St. Lawrence County berries under attack from Asian fly
By PAUL HETZLER
What looks attractive, smells great, has super antioxidant properties, and flies? It could be a new superhero, I suppose: Antioxidant Man, a caped figure of superior hygiene who heals people through the use of antioxidant therapy. Maybe I’ll pitch it to Marvel Comics.
The answer of course is berries. In particular, strawberries, raspberries, blueberries, and blackberries. Whether wild or domestic, these native fruits are icons of the season; bright, mouth-watering gems with which we stuff our faces and pie crusts. Or make into jams and jellies to preserve a taste of summer for those long cold months. Berries are a pastime, a passion, and for produce growers, a paycheck.In point of fact, berries could use a superhero right about now to fend off a new invasive pest. In 2008, an Asian fly called the spotted-wing drosophila (SWD) was introduced on the West coast, and in 2011, the first few individuals were found in New York State. I suspect these were scouts, because somehow in 2012, hordes of SWD showed up across NY, destroying entire crops of raspberries and blueberries.
The arrival of the spotted-wing drosophila changed everything for commercial and backyard berry growers alike. Late-season fruit such as blueberries and fall raspberries are especially hard-hit because SWD populations build throughout the season. From 2012 through 2015, SWD found its way to northern NYS by mid- to late August. In 2016 it arrived in late July, and this year we discovered it in the first week of July. The reason for this trend is unknown as yet.
“Drosophila” sounds exotic, but it is merely the Latin genus name for fruit flies. Since fruit flies have been around forever (give or take a few days), you’d think one more species would not matter. But the spotted-wing drosophila is different—they are not your grandmother’s fruit flies. OK, let me rephrase that. Unlike respectable, old-fashioned fruit flies that go for spoiled fruit, the spotted-wing drosophila does not wait for berries to ripen and get soft. The SWD comes equipped with a sharp saw and a bad attitude.
The female has a saber-like ovipositor lined with a double row of sharp, sclerotized (hardened) teeth. Under magnification, her egg-laying equipment literally looks like a tiny pruning saw. She uses this formidable tool to break the skin of unripe berries and insert eggs. Strawberries, blackberries and raspberries are easiest to penetrate, but cherries and blueberries are no problem for them, and even grapes are fair game. As the fruit begins to turn color, tiny maggots are maturing inside. Other fruit flies are attracted to rotting fruit; SWD makes fruit rotten.
In raspberries, signs of SWD activity include fruit that may be darker in color or feel softer than usual. Berries may have poor or “off” flavor, or fall to the ground prematurely. Fruit brought indoors may begin to spoil faster than normal, even overnight. Droplets of juice on the berry, or on the plant when it is plucked, are other clues. Sometimes berries will seem to “deflate” or dry out. As the infestation progresses, you may even see the adults at dusk or early in the morning.
The male spotted-wing drosophila stands out from other species because he has one spot on each wing. At rest you can usually see this without magnification (it helps if the fly is resting as well). Females have no such markings, but can be identified by the aforementioned ovipositors. A dissecting scope or good-quality loupe is required for this, however. The white larvae range from one to three millimeters long, or in non-metric terms, from small to really, really small.
SWD can breed in a wide variety of non-crop hosts like juneberry, elderberry, dogwood, buckthorn, honeysuckle, and even nightshade. In very warm weather there can be one generation of SWD per week, with the eggs hatching in as few as twelve hours. As the weather cools down, of course, this takes longer. Eggs and larva become inactive at about 35F, and 33F may even kill some of them. Like many of us, SWD drastically reduce their activity at temperatures above 90 degrees.
Even after five years, much is still unknown about SWD. We think they are not very cold-hardy, and do not overwinter in northern NY, but this has not been proven. They may migrate north over the season, and/ or be blown here on storm fronts. We do know that some of them arrive by truck.
Fruit shipped early in the season from southern California, Florida or other warm locales comes with a free supply of SWD eggs and larvae. It’s unavoidable. Even though commercial berries are now sprayed far more frequently and heavily than ever before, SWD is impossible to completely control. If not for this movement of SWD in produce, it is possible that northern NYS might not see SWD until much later in the season.
There is no getting rid of SWD, but you can fight them. For soft fruits like raspberries, pick them when they are almost, but not quite, ripe. Then refrigerate them as soon as possible. Stomp on berries that fall to the ground so they will dry out faster and not incubate more SWD. If you are not a licensed pesticide applicator, there are very few pesticides options available. Some over-the-counter sprays like carbaryl can remain toxic for a week or more, and should not be used.
Exclusion netting is expensive, but it is the best solution we have at the moment. Cover your berry patch just after petal drop with a fine-mesh product such as mosquito or “no-see-em”netting. Commercial growers have worked fast to develop a viable, long-lasting product. Right now the only manufacturer in North America is a factory near Montreal, and their exclusion net is made one month out of the year, strictly by order.
Many efforts are underway to combat this new threat to small fruits. Areas of study include new insecticides, as well as biological controls such as predatory insects, and pathogenic fungi and bacteria. At this point, though, the best defense is a good defense. Or a superhero.
For more detailed information, visit http://www.fruit.cornell.edu/spottedwing/ or call your local Cornell Cooperative Extension office. For information on exclusion netting, see goo.gl/knTzFZ
Paul Hetzler is a forester and a horticulture and natural resources educator with Cornell Cooperative Extension of St. Lawrence County.