Planet Earth

Fly Wars

California spends tens of millions to defend its crops against the voracious medfly. But one entomologist says the defense is based on sloppy science.

By Mark WheelerFeb 1, 1993 6:00 AM


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Since 1975 a tiny, pestilent, blue-eyed fly has been found in California 12 times. So far it has cost the state lots of money--more than $170 million in eradication efforts--and lots of bad press, mostly because of the aerial spraying of the pesticide malathion over urban neighborhoods. Lately, for officials in the California Department of Food and Agriculture, it’s cost lots of angst, thanks to an entomologist who’s gone public with his charge that in dealing with little Ceratitis capitata, also known as the Mediterranean fruit fly, the CDFA has been engaging in some woefully bad science.

The medfly is one of agriculture’s most destructive pests, a voracious eater that is attracted to more than 250 different types of fruits and vegetables. In tiny, nearly invisible holes drilled into the fruit’s skin, the female medfly lays up to 1,000 eggs in her average 40-day life span. Those eggs turn to larvae, which then dine on--and destroy--the fruit’s pulp. California’s $17-billion-plus agricultural economy is particularly vulnerable to the medfly: the state’s climate is temperate, virtually matching that of the insect’s Mediterranean homeland, and its flora is extensive, offering the fly more than 31 million acres of commercial farmlands and innumerable gardens and fruit trees.

Little wonder, then, that California is intent on keeping the medfly from ever establishing residency. And according to the official claims of both the CDFA and its federal counterpart, the U.S. Department of Agriculture, the state has indeed been successful. Yes, the fly is occasionally found in California, officials admit. But each appearance, they say, is the result of medfly eggs or larvae hitchhiking inside fruit illegally mailed or carried into the state. Mostly, they believe, that fruit comes from places like Hawaii and Central America, where the medfly is endemic. Whenever the flies are found, the CDFA says, either at one of its 16 inspection stations located on major roads leading into the state, or inside flytraps located throughout the state’s rural and urban areas, officials immediately take measures to eradicate the would-be invaders. They spray the area with malathion, strip fruit from infested trees, and release hordes of sterilized flies that they hope will breed the wild flies out of existence.

In 1990, though, James Carey, an entomologist at the University of California at Davis, went before a session of the California State Assembly to say the state agriculture agency had got it wrong. The medfly’s numerous appearances in the state aren’t a series of random introductions at all, he claimed, but are instead different sightings of the same low- level populations that have existed for years. The medfly, in other words, despite California’s best attempts at discouragement, has found the area so attractive that it’s decided to stay.

Carey created quite a stir: foreign importers of California fruit blanched, newspapers ran editorials critical of CDFA policy, and agricultural officials went ballistic. It didn’t help that they considered Carey to be one of their own--he sits on the agency’s own Mediterranean Fruit Fly Science Advisory Panel. As the agency told the press, the four other members of the panel had listened to Carey since 1987, and three had simply rejected his hypothesis as speculative. Carey, for his part, says that any research he presented that disagreed with the CDFA’s position was simply disregarded. He also points out that the three who disagreed with him are USDA scientists who have to take up the party line.

Why this is more than a bit of foofaraw among fellow Ph.D.’s arguing entomological fine points is, of course, purely a consequence of the fly’s economic wallop. Because much of California’s produce is shipped overseas, especially to Japan and other Pacific Rim countries, federal and state officials have always gone to great lengths to reassure buyers that California is medfly-free and its fruit pure. If importers came to believe that the medfly is a permanent California resident instead of an occasional tourist, they would place permanent quarantine restrictions on the state’s fruit. Growers would have to install expensive equipment to kill any possible larvae by freezing; they also might have to use high quantities of expensive pesticides, which carry the additional stigma of being anathema to California’s environmentally conscious denizens. No one knows precisely how much this would cost, but it wouldn’t come cheap--a single medfly infestation in 1981 cost California’s growers $40 million.

Carey is uncomfortable, he says, playing the role of troublemaker and being embroiled in political bickering. So why is he doing it? Because I’m a scientist, he answers simply. And as a scientist, I’m just convinced their approach is wrong. They’re trying to defend a position rather than address the science. The question should be ‘Do the data support or refute the established hypothesis?’

