Spotted-Wing Drosophila

View male/female Identification Card (PDF)

Adults and maggots (larvae) closely resemble the common vinegar fly, Drosophila melanogaster, and other Drosophila species. Adults are 0.08 to 0.12 inch (2–3 mm) flies with red eyes.  The thorax and abdomen are pale brown with black stripes on the abdomen. The two most distinguishable traits are a black spot on the tip of each wing and a dark band encircling the front legs on male flies. The female does not have spots on its wings and bands on the legs, but their ovipositor (egg-laying organ) is heavily serrated, unlike other common Drosophila species. Larvae are small, up to 0.14 inch (3.5 mm), with a white cylindrical body, and black mouthparts. One to many larvae may be found feeding within a single fruit. After maturing, the larvae pupate inside, partially inside, or completely outside of the fruit.

Spotted-wing drosophila can be distinguished from the western cherry fruit fly, Rhagoletis indifferens, by comparing anatomical features of the larvae and wing patterns of adult flies. Western cherry fruit fly adults have a dark banding pattern on their wings and are much larger (5 mm) than spotted-wing drosophila adults. 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 commissioner's office.

Like other vinegar flies, spotted-wing drosophila has a short life cycle (one to several weeks depending on temperature) and may have as many as ten generations per year. Large numbers of flies can appear quickly because of this rapid developmental rate. Large numbers of spotted-wing drosophila inflict severe damage to susceptible crops, including cherry.

In California, spotted-wing drosophila may be overwintering in vegetation surrounding agricultural fields in the wintertime and then reinvades the fields once green fruit begins to change color in the spring. In San Joaquin Valley cherry orchards, flies appear in two peaks: an early-season peak (spring to early summer) and a late fall peak (November). The early peak generation causes significant damage to ripening cherry fruit. During the summer, adult activity is minimal due to the high temperatures and low humidity. In contrast, flies in the higher elevations of the Sierra Nevada mountain range are active during the summer but not in spring and fall.

Spotted-wing drosophila is found in many California counties infesting primarily fruit with thin skins, such as ripening cherry, raspberry, blackberry, blueberry, and strawberry. Cherry fruits become susceptible to spotted-wing drosophila attack as soon as the fruit color begins to change (i.e., from green to yellow) and the susceptibility increases as ripening progresses through to harvest.

Unlike other vinegar flies that occur in California, spotted-wing drosophila attacks healthy ripening fruits as well as damaged or rotting fruits—all other vinegar flies (or drosophilids) go after decayed or overripe fruits after harvest. The female's serrated ovipositor is strong and able to penetrate soft-skinned fruit to lay eggs just under the skin, creating a small depression (often called a sting) on the fruit surface. The female deposits eggs on many fruits and each clutch of eggs numbers from one to three eggs. Many larvae within a single fruit are possible because many females may visit the same fruit to lay eggs. In most instances, the breathing tube of the egg can be seen on the fruit surface when inspected using a dissecting microscope or hand lens.

Eggs hatch and the larvae 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. Fruits injured by oviposition and feeding become an easy target for several other pests, such as common vinegar flies and fungal and bacterial infections, which are otherwise not a threat to intact fruits.

Cultural practices can reduce the overwintering population and attractiveness of the orchard for this pest. There are early indications of spotted-wing drosophila populations with resistance to some commonly used insecticides such as spinosad in berries in coastal California. Therefore, the judicial use of available insecticides and other monitoring and management options is critical for long-term successful spotted-wing drosophila management.

Over-ripe fruits that remain on the trees after harvest serve as a food source for spotted-wing drosophila and contribute to increasing numbers of this pest in the orchard. Therefore, timely harvest is important. After harvest, remove all leftover fruits from the trees. Consider destroying rotten fruits on the ground by crushing or flail mowing if other susceptible crops are grown within the region, including after the harvest of early varieties if these fruits pose a risk to later-harvested varieties in the same orchard.

Reducing relative humidity can create an environment that is less favorable to spotted-wing drosophila. Cherry orchards on drip irrigation tend to have less problems with spotted-wing drosophila than those with sprinklers. Likewise, weedy orchards have more problems than orchards with no ground vegetation or where the vegetation is tightly mowed. Prune trees to open up the orchard canopy for greater sunlight penetration. Manage irrigation to minimize oversaturation of the soil and manage the soil surface to be flat to avoid opportunities for puddles.

