CANTON, N.Y. — Ever since a tiny Asian fly called the spotted-wing drosophila (SWD) “discovered” us in 2012, growers of cherries, raspberries, blueberries, and other small fruits have battled this fruit-wrecking pest. Though it’s “just” a fruit fly, SWD are not your grandparents’ fruit flies. Wait, that sounded awkward. Old-school, respectable fruit flies gently push their eggs into rotten fruit. SWD, which come equipped with sharp saws and bad attitudes, don’t wait for fruit to go soft.
The female has a saber-like ovipositor with sharp, sclerotized (hardened) teeth. She uses this formidable tool to break the skin on unripe berries—strawberries, raspberries, and blueberries are favorites—and insert eggs. As the berry starts to turn color, tiny maggots are maturing inside. Other fruit flies need mushy fruit to lay eggs; SWD makes fruit mushy.
Signs of SWD in raspberries include fruit which are darker and squishier than normal, have poor flavor, fall to the ground prematurely, or “deflate” and dry out. Once picked, infested fruit spoils very faster, even overnight. Juice droplets on the fruit, or on the plant after the berry is plucked, are other clues. At dusk or early morning you may even see adult flies checking out the fruit.
With one spot on each wing, the male SWD stands out from other species. Females have no wing spots, but can be identified, under magnification, by their spike-tooth ovipositors. Spotted-wing drosophila larvae are white, and about 1/32” to 3/16” long.
SWD breed in loads of wild fruit such as elderberry, dogwood, buckthorn, honeysuckle, and even nightshade. In warm weather they can have about one generation per week, with eggs hatching in as few as 12 hours. Cool weather, of course, slows them down. Eggs and larva become inactive at about 35F, and at 33F some may even be killed.
Initially it was believed SWD were not cold-hardy, but that is now in question. No one is sure if they simply emerge later than other fruit flies, blow in from the south, or if their major route of infestation here is via produce shipments. Early-season berries shipped from warmer locales come with a free supply of SWD eggs and larvae. It’s unavoidable. Although commercial berries are now sprayed more frequently than ever, SWD cannot be entirely controlled.
We can’t eradicate SWD, but we can reduce their impact. Pick berries less ripe than you’d normally select, and refrigerate right away. Stomp on berries that fall to the ground so they dry out and don’t continue to breed flies. For homeowners, there are few pesticide options. Some common products like carbaryl can remain toxic for 7-10 days, and shouldn’t be used on berries.
Fortunately, innovative 2017 research on attract-and-kill methods, and a new SWD-exclusion netting system, may hold the key to SWD control. You can learn more about field-proven control options at an upcoming Cornell Cooperative Extension class featuring Dr. Juliet Carroll from the NY Integrated Pest Management program, Peter Jentsch from Cornell’s Hudson Valley Jentsch Lab, and Dale Ila Riggs, berry farmer and president of the NY Berry Growers’ Association.
The class will be held on Thursday, March 15, 2018 from 8:30 AM to 3:00 PM at the Best Western Inn in canton. The cost is $20. 00 which includes lunch and materials. You can register at stlawrence.cce.cornell.edu/events or call (315) 379-9192. For more information, email ph59@cornell.edu.
–Paul Hetzler
Cornell Cooperative Extension
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