Medfly entrenched in California, study finds

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Feared and despised by California’s $43.5-billion agricultural industry, the Mediterranean fruit fly is seen as a potentially devastating foreign invader who routinely hitchhikes across the border in smuggled fruit.

But a new study argues that the infamous Medfly has established permanent residence in the Golden State — even after decades of diligent spraying, trapping and biological attacks by state officials, who say they have eradicated the pest.

“The invasion is complete and it’s irreversible,” said study coauthor James Carey, an entomologist at UC Davis. “It’s like a slow-moving cancer.”


The study, published in the British science journal Proceedings of the Royal Society B, incorporated research conducted at the University of Thessaly in Greece as well as data collected on 5,500 adult tropical fruit flies captured in California from 1950 to 2012.

Based on the geographic pattern and frequency of fruit fly outbreaks, researchers concluded that the Medfly and at least four related species gained a foothold here more than 20 years ago and had increased their range as they adapted to the environment. Among the Medfly cousins that have taken root are crop-damaging Mexican and Oriental fruit flies, the authors reported.

“Despite the 250-plus emergency eradication projects that have been directed against these pests by state and federal agencies, a minimum of five and as many as nine or more ... species are established and widespread,” they wrote.

Carey and colleagues said their findings suggested that state and federal officials needed to re-strategize their long-running war against fruit flies.

Among other things, they said, the California Department of Food and Agriculture should increase its trapping and monitoring of fruit flies throughout the state and should formulate long-term plans for dealing with an entrenched and growing fruit fly population, rather than one made up of occasional visitors.

“The bad news is that they’re here,” Carey said. “The good news is they’re still at low levels.”


Uncontrolled growth of tropical fruit flies, which lay their eggs in hundreds of varieties of fruits and vegetables, could carry significant economic consequences for the state. Local consumers would be dismayed to find maggots in their fruits and vegetables. Foreign and domestic trading partners might embargo California produce to prevent the flies from invading their agricultural fields and backyard gardens.

A top state official, responding to the new report, contended that California’s fruit fly control programs were effective and environmentally friendly.

“We’ll give this paper to our science advisory panel and have them read it and evaluate it and give us their recommendations,” said Robert Leavitt, director of plant health and pest prevention services at the Department of Food and Agriculture. “Quite frankly, I don’t think it’s going to change anything on the ground.”

One issue addressed in the new study is the definition of eradication.

Currently, a species of fruit fly is considered eradicated if it fails to appear in collection traps for three generations — about eight months.

Carey says this lack of trapped flies usually coincides with winter, when the insects aren’t absent from a region, just harder to come by. This seasonal fluctuation, he says, has been reported by state officials as periodic outbreaks and eradications.

State officials have said recurrent outbreaks in the Los Angeles Basin and the San Diego area are the result of overseas travelers bringing infested, contraband fruit into California.


“Border protection does intercept pests, not just fruit flies, but lots of pests,” Leavitt said. “We’re pretty sure that that is a pathway.”

But Leavitt acknowledged there was room for debate over the definition of eradication. “There are some philosophical discussions we can have,” he said, “but practically there’s no real difference. We’d be doing the same programs, because they work.”

California has a long and complicated history with tropical fruit flies, the Medfly in particular. Outbreaks in the 1980s and ‘90s led to the controversial aerial spraying of malathion, an insecticide that attacks the fly’s nervous system — and which raised human health concerns.

The state abandoned spraying in the 1990s when an international science panel recommended year-round release of sterilized male Medflies instead. When such lab-altered flies mated with wild female flies, no offspring resulted. Over successive releases, Medfly populations grew smaller and smaller.

Today, sterilized fruit flies are the “workhorses” of Medfly and Mexican fruit fly control programs, according to agriculture officials. The state also still uses insecticide-laden bait traps.

E. Leon Spaugy, who served as Los Angeles County agricultural commissioner from 1988 to 1998, oversaw the spraying of more than half the L.A. Basin during his tenure. Spaugy said that even then, there was debate among members of the science advisory panel over whether the insects had established themselves in the state.


“We had sporadic outbreaks every year, and the science advisory panel were never in agreement on much of anything, except the Medfly was a very detrimental insect,” he said.

It was in the 1990s that Carey first argued that tropical fruit fly colonies had established themselves in California.

This time around, Carey says he has more than 20 years of additional data to back up his claim.