The newest agricultural pest in California, the cherry vinegar fly (CVF), had a bad reputation even before it had a name, and that reputation is growing.
CVF is a cousin in a large family of tiny, gnat-size fruit flies. It has wiped out sweet cherry varieties in the Central San Joaquin Valley and halted harvest of the more popular Bing cherries in Santa Clara County because of heavy damage. It has caused significant damage to strawberries in the Watsonville area.
University of California, Berkeley entomologist Bob Van Steenwyck, says the pests from the drosophila fly family could pose a major threat to all soft fruit in the state.
“It is speculation, but there is an indication that the cherry vinegar fly will go to grapes, plums, nectarines and other soft fruit,” he says. Apricots are another soft fruit it may damage. He does not expect the new pest to damage apples or pears. They are harder than the grapes and stone fruit.
It is not speculative to say it will damage cherries, strawberries and cane berries — but rather a painful, proven fact.
“It looks like it has a wide host range in the state. It is already all over the state. We could have our hands full with this, and it came on so suddenly,” Van Steenwyck notes. He saw it for the first time this spring as did many others in the state. It has already spread from Southern California to coastal areas to Northern California.
“We know it has been here at least a year (undetected) and there is some anecdotal evidence that it has been in California for three years. Nevertheless, the spread this year is alarming.” The UC entomologist compared the spread to that of the olive fruit fly, which spread from San Diego to Redding in about five years.
First discovered in damaging numbers almost simultaneously in Santa Clara and Stanislaus counties, it has since been identified in Los Angeles, Santa Barbara, Santa Cruz, Fresno, Yolo, Merced, Yolo and San Joaquin counties.
UC Cooperative Extension (UCCE) Farm Advisor Bill Coates, Hollister, received a mid-June call from a San Jose homeowner who said the cherry vinegar fly was infesting an ornamental plum at a residence. “I have not checked it out yet, but according to the call the plum was next to a cherry tree that was infested with the cherry vinegar fly,” he says.
A California Department of Food and Agriculture taxonomist, Dr. Martin Hauser, did not find reference to the new pest anywhere in the U.S. when Coates sent him the first samples.
The initial CVF discovery in Santa Clara County came from a pest control adviser (PCA). In this case Coates said the fruit appeared undamaged except for “stings” on the fruit surfaces, but upon slicing open the fruit, the cherries were filled with maggots.
Early varieties like Early Burlat and Black Tartarian were extensively damaged in Santa Clara County. Coates said some growers were forced to suspend Bing harvest because of extensive damage from CVF. Others with lighter infestations continued harvesting, but were forced to sort out possible infested fruit, said Coates.
Kathy Kelley Anderson, UCCE farm advisor in Stanislaus County, also found early season varieties like Ruby and Brooks infested and unharvestable in her county. She had not found it in Bing variety through early June.
The field person who discovered the damage was with a large packer, she said. “He has been working with cherries for four years and had never before seen the damage we saw.
“Growers are quite concerned,” she says.
Initially the fly was identified as a drosophilis 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 CVF’s wingtips. 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.
“It is much smaller than the walnut husk fly,” added Coates.
The common vinegar fruit fly is not found on sound fruit — CVF is.
One particular fruit fly, melanogaster, is used extensively in laboratory and field experiments on genetics and evolution because it is easy to raise and has a short life cycle.
The CDFA taxonomist successfully traced the cherry vinegar fly Coates and Anderson found to Japan, China, Korea, and Thailand. It recently has been identified in Hawaii and Spain. It is scientifically noted as Drosophilia suzukii.
“We do not want to confuse this new pest with the common vinegar or fruit fly or the Western cherry fruit fly that is found the Pacific Northwest,” Coates says. Coates works in tree fruit and nut crops in San Benito, Santa Cruz, Santa Clara and Monterey counties.
“We are calling it the cherry fruit fly or cherry drosophila. They do not want to call it just a fruit fly,” says Coates. CDFA gave it the name.
Van Steenwyck says what makes the CVF such an ominous pest is that it attacks fruit just as it softens on the eve of harvest.
“It is quite ugly when it comes in. I was on one orchard where 30 percent of the fruit had collapsed just ahead of harvest,” he said. Once the fly punctures the soft fruit skin and lays its eggs, it can be as few as two days before the fruit starts to collapse. CDFA has found infested cherries in supermarkets.
Van Steenwyck said a PCA or grower can identify damage by a small depression at the bottom of the fruit. This is the oviposition site. It is particularly evident on lighter colored cherries.
Cherries are particularly vulnerable because they use in-field pollinator trees that mature earlier than the desirable Bings.
“Pollinator rows can act like ‘typhoid Marys’ by being reproduction sites that can cause severe problems with the later and more popular Bing variety,” the entomologist says.
The UC scientist says while serious, he does not want growers and PCAs to panic until researchers learn more about the life cycle and control measures for this new pest.
“We don’t want growers to spray without it being warranted,” he says. “Right now we are guessing and have no good data on the fly and how to control it. There are a bunch of us jumping on this problem.
“I am sure there are people who want to do a revenge spray for the damage that has been caused. It might make them feel better, but we need to be cautious about spraying needlessly,” he says.
Growers are likely to get another shot at the newest ag pest in California next year, according to Van Steenwyck.
As word spread about the new pest, growers sprayed unharvested orchards to prevent damage. Coates said GF120 provided some control of lighter infestations, but not heavier infestations.
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