Spotted-Wing Drosophila

(View male/female identification card)

Spotted-wing drosophila is found in many California counties infesting ripening cherry, raspberry, blackberry, blueberry, and strawberry fruits; it has also been observed attacking other potential hosts such as grape, peach, boysenberry, varieties of Japanese plums, plumcots and other soft-fleshed fruits. 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 0.08 to 0.12 inch (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. The female does not have spots on its wings, but their ovipositor (egg-laying organ) is very large and serrated, unlike other common Drosophila species. Larvae are small [up to 0.14 inch (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.

Like other vinegar flies, spotted-wing drosophila 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 numbers 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 decreases at temperatures above 86°F, and egg laying stops at about 91°F. In California, it appears to overwinter in vegetation surrounding agricultural fields in the wintertime, and then reinvades in very low numbers once fruit are apparent again in the spring.

Unlike other vinegar flies that occur in California, spotted-wing drosophila attacks healthy ripening fruit as well as damaged or rotting fruit. The female's serrated ovipositor is strong and 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 deposits eggs on many fruits. Multiples of larvae within a single fruit are quite possible because many females may visit the same fruit to lay eggs. As fruit integrity is compromised by spotted-wing drosophila's activities, common vinegar flies (i.e., Drosophila melanogaster) may also lay eggs 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 can be used for several weeks without changing the liquid.

To make the bait solution:

1. Mix 12 oz of water with 0.25 oz of baker's yeast (e.g., Fleischmann's) and 4 teaspoons of sugar.
2. 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).
3. Transfer 3 to 4 fl oz of the liquid to a 500-ml Nalgene bottle or other container of low height that has four or more 7/16-inch diameter holes drilled into the lid. (The idea is to use a container that is low enough that the opening is well below the plant canopy.) 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.

While some jars or bottles can be hung with a wire in the shady, cooler areas of the field or farm, others should be placed directly in the strawberry field itself. It is important that the traps be placed in the shady canopies of the strawberry plants. Check traps at least weekly and count and remove the flies.

A successful management program will focus on reducing breeding sites and controlling flies before they lay eggs because to date, there are no effective tools for controlling maggots within fruit. The fruit seems most susceptible to attack after it has colored and developed some sugar.

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 the numbers of this pest. This can be especially important if a nearby susceptible crop will soon be ripening.

Even though spotted-wing drosophila lays eggs in newly ripening fruit, they readily infest older, overripe fruit as well. Extending harvest intervals, as may occur for the processing crop, will result in larger numbers of spotted-wing drosophila, more fruit damage, and a greater risk for future infestations of the new crop.

If monitoring indicates pest presence, apply a spray to protect the fruit. If monitoring indicates high numbers of spotted-wing drosophila early in the season, an earlier spray to lower numbers may be warranted in addition to harvest applications.

Pesticides such as malathion, pyrethroids, and spinosyns have been shown to be very effective in reducing numbers of spotted-wing drosophila, but they are seldom needed in California strawberries unless many flies are present. Fruit tend to be at risk primarily when sanitation is lax and over-ripe fruit are present or because of extended harvest intervals. Horticultural oils do not show much promise as a control agent.