LARGO, Md. — Spotted wing drosophila, an invasive, destructive pest originating from eastern Asia, has recently been found in Maryland infesting blackberries and raspberries.
Damage to late season fruit crops such as fall raspberries, late-season blackberries and day-neutral strawberries was first discovered in Pennsylvania and Maryland in 2011. Other crops that have suffered damage include blueberries, summer blackberries and raspberries, cherries and even grapes.
Tree fruits are also vulnerable with cherries the most likely to suffer heavy damage.
Gerald E. Brust, integrated pest management vegetable specialist with the University of Maryland, said that although they have caught the the insect in grapes, he has yet to see where they did a lot of damage. “We’ve even found them in peaches, but not as much,” Brust said.
He suspects the pest hitched a ride to the U.S. via imported fruit from China.
“They are using the same pesticides we do,” he said, “But the infestation has been worse here because it surprised farmers and took off so quickly, whereas their SWD have been a problem for a long time in Korea, Japan and Philippines, where small fruit growers have learned to live with it.”
Brust recommends using all available means of management, including cultural and chemical options. “Ideally, cultural management is the best, but once they get started, cultural control is not going to cut it,” he said. Because they are so small, one-tenth of an inch in length, it’s “very easy to miss them until they’ve done damage.”
The first year the pest appeared in Maryland, “they shut down a lot of blackberry and raspberry fields. The cost to farmers was probably in the millions,” he said, adding that some farmers, especially organic farmers, have lost an entire crop to the bug.
“One problem is that we don’t have a good trap to catch them when they first enter the field,” he said.
At first producers baited make-shift or commercial traps with apple cider vinegar, which will attract spotted wing drosophila. “It works better to use white sugar and yeast — they are strongly attracted to fermenting yeast. The flies smell the gases they give off. It’s messy and you have to change it every week but it works better than the apple cider vinegar,” he said.
Brust said one of the best things to do is watch fruit as it starts to ripen. “If you find punctures, open the berries and look for maggots,” he said, then treat the crop with an appropriate pesticide. Treatment is most effective before the pests take hold.
Since other kinds of flies may also attack fruit crops, he said it can be hard to tell if it’s the spotted wing drosophila.
The maggots and even the larvae are extremely small and the physical differences between species subtle, he said.
As with other insect pests, Brust said it is important to eliminate fruit that has fallen to the ground and any infested fruit remaining on plants. This will help reduce populations of flies that may infest later ripening crops or varieties or even next year’s crop.
Adult spotted wing drosophilas look like many other drosophila species, also called vinegar flies.
This pest was first found on the West Coast of the U.S. in 2008. In 2010, it was found in the Midwest, Florida and Pennsylvania, primarily in strawberry or grapes. This rapid spread was most likely due to human intervention.
Winter doesn’t kill the insect.
“SWD go into the forest and nest in heavy ground clutter. Population dips because some of them don’t make it through the winter. But they become active in March and it’s in July before we see them,” he said, adding the only thing they don’t like is dry conditions.
“I am going to try sampling a number of berries that may be infested and put the samples in the fridge. Maggots will come out of the fruit because they don’t like the cold, but still very hard to identify them with certainty unless you are a fly expert,” he said.
DNA testing is challenging because it’s so easy for samples to be contaminated. Brust said that if you put the maggots in a bag with juice and let them pupate they are easier to identify. “You can be much more accurate.”
Adult flies become active again in early spring. Ideal conditions for the insect are high temperatures between 62 and 83 degrees, high humidity and available fruit. Laboratory studies show that as the temperature rises above 86 degrees, adult males become sterile. This species can complete one generation in as short as 12 to 14 days, producing 10 to 12 generations in one growing season along the Mid-Atlantic.
They are also relatively immune to predation. “Although some species do eat them, they reproduce so quickly that predation does not have an impact,” he said. But it is susceptible to infection from Beauveria bassiana, a fungus that attacks many different insect species. One commercial product on the market includes a strain of B. bassiana that reportedly works specifically against spotted wing drosophila.
The important distinction between this and other types of vinegar flies is that this pest attacks healthy, ripening fruit as well as damaged or rotting fruit. Any thin-skinned fruit is vulnerable. Females lay one to three eggs each on multiple fruits visiting the same fruit again and again over time. As a result there may be larvae of different sizes within a single fruit.
Smooth-skinned fruit like blueberries and cherries often exhibit an ooze of fluid on the surface of the fruit. As the sugar content of fruit increases, female flies lay more eggs and the survival rate of the larvae increases.
Once inside the fruit, larvae is unaffected by foliar sprays. As fruit begins to overripe on the plant, other drosophila species will begin to invade. In Maryland, on raspberries and blackberries, 60 to 70 percent of the adults captured in traps in areas of overripe fruit were not spotted wing drosophila.
Brust said growers should be sure to correctly identify the pest as the harvest season progresses and not assume every fly around fruit or in a trap is the spotted wing drosophila.
Chemical sprays, he said, must be applied before the insect begins to lay eggs in fruit. Several types of pyrethroids and neonicotinoids have been found to effective at reducing damage and lowering adult populations.
“Read the label,” he said. “Some chemicals can be used on some fruit, but not others and pre-harvest intervals can also vary by fruit crop.” In many cases, some insecticides are not approved for the crop you want to protect, while others are not registered for certain crops because they have seen deformed fruit in testing and there is no recourse with the manufacturer if it fails to work.
Agriculture experts say that after treatments begin, the adult pest populations may not be reduced to low numbers, but new damage to fruit is greatly reduced.