The Mediterranean fruit fly infestation detected in the northern Solano County city of Dixon in mid-September may be just one of many isolated pockets of medfly infestations in California, says noted UC Davis medfly expert James Carey.
"This is really serious because the invasion process is so insidious," said Carey, a UC Davis professor of entomology who has published widely on what is considered the world's worst agricultural insect pest.
Carey suspects that the medfly has been multiplying and spreading undetected — like cancer — for years.
"It may be a symptom of a much larger problem," Carey said. "But any way you look at it, this is the first really big outbreak in the Central Valley. We should look at this as a chronic, cancerlike process rather than a flu-like event."
The initial find of four medflies on Sept. 10 in a single trap at the intersection of C and Washington streets marked the first-ever discovery in Solano County and the second in the 400-mile-long Central Valley.
The only medfly previously captured in the Central Valley, Carey said, occurred at the peak of the Bay Area medfly outbreak in 1982 in the city of Stockton. With Dixon, that makes 160 cities in California with medfly captures, Carey said.
"It's like a puzzle," Carey said. "You find one here and over there and you need to consider the possibility that they're all connected and not isolated outbreaks. For example, the olive fruit fly was first discovered in 1998 in California, but within a few years was found up and down the state. How did it get statewide status within a few years? It's because it's been here for a long time."
"I wouldn't be totally shocked, if at some point, the medfly is found in multiple regions of California," Carey said. "Just as most of cancerous growth occurs at subdetection levels, we may be dealing with scores of medfly pockets scattered around the state, all of which are at subdetection levels."
Within a five-day period, from Sept. 10 to 14, ag officials detected 12 adults at five sites, and 33 larvae infesting a single peach tree in a backyard, all within the Dixon city limits.
"It's definitely breeding, but it's all within the city so far," said Kevin Hoffman, primary state entomologist with the California Department of Food and Agriculture. A 114-mile radius quarantine took effect on Sept. 17 and is expected to last nine months to a year.
The 114-mile radius quarantine is expected to wreak economic havoc on growers, shippers and processors of fruits, vegetables and nuts, and in turn, will adversely affect consumers. Dixon is in the throes of tomato and walnut harvesting. The owner of a 65-acre organic produce farm that ships to 800 clients said he may lose $10,000 a week in potential sales.
DNA testing on the first five medflies revealed they are of the BBBB strain, commonly found in Hawaii and Venezuela, said state plant health director Helene Wright of the USDA Animal and Plant Health Inspection Service.
Wright said this is the third occurrence of BBBB strain detected in California: a single medfly in Santa Ana in 1993; and medflies in La Jolla in 1998. Hawaii and Venezuela have only the BBBB strain, but it is also commonly found in Africa, South America, Madeira and the Azores.
"We are trying to track the potential pathways," she said.
Carey said the key pathway is apparently not by air travelers. His research published in 1991 in Science magazine indicated that USDA/APHIS in Honolulu found only "two to three medflies in five years or so despite 100 percent screening of bags and carry-ons."
Carey said it is too early in this outbreak and there are too few points to start connecting the dots.
The pest, smaller than a housefly, is considered the world's worst agricultural insect pest because of its wide distribution, wide range of hosts (its larvae infest more than 260 fruits and vegetables) and its ability to tolerate cool climates.
Since the initial find on Sept. 10, the CDFA Medfly Action Plan kicked into gear. Since then, ag officials have:
-Stripped all fruit from trees within a 100-meter radius of all medfly finds. -Ground-sprayed the organic compound Naturalyte (the active ingredient is Spinosad, a naturally occurring product of a soil bacteria) within a 200-meter radius of all medfly finds. -Set 1,700 fruit fly traps within an 81-square mile grid in all of Dixon and the surrounding area from near the Yolo County border to Midway Road. -Began aerially releasing 1.5 million sterile male medflies (dyed pink for easy detection) over a 12-square-mile area on Sept. 14, with weekly releases of 3 million medflies scheduled for at least 9 months. -Set up a command center, with four portable buildings and a task force of 25 on the Dixon May Fair grounds for what is expected to be a one-year stay. -Established a 114-square mile quarantine zone of fruits, vegetables and nuts in the area.
"CDFA is doing exactly the right thing," Carey said. "You've got to react immediately, of course. But once things are under control, they need to thoughtfully consider the nature of invasion so they make sure they are treating the disease and not just the symptoms."
CDFA's quick emergency response follows standard procedures that have served them well in the past, said UC Davis entomologist Frank Zalom, a member of the state's Exotic Pest Eradication Task Force in 1995, which addressed the extensive medfly outbreak in the Los Angeles area.
"One of the things we really worry about is that it has so many crop hosts and the impact of potential quarantines on different crops," said Zalom, who studied the overwintering capabilities of medflies in Barcelona, Spain.
Earlier medfly infestations in California — the seven-county Bay Area infestation in 1980, and the 1989-90 and 1993-94 infestations in Southern California — involved the aerial spraying of the controversial pesticide malathion.
"The spraying of the spinosad bait is a safe approach," Zalom said. "It has been used successfully by our growers to treat olive fruit flies, and it has been used successfully in Hawaii to suppress (the pest) there."
The pest, first detected in California in 1975, prefers thin-skinned fruits like peach, nectarine, apricot, avocado, grapefruit, orange and cherry. The pest's permanent presence in California could result in estimated annual losses of $1.3 billion to $1.8 billion, according to CDFA Secretary A.G. Kawamura.
The release of sterile flies has dramatically reduced the number of infestations, said Steve Lyle, director of the CDFA's Office of Public Affairs. Between 1987 and 1994, the state recorded an average of 7.5 medfly infestations each year in California. Since the inception of the Preventive Release Program in 1996, the tally is just five infestations statewide, Lyle said.
The sterile male flies "have a proven track record in southern California of breeding with wild females to help achieve eradication," Lyle said. "The females breed once and if they breed with a sterile male that ends their reproductive activities."
For more information about the Mediterranean fruit fly or to report discoveries of suspected infested fruit, call the CDFA toll free pest hotline: (800) 491-1899 or access the CDFA Web site at http://www.cdfa.ca.gov/.