At the heart of the matter is the CDFA’s insistence that each appearance of the medfly is an isolated one. With each find, they eradicate. End of story. But Carey believes that California’s medfly population is part of a broader, inexorable phenomenon known as biological invasion, a process in which an animal or plant is introduced into a new geographic area by humans. This usually happens by accident, but sometimes it is intentional: an attempt to use one insect to control another, for example, or perhaps to introduce a new breed of sport fish for anglers. In time the introduced species adapts to its new environment, its population grows, and it begins to spread to other areas. Carey believes this is precisely what has happened with the medfly. And instead of simply lurching from outbreak to outbreak with no long-term plan, he says, the CDFA should be studying the invasion process to gain a better understanding of the insect.

CDFA officials are having none of it. In the past five years, they say, Carey hasn’t presented any new evidence to back up his claim; his hypothesis, they insist, is wide open to interpretation. Isi Siddiqui, an assistant director of the CDFA who’s in charge of the medfly program, dismisses him out of hand: Does Dr. Carey have any new data? he asks angrily. Because I returned from China last summer, a potential $200 million market for California, and they won’t buy our produce based on this irresponsible perspective from Dr. Carey.

Siddiqui’s annoyance is understandable. Carey, since his 1990 appearance before the state assembly, has gone on to charge that the CDFA is being swayed by political economics. Each time a medfly outbreak occurs, the CDFA declares it an emergency situation. This makes the financially strapped agency eligible for supplemental state and federal emergency funds. If the agency were to declare that the medfly is in California to stay, it would lose access to those dollars, and the additional costs of controlling the medfly population would then have to be borne by the state’s growers, who maintain a powerful lobbying presence in the capital, Sacramento. In addition, Carey says, the CDFA is under inevitable political and economic pressure to declare eradication quickly so any quarantine restrictions can be lifted.

The pest behind all the fuss is a mere quarter-inch fly with a typical insect metamorphosis. Once laid inside a host fruit, the eggs take approximately two days to hatch into larvae. Larval development time ranges from 7 to 12 days, depending on the host (7 days for tomatoes, for example, 12 for grapes). The larvae eat their way through their host, then burrow into the ground, where they form pupae. The pupal period lasts about 12 days before a full-grown fly emerges. Under normal circumstances, medfly populations in total are capable of growing at a rate of 10 to 20 percent a day.

But when an invading species is introduced to a new environment, Carey says, the normal reproductive cycle is disrupted, and it becomes subject to multiple obstacles that keep populations low. An insect may need years to adapt to new surroundings, even ones as inviting as California’s.

Carey notes that this points up the logistical problems of the CDFA’s insistence that every medfly introduction is caused by hitchhiking larvae. No doubt random introductions occur, he says, but they are very rare; each time, larvae would have to somehow escape the garbage disposal or garbage can, crawl out of the fruit, burrow into the ground to pupate, then emerge and find food and a mate--all in a new and hostile environment. My goodness, that’s almost beyond belief, he says. After all, how much fruit have you thrown out the door lately? I mean, are people raising pigs in their backyards or what? Far more likely, he believes, is that the medflies are always around, but in concentrations too low to be detected easily.

There are four main phases to the invasion process, Carey says: introduction, colonization, naturalization, and spread. In the first phase, all you need are enough insects to allow phase two, colonization, or population growth, to occur. Theoretically, an introduction occurs any time a fly enters the state, he says, but realistically an introduction should be defined as having at least one fly make it through one reproductive cycle--from larva to adult.

Conservation biologists talk of having a minimum viable population, which means that you must have sufficient genetic diversity so that if some flies die, there will be enough variation for the population to survive and later adapt. There’s no diversity with a single female. This isn’t like planting a seed and nursing it along. Even when insects are introduced intentionally, for biological control, Carey says, thousands of them must be released to make sure they get established. And even then, most biocontrol releases don’t take.

The other three steps, colonization, naturalization, and spread, have similar restraints. While most invasion biologists define a colonization as achieving one reproductive cycle--that is, the average four-week egg-to-adult period--Carey uses that definition for a successful introduction. A true colonization, he believes, would involve an introduced population surviving at least an entire year by engaging in multiple reproductive cycles. For example, suppose there is an introduction. The flies could be in an area and season where the weather is warm and there’s abundant host fruits--hog heaven for a medfly--but then comes change, a colder, wetter season absolutely barren of fruit. The odds against a medfly’s surviving such environmental challenges, he thinks, are overwhelming, and he attributes any such survival to an aberrant superfly or just plain old luck.

