"Worms" in Your Berries? There's a New Pest in Town
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---Roswitha ("Ros-veta") in Emmaus, PA
A. This question hit really close to home, as her garden is less than ten miles away from mine. Luckily, I have not yet had any worms in my berries. But I think about it all the time—especially after I learned that local organic farming friends of mine chose to pull out and destroy all their canes because of the same problem— the Spotted Wing Drosophila ("dra-sof-a-la"); a tiny new insect from Asia.
Most people would call this nasty pest a 'fruit fly', but its technically a "vinegar fly". More important than mere characterizations, the females have a hypodermic-like appendage they use to shoot their eggs into the interior of fruits that are just beginning to ripen, especially blueberries, strawberries and raspberries.
Eventually, little 'worms' hatch out. They eat some of the fruit, but they live so deep inside that they're generally not discovered until the fruits are harvested and ready to go on top of your Cheerios, which, yes is beyond gross. The only thing that might be worse is if you're a small-scale market farmer and the 'worms' start crawling out while potential customers are examining your wares.
Now, we're calling them "worms"—which is part of the common name for many pest caterpillars. But these are the immature form of flies; so they're actually maggo….
…You know; let's just call them 'larvae' and keep what audience we have left. Now: According to the Michigan State University Extension , the tiny but distinctively-colored flies first showed up in California in 2008, have become widespread, and are seriously hard to control because of the way the female literally shoots her eggs into the center of ripening fruit.
The first thing that home gardeners (heck; and professionals) should do is set up traps. You'll find lots of detail in THIS LINK from Michigan:. Essentially you hang a yellow sticky trap inside a big cup that has tiny holes punched into the lid and liquid bait in the bottom. The bait is a mixture of water, yeast and sugar that ferments and attracts the flies. Checking the traps lets you know when the flies have arrived in your garden.
Now, at least in Michigan, these pests typically show up fairly late in the season, so it's crucial to not follow the common advice to mow 'double bearing' raspberry canes back to the ground in the fall. There's a good chance that the early harvest the following Spring from the 'second year canes' you saved might be unaffected. Conversely, some growers might want to deliberately sacrifice the traditionally smaller fall crop so as to not give the pests a breeding ground. (Read through our past raspberry articles for details on the two distinctly different types of this crop.)
Now: what about strawberries and blueberries?
As their name more than implies, 'June bearing' strawberries are harvested early in the season, before the flies typically appear—making them a much better choice for this battle than 'everbearing' varieties, which ripen up continually throughout the season. Gardeners in regions under attack by the nasty little fly may also wish to choose early-ripening blueberry varieties, and/or types that have the thickest skins, as these pests prefer thin-skinned fruits.
No matter what, keep the yeasty bait in your traps fresh; and if you start to catch the pests, pick ripe fruits promptly and immediately refrigerate the harvest. Cold slows the development of the eggs—and freezing might kill them. In addition, get rid of any visibly-damaged fruit by putting it in sealed plastic bags and letting it sit out in the sun.
Are there any safe sprays for this pest?
There are approved insecticides for both chemical and organic production, but they're all toxic to beneficial insects and the bees that are essential for pollination, which makes this kind of control tricky. Raspberries, for instance, always have new flowers opening up. But some blueberries have more of 'a flush of flowers followed by a lot of fruit', so I'd certainly consider spraying them with one of the spinosads—an organic insecticide that's approved for use on this pest and otherwise harmless—as long as all the flowers are gone. (Maybe pull off the last few flowers to be able to spray 'bee safely' and save the ripening berries.)
Screening is technically possible, but you'd need a very tight mesh row cover with ridiculously small openings; these flies are VERY small. And it would have to be sealed tight, like in a well-managed hoop house. And you'd have to introduce honeybees or bumblebees for pollination.
So then: What about just trying to intercept them all with a huge number of yeast traps?
Michigan actually suggests this for small gardens; and its worth a try. Start with just a few traps early in the season, and when you verify that you're capturing Spotted Wing Drosophila (ID details are in the linked articles), pile on. Because the actual traps are home-made by you and yeast and sugar are cheap, the biggest expense is your time.
And be sure your harvest goes right into the fridge, so any already-laid eggs don't hatch.
"Gross! We'd be eating fly eggs!"
Yeah. You'd be shocked at how much of your food contains a little rogue protein. They won't harm you and they won't breed inside you.
Note: Although we use the Michigan State Extension as our primary resource for this article (they are seriously on top of this pest), the topic has been covered by many State Extension services (although maybe not as thoroughly as Michigan.) It might make sense for you to also read articles generated by the states closest to you—especially for issues of timing and emergence.