Spotted wing drosophila 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) 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 may pupate in the fruit or exit to pupate elsewhere.
The spotted wing drosophila can be distinguished from the western cherry fruit fly, Rhagoletis indifferens, by comparing anatomical features of the 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 been found in California. If you suspect you have a western cherry fruit fly, take specimens to your local agricultural commissioner's office.
At this point not much is known about the life cycle in California; however, like other fruit flies it appears to have a short cycle (one to several weeks depending on temperature), and may have as many as 10 generations per year. This rapid developmental rate allows it to quickly develop large populations and inflict severe damage to a crop.
In Japan, 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 rather than 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. 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 also provides an entry site for infection by secondary fungal and bacterial pathogens.
Spotted wing drosophila may be monitored with a variety 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 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:
- Attractant bait sprays
Sprays targeting adult flies, such as GF-120 (bait plus insecticide) or NuLure (bait only that is mixed with an insecticide) can be applied. Because these are baits, 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, these materials need to be re-applied, perhaps at weekly or biweekly intervals to be effective. Traps will need to be monitored to assure that adult fly suppression has been achieved.
Infested fruit that remains in the field or orchard allows eggs and larvae to fully develop and serves as a source of more flies. Remove all infested, ripe fruit from the crop site and destroy, either by burial or disposal in a closed container.
- 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.
This information was gleaned from the University of California Integrated Pest Management Web site.