He admits that much of this is a line drawing exercise, since more research is needed on these early stages. Still, he believes it a useful way to classify. For example, the distinction between the phases of colonization and naturalization--when the flies adapt to a new climate, new food, and new predators--is a narrow one. Naturalization is the process of microevolution, says Carey. If different pesticides were used, for example, the medflies would need to adapt to them. There may be different hosts--perhaps mangoes where they came from and citrus over here; temperatures are different; there may be different predators. All this is a selection process that can take many years to adapt to. At that point the last stage occurs, in which the population has grown enough to begin spreading to new territory--again a slow process, since the medfly’s proclivity is to stay close to its original colony. Significantly, Carey says, it’s at this last stage that the population finally grows large enough to be detected.

The concept of biological invasion was introduced in 1958 by British ecologist Charles Elton, but the phenomenon is as old as human transport, and it has flowed mostly in a westerly direction. European insects invaded the New World more frequently than the reverse. The reason was not superior genes but simply an early preponderance of east-to-west travel.

Carey identifies three major jumps the medfly has made. The first, probably during the Middle Ages, took it from equatorial Africa, up the west coast of the continent, to the Iberian Peninsula. Spain and Portugal appear to be the source for the rest of the Mediterranean and for a second jump to the Americas, says Carey. The flies probably traveled aboard ships using the transatlantic trade routes during the seventeenth and eighteenth centuries. He thinks they may have ridden along inside fruit intended to help sailors prevent scurvy.

Once they landed in the Americas, probably in Brazil, it took years for colonization and naturalization to occur. Eventually, though, the medflies crept north to Central America, where today they’ve established a beachhead at the Guatemala-Mexico border. In the meantime they managed to make their third jump: sometime in the early to middle nineteenth century the medflies made their way to Australia and Hawaii.

Carey says a genetic analysis done by researchers at the Agricultural University of Athens supports this scenario. After comparing medflies from Africa, the Mediterranean, Central and South America, and Hawaii, the researchers tentatively concluded that the Iberian Peninsula was indeed the original source for the rest of the Mediterranean and the spread to South America. It also showed little difference between Central and South American medflies. To Carey, this suggests there was one common founding population in the Americas and is evidence that a single population was introduced, then diffused.

That’s why the California medfly problem is part of a global invasion, a long-term, chronic, and insidious problem, he says. These aren’t just spores falling out of the skies; this is diffusion throughout the Americas. It’s probably taken a century or so for the medfly just to move across Brazil. So we’re dealing with a 200- or 300-year invasion of the Americas; and California is now the northern tip of this whole spread.

The CDFA does not contest Carey on ancient medfly history. Where we differ with Jim is not on the process of invasion; we agree that happens, says CDFA entomologist Bob Dowell. But we maintain it happens in the absence of significant environmental resistance, and that’s us--the active efforts of the CDFA and the USDA. Jim hypothesizes a limited number of invasions followed by a spread throughout the entire area. We believe the insect has invaded and begun to colonize, but we find it and eradicate it. That’s followed by another invasion, and we start all over again.

Carey notes how frequently the medflies have been found in the state since the first finding, in 1975: every single year from 1980 on, except 1983 and 1985. Just how, Carey asks, is the CDFA defining eradication? He’s not saying the state’s efforts have no effect. Look, the CDFA’s eradication methods do work, he says. Malathion spraying and fruit stripping do kill flies. But even there, logistical problems exist. Just gaining access to people’s backyards--what if they’re not home, what if the gates are locked or there’s a dog back there--we are talking about hundreds of individual fenced backyards where in every case you have to explain to the occupant what you want to do. It’s not unlikely a small colony could be missed. (Carey notes that during ground spraying last September, 10 percent of the residents in the affected areas refused to allow access to their backyards.)

