Spotted Wing Drosophila, Drosophila suzukii: A New Pest in California
Provisionary Guidelines: Management of Spotted Wing Drosophila in Home Garden Situations
DESCRIPTION OF THE PEST
Spotted wing drosophila (SWD), Drosophila suzukii, has recently been found in many California counties infesting ripening cherries, and in coastal areas infecting ripening raspberry, blackberry, blueberry, and strawberry crops. It has also been observed occasionally attacking other soft-flesh fruit such as plums, plumcots, nectarines, and figs when conditions are right. 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. See the identification key to Drosophila suzukii from Oregon Department of Agriculture for help distinguishing this pest from other flies, and the Male/Female Identification Card for additional photos of adult males and females.
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. Adult spotted wing drosophila females also have a very prominent ovipositor. 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.
Research studies to define the life cycle in California are still under way; however, like other vinegar flies SWD 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. In California’s inland valleys the adult flies are most active during spring and fall when temperatures highs are between 60 and 80º F, especially when temperatures are humid and food is available. In laboratory studies at constant temperatures, 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. Once 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 surface of smooth skinned fruit like cherries and blueberries. Damage can provide an entry site for infection by secondary fungal and bacterial pathogens, but this is not always the case.
Spotted wing drosophila attacks ripening fruit and unfortunately is often not noticed in backyard fruit crops 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, to remove fruit with stings on the surface. Place infested fruit in a sturdy, sealed plastic bag and dispose of it in the trash. Several cultural practices may be useful for reducing problems on trees and berries. If using insecticides, it is important to monitor for flies to be sure treatments are made before fruit is attacked.
Varietal Susceptibility. Some cherry varieties may be more susceptible to SWD than others but more research is required. Usually the earliest harvested cherries get the least damage. Among the berries, raspberries appear to be the most susceptible; blackberries and strawberries are also susceptible in coastal climates under very moist conditions when fruit is not harvested frequently. Blueberries are also quite attractive to SWD in moist, coastal environments but less so where moisture is lacking and temperatures are high. All blueberry varieties appear to be susceptible.
Sanitation. Eliminating any fruit that has fallen on the ground and any infested fruit remaining on plants in the garden may reduce populations of flies that might infest next year's crops or later ripening varieties. Infested fruit may be placed in a sealed bag in the trash, or it can be buried. Composting may not be a reliable way to destroy eggs and larvae in fruit.
Netting. Netting may be useful to keep flies from attacking fruit on blueberries and other small fruit or possibly branches on small trees. However, the netting must be applied before fruit begins to ripen so that flies will not be caught inside the net. Netting must be secured at the bottom so flies cannot enter, and the mesh size should be very small.
Early Harvest. Early harvest can be important in reducing exposure of fruit to the pest. Begin harvest as early as you can and continue to remove fruit as soon as they ripen.
Trapping. Trapping as discussed below under "monitoring" has not been shown to effectively reduce populations of SWD in backyard trees. Trapping is important for monitoring for the pest.
Monitoring for SWD. It is very important to observe your fruit regularly as it begins to ripen. In some cases, this will allow you to harvest before problems are serious. Monitoring will also help you time insecticide applications for greatest effect. On cherries and blueberries start checking fruit for damage (punctures created when the female lays eggs in fruit, prematurely rotting fruit) as soon as fruit begins to develop any pink color. SWD stings are tiny—a magnifying glass may help you to see them. You can gently squeeze the fruit to see if juice leaks from the small punctures—this can indicate presence of the pest. Pull open suspect fruit to see if there are larva inside. If you find infected fruit you should either harvest all the fruit immediately or spray to prevent the damage from increasing before harvest. The infestation level can increase quite rapidly if left untreated/unharvested. Remove and destroy infested fruit as you monitor. Stings are not readily visible on berries so it is difficult to detect an early infestation by monitoring the fruit for damage.
You can also use homemade traps to monitor for SWD. Make traps out of one quart plastic yogurt (or similar) containers (with a lid). Drill 16 holes that are 3/16 inch in diameter around the top of the container. Bait the trap with about 1 or 2 inches of white wine or apple cider vinegar; add a drop of unscented dishwashing soap to break the surface tension so the flies will drown. Hang the trap in the shade in your cherry tree or near your berries in early May or well before fruit begins to ripen. Check the trap weekly for small flies with dark spots at the tip of their wings floating in the fluid. These are male SWD and will confirm that you have the pest. Put fresh apple cider vinegar or wine and a drop of soap in each week.
Insecticides. Because this pest is so new to California, there has been limited research on treatments to manage SWD. Sprays must be timed to kill adults before they lay eggs as they will not control larvae already in the fruit.
The organophosphate insecticide malathion will control SWD, but malathion is very toxic to bees and natural enemies of other pests in the garden, so care must be taken to keep the application on the target plant and to avoid drift and runoff. Improper application can also result in injury to cherry trees. If monitoring indicates a need to spray, the application should be made as soon as the fruit just begins to turn from yellow to pink. This should be about 2 to 3 weeks before cherry or berry harvest. In the case of indeterminate fruiting berries such as raspberries or strawberries, sprays may need to be repeated to keep SWD populations low during their prolonged fruiting period in summer and fall. Because of the potential negative impact of malathion in the garden, use it only where you are certain you will have a SWD infestation, either because you had a problem last year or from trapping and positively identifying insects as SWD this season.
An alternative to malathion with fewer negative environmental effects is spinosad (Monterey Garden Insect Spray); however, it is not quite as effective against the fruit fly adults as malathion. To get satisfactory control two sprays may be required; the first applied as the fruit just begins to turn pink and the second applied 7 to 10 days later. Additional sprays may be needed for berries with a prolonged fruiting period. Be sure to wait the interval specified on the label before harvesting the fruit. Some spinosad products are sold to be applied with a hose-end sprayer, but a compressed-air sprayer will give more reliable coverage.
Contributors to the first edition: W.W. Coates, UC Cooperative Extension, San Benito County; F. G. Zalom, Entomology, UC Davis
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