Gearing Up for the Cherry Maggot Battle

"Cherry Maggots Come To Town" would be a great name for an indie band. Unfortunately, this is one insect you don't want performing in your yard. The spotted wing drosophila (SWD), is responsible for the little worms that backyard gardeners are starting to find in their cherries, and to a lesser extent in raspberries, strawberries and blueberries. First spotted in Santa Cruz County in 2008, this pest is now throughout the West Coast, and has been spotted recently in traps in Sacramento County and the surrounding area.

According to Sacramento County Farm Advisor Chuck Ingels, the adult SWD is small (2-3 mm), resembling a gnat that might be found on that old piece of fruit on your kitchen counter. This one, though, has a sharp ovipositor (sort of a needle-like, egg-laying device) that penetrates ripening fruit...such as your backyard cherries, that are developing now for a May-June harvest.

 Adult male spotted wing drosophila, Drosophila suzukii, (2-3 mm long) has a dark spot on each wing tip.
Photo by Martin Hauser.

Adult female spotted wing drosophila, Drosophila suzukii (2-3 mm long).
Photo by Martin Hauser.

This pest might easily be confused with a vinegar fly or the western cherry fly. 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. 

Here is a detailed description of the SWD, according to an excellent report produced by the  UC Integrated Pest Management Website on Spotted Wing Drosophila: "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."

Among you are those who are starting to ask the question, "What garden insecticide can I buy to stop this pest...."

Let me stop you right there. The cure may be worse than the problem. Limited research has been done on this new invader to California, and the chemical that so far has been found to best control the adult spotted wing drosophila is deadly to one the best "garden good guys" around, the honeybee.

The UC IPM Website says: "Although malathion...has been shown to control the adult SWD, coverage would need to be so thorough throughout the entire tree. 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 tree and avoid drift and runoff. Improper application can also result in injury to the tree. Application should be made about 2 weeks before harvest. Sprays must kill adults before they lay eggs. Malathion will not control larvae in fruit. 

An alternative to malathion with fewer negative environmental effects would be spinosad (Monterey Garden Insect Spray); however, it is not believed to be as effective against the fruit fly adults as malathion. Two sprays may be required at about 14 days and 7 days before harvest to get satisfactory control. As with malathion, all foliage and fruit on the tree must be covered with the spray. Partial coverage will not be effective. A compressed air sprayer will give more reliable coverage than a hose end sprayer."

Two better, safer options: dispose of infested fruit and set up traps, reports the UC IPM site: 

"Spotted wing drosophila attacks ripening fruit, and unfortunately is often not noticed in backyard trees 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, removing fruit with stings on the surface. Place infested fruit in a sturdy, sealed plastic bag and dispose of it in the trash. Also remove any fruit that has fallen on the ground and any infested fruit remaining on trees—this may reduce populations of flies that might infest next year's crops or later ripening varieties. In addition to placing infested fruit in the trash, it can also be buried. Composting may not be a reliable way to destroy eggs and larvae in fruit."

The report from UC goes on to say, "There is some evidence from research done in Japan in the 1930’s that traps can be used to manage SWD in backyard trees. Some trials on this method will be undertaken this summer. To try this method for yourself, make traps out of one quart plastic yogurt containers (with a lid). Drill 16 holes that are 3/16-inch in diameter around the top of the container. Bait each trap with a solution of 1/4 cup grape wine plus 1/4 cup water plus 3/4 teaspoon of molasses. In early May, about 1 month before harvest, hang 3 to 5 traps in a shady spot on the lower branches of the tree. Keep the traps up until harvest is completed. Refresh the bait if needed. These traps can also be used to detect SWD in your garden earlier in the season. Bait the trap with about an inch or 2 of white wine. 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."

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." 

And, as that would imply, choosing cherry trees with an early harvest date may minimize problems with the spotted wing drosophila. "Usually the earliest harvested cherries get the least damage," says that UC report. Here in the Sacramento and Northern San Joaquin valleys, early ripening cherry varieties include Minnie Royal, Black Tartarian, Royal Lee, Craig's Crimson and Royal Ranier, according to wholesale fruit tree grower, Dave Wilson Nursery.

The bottom line: good sanitation - cleaning up and disposing of fallen fruit and vegetables - goes a long way to thwarting many backyard pests.