Consider removing wild host plants such as bush honeysuckles, wild cherries, buckthorns, dogwoods, and wild berries near the orchard when feasible.

Cultural controls and sprays of the Entrust formulation of spinosad, pyganic, azadirachtin, and Chromobacterium subtsugae are acceptable for use on organically grown cherries.

It is very important to track spotted-wing drosophila activities in the orchard regularly throughout the season because the economic damage caused by this pest can become severe quickly.

Adult trapping

Use liquid bait traps or sticky cards. Liquid-based baits and synthetic lures are commercially available. Alternatively, traps can be constructed locally, using tall, round plastic containers (16–32 ounces; e.g., a peanut butter jar). Use apple cider vinegar or other type of commercial or home-made bait as the attractant with 1 to 2 inches liquid at the bottom to drown the captured flies. To break surface tension of the liquid add a drop of unscented dish soap. To make home-made bait, 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 (in an open or loose-lidded container, as quite a bit of gas is formed during the fermentation process). Container traps should have either a series of smaller holes or a bigger screened hole (about 1 inch in diameter) to allow fly entry, but minimize the entry of bigger-sized insects such as bees or wasps. Hang traps on the shaded side of the tree. Use a minimum of two traps: one at the orchard edge and one inside the orchard. Hang the traps within reach or at eye-level. Because these traps may also capture other species of Drosophila, make sure to carefully identify the trap captures to confirm the presence of spotted-wing drosophila. Use the Male/Female Identification Card (PDF) and a hand lens or dissecting microscope to aid in identification.

Fruit sampling for larvae

In addition to placing traps for monitoring, look for signs of spotted-wing drosophila damage on sample fruits collected from the trees. Collect 100 to 200 sample fruits from a minimum of five trees from the orchard. To monitor for larvae in fruit, take 1 to 2 cups (about 0.4–0.9 lbs) of ripe cherries from your samples, lightly squeeze them, and put them into plastic bags. Depending upon the ripeness level, you may need to adjust the pressure to release the larvae, without killing them physically. Dissolve 7 lbs of brown sugar in 5 gallons of warm water. Place the squeezed fruits in a shallow container, cover with the sugar solution, and stir frequently. Inspect the solution after 10 to 15 minutes for larvae exiting from the fruits while they are still live and active. You need good lighting and a hand lens (20­–30x) to see smaller larvae. Record the numbers.

Pesticides

Since biological control methods are not yet available for spotted-wing drosophila, use of insecticides is often necessary when cherry fruits become susceptible (i.e., from color development to harvest). There can be differences among stages of maturity in susceptibility to spotted-wing drosophila oviposition (egg laying) and development. If monitoring indicates high numbers of spotted-wing drosophila in the traps and the fruit of your cherry variety becomes susceptible (i.e., fruit color begins to change from green to straw), that is the right timing of the first insecticide application.

Insecticides may need to be applied 2 to 3 times at 7- to 10-day intervals during the period when ripening fruit is susceptible to spotted-wing drosophila. Several groups of insecticides, particularly pyrethroids, malathion, spinosyn, and some diamides are effective against this pest. Although selected neonicotinoids resulted in good mortality of eggs and larvae in some trials, their efficacy is relatively weak against spotted-wing drosophila adults.

When selecting an insecticide, consider efficacy of the pesticide against spotted-wing drosophila adults and the preharvest interval (PHI) to avoid exceedance of the maximum residue limits (MRL). In the case of cherries grown for export, the selection of insecticides is primarily dictated by the MRLs specified by importing countries and agencies.

Since multiple insecticide applications are required for effective spotted-wing drosophila management, selection and rotation of active ingredients during the season is critical to minimize the potential risks of secondary pest outbreaks, adverse impacts on natural enemies, and preventing the development of pesticide resistance. For effective protection of the fruits from spotted-wing drosophila, full spray coverage is essential. 

• 2014 Recommendations for Sweet Cherry (PDF)
• Male/Female Identification Card (PDF)
• Key to Identifying spotted-wing drosophila, Drosophila suzukii (PDF)
• Susceptibility of small fruit and cherries to SWD (PDF)