Spotted Wing Drosophila, Drosophila suzukii: A New Pest in California

Adult female.

Adult male.

Oviposition scar.

Maggot in fruit.

DESCRIPTION OF THE PEST
Spotted wing drosophila, Drosophila suzukii, has recently been found in many California counties infesting ripening cherry, raspberry, blackberry, blueberry, and strawberry crops; it has also been observed attacking other soft-flesh fruit such as boysenberry, varieties of Japanese plums, plumcots, and nectarines. Adults and maggots closely resemble the common vinegar fly, Drosophila melanogaster, and other Drosophila species that primarily attack rotting or fermenting fruit. The spotted wing drosophila, however, readily attacks undamaged fruit.

Adults are small (2-3 mm) flies with red eyes and a pale brown thorax and abdomen with black stripes on the abdomen. The most distinguishable trait of the adult is that the males have a black spot towards the tip of each wing. Larvae are tiny (up to 3.5 mm), white cylindrical maggots that are found feeding in fruit. One to many larvae may be found feeding within a single fruit. After maturing, the larvae partially or completely exit the fruit to pupate.

The spotted wing drosophila can be distinguished from the western cherry fruit fly, Rhagoletis indifferens, by comparing anatomical features of the maggots and wing patterns of adult flies. Western cherry fruit fly adults are much larger (5 mm) than the spotted wing drosophila adults and have a dark banding pattern on their wings. The western cherry fruit fly, which is a quarantine pest, occurs in Washington and other states but has not established in California. If you suspect you have a western cherry fruit fly, take specimens to your local agricultural commissioners' office.

At this point not much is known about the life cycle in California; however, like other vinegar flies it appears to have a short life cycle (one to several weeks depending on temperature), and may have as many as ten generations per year. This rapid developmental rate allows it to quickly develop large populations and inflict severe damage to a crop.

In Japan and in coastal California the adult flies may be captured throughout much of the year. They are most active at 68°F; activity becomes reduced at temperatures above 86°F, and adult males become sterile.

DAMAGE
Unlike other vinegar flies that occur in California, spotted wing drosophila attacks healthy ripening fruit as well as damaged or rotting fruit. The female ovipositor is very large and serrated, so it is able to penetrate the skin of soft-skinned fruit and lay eggs just under the skin, creating a small depression ("sting") on the fruit surface. Each clutch of eggs is from one to three, and the female will oviposit on many fruit. Multiples of larvae within a single fruit are quite possible because many females may visit the same fruit to oviposit. As fruit integrity is compromised by spotted wing drosophila's activities, common vinegar flies (i.e., Drosophila melanogaster) may also oviposit in the damaged fruit.

Eggs hatch and the maggots develop and feed inside the fruit, causing the flesh of the fruit to turn brown and soft; sunken areas that exude fluid often appear on the fruit surface. Damage can provide an entry site for infection by secondary fungal and bacterial pathogens, but this is not always the case.

MANAGEMENT
Spotted wing drosophila may be monitored with a variety of traps. Liquid traps, such as the Rescue Fly Trap, can be filled with about one inch of apple cider vinegar to monitor for this pest. Yeast and/or bananas slices may also be added to the liquid. Because these traps may also capture other species of Drosophila, check the trap captures to confirm the presence of male flies with spotted wings.

In cherries, monitor the early ripening pollinizer crops (e.g., Black Tartarian or Early Burlat) for damage so that management steps can be taken before the main cherry crop becomes susceptible.

While no set management program has yet been determined for spotted wing drosophila, a successful one will need to focus on controlling flies before they lay eggs and reducing breeding sites. There are no effective tools for controlling maggots within fruit. Three essential parts of a management program will likely include:

1. Attractant bait sprays and pesticides. Attractant-based bait sprays targeting adult flies such as GF-120 (bait plus insecticide) can be applied. Because it is a bait, coverage is not as important as keeping the bait attractive. Applications made at low volumes and with large droplet size across the production field or orchard and border areas can be useful in reducing fly populations while minimizing effects on predators, parasites, and honeybees. However, because the efficacy of any bait and toxicant decreases over time, this material need to be re-applied, perhaps at weekly or bi-weekly intervals to be effective. Traps will need to be monitored to assure that adult fly suppression has been achieved. Heavy fly infestations may require more frequent applications because the bait may be consumed faster than with lower populations.
   Pesticides such as malathion, pyrethroids, and spinosyns have also been shown to be very effective in reducing numbers of spotted wing drosophila. Horticultural oils do not show much promise as a control agent. As always, applicators should be aware of restrictions in using pesticides, especially the effects that they may have on non-target organisms such as predators, parasites, and honeybees.
2. Sanitation. Infested fruit that remains in the field or orchard serves as a food source and allows eggs and larvae to fully develop and serves as a source of more flies. When feasible, removing ripe, overripe, and rotten fruit from the crop site and destroying, either by burial or disposal in a closed container can help to reduce populations of this pest, which can be important if a nearby susceptible crop will soon be ripening.
3. Area-wide management. In looking at other successful programs of fruit or vinegar fly management, it is clear that using the above practices over a wide area is essential. It is important for every grower within a fly-infested area to participate, because a single, unmanaged field or orchard will serve as a source of infestation to nearby susceptible crops.

Home Garden Situations. Spotted wing drosophila attacks ripening fruit, and unfortunately is often not noticed in backyard trees until fruit is being harvested. Sprays at this time will not protect the crop, because maggots are already in the fruit. If only some fruit are infested, you can salvage some of the crop by harvesting the crop immediately and sorting, removing fruit with stings on the surface. Place infested fruit in a sturdy, sealed plastic bag and dispose of it in the trash. Also remove any fruit that has fallen on the ground and any infested fruit remaining on trees—this may reduce populations of flies that might infest next year's crops or later ripening varieties. In addition to placing infested fruit in the trash, it can also be buried. Composting may not be a reliable way to destroy eggs and larvae in fruit.

The GF-120 NR Naturalyte, which contains attractant bait plus spinosad insecticide, is available to home gardeners in some farm chemical distributor stores and other outlets. This product, which has been partially effective in commercial orchards, may also be useful for home gardeners in areas where the spotted wing drosophila has been a problem in the past. GF-120 kills adult flies before they lay eggs and must be applied preventively before fruit are infested. No research-based guidelines are available for when to begin treatment, but sprays should probably begin 2 to 3 weeks before harvest or when the first sting is noticed on fruit. Repeat the application after 7 days. Because the fly is attracted to the bait from several yards away, thorough coverage is not necessary. A hand held spray bottle adjusted to a very large droplet (about 3/16 to 1/4 inch diameter) can be used to apply a squirt to the inner canopy every few yards. It is not necessary to spray the fruit. For each tree to be treated, mix 2 tablespoons of bait with 4 tablespoons of water. Shake thoroughly and apply as directed. If you have a very high population, you may want to use up to 6 tablespoons of bait diluted with 12 tablespoons of water per tree. Or if you are in a hot, dry climate, you may want to dilute the bait with twice the amount of water specified.

Authors:
M. P. Bolda, UC Cooperative Extension, Santa Cruz County
W. W. Coates, UC Cooperative Extension, San Benito County
J. A. Grant, UC Cooperative Extension, San Joaquin County
F. G. Zalom, Entomology, UC Davis
R. Van Steenwyk, Insect Biology, UC Berkeley
J. Caprile, UC Cooperative Extension, Contra Costa Co & Alameda County
M. L. Flint, UC IPM Program, UC Davis

Top of page