Spotted wing drosophila, Drosophila suzukii, was officially identified as a pest on the cherries earlier this year. It has caused significant damage in Santa Clara County as well as several other cherry producing areas. In addition to cherries, the spotted wing drosophila has been found infesting the fruit of raspberry, blackberry, blueberry, and strawberry plantings on the central coast.
Although it has not been reported on grapes in the field, in the lab it has been shown to feed on grapes and tomatoes. There have been reports of spotted wing drosophila feeding on grapes in Asia, which is where the fly originated. Currently, spotted wing drosophila has been found in 21 California counties, as well as locations in Washington and Oregon.
Adults and maggots closely resemble the vinegar or pomace fly, Drosophila melanongaster, which are attracted to damaged and bunch rot infested clusters in the vineyards or fermenting grapes and pomace at the winery. Adult spotted wing drosophilae are two to three millimeters long with pale brown bodies and black horizontal stripes on the abdomen. The adult male has a black spot on the end of the wings.
Unlike the vinegar fly, the spotted wing drosophila has the ability to penetrate the skin of healthy undamaged soft fruits and lay eggs inside. The female has a serrated ovipositor that gives it the ability to pierce the skin of fruits. Once the eggs hatch the developing maggots turn the flesh of fruits brown and soft; they can cause fluid leakage to the berry surface. Feeding damage also provides an entry wound for the secondary fungal and bacteria pathogens to infect produce fruit rot.
Spotted wing drosophila prefers high humidity and moderate temperatures, conditions that are common to the Central Coast. The adults are most active at 68 F; activity is reduced at temperature above 86 F.
Similar to other fruit flies, the spotted wing drosophila has a short life cycle that can be one to two weeks, depending on temperatures during the fruit ripening period. At optimum temperatures, these insects are able to quickly develop large populations and cause significant fruit damages.
The insect may have as many as 10 generations per year under California conditions. Winter cold does not appear to limit the flies’ survival in China and northern Japan where spotted wing drosophilae are well established.
Spotted wing drosophilae can be monitored with a number of traps. Liquid traps, such as the Rescue Fly Trap, can be filled with about 1 inch of apple cider vinegar to monitor for this pest.
Yeast and/or banana slices may also be added to the liquid. These traps may capture other species of Drosophila; check the trap captures to confirm the presence of male flies by identifying the spotted wings.
Researchers are currently evaluating monitor and control methods for spotted wing drosophila. If the fly is found in the traps then the fruit should be monitored carefully for egg-laying punctures and larval damage to the fruit. Experience with our fruits suggests that females do not lay eggs until the fruit is nearly ripe.
Time will tell if spotted wing drosophila will become a pest of the grape industry in California.
Due to this insect’s preference for cooler conditions the risk may be higher in coastal productions areas. Spotted wind drosophilae have been found in traps on the edge of a Central Coast vineyard adjacent to a cherry orchard, but currently they have not been observed on grape berries. Vineyards adjacent to areas where susceptible soft fruits are grown (cherry, strawberry, cane berries) may be at greater risk of having spotted wing drosophila populations.
For more information on spotted wing drosophila: http://www.ipm.ucdavis.edu/EXOTIC/drosophila.html.