Spotted-Wing Drosophila

Male/Female Identification Card (PDF)

Spotted-wing drosophila 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. The abdomen has black stripes. 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.

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.

Spotted-wing drosophila may be monitored with a variety of traps. In the berry production districts of the Central Coast of California, one of the most successful trapping methods has been a yeast-sugar-water mix in a jar or bottle trap. Beyond the capability to consistently trap spotted-wing drosophila, this mix is sufficiently clear to easily distinguish the flies, and it also can be used for several weeks without changing the liquid.

To make the bait solution, mix 12 oz of water with 0.25 oz of baker's yeast (e.g., Fleischmann's) and 4 teaspoons of sugar. Allow the solution to ferment for a day or so then transfer 3 to 4 fl oz of the liquid to a 500 ml Nalgene bottle that has four 7/16-inch diameter holes drilled into the lid. Flies enter the bottle through these holes and while there is the possibility of flies escaping back out through the holes, most eventually land in the liquid and drown.

Hang the jars or bottles with a wire in the shady, cooler areas of the field or farm. Check traps at least weekly and count and remove the flies.

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:

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, either alone or with a commercial bait added, 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 nontarget organisms such as predators, parasites, and honeybees.

The fruit seems most susceptible to attack after it has colored and developed some sugar. If monitoring indicates pest presence, apply a spray to protect the fruit during this time. If monitoring indicates a high population earlier in the season, an earlier spray to lower populations may be warranted in addition to a preharvest application.

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.

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.

• Male/Female Identification Card (PDF)
• Key to Identifying Spotted-Wing Drosophila, Drosophila suzukii (PDF)
• Susceptibility of small fruit and cherries to SWD (PDF)