Scientists Grow Skeptical About Eradicating Medfly

Despite repeated aerial pesticide sprayings, there are growing doubts among scientists about whether the Mediterranean fruit fly can be eradicated soon, if at all, from Southern California.

The most pessimistic scientists say the farm-rich state must begin to face the challenge of learning to live with the Medfly. “I don’t think they will be able to (eradicate),” said Kenneth Hagen, a professor of entomology at UC Berkeley. Hagen, whose research led to the development of a bait for the pest, advised the state on its Medfly war in the early 1980s.

State officials have consistently maintained that the pest will be eradicated, with members of a scientific panel directing the effort expressing varying degrees of optimism. But several scientists outside the program said they were dubious, and even a government official close to the effort privately expressed “cautious pessimism” about winning the battle.

There are several reasons for the mounting concern: the continuing discoveries of more flies outside the eradication zone; the region’s warm weather, which leads to more active breeding; the presence of a large international airport and frequent travelers to places where the fly is endemic; the region’s wide variety of hosts on which the flies can feed, and even the Santa Ana winds, which can carry a fly eight miles.

Two of the state’s five scientific panel members openly concede that eradication this time will be tougher than it was in Santa Clara County, the site of the state’s worst previous infestation, during the early 1980s. But these panelists, like their colleagues on the board, say the skeptics underestimate the state’s resolve and the success of past eradication campaigns.

If the pest became permanently established in the Los Angeles Basin, it probably would reach agricultural fields eventually. To export produce, growers would have to spend millions of dollars to treat the infested fruit, and some growers could be forced out of business or would have to relocate.

Many citizen groups and public officials, unhappy about as many as 12 aerial sprayings over their communities, complain that not enough is known about the pesticide malathion to declare it safe both for people and the environment. Proponents of the $25-million eradication effort counter that the amount of potentially harmful pesticides entering the environment will be substantially greater in the long run if the current program fails.

But even if it succeeds, scientists say, the Medfly is certain to be introduced again by travelers or in the shipment of fruit through the mail, requiring future pesticide applications. Aerial spraying to combat the Medfly was done in Southern California in 1980, 1981, part of 1982, 1987, 1988 and 1989. In 1983 and 1984, pesticide also was applied from the air but for another pest, the Mexican fruit fly.

State officials initially underestimated the scope of the current infestation. During the last several months, repeated discoveries of new flies forced officials to escalate the spraying campaign and expand the borders of treatment. More than 300 square miles are now expected to be sprayed with malathion, and future finds could expand the borders further. The spraying is now concentrated in Los Angeles County and in parts of Orange and San Bernardino counties.

The spread of the fly has been blamed in part on the failure of the state to install enough traps early on to detect the pest and to mount an effective public information campaign. Residents are not supposed to take fruit out of a 728-square-mile quarantine zone or to bring in fruit from places where the fly is endemic, such as Hawaii. Even fruit that looks perfectly whole can be infested. Police also are investigating an anonymous claim that the flies have been released deliberately.

UC Berkeley professor Hagen attributes the success of the 1981-82 Medfly war, which was primarily confined to Northern California, at least in part to cold winter temperatures there. The fly reproduces less often in cold weather.

“I’ve been following the temperatures down there, hoping there might be some cold nights, but they (the state) are not going to have that helping them,” Hagen said. “And I don’t think they can rely solely on the spray.”

Susan Opp, an assistant professor of biological sciences at Cal State Hayward, said the sheer size of Southern California’s infestation and the possibility of a continuing spread makes her dubious about the success of eradication.

“I have my doubts--(more than) 250 square miles--that it will work,” said Opp, who has done research on Medflies. “It is very hard to tell at this point, but it’s very difficult to inundate that large an area with pesticide and with sterile flies.”

Despite such doubts, Roy Cunningham, a U.S. Department of Agriculture researcher who heads the state’s scientific advisory panel, said he is “confident” that the pest will be driven from the region. “I think we have a tough row to hoe but I’m confident we will (eradicate),” he said.

Because the state was able to eradicate the fly in the early 1980s, when the infestation was scattered over eight California counties covering more than 1,300 square miles, it can be beaten in Southern California, Cunningham and other eradication supporters say.

Cunningham concedes that Southern California’s warm weather may cause flies outside the spray zone to reproduce more quickly and the infestation to spread more rapidly. But within the 300-mile eradication zone, the warm weather will be an advantage, he said.

The malathion bait kills only adult flies and works more efficiently when the population turns over more quickly. In its earlier stages of development, the insect is underground or in the fruit and therefore protected from the pesticide.

Fellow panel members Carrol Calkins, a U.S. Department of Agriculture research leader, and Richard Rice, a UC Davis entomologist, share Cunningham’s commitment to eradication, although both believe it will be tougher in Southern California than it was in Santa Clara County.

In addition to having warmer weather, Southern California has more kinds of fruit, more fruit on back yard trees throughout the year and more movement of people within the region and from places where the fly is endemic, the two panel members said.

“I think the L.A. infestation is worse because we can’t get a handle on it, and it’s in much greater danger of becoming established in L.A. than it was up there in the San Jose area,” Calkins said.

The difficulty in eliminating the Medfly this year might depend on how long the fly has been in Southern California. Some scientists suspect the current infestation stems from an introduction several years ago, and a fly population is already firmly established. Others believe the fly was newly introduced in 1989. Rice contends that no one will ever know for certain.

