By Ricardo Bessin Extension Entomologist
We have been working with producers in a number of Kentucky counties to monitor the spotted wing Drosophila (SWD). The first traps positive for SWD were in Breathitt and Bourbon Counties but since the start of July, trapping locations in seven addition counties have been positive for SWD. SWD is a serious threat to small fruit production. The adults lay eggs under the skin of otherwise sound fruit. This can result in small maggots in the fruit at harvest or just after harvest. Blueberries, blackberries, and raspberries are very susceptible to this new invasive pest. In addition to damaging a large percentage of a crop, this pest also has the potential to upset customers and ruin markets.
So far we have recorded SWD in Breathitt, Breckinridge, Bourbon, Casey, Larue, Warren, Daviess, Caldwell, Crittenden, Letcher, McCracken, and Meade counties. So it isn’t restricted to one region of the state, it is in the western, central, and eastern parts of the state. This past year we have recorded SWD only from Warren and Daviess Counties. We are finding it again in these two counties, as well as the additional new counties.
In other states where the SWD has become established producers that have detected SWD often need to spray periodically during the harvest period to reduce losses. Because of this, producers need to coordinate their harvests and spray timing in order to comply with pre-harvest intervals (PHIs) with the respective insecticides. As the small fruit crops are harvested several times a week, this limits the types of materials that can be used. Insecticide trials at Michigan State have indicated that some insecticides in the organophosphate, pyrethroid,and spinosyn classes have good activity against SWD. The neonicotinoids have not been effective.
Several states have developed extension recommendations and have factsheets available online. I would recommend the following for blueberry and blackberry/raspberry producers. They list monitoring methods (that we have been using) and available insecticides.
There are several cultural controls that may be helpful. The first is timely and clean harvests. This includes removal of overripe and damaged berries. Burial of infested fruit has not been effective as the SWD can still emerge. Placing fruit in clear bags that are sealed and left in the sun will kill any SWD that emerge. Removal of wild hosts (brambles, poke, honeysuckle, wild grape) near commercial plantings may help.
When SWD has been detected on a farm, growers may need to begin insecticidal control. But insecticides need to be applied correctly in order to be effective. They need to be in place prior to oviposition (egg laying), coverage needs to be thorough as the adults often hide in the dense portions of the canopy. So, higher pressure and higher spray volume will be needed to reach these difficult to reach spots. Even the best of the insecticides will not consistently last more than 7 days, so at minimum, weekly applications will be needed. With heavy rains, sprays may need to be reapplied. Producers should also rotate among insecticides with different modes of action to prevent/delay resistance. A general rule of thumb is to switch modes of action with each new pest generation, which would mean switching to a new IRAC insecticide group. Since this pest can complete a generation in less than two weeks, we need to have multiple types of insecticides available.
By Ricardo Bessin and Patricia Lucas, Extension Entomologist and Agriculture Extension Specialist
In September 2012 the USDA Animal Plant Health Inspection Service confirmed Spotted Wing Drosophila (SWD) from the samples submitted from fruit fly traps located in Daviess and Warren counties in western Kentucky. This invasive Asian insect is a very serious pest of softer skinned fruits including blackberry, raspberry, grape, strawberry, blueberry, cherry, mulberry, and peach. Other harder fruits, such as apple, pear, and tomato, can be attacked if the skin is damaged. The SWD also will lay eggs in and complete its development of several weedy or wild hosts.
The SWD is a tiny fly (about 1/10 inch long) with red eyes. It is very similar to the common fruit fly or vinegar fly that occurs throughout the state. The adult male has a dark spot near the tip of each wing and two dark ‘bands’ on each front leg. The female must be identified by her unique ovipositor (egg-laying device) at the end of the abdomen.
Common fruit flies are associated with damaged, overripe, or rotting fruits and vegetables. However, the female SWD has a stout, toothed ovipositor that enables her to insert eggs under the skin of ripening fruits that are otherwise healthy and sound. Generally, soft-skinned fruits become vulnerable to SWD attack as they begin to ripen and turn color in the final 7 to 10 days before harvest. The larvae tunnel and feed under the skin of the fruit and are about 1/8 inch long when mature. There is often a sunken area at the site of egg laying and damaged fruit may appear to collapse from the internal damage and rots.
The short (about 10 day) life cycle of the SWD combined with the female’s ability to lay 200 to 600 eggs can result in very high populations by the end of the season. Early season crops like strawberries are exposed to small SWD populations but fall-harvested crops will be ripening when populations of this fruit fly are at their peak.
Last Updated: This page is maintained by Lee Townsend, Department of Entomology, University of Kentucky. Please send questions or suggestions to: Lee.Townsend@uky.edu