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Have you cut into a blueberry, raspberry or other fruit, only to be disgusted when you saw a tiny white "worm" inside? Odds are, you saw the larvae of Spotted-wing drosophila (SWD), Drosophila suzukii (Matsumura) (Diptera: Drosophilidae). SWD is an invasive species of fruit fly that first appeared in California in 2008, and spread rapidly across the United States. It was in Georgia by 2010.
SWD attacks many soft-skinned fruits such as blueberries, blackberries, raspberries, strawberries, cherries, even peaches. U.Ga. estimated that it has done $255 million in damage to the Georgia blueberry crops since 2010, annual crop losses of up to 20%.Total U.S. losses are estimated at more than $718 million annually.
Adults are small flies about 1/16 to 1/8 inch long with red eyes and a pale brown thorax and abdomen with black stripes on the abdomen. The most distinguishable trait of SWD males is a black spot towards the tip of each wing. The females do not have spots on wings but have a very prominent, sawlike ovipositor for laying eggs in fruit. That enables them to make an incision in the fruit and insert an egg.
The larvae are tiny, white cylindrical maggots a little longer than 1/8 inch when full grown. One to several larvae can be found feeding within a single fruit. After maturing, the larvae partially or completely exit the fruit to pupate.
UGA researchers say that warm winters, low night-temperatures lingering late in the season, and excessive rain events may make SWD infestations worse. Frequent rains during the harvest above all increases the risk of fly infestations in blueberries.
Blueberries are also attacked by the common, blueberry maggot (BBM), and both SWD and BBM infestations may be present in the same field.
Researchers say that the eggs and larvae can't survive freezing and 24 hours at or below 28 F degrees will kill 3/4 of the adult flies. High heat appears to have the same effect. temperatures above 90 F for 24 hours or longer are also said to kill the flies. This means the SWD can live year-round in Florida and the warmer, southern parts of Georgia, Alabama, Mississippi, California, Texas and coastal areas of Oregon. In colder northern areas, SWD is most active from mid-June to August.
The SWD are typical active in summer during the cooler part of the day, at dawn and dusk, as their preferred temperature is 68 F, but they are active when the temperatures range between 59 and 70 F. Their preferred temperature is 68 F.
Eliminating any fruit that has fallen on the ground and any infested fruit remaining on plants in the garden can reduce populations of flies that might infest next year's crops or later-ripening varieties. Infested fruit can be placed in a durable plastic bag, sealed, and placed in the trash.
Composting or burying is not a reliable way to destroy eggs and larvae in fruit. Solarizing fruit under clear plastic in the sunshine has been quite successful in killing flies in fruit in preliminary studies performed in Oregon.
Spinosad is said to be effective on SWD. The application should be made as soon as the fruit just begins to turn pink. You should make a second application 7 to 10 days later. And with berries that produce over a longer period, such as raspberries, or strawberries, sprays might need to be repeated weekly. Of course, follow the directions and intervals specified on the pesticide label regarding sprays and harvesting fruit.
A fly eating fungus, Beauvaria bassiana, under the brand name "balEnce", has been developed and sold commercially in the US. This is a strain that is specific to flies. It works well against Drosophila but is very expensive.
The organophosphate insecticide malathion control spotted wing drosophila, but it's a regular chemical pesticide that is toxic to bees and beneficial insects, so I don't use it. Besides, what's the point of growing your own fruit, if it still has pesticides on it?
U.Ga says that using traps to control SWD not been shown to be effective to reduce populations of SWD in backyard trees. Trapping, they say, is important for monitoring for the pest.Regardless, I'm going to try some homemade traps this summer, just to see.
Fine netting (grid openings 1 mm or less - keep in mind, there are 25 mm per inch, so that's much less than 1/16 of an inch!) over whole plants or canes can be useful to keep flies from attacking fruit on blueberries and other small fruit and possibly on branches on small cherry trees. However the netting must be applied before fruit begins to ripen so that flies will not be caught inside the net. Netting must be secured so flies cannot enter, and the mesh size should be very small, like the size of the screen on your windows, such as 0.98 mm mesh used for screening out no-see-um flies.
Early harvest of fruit can be important in reducing exposure of fruit to the pest. Begin harvest as early as you can and continue to remove fruit as soon as they ripen.
Monitoring for SWD with traps , as well as checking the fruit regularly as it begins to ripen is important. That way, you can start harvesting early, before the fruit is badly damaged. .
Start checking fruit for damage (such as rotting fruit or incisions caused the female lays eggs in fruit) as soon as fruit begins to develop any color.
Oregon State University has a nice explanation of how to build your own SWD monitoring traps. Below is a brief synopsis of it:
The researchers are looking at natural enemies and fungi, bacteria, etc that attach only the flies.
Above is the
2020 version of
the Ball Blue Book