In his office, Carey points to three maps of southern California; each has arrows branching off in different directions. On the maps Carey has marked each medfly infestation. I laugh at myself a little bit over these, he says with a nod toward the maps. Here we are in an age of mitochondrial DNA analysis, and I’m using hand-drawn maps to make my case. But look, he says, pointing to a spot near the ocean. Here’s the first find, in ’75 in Culver City. Now if you look at the subsequent finds, you see a steady progression to the east, with forays to the south into Orange County and north into the San Fernando Valley. The flies couldn’t go west because they were blocked by the ocean. Eventually, Carey says, the flies crossed the L.A. basin to the city of Whittier, where a major infestation occurred in 1989. Diffusion is a series of fits and starts; these bugs are subject to topographical factors too. Behind Whittier sit the Puente Hills, which, Carey believes, channeled the flies north, then east. It took them a while to make it past those hills, but in 1990 they finally spread into the cities of Riverside and San Bernardino. He also believes they’ve been slowed from a further spread north by the Santa Susana and San Gabriel mountain ranges, which rim the L.A. basin to the north.

These steady, near-straight-line progressions can’t be random introductions from people carrying in infested fruit; if that were the case, there’d be medflies scattered all over the map. Instead, he says, new medfly outbreaks almost always appear in roughly the same neighborhoods as prior outbreaks.

Last fall a number of new medfly sightings took place throughout the state. To Carey, each fit the pattern. In September, outbreaks in Pasadena, Griffith Park, and Duarte sat in that rough line heading east. A discovery in South-Central L.A. was just a few blocks from a 1991 outbreak in Chinatown, where eradication efforts just ended last July. In November more outbreaks occurred in the southern California towns of Artesia and Pico Rivera, and in Oceanside, in northern San Diego County. Up north an outbreak last July in San Jose was close to outbreaks of 1980 and 1989. These are all examples of diffusion, Carey believes, further evidence that with each discovery, the flies are not eradicated but knocked down to numbers so small as to be undetectable.

The CDFA and USDA, on the other hand, attribute the geographic clustering to the same people sending fruit to the same families at the same locations. If that were true, asks Carey, then why do the majority of infestations occur in the summer, corresponding to the seasonal patterns-- greater numbers and activity in the spring, quiescence in the colder months--that medflies follow in their native Mediterranean region?

Siddiqui defends the reintroduction argument by pointing out that California spends $14 million a year on trapping exotic pests: there are more medfly finds, he says, because of improved detection methods. Previously a single fly may have hitchhiked in, then died out, and we might never have known about it. Now we may be detecting them much earlier. Also, keep in mind there are many more people coming into the state from countries where the medfly is endemic.

The CDFA’s defense starts with a network of triangle-shaped flytraps that are laced with trimedlure, a chemical odorant that’s supposed to mimic the medfly’s sex pheromone. Whenever a single medfly is caught, more traps are added to the area. If two or more flies are then found within a one-mile radius, the CDFA moves in with its eradication measures.

One such measure, the aerial spraying of malathion from helicopters, has not been used since 1990, despite its being the most inexpensive and effective eradication tool the CDFA has. Intense public protest about such broad-based spraying, beginning in 1982, may well have played a role in that decision. Thus trapping, ground spraying, and sterile flies are now the agency’s chief tools for detection and eradication. Yet these too have been criticized, and not just by Carey. Researchers Ken Kaneshiro and Richard Rice have also found themselves caught up in the fly wars. Kaneshiro, an evolutionary biologist at the University of Hawaii, has received grant money from the USDA for his medfly research; Rice, like Carey an entomologist at the University of California at Davis, also sits on the state’s science advisory panel and is the lone member who is at least open to Carey’s ideas. While Siddiqui likes to point out that the CDFA has tripled the number of traps it puts in urban areas to 15 per square mile (with a total of 48,000 traps statewide), Kaneshiro and Rice both say that’s still not enough. Kaneshiro, for instance, found that a trap less than 10 meters from the point of release of 140 marked sterile flies did not catch a single one within three days.

The big stumbling block is the weakness of trimedlure, the attractant. Chemically we haven’t been able to duplicate the medfly pheromone, and we don’t really know why, says Rice. It could be the medfly’s pheromone isn’t that stable naturally; as soon as you synthesize the chemical in the lab, the moment it’s exposed to air and sunlight, it begins to break down.

Where the traps are placed is critical, too, says Kaneshiro. His research shows there are only certain arenas where males and females congregate to mate. In a grove of 118 potential host trees on Maui that he uses as a lab, he’s found that the same four trees serve as mating arenas. Rice thinks it’s possible the male flies recognize the weakness of their own pheromone: It could be the flies congregate because it’s the only way to attract females--they bunch together to produce enough of the pheromone.

