The latest insect to threaten California agriculture has had at least three names since it was officially identified as a pest just a few months ago.
The cherry vinegar fly/cherry drosophila is now officially known as spotted wing drosophila (SWD). (Apparently cherry growers did not like the first two monikers because they were associated too closely with their crop. The California Department of Food and Agriculture renamed it.) However, it is cherries where SWD (Drosophila suzukii) has caused the most damage so far.
Although it has not been officially recognized as a threat to other soft fruit, a veteran University of California, Berkeley entomologist, Bob Van Steenwyck, will not be surprised if it moves beyond cherries to other soft fruit. It has in other countries.
According to the UC entomologist, SWD has fed on grapes and tomatoes in the lab at UC Davis. There have been reports of damage in backyard plums and other fruit. However, it has not been confirmed in commercial orchards.
“Will it damage grapes? That is the $64,000 question. I believe it will hit grapes as they start to sugar. I think, however, it will not be a problem in the Central Valley because it is too hot for suzukii. I think the problem will be in the cooler (wine grape producing) areas of the state,” he said. “It likes cooler climates.”
So far SWD has caused economic damage to sweet cherries, strawberries, blueberries, raspberries, blackberries, Ollalieberries and in backyard Santa Rosa plums in the San Jose area.
It has been positively identified from traps in 21 California counties and suspected in another five.
“It has been found from San Diego to Humboldt counties in California and in Washington, Oregon and British Columbia – all along the West Coast of the U.S.” reports Van Steenwyck.
“Everyone is buzzing about the spotted winged drosophila ... there are thousands of eyes aware of the pest and checking traps,” said the UC entomologist.
And there is reason to be on high alert.
It can cause widespread damage quickly at harvest time, especially in cherries. When severity of the pest’s damage became evident earlier this summer, some growers stopped harvesting the crop, fearful it would reach consumers.
Coates reports that one grower in his coastal area was sorting cherries on a very slow moving sorting belt, throwing out any blemished fruit.
“You don’t want consumers biting into a cherry and finding maggots,” Coates says.
Coates says if the pest persists, growers may be forced to fumigate before shipping to markets.
Coates said the spread of this new pest has been amazingly rapid “and we know probably only about 10 percent of what we need to know about it.” A concerted research effort is underway to learn more about the new pest.
Field reports are coming in rapidly and some are perplexing.
Van Steenwyck said UCCE Contra Costa Farm Advisor Janet Caprille reported heavy damage in cherries from SWD in a “you-pick” farm at Brentwood, Calif. However, she did not find SWD in any of the closely adjacent peaches or nectarines on the same farm where damage would be expected, “particularly since fruit in U-pick operations is typically more ripe and therefore softer than in a commercial orchard. That is perplexing.”
SWD females have a “raspy” or serrated ovipositor and can easily penetrate the skin of soft fruit, laying eggs inside. The fruit quickly collapses.
The female’s puncture wound on a cherry is readily visible with fluid running down the side of the fruit.
It is not that visible on strawberries, according to Coates.
Santa Cruz County UCCE Farm Advisor Mark Bolda has received reports that PCAs are picking up more SWD in moist areas than in dry areas. A PCA also reported the heavy infestations in strawberries apparently from a nearby field of caneberries.
“This is a major pest for cherry growers in the coastal area. Growers do not spray a lot on the coast … maybe occasional brown rot or powdery mildew spray and maybe something for black cherry aphid.
“This has the potential to be a very serious pest for cherry growers,” he added.
He said it poses more of a threat than the light brown apple moth (LBAM). “So far LBAM has been a very, very minor (field) pest. It is a quarantine pest, but it has caused no damage. SWD is definitely a damaging ag pest that has the potential to be a continuing pest.”
Areas of the state are now under LBAM quarantines as the state attempts to eradicate it before it spreads into commercial crops.
There will be no quarantine for the SWD. One reason is that it is already widely established in California. Secondly, it is already a pest in California’s major trading partners in Japan and the rest of Asia.
SWD is one of 3,000 drosophila species identified worldwide. Here the common vinegar fly is the one associated with rotting fruit. It never damages fresh fruit. SWD does.
The lifecycle of SWD is 12 to 15 days at 65 degrees, according to lab work done at UC Davis. SWD can have 10 generations per year and probably three generations on one crop of sweet cherries.
“There are no peaks like with the codling moth,” says Coates. “There can be all sorts of generations in a field at one time.”
Fruit is most susceptible as it softens and approaches harvest, noted Coates. To head-off SWD problems in his area, Coates is recommending growers next season check pollenizers closely for SWD damage before Bing cherries ripen. “These pollenizers ripen 10 days earlier than Bings. I am suggesting people watch traps closely ahead of the Bings and when the pollenizers start to ripen.”
For low to moderate early-season populations, the scientists are recommending GF 120 or NuLure. For higher populations, the researchers are looking at the use of stronger materials like malathion of spinosid along with lure baits.
“We hope by next spring we can have control measures worked out,” said Coates.
Initially the fly was identified as a drosophila fruit fly or vinegar fly, a common fly that infests overripe or rotting fruit, but never found on fresh fruit on the tree. This fruit fly is the same one that is found in homes circling fruit left out too long. It is more a nuisance pest rather than an economic pest.
Van Steenwyck says it is also a common fruit fly in agriculture where there is fruit rotting on the orchard floor or around packinghouses.
“That is going to make the CVF easily mistaken for the common vinegar fly, since fruit flies are everywhere,” he says. The only distinguishable difference is a dark spot along the front edge of the male’s CVF wing tips. Otherwise, the fly has the usual Drosophila-like red eyes, pale brown thorax, and pale brown abdomen with black horizontal stripes. It is small, reaching only 2-3 millimeters in body length.
“It is more like a gnat than a fly,” said Van Steenwyck.