About 300 growers, pest control advisors, and other industry personnel attended a hearing July 8 in Stockton to learn more about this new pest threatening cherries, berries, and other thin-skinned crops in California.

A serious new pest of sweet cherries, berries, and potentially other thin-skinned fruit crops has California growers worried. Several hundred growers, pest control advisers (PCA), and other industry personnel attended a hearing in Stockton last month to learn more about the new pest. Unfortunately, the pest is so new to the state that the hearing provided more questions than answers, starting with what to call it. (See “The Name Game.”)

What they do know is the pest, Spotted Wing Drosophila (SWD), was originally called a vinegar fly because it is similar to other drosophila that go after rotting fruit, the California Department of Food and Agriculture’s (CDFA) primary state entomologist, Kevin Hoffman, told the audience. But this pest also attacks ripe and ripening fruit once the fruit reaches an as-yet unknown Brix.

Originally from Asia, SWD’s now established in Hawaii and Spain. Just months after the initial finding in California, SWD’s now in 21 counties, from Napa to San Diego. Because of the widespread infestation, Hoffman said the CDFA has given SWD a permanent pest designation of “C,” meaning the state will take no control or regulatory action. And, he added, USDA likewise has no plans for action.

In addition, there are no quarantine implications, says the CDFA’s Courtney Albrecht. The SWD and its cousins have never been a problem before. “When we do a (computer) search for pests of concern worldwide, drosophila is not found,” she said. “It does not appear to be a pest on anyone’s radar screen.”

Other Crops At Risk?

Many growers at the hearing wanted to know if other thin-skinned fruits were at risk, especially winegrapes. Hoffman said it would be premature to speculate on what crops — other than cherries — will be affected. Because the SWD is so new to the U.S. — or at least it’s thought to be new; entomologists say that it may have been around for a while, it just wasn’t doing much damage — no one knows much about it. Chief among these unknowns is what crops are at risk.

What is known is that, like its cousins, SWD goes after rotting fruit of all types. But unlike most of the approximately 3,000 other types of Drosophila, it goes after ripe and ripening fruit, as well. The pest damages the fruit by depositing eggs into the fruit that hatch in up to 72 hours. The females have a saw-like ovipositor to deposit eggs into the ripening fruit, but they are only able to lay eggs in thin-skinned fruit. The fruit damaged in California has been limited to raspberries, strawberries, and cherries.

In Asia SWD has been reported damaging primarily cherries, according to Hoffman. However, it has also been found to infest such thin-skinned fruits as apples, blueberries, peaches, persimmons, plums, and grapes. It was that last fruit, grapes, that was of very keen interest to the many growers and pest control advisers who attended the July information hearing.

After all, the California winegrape industry is huge, and this year’s crop will be the first to ripen since the SWD was found to do serious damage. “We didn’t have it last year at harvest time,” says Larry Whitted, a PCA with San Joaquin Sulphur in Lodi, an area that has developed a wine industry with a total economic impact of $5 billion. “We have to be prepared that it may damage grapes.”

Raising Red Flag

Whitted, who has been a PCA for nearly 30 years, noted that Hoffman and other entomologists simply don’t know if SWD will go after ripening grapes. However, he did point to the testimony of Arturo Kopp, of the Department of Evolution and Ecology at the University of California-Davis. In his laboratory, Kopp put some SWD females in a container with Thompson seedless grapes, and he said it took just two to three minutes for the SWD to puncture the grape skins.

“Sometimes insects behave differently in the lab, but that’s certainly a red flag,” says Whitted. “That’s definitely a warning there.”

Complicating the winegrape picture is the fact that SWD does not like very hot, dry weather of the type found in the San Joaquin Valley in the summer. However, varieties such as Cabernet Sauvignon aren’t harvested until October or November when temperatures are much cooler. In addition, vintners are increasingly calling for greater “hang time.” Grapes that used to be harvested at 23 Brix several years ago are now harvested at 28 Brix, says Whitted, and the sweeter grapes would logically be more attractive to the pest.

About the only sure thing is that winegrape growers and PCAs are really going to be on their toes this coming fall, says Whitted. “We’ll have to keep scouting all through harvest, and if we start to get damage, we may have to do bait sprays, something we’ve never done on winegrapes,” he says. “Most people are guessing it’s not going to be that bad, but nobody really knows what’s going to happen. It sure has people nervous.”

For the latest on this pest, check out Mark Bolda's blog here.