There is also a need to learn the environmental parameters of these mating arenas, says Kaneshiro. That may be part of the problem with California’s detection. The CDFA tends to hang its traps on the best larvae hosts. But that’s not where the flies go to mate, and it may be another reason that small populations go undetected.

He’s also suspicious of the efficiency of the CDFA’s sterile flies: The USDA once dropped a load of sterile flies on top of our research grove, and we observed what the sterile flies did in relation to the wild population. We found that 95 percent of them were going to the wrong place. Kaneshiro suspects that males bred and sterilized in the lab lose their ability to find the natural mating arenas. His later research confirmed his earlier visual observations; he has found that fewer than 2 percent of sterile males participate in matings with wild females. In the natural population we know that 30 percent of the males do more than 50 percent of the matings; I call them the stud males. On the other end of the scale, 30 percent of the males won’t mate at all--they’re the duds. And, he says, lab males tend to be more dud than stud. Sexual selection is very powerful in the field, says Kaneshiro. We’ve observed that 90 percent of courtships end in the female rejecting the male. That tells us there is very strong female choice going on.

As for the traps, the CDFA and USDA have always acknowledged their medfly traps are weak; indeed, over the past two years USDA entomologists Roy Cunningham and Derrell Chambers have headed a $1.5 million CDFA effort to develop a better trap, one that could be used not only to find the flies but to help get rid of them. They think they’ve found it in a trap that employs a flypaperlike outer coating laced with a substance called ceralure. Ceralure lasts longer than trimedlure in the field; unfortunately it is also four to five times more expensive. But Cunningham believes it can be effective enough, with further refinement, to make this trap an eradication tool. Carey is more cynical; he believes ceralure isn’t much better than trimedlure and that the trap won’t work at getting rid of the flies.

You can’t even mass-trap these things across a single orchard, much less over hundreds of square miles, he says. But it’s good politics, because the CDFA can tell the people of California it’s looking for an alternative to pesticide spraying. Carey and others have been saying for some time that money would be better spent comparing the genetics of the California medfly with that of flies from other countries, in an effort to pinpoint the medfly’s origin. Last summer the CDFA finally did just that.

Unfortunately, the results are so far anything but clear. One study found that flies from southern California outbreaks in 1989 and 1991 did not genetically match Hawaiian flies but were similar to flies from a number of Central and South American and African countries--thus casting doubt on the CDFA’s contention that one major medfly pathway was larvae- contaminated fruit mailed illegally from Hawaii. A second analysis comparing those southern California flies with flies trapped in northern California in 1992 found them all to be the same--thus supporting a single- population scenario--but it couldn’t rule out Hawaii as their possible source.

In the end, the medfly may prove to be too formidable a foe for everybody involved. For now, the CDFA’s Dowell still believes in his agency’s approach: At any given time, he says, we’re fighting multiple infestations, and not just the medfly. That’s our job; we’re a frontline action agency, not a research agency. When one of these critters reaches up and bites us on the butt, we don’t have the luxury of an academic to sit back and say, ‘Let’s do the research.’ We have to respond the best we can with what we know. Yet even he implies that it may be time for the CDFA to reevaluate its position. In referring to last summer’s outbreaks, Dowell says, The department’s perspective is that these are all reintroductions. But I’d like to sit down with the science advisory panel and shoot the bull about this at some point. It’s becoming harder to view these as a ‘sole source.’

Ironically, all sides agree on one thing--more genetic research is needed, to either pinpoint where the medflies are coming from or establish once and for all that a California medfly strain now exists. As for Carey, catch him on his bad days and he admits he’s tempted to drop all this to focus instead on his other research interests (see sidebar). But he remains adamant that the CDFA needs to expand its medfly thinking.

By late November 1992 so many medflies were being found in southern California that the CDFA was dropping sterile flies over a 485- square-mile swath. To Carey, the numbers speak for themselves. Right now this thing is confined to the urban areas, he says. But the only thing holding it back in the south are the mountains; nothing’s holding it back in the north except time. The state needs to change its strategy from that tinderbox mentality to develop a long-range plan to deal with insect invasion. So much is at stake here that we cannot neglect the science to save face.

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