible terrible little tiny pest called spotted wing drosophila.

So as it happened, I was out trying to prevent the birds from decimating my just-now-ripe cherry crop with bird netting. The anticipation for those sweet ripe cherries was building.  In just days, those beauteous little red fruit would be turning that deep shade of burgundy and I would be popping them right into my mouth.

But then, as I went to harvest, I noticed the telltale signs. A little mushy spot on the fruit and darn if every single fruit weren’t damaged with that mushy little spot — and in some cases, the whole fruit was mush.  I inspected the fruit with dismay… my trees had finally become infested with the recently introduced pest called spotted wing drosophila. What a major disappointment.  Even more disappointing is the fact that not only are ripening cherries a favorite of this fruit fly but so are ripening black berries, raspberries, and blueberries — all of which are now getting ripe in my garden.

Spotted wing drosophila is a fruit fly and it was first found in many California counties in 2008.  I’ve been pretty lucky not to have had it until this year. 

These flies and the damage they do often go unnoticed until fruit is being harvested. Sprays at this time will not protect the crop, because maggots (or the fruit fly larva) are already inside the fruit.

If a small percentage of fruit is infested, you can salvage some of the crop by harvesting immediately and sorting and removing fruit with stings on the surface.

Now that I know that my trees have SWD, I will need to take preventive action next year before the fruit begins to change color from yellow to pink, which for me is in early May. 

Some options for management include draping the trees with a fine mesh netting that is less than .98 mm. along with early harvest. You can also monitor for populations and then treat with an appropriate pesticide at the right time to prevent infestation.

It’s important to start checking your fruit for damage (that is, prematurely rotting fruit or punctures in the fruit created when the female lays her eggs) as soon as it begins to develop any pink color. If you find infected fruit, you should either harvest all the fruit immediately or spray to prevent the damage from increasing before harvest. If you don’t treat, the infestation level can increase pretty quickly to implicate the entire crop.

To help reduce the infestation the following season, it’s a good idea to remove and destroy infested fruit. It is best to dispose of it by putting it into a durable plastic bag and putting it in the tras — not the compost pile. 

If monitoring indicates a need to spray, the application should be made as soon as the fruit just begins to turn from yellow to pink. This should be about 2 to 3 weeks before cherry or berry harvest. You may need a second application 7 to 10 days later. The insecticide spinosad (e.g., Monterey Garden Insect Spray) is effective and has the least negative environmental effects of currently available products.

How to trap for SWD:

You also can use traps to detect and monitor SWD adults. Commercial fruit fly traps are available or you can make traps out of 1-quart plastic yogurt (or similar) containers that have a lid.  Drill 10 to 16 holes that are 3/16-inch in diameter around the upper side of the container for fly entry. Bait the trap with 1 to 2 inches of pure apple cider vinegar; Add a drop of unscented liquid dishwashing soap to break the surface tension so the flies will drown. Hang the trap in the shade in your cherry tree or near your berries in early May or well before fruit begins to ripen.

Check the trap weekly for small flies with dark spots at the tip of their wings floating in the fluid. These are male spotted wing drosophila and will confirm that you have the pest. Put fresh apple cider vinegar and a drop of soap in each week. When you see adults, it is time to apply your protect spray. For more specific information on SWD management, visit the University of California's Statewide Integrated Pest Management Program.