“Once you have an established population--and I would say the evidence in L.A. is that it is pretty well established--it (eradication) is very unlikely,” said Daniel Simberloff, a Robert O. Lawton Distinguished Professor of biological sciences at Florida State University. “That’s just an enormous area, and even if they dump very large amounts of malathion, they will not be able to get complete coverage.”

How Long a War?

The infestation could stem from a new introduction, but there have been so many infestations in recent years “that I would have to suspect that a population has established by now,” said Simberloff, who has studied insect invasions.

Some scientists also fear that the Medfly could become resistant to malathion. Resistance occurs when a few insects are able to tolerate significantly larger doses of a pesticide than the normal population and then pass on their resistant genes to succeeding generations.

Several kinds of insects have become resistant to malathion, and in Israel, where the insecticide is used regularly to kill Medflies, “some possibility of resistance of a very minor nature” has been discovered, according to George Georghiou, professor of entomology at UC Riverside and an expert on resistance.

But full-scale resistance is less likely in the Medfly than in other insects because the fly feeds on a bait filled with malathion, consuming a strong dose. Moreover, eradication programs use sterile flies that presumably would mate with any malathion survivors, Georghiou said.

Confident that resistance will not develop and assuming that the pest will be eliminated, the state’s scientific advisers nevertheless disagree over when the war will be won.

Barring “really bad developments,” Cunningham said, the pest should be eradicated within the treatment zone by spring or early summer. Rice, however, said he doubts that the fly will be eradicated by June. Asked if the Los Angeles Basin could be rid of the pest by next autumn, he replied, “possibly.” He refused to estimate the chances of eradication by then or to speculate on whether aerial spraying might be extended into summer.

Complicating the outlook further is the likelihood of new introductions of flies later this year, a possibility that no one discounts.

Cunningham, for one, is not daunted by such a prospect. Citing the case of the Oriental fruit fly, which has been introduced repeatedly over the past several years and repeatedly eradicated, Cunningham maintains that new Medfly introductions should not be discouraging. In cost, however, eradication of the Oriental fruit fly is dwarfed by what is required to beat back the Medfly.

The U.S. expects to have enough sterile flies available within two years to eradicate future Medfly infestations with a minimal amount of aerial spraying. The cost of breeding sterile flies, however, significantly exceeds that of using malathion.

Because of the costs and the detrimental effects of aerial spraying, UC Berkeley entomology professor Donald Dahlsten believes the time has come to plan for the Medfly’s presence in California.

“I think with something like the Medfly that is continually being introduced we need to start looking down the road as to what we are going to do when it gets into our agricultural areas,” said Dahlsten, chairman of the campus’ Department of Conservation and Resource Studies. “These aerial bombardments of urban areas is just not a feasible alternative.”

Monitoring by the state during the 1981-82 Medfly campaign showed that aerial sprays killed off such insects as honey bees, ladybugs and butterflies and produced outbreaks of harmful pests, such as aphids, in gardens. The spray also killed some species of fish when it was applied during rainy weather.

Epidemiological investigations, however, failed to turn up any persuasive associations between the aerial sprays and human health problems, such as premature births or birth defects. The state is doing no similar monitoring during the current campaign, although the Los Angeles Unified School District has advised school nurses to be alert to student absences that might be linked to spraying.

Agricultural officials believe the disturbances caused by aerial spraying are minimal compared to the costs of living with the Medfly. A resident Medfly population would require growers to undertake costly treatment of fruit before it could be exported.

Mediterranean countries, including Israel, grow fruit with the Medfly but none of those nations have a market as large or diverse as California’s. The state’s fruit crop is valued at about $1.5 billion annually, with exports reaching $429 million in 1988.

Treatment of infested fruit includes exposing it to hot or cold temperatures, fumigating it or even irradiating it. Some scientists believe natural predators might also be effective in managing a resident fly population and others find hope in the possibility of one day discovering genetically engineered diseases that could kill off the pest.

Cost of Not Spraying

But the state of California, bent on eradication, has no plans for coping with an endemic population. The only state study to outline a plan to cope with the pest was done in the early 1980s and assumed use of a pesticide that has since been banned.

That study, using 1979 dollars, estimated the cost of building fumigation and cold storage facilities and upgrading packing and shipping facilities at $54.4 million. Recurring costs, including the use of additional pesticide and fumigation, were estimated at between $316.3 million and $756.1 million annually.

During the 1981-82 infestation, Japan refused to accept untreated produce that is susceptible to the Medfly, which in California includes more than 50 kinds of fruits, vegetables and nuts. Growers estimated that the embargo cost them $100 million in produce they could not sell, the same amount it cost government to eradicate the pests then.

Faced with such a troubling future, the state’s scientific advisory panel sees no alternative to pursuing eradication until it succeeds. “We’re only four or five months into this, and you can’t give up,” said James R. Carey, a UC Davis entomologist. “There is way too much at stake economically to consider not eradicating.”

Fellow panelist Rice agreed.

“We don’t have any alternative but to eradicate it,” Rice said. “So my feeling is to think positively and just do it.”

STUDENT CHECKUPS--School nurses will monitor L.A. schoolchildren after malathion sprayings. B1