By the end of the month, grape growers should have a better idea of how — if at all — the newest pest threat to California’s fruit orchards might affect their vineyards.
The gnat-size insect, initially called cherry vinegar fly (CVF), has been identified as Drosophila suzukii and has been renamed spotted wing drosophila (SWD).
Unlike its cousin, the vinegar fly, which attacks spoiled and rotting harvested fruit in places like warehouses and kitchens, SWD attacks healthy soft fruit as it ripens on the tree. SWD was reported in California for the first time last year in the central coast area.
The fast-spreading insect has since become established in 21 counties, from Southern California to coastal areas and Northern California, where it has damaged strawberries, raspberries and cherries. This year it has also devastated sweet cherries in the central San Joaquin Valley and brought the Bing cherry harvest in Santa Clary County to a halt. And it has been found in blackberries, blueberries and raspberries.
“The fly likes soft-skinned fruit with fairly high sugar content, which it can easily puncture to lay eggs,” explains Paul Verdegaal, University of California Cooperative Extension (UCCE) farm advisor for San Joaquin County. “The concern is that it could spread to other crops as they ripen. It attacks certain crops in some areas, but not in others.”
Verdegaal helped organize a July 8 meeting in Stockton to alert growers to the new pest. More than 200 grape growers, peach and other fruit producers and PCAs attended the event, where UC Davis researchers, farm advisors and California Department of Food and Agriculture (CDFA) representatives discussed the current status, potential hosts and outlook for the pest.
The fly is found in China, Korea, Japan and Thailand, on cherries, grapes and peaches, as well is in other areas of the world, such as Spain and Hawaii, Verdegaal reports.
“It has been reported as a pest in Japan on cherries and blueberries, but it hasn’t been considered a major pest that requires monitoring or quarantines,” he says. “Also, it has been found at California border check stations on fruit coming from Colorado, Oregon and Canada. But, we don’t know if the fruit was grown in those areas or if it originated in infested parts of California and was brought back.”
So what’s the threat of SWD to California’s grape crop?
“Right now, no one knows for sure,” Verdegaal says. “In the lab, the fly can attack grapes, but there are no conclusive findings about what it will do to them in the field.”
The pest prefers warm, moist conditions, he notes, but how it will do in relatively hot, dry vineyard conditions is still a guess.
“Conditions have to be just right,” Verdegaal says. “The fly has never been reported as a major pest on cherries in Asia. But, as we’ve seen this year, for some reason, it can cause major damage to cherries here in California.”
In early experiments in San Benito and Santa Cruz counties, a liquid bait of spinosad, a naturally-based insecticide, or malathion seems to have reduced SWD populations somewhat. But, that doesn’t necessarily indicate how effective this approach would be for controlling the pest in the long term, he notes.
One challenge to controlling SWD with a spray is its short reproduction cycle — the fly can complete a generation in 12 to 15 days, Verdegaal says. It is thought that it can produce 10 generations a year and probably three generations in a crop of cherries.
A better approach might be a combination of baits, biocontrol and a spray program, he says.
Currently, neither the CDFA nor the USDA is recommending any regulatory action, such as